U. S. DEPARTMENT OF COMMERCE

BUREAU OF STANDARDS

FILTERS FOR THE REPRODUCTION

OF SUNLIGHT AND DAYLIGHT AND

THE DETERMINATION OF

COLOR TEMPERATURE

By RAYMOND DAVIS and K. S. GIBSON

MISCELLANEOUS PUBLICATION, BUREAU OF STANDARDS, No. 114

U. S. DEPARTMENT OF COMMERCE

R. P. LAMONT, Secretary BUREAU OF STANDARDS

GEORGE K. BURGESS, Director

MISCELLANEOUS PUBLICATION, BUREAU OF STANDARDS, No. 114

FILTERS FOR THE REPRODUCTION

OF SUNLIGHT AND DAYLIGHT AND THE

DETERMINATION OF COLOR TEMPERATURE

By • RAYMOND DAVIS

K. S. GIBSON

January 21, 1931

UNITED STATES

GOVERNMENT PRINTING OFFICE WASHINGTON : 1931

For sale by the Superintendent of Documents, Washington, D. G Price 45 cents

M114

FILTERS FOR THE REPRODUCTION OF SUNLIGHT

AND DAYLIGHT AND THE DETERMINATION OF

COLOR TEMPERATURE 1 By Raymond Davis and K. S. Gibson

ABSTRACT

A large number of filters, as listed in the introduction of the paper, have been

developed for use in photographic sensitometry, colorimetry, and photometry.

They are reproducible from specification and may be prepared in the laboratory.

They consist of a 2-compartment cell with three borosilicate crown glass windows, the two compartments being filled, respectively, with solutions A and B, having compositions as follows: Solution A

Copper sulphate (CuS04 .5H>0) g__ c Mannite (C6 H8 (OH) 6 ) g_ _ c Pyridine (C5 H5 N) cc__ 30. Water (distilled) to make cc__ 1,000

Solution B

Copper sulphate (CuS04 .5H2 0) g__ Ci Cobalt ammonium sulphate (CoSO^NH^SO^H^O^g-. c2 Sulphuric acid (specific gravity 1.835) cc._ 10. Water (distilled) to make cc__ 1,000

The values of c, Ci, and c2 vary from one filter to another. The design of the filters has been based upon extensive spectrophotometrie measurements of the component solutions and cell, including a study of Beer's law over the ranges of concentrations used, and upon methods of computation

giving accurate color matches and relatively good spectral energy matches

between the source-and-filter combination and the energy distribution which it was sought to duplicate. Detailed studies have been made of the reliability of the data, the spectrophotometrie reproducibility of the chemicals, the permanence of the solutions, their temperature coefficients, and various other factors

of importance in the preparation and use of the filters. The paper contains 33 figures and 26 tables, giving among other things the energy distributions and fundamental spectrophotometrie and colorimetric data used in the computations, auxiliary spectrophotometrie and colorimetric information obtained during the investigation, and the trilinear coordinates of the various source-and-filter combinations and of the Planckian radiator from

1,600° to 20,000° K. computed on the basis of the adopted mean sun as the

"neutral" stimulus. The filters are further described in 38 charts, each chart

giving the chemical formula, the spectral transmission, and the light transmission

of the filter, as well as the spectral energy distribution of the source, that of the

source and filter combined, and the energy distribution which serves as the ideal in that particular case.

1 Partial reports of this investigation have been published as follows:

1. Davis and Gibson, Reproducible Liquid Filters for the Production of "White Light" (abstract),

J. Opt. Soc. Am. and Rev. Sci. Inst., 14, p. 135; February, 1927.

2. Davis and Gibson, Reproducible Liquid Filters for the Determination of the Color Temperatures of Incandescent Lamps (abstract), Phys. Rev. (2), 29, p. 916; June, 1927.

3. Davis and Gibson, Filters for the Reproduction of Sunlight and the Determination of Color Temperature (abstract), J. Opt. Soc. Am. and Rev. Sci. Inst., 16, p. 332; May, 1928.

4. Gibson and Davis, Methods for Determining the Color of Sunlight and Daylight (abstract), J. Opt.

Soc. Am. and Rev. Sci. Inst., 18, p. 442; June, 1929.

5. Davis and Gibson, Artificial Sunlight for Photographic Sensitometry, Trans. Soc. Motion-Picture

Engineers, 12, No. 33, pp. 225-236; 1928; Proc, Seventh International Congress of Photography, July,

1928, pp. 161-173.

6. A Filter for Changing the Color Temperature of 2,848° K. to Average Daylight, B. S. Tech. News

Bull., No. 138, pp. 143-144; No. 139, p. 156; 1928.

7. Filters for the Reproduction of Sunlight and the Determination of Color Temperature. (Proposed Working Standards for Sensitometry, Colorimetry, and Photometry.) Standards Yearbook, pp. 188-191

;

1928; also Standards Yearbook, pp. 138-141; 1930, 1
2 BUREAU OF STANDARDS

CONTENTS

Page

I. Introduction 3

II. Characteristics of white-light niters 7

1. Previous investigations 7

2. Desirable features 12

III. Spectral energy distributions ._ 14

1. Noon sunlight at Washington 14

(a) Abbot's data 14

(6) A standard for sensitometry 16

2. Sunlight outside the earth's atmosphere—a standard of daylight 17

3. The Planckian radiator or black body 18

(a) An equivalent of incandescent illuminants—color temperature 18

(6) Energy distributions, 2,000° to 20,000° K 20

(c) Use of high color temperatures 25

4. The acetylene flame 25

IV. Component materials of filters 26

1. Choice of materials 27

(a) Aqueous ammonia solution of copper sulphate 27

(6) Aqueous pyridine solution of copper sulphate with

mannite—solution A 27

(c) Aqueous acidified solution of copper sulphate

solution Bi 30

(d) Aqueous acidified solution of cobalt sulphate 30

(e) Aqueous acidified solution of cobalt ammonium

sulphate—solution B2 31

(/) Mixture of solutions Bi and B2—solution B 31

(g) Cell for holding filter solutions 31

(h) Definition of Davis-Gibson filters 31

2. Purity of chemicals used 32

(a) Copper sulphate 32

(6) Cobalt ammonium sulphate 34

(c) Pyridine 36

(d) Mannite 37

(e) Ammonia 38

V. Methods of computation 38

1. Spectrophotometry terminology 38

2. Standard spectrophotometric data 40

3. Beer's law correction for solution A 43

4. Design of filters of Series I—2,300°-4,000° K. to mean

noon sunlight 44

(a) To obtain spectral energy match 44

(6) To obtain color match . 46

(c) To compute light transmission of filters 53

5. Design of filters of Series 11—2,848° to 3,500°-10,000° K_ 54

6. Design of miscellaneous filters 61

(a) Acetylene to mean noon sunlight 61

(6) 2,848° K. to noon sunlight at summer and winter

solstices 63

(c) 2,848° K. to Abbot-Priest sunlight 63

(d) 2,848° K. to sunlight outside earth's atmosphere

variation with color temperature of source 64

(e) 2,450° to 3,500° K. and to 6,500° K.—variation with

color temperature of source 66

VI. Use of the filters 69

1. In photographic sensitometry 70

(a) International unit of intensity for sensitometry-- 70

(b) Comments on use of the filters 71

2. In colorimetry 72

(a) Neutral stimuli 72

(6) Methods for measuring the color of natural sunlight and daylight 72

(c) Standards for grading daylight glasses and artificial white-light illuminants 76

(d) Calibration of incandescent electric lights in terms of color temperature 76
REPRODUCIBLE WHITE LIGHT FILTERS 3

VI. Use of the filters—Continued. Page

3. In photometry—comparison of intensities of incandescent

electric lights of different color temperatures 78

4. In astronomy—determining the color temperatures of the

stars 79

VII. Spectrophotometric data .__ 80

1. Apparatus and methods 80

(a) Visual method 80

(b) Auxiliary methods 80

(c) Miscellaneous 81

2. Standard filter components 81

(a) Two-compartment glass cell with distilled water. _ 82

(6) Solution A' 82

(c) Solution B/ 83

(d) Solution B 2

' 85

(e) Solution B' 86

3. Permanence of solutions 86

4. Temperature effects 88

(a) Solution A 88

(6) Solution B 89

5. Variation of pyridine content in solution A 92

6. Beer's law 93

(a) Solution A 93

(b) Solution B 97

7. Reliability of the spectrophotometric measurements 98

(a) At the Hg and He wave lengths 98

< (6) From 350mM to 720mM _ 102

8. Spectrophotometric reproducibility of the chemicals 102

9. Auxiliary data 110

(a) Aqueous pyridine solution of cobalt sulphate 110

(6) Aqueous a-picoline solution of copper sulphate with

mannite 110

(c) Aqueous solution of nickel sulphate 112

(d) Aqueous solution of pyridine 113

(e) Aqueous solution or sulphuric acid 113

10. Use of solutions and data in spectrophotometric calibration. 114

VIII. Examination and trial of completed filters 115

1. Spectrophotometric measurements 116

2. Colorimetric computations 1 119

(a) Spectrophotometric uncertainty 122

(6) Deviation from theoretical concentrations 123

(c) Deviation from theoretical pyridine content 123

(d) Deviation from standard temperature of 25° C 124

(e) Change with time 125

(f) Change with C2 in Planck's equation 125

(g) Experimentally determined energy 126

3. Effect of pyridine content on resulting energy distribution. 127

4. Preliminary tests of the color of noon sunlight and daylight at Washington 129

5. Determination of color temperature of incandescent lights. 131

6. Test of acetylene-to-mean-sun filter 132 IX. Preparation of Davis-Gibson filters 134

1. Selection of chemicals 134

2. Preparation of A and B solutions 134

3. Preparation and filling of cells 135 X. Charts describing Davis-Gibson filters 136 XI. Bibliography 163

I. INTRODUCTION

Both in photographic sensitometry and in colorimetry the need for a practical and reproducible working standard of "white light" is well recognized, although there is as yet no universal agreement re- garding a precise definition of white light itself. It is generally conceded, however, that the spectral energy distribution of such white

light must approximate that of daylight, noon sunlight, or a Plane-
4 BUREAU OF STANDARDS

kian radiator or "black body" at some color temperature between

4,000° and 8,000° K. (For the sake of brevity in the present paper,

the authors have used the expression " white light" where it is desired not to be specific, but to refer to radiant energy having a spectral

distribution approximating these.)

In the meantime the need is being met by the use of arbitrary

working standards. In the description of a color in terms of dominant wave length, purity, and brightness, or in terms of the three

excitations, whether such description is by way of computation from

spectral distribution or by direct observation, a standard " white,"

often termed a " neutral" stimulus, is necessary. For such computations, Ives (1)

2 has proposed and used the spectral energy distri- bution of a Planckian radiator at 5,000° K. Likewise, Priest (2, 3)

has devised, both for computation and direct experiment, an artificial sunlight which has been extensively used as a " neutral" stimulus

in various investigations at the National Bureau of Standards.

Further information regarding the need and uses of standard white

light may be found in a report (4) of the Committee on Colorimetry

for 1920-21, Optical Society of America, and also in a paper by

Guild (5). It may be emphasized that, while the establishment and

general adoption of a standard "neutral" stimulus satisfactory in

all respects to all concerned is of fundamental importance, valuable

contributions have been and are being made to the science of colorimetry by the use of standards such as those noted above, which are not in universal use. It may be further noted that, while any reasonable energy distribution may be readily used in colorimetric computations as a standard white, the accurate reproduction of such dis- tribution in the laboratory may not be possible.

In photographic sensitometry the most important factor in specifying the characteristics of photographic negative materials is that

referred to as speed. This has been emphasized in a report (6, 7) of

the Committee on the Unit of Photographic Intensity, Optical Society

of America. It was there pointed out that relative speeds measured

by incandescent light in the laboratory were radically different from

the relative speeds obtained under the sunlight illumination to which

the great majority of photographic negative materials are exposed.

This is a result, of course, of the widely varying spectral sensitivities of the different emulsions. The ordinary emulsions are sensitive only

to the blue and violet (besides, of course, the ultra-violet), the orthochromatic to the violet, blue, and yellow, and the panchromatic in

varying degrees to the whole visible spectrum.

In 1925 the Sixth International Congress of Photography tentatively adopted (6, 8) two standards of energy distribution for use in

sensitometry; one a color temperature of 2,360° K. (nominally the

acetylene flame), and the other a color temperature of 5,000° K.

(representing sunlight) to be obtained by means of a specified light

filter and a 2,360° K. source as recommended by the American committee. The recommendations 3 were as follows (6, 8):

2 Numbers in parentheses, followed occasionally by page references, refer to the Bibliography at the end

of the paper.

3 The action taken by the Seventh International Congress of Photography in 1928 is considered later in Section VI, 1.
REPRODUCIBLE WHITE LIGHT FILTERS 5

1. It is recommended to use, for sensitometric work, a light source of which the

radiation is identical with that of a black body at 2,360° (absolute)

.

The intensity of this source shall be expressed in candles. This intensit}T shall be measured visually by ordinary photometric procedure in comparison with

the usual photometric standards.

2. In case it is considered necessary to use radiation representing approximate!}' the radiation of a black body at 5,000° absolute (average dayhght day)

there shall be placed before the light source provided in the preceding paragraph,

a colored filter suitably chosen.

3. It is provisionally recommended to use for this purpose a filter composed of two distinct liquids, each of a thickness of 10.0 mm. One of these liquids shall be an aqueous solution of which 100 cubic centimeters shall contain 3.00 grams

of pure crystallized copper sulphate CuS04 .5H2 and 5 cubic centimeters of a

solution of sulphuric acid containing per liter 50 grams of pure sulphuric acid reading 66° Baume. The other liquid shall consist of an aqueous solution of copper ammoniacal sulphate containing three times the quantity of ammonia

necessary to give the compound Cu(NH3 )S04 ; 100 cubic centimeters of this solution should contain a quantity of copper corresponding to 0.50 gram of pure

crystallized copper sulphate CuSt>4.5H2 0; this solution should not be prepared

before use for less than a day nor more than a month.

The cell shall be made of three sheets of white polished glass, the thickness of each sheet being not less than 2 mm nor greater than 4 mm. The cell used should

be closed. This compensating filter was hurriedly devised (at the Research

Laboratory of the Eastman Kodak Co.), as the time was very limited

and the American committee was unanimous in its opinion that daylight or sunlight quality should be used in determining the speeds of

photographic emulsions. While this filter did not give the 5,000° K.

energy distribution with the desired accuracy, it was distinctly better

for the purpose than the 2,360° K. source alone. It was suitable

for temporary use, but was not acceptable as a permanent Inter- national Standard.

The problem of devising suitable niters for converting the color

and spectral energy distribution of incandescent light sources to those

of sunlight, 5,000° K., or other possible white light, was of interest to the authors not only as members of the American committee noted

above, but also in connection with the work of those sections of

the National Bureau of Standards devoted to photographic and

colorimetric research. The investigation has developed greatly since

its inception, and the scope of the material herein presented may be

outlined as follows:

1. A series of niters has been developed by which the spectral

energy distribution of a Planckian radiator at any color temperature

from 2,300° to 4,000° K., inclusive, may be converted into a close

approximation of the spectral energy distribution of mean noon sunlight at Washington. The data cover the range from 350nux to 720nnz. Light transmissions have been computed for all the filters.

2. By designing these filters so that they all give an accurate color

match with the adopted mean sunlight, in addition to a close energy

match, they may be used to calibrate the intensity and color temperature of incandescent lamps at any color temperature above

2,300° K., provided a source is available calibrated for intensity and

color temperature at some one color temperature within this range.

The comparison of intensities of incandescent lamps at different color

temperatures may thus be made under conditions of exact color match.

3. A second series of filters has been developed for converting the

color of a Planckian radiator at 2,848° K. to that of any Planckian

radiator between 3,500° and 10,000° K., inclusive, keeping the energy

distribution as closely matched as possible.
6 BUREAU OF STANDARDS

e 4. Seven additional filters, not in the above series,rare also given,

viz: (a) A filter for converting 2,848° K. to the color of the sun

outside the earth's atmosphere, which satisfactorily represents the

color of overcast sky, this filter thus affording a reproducible daylight

standard. Bv varying the color temperature of the light source from 2,300° to 3,000° K. the filter will closely match the colors given

by daylight and noon sunlight; (6) a filter for converting 2,848° K.

to Abbot-Priest sunlight; (c) a filter for converting acetylene to mean noon sunlight at Washington; (d) and (e) two filters, one for converting 2,848° K. to noon sunlight at the summer solstice at Washington and the other for converting 2,848° K. to noon sunlight

at the winter solstice at Washington; (f) and (g) two filters designed

to cover a wide range of black-body color temperatures by varying

the color temperature of the fight source between the limits of 2,000°

and 3,100° K. With this variation one of the filters covers the blackbodv scale from 2,650° to 5,000° K., and the other from 4,000° to 19,000° K.

5. Spectral transmissive data are given on the standard component

solutions which should be of value in checking and calibrating

spectrophotometric apparatus. Various other detailed information

is given concerning these solutions which is of permanent value

independent of their use in the filters.

6. Tables of energy and visibility data are presented which are convenient for general reference, viz: (a) The relative spectral

energy of various phases of sunlight, from Abbot's data; (b) the

relative spectral energv of a Planckian radiator, from 350m/z to 720nui, at various intervals from 2,000° to 20,000° K., the values

being taken or computed from the tables and curves (9) of B. S. Miscellaneous Publication, No. 56; (c) the internationally adopted

(10) visibility data as recommended by Gibson and Tyndall (11, 12).

7. Tables are presented from which the trilinear coordinates

(r, g, b) may be obtained for a Planckian radiator at any temperature

from 1,600° to 20,000° K., on the basis of the adopted mean sunlight

as standard "white." The excitations and the method used in

these computations are given so that the r, g, and b coordinates may

be computed for any desired energy distribution.

The division of work between the photographic and colorimetry

sections of the bureau has been, in general, as follows: The former

has been responsible for the selection of the materials best suited for the purpose, for devising methods for the determination of the

correct proportions of the ingredients~ (filter design), and for the very

extensive computations necessary to the final solution of the problem; the latter has been responsible for the energy distributions, and

for the fundamental spectrophotometric data on the solutions, in- cluding the miscellaneous studies of Beer's law, temperature effect,

permanence, etc. The authors take pleasure in making the following acknowledgments

:

To Hugh K. Clark for general assistance throughout the investigation, this consisting of extensive computations and reduction of data,

preparation of graphs for publication, and preparation of chemicals

and solutions; to E, Wichers for advice regarding copper and cobalt
REPRODUCIBLE WHITE LIGHT FILTERS 7

salts; to A. Isaacs for the chemical analysis of such salts and

solutions; to Burt H. Carroll for advice, analysis, and titrations used

in the study of pyridine and picoline; to Mabel E. Brown for assistance

in spectrophotometric measurements and computations; to Irwin G.

Priest for his interest and encouragement in the development of the

filters; and to others, as noted at various places in the paper.

II. CHARACTERISTICS OF WHITE-LIGHT FILTERS

1. PREVIOUS INVESTIGATIONS

The problem of absorbing the excess energy of the longer wave

lengths in common incandescent illuminants has been a subject of

investigation for many years. In general, such research and development have had one of two objects in view.

In one case the problem has been the commercial production and

utilization of white light. "Artificial daylight" is a term used to advertise a wide variety of illuminants vanning in the color of their

light at normal operation from yellower than noon sunlight to the

blue of north skylight and produced by bluish "daylight" glass in one form or another used as a filter for the yellow light of the incandescent source. Those interested in the development of this kind of

glass should refer to papers b}T Ives (13), Luckiesh and Cady (14),

Brady (15), and Gage (16) and to the bibliographies given in the first two of these papers. Combinations of glass and dyed gelatin were

tried in some cases, but the daylight glasses finally evolved have

proved satisfactory for general illumination purposes without the

use of any additional material. A recent survey of the present

status of artificial white-light illumination may be found in a paper

by Macbeth (17). The filters described in the present paper were

not designed for general illumination but for scientific purposes.

They will, however, serve in an excellent manner as standards for

the testing and calibration of daylight glass and artificial white-light

illumination.

In the other case the problem has been the development of permanent and reproducible filters for laboratory use in photographic sensitometry, colorimetry, or photometry. Lacking more suitable filters, daylight glass has often been used for scientific purposes. It is not

sufficiently reproducible, however, for many kinds of work and the

spectral distribution which it gives often deviates more widely from

the ideal than is desirable. Various other attempts have been made

to devise filters which could be prepared in the laboratory from

specification, and which would be satisfactory as regards permanence,

convenience, and spectral distribution. The following brief resume

will illustrate what has been attempted and what is available along

this line. One of the earliest attempts, apparently, is that of Mees and

Sheppard (18, p. 292; 19, p. 314) who devised filters for converting

the energy distribution of the acetylene flame to that of daylight.

Their method for designing the filters was to take photographs on

panchromatic plates in a Tallent spectroscope, altering the composition of the filter placed in front of the acetylene burner until the
8 BUREAU OF STANDARDS

resulting energy gave, as nearly as possible, the same curve as daylight. Their final filter was 1 cm each of the following two solutions:

A. Gentian violet g__ 0. 002 Acid green g__ . 001 Mandarin orange g__ . 001 Rose bengal g__ . 004

Water to make cc__ 100

B. Copper acetate g__ 1.500 Water to make cc__ 100 This filter was apparently unsatisfactory as regards stability and

the accuracy with which it reproduced the ideal spectral distribution.

A later filter (20) is noted in which only dyes are used and which was

considered permanent. The composition of the filter is not given,

however, and the practical difficulties of preparing the filter were

said to be considerable.

The present Wratten filters (21), made of dyed gelatin—No. 78

" Tungsten to daylight (visual)," No. 78AA " Tungsten (Mazda C)

to daylight," and No. 79, "Acetylene to daylight"—are doubtless

the result of this earlier work. They are classed as " stable" or "moderately stable." Illustrations of the energy distributions obtainable with these filters are given below. (Fig. 1.) A dyed gelatin filter for converting 2,200° to 5,000° K. has been

described by Naumann (22). In its final form, superseding previously

published formulas, the filter is composed as follows

:

Grams Thionine blue 0. 18 Methylene blue . 08

Crystal violet . 12 Fast red D .03

Rapid filter red I . 04 Orange II . 14 Tartrazine . 03 These dyes are dissolved separately and the solutions mixed and

prepared for coating by addition of gelatin. The formula is designed

to cover 1 m2 of filter surface. Small scale density curves of this and the author's previous filters are given, but, otherwise, no quantitative data are supplied from which one could derive the spectral

energy distribution or color of this filter. It is said to be somewhat

bluer than daylight in spite of the fact that it was intended to dupli- cate 5,000° K. Questions of reproducibilityand permanence are not

adequately considered.

A different type of filter, developed by Pfund, is described by

Sinden (23) as a liquid-cell having two distinct compartments, one

of which contained an aqueous solution of ammoniacal copper sul- phate and the other an aqueous solution of the sulphates of copper

and cobalt. The filter was used with a lamp operated at 3,000° K.,

and an actual comparison showed that this combination gave no

appreciable color difference from noon sunlight at Baltimore, in

April. Its energy distribution compared with that of the O. S. A.

average noon sunlight is briefly illustrated. No details regarding

proportions of the ingredients were given. It would appear that this

filter was designed by trial and error procedure and the exact proportions have apparently never been published.

More recently Guild (24) has described two tentative filters devised

by his colleague Young, using materials similar to those in Pfund's

filter. One of these is for converting 3,260° to 3,000° K. and the
REPRODUCIBLE WHITE LIGHT FILTERS 9

other for converting 3,000° to 5,000° K. for use as a standard white

for colorimetry. Then compositions are given as follows:

2,360° to 3,000° K:

Solution A—Copper sulphate g__ 1. 41 Ammonia (density 0.90) cc__ 141

Distilled water to cc__ 1,000

Solution B—Copper sulphate g__ 11 Cobalt sulphate g__ 8. 5

Distilled water to cc__ 1,000

3,000° to 5,000° K:

Solution A—Copper sulphate g__ 2. 30 Ammonia (density 0.90) cc__ 230

Distilled water to cc__ 1, 000

Solution B—Copper sulphate g__ 13 Cobalt sulphate g__ 12

Distilled water to cc__ 1, 000

Solutions A and B are contained in separate compartments of a double cell, of which the walls are of optical glass having very high

and as nearly as possible nonselective transmission. The thickness

of each solution is 10.00 mm. It is emphasized that the figures

given are provisional and that some modification must be looked for in a subsequent publication, when "full particulars of the actual

energy distribution, etc., will be given." Such publication has apparently not yet appeared.4 Guild's filter is, however, now being

sold by Hilger (25) under the name of "Standard artificial daylight." In 1918 Priest (2) proposed the use of a rotatory dispersion filter for obtaining artificial noon sunlight from the gas-filled tungsten lamp

operated at 15.6 lumens per watt. He shortly after this changed the

specification of the source to a lamp at a color temperature 2,848° K.,

this being a more precise form of specification having the same intent.

The resulting energy distribution has been designated as "AbbotPriest sunlight." Since this proposal the filter has been constructed

and in service at the National Bureau of Standards for some time.

The filter consists (3) "of a crystalline quartz plate between ' crossed

nicols/ the plate being 0.500 mm thick and placed so that the path

of light is parallel to the optic axis. These parts are built in a cell which is inserted as a unit near the eye in the optical train of the

colorimeter. The effect is that of a blue filter having a spectral

transmission such that the spectral distribution of energy delivered

to the observer's eye approximates very closely to average natural noon sun at Washington." The desired reproduction was considered more precise than had previously been obtained by blue glass, dyed

gelatin filters, or solutions of selective absorbents.

Artificial white-light filters available commercially include:

1. The Wratten No. 78, No. 78AA, and No. 79, as noted above,

manufactured by the Eastman Kodak Co. These filters are composed

of dyed gelatin cemented between optical glass flats. 2. Guild's filter as sold by Hilger.

3. Daylight glass, particularly the Luckiesh "Trutint" and the

Corning "DayHte" glass. The thickness of glass necessary to obtain the nearest approximation to the desired energy distribution

4 A partial revision of the specification has, however, been made and is given in the Appendix to a paper by W. D. Wright, A Redetermination of the Trichromatic Coefficients of the Spectral Colors, TransOpt. Soc, 30, pp. 141-164; 1928-29. The auxiliary filter for rating the gas-filled lamp is unchanged, but

the color temperature obtained when it is used with a 2,360° K. source is now " believed to be very close to 2,900° K. " The "white light" filter has been changed in that the copper and cobalt sulphates in solu- tion B are now given as 15 g each instead of the 13 and 12 g above given. The color temperature obtained with this filter (with source calibrated by means of the auxiliary filter) is now considered to be " approxi mately 4,800° K."
10 BUREAU OF STANDARDS

depends, of course, upon the color temperature of the source to be

used with the glass as well as upon the spectral transmission characteristics of the glass itself. Corning "Daylite" glass has been used

by Davis in his work on photographic sensitometry (26). The particular sample used converted the color of a gas-filled lamp operated at 2,810° K. to that of noon sunlight.

The spectral distributions which may be obtained by some of the

foregoing filters are shown in the accompanying figures. Figure 1 shows the computed energy distributions of the three Wratten niters,

based on the published spectral transmission data (21) for them.

With Wratten No. 78 the energy for 2,360° K. was used, with No.

78AA that for 2,900°K., and with No. 79 the energy data of Coblentz

(27) for acetylene. Mean sunlight,5 daylight, and 5,000° K. are also included for comparison.

is ioo

O O O Wratten 78

x x x Wratten 78 A A

• • • Wratten 7 9

—

<

T-

* - € - - <

/ f \ •si 500 Q°K

c I 5

t

c 1 1 /

r^ y / /

t t

Sun Outtida t " -i r- Carth s Atmosphere > > \ t Mean Sunlit ht /

/

I 350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/z) Figure 1.

Relative energy distributions obtained with Wratten filters as indicated, compared to 5,000° K., mean sunlight, and sun outside

earth's atmosphere (which approximates average daylight) With No. 78, the energy for 2,360° K. was used; with No. 78AA, that for 2,900° E; and with No. 79, the energy distribution of acetylene was used. In Figure 2 is shown the relative energy distribution obtained with

the filter tentatively adopted by the Sixth International Congress of Photography in 1925. The data were computed using a 2,360° K.

source (black-body distribution) and a spectral transmission obtained

by the writers on a solution made up in accordance with the formula

already given using 36 cc of ammonia. Also on the graph are plotted

5,000° K. (the ideal in this case) and mean sunlight for comparison.

Figure 3 illustrates Abbot-Priest sunlight as calculated (by Priest)

for the rotatory dispersion filter. For comparison mean sunlight and

5,000° K. energy curves are included.

In Figure 4 are shown the energy distributions obtained with two

samples of Corning "Daylite" glass from different melts. The thick5 The expression "mean sunlight" (or "mean sun") refers specifically in all cases to a certain energy dis- tribution, or its resulting color, as defined in Sec. Ill, 1.
REPRODUCIBLE WHITE LIGHT FILTERS 11

nesses of the glass and the color temperatures of the source are noted.

The spectral transmission measurements were made at the bureau. oo

150

I25

O)h ioo

75

50

25

I I

-i

-i 5—

SOOO°K u

—J

( ) M M y ..;

i H < r =U ) — I

I -A —

<

5-

-4 i- s 4 - r I /

f

nlLoAC X /

/

\

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/z) Figure 2.

Relative energy distribution obtained with 2,360° K. source and filter tentatively proposed by the Sixth International Congress of Photography (shown by circles) compared with mean sunlight and

5,000° K. 50

I25

O)v IOO

c

UJ 75

5

25

m

SunlLyh

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (jt\jj) Figure 3.

Relative energy distribution of Abbot-Priest sunlight (shown

by circles) compared with mean sunlight and 5,000° K. For comparison the curves for mean sunlight and 5? 325° K. are also
12 BUREAU OF STANDARDS

2. DESIRABLE FEATURES

In designing filters for scientific and technical purposes, such as the

production of standards of light quality, the following properties

would be desirable: (1) Accurate reproducibility from specification,

(2) spectral transmissions which are unaffected by time and temperature, (3) spectral transmissions such that the energy distribution

obtained is (a) exactly that sought and (b) an exact color match

therewith regardless of whether or not (a) is fully satisfied, (4) the

highest transmission of light consistent with (3), and (5) other physical

properties (size, shape, durability, etc) which facilitate use of the

filters. It is not to be expected that filters can be constructed which will embody all these properties. No known absorbing materials exist, for example, by means of which it would be possible to get (3 (a)) a

Si ioo

Carntng Day/c^t Glass

) -

O o

ei

el .5

i

c 1 -

i - H

1 " >—

f

;

• t

•A

1 —

—C 1y

V7 t

V

u

/

M an 5 / / \

- -

350 400 450 500 550 600 650 700 750

Wave Length millimicrons (m/u) Figure 4.

Relative energy distribution obtained with Corning daylight

glass as indicated compared with mean sunlight and 5,325° K.

The thicknesses of glass and color temperatures of source were: Melt of 1918, 3.59 mm and

2,900° K.; melt of 1925, 4.75 mm and 2,810° K. perfect match in energy distribution. Careful consideration must be

paid to the special purposes of a given filter before the sort of compromise which is most advantageous can be determined. A compromise

which would ordinarily be advantageous is to adjust the unavoidable

deviations in energy distribution so as to obtain (3 (&)) an accurate

color match and, at the same time, (4) a reasonably high transmission

of light. Likewise, the lack of an absorbing medium, strictly permanent and completely independent of temperature, is not practically

important if a reasonably permanent medium is available having

known temperature coefficients of spectral transmission, because such

variations in spectral transmission as may occur can be taken into account. It should be obvious that the minimum requirement (in

the past often unfulfilled) in this respect is that the spectral trans- mission of the filter be accurately known, including the effects of as;e and temperature.
REPRODUCIBLE WHITE LIGHT FILTERS 13

From the standpoint of convenience and permanence probably

nothing is superior to the various daylight glasses. However, the

resulting energy distributions depart, in general, to an undesirable

extent from the ideal sought, and, as already illustrated in Figure 4,

glass from different formulas or different melts from the same formula

vary considerably in this respect. Even the best samples show excess transmission of energy in the yellow-green and usually also in the red.

There seem to be no data available regarding changes in transmission

with temperature, although cobalt blue glasses do not have very large

temperature coefficients over most of the visible spectrum. As

ordinarily used with incandescent light, the transmission is reasonably

high (10 to 20 per cent). While satisfactory in many respects then,

the principal objection to daylight glass is that it can not be accurately

reproduced from specification. This outstanding difficulty, together

with its spectral irregularities in the yellow-green and red, would seem

sufficient to prevent the general adoption of any of the known daylight glasses as working standard filters for the production of white

light.

Dye solutions and dyed gelatin films likewise can not be considered

reproducible from specification, the dyes themselves being, in general,

not accurately reproducible in their spectral characteristics. Furthermore, the absorbancy of dyes (coloring strength) is, in general, so great that it is difficult to make weighings with the desired precision.

When spectrophotometric data have been given to an extent sufficient

for reliable computation, the resulting energy distributions have been

hardly as good as those obtained with daylight glass. A certain

stability is claimed for the Wratten filters, but the effects of temperature variation are not given. The use of dyes can not, in general, be

recommended, inasmuch as in solution they are not as satisfactory as solutions of inorganic salts (see below) and in dyed gelatin films are not as satisfactory as daylight glass.

The quartz-nicol rotatory dispersion filter described in the previous

section is reproducible from specification (although the spectral

selectivity of the nicol prisms has been ignored in the computation)

and its spectral transmission is accurately known (subject to the same limitation). Its reproduction of sunlight is very satisfactory

from a colorimetric standpoint, but its relatively high transmission

in the violet and ultra-violet renders it less suitable for photographic

purposes. Its light transmission is very low (about 1% per cent),

and the small dimensions of nicol prisms limit its usefulness practically

to the eyepiece of optical instruments. Its temperature coefficient

is not given.

Solutions may not at first be considered very convenient to use, but, on the other hand, liquid filters are more readily prepared in the laboratory than any other type. Once prepared and inclosed in a suitable

glass cell, they are not inconvenient to handle. The extent to which

the desired energy distributions are attained with our liquid niters

is brought out in the present paper, together with data regarding

reproducibility, permanence, change with temperature, etc. The

light transmissions compare very favorably with those of other filters. It is apparent from the brief discussion given in this and the previous

sections, considering the variety of materials and methods which have

been used and the degrees of approximation to the desired energy

distributions which it has been necessary to accept, that the design
14 BUREAU OF STANDARDS

of satisfactory white-light filters is not a simple undertaking. The

reason is not hard to find. No single absorbing material has been

found which has the desired spectral characteristics and the number

of those which even crudely approximate the desired selectivity is apparently limited.

In general, the first approximation has been secured by the use of some blue coloring material; and additional materials have then

been added, to the extent considered necessary, to correct for spectral

deficiencies of the principal material. Considering the type of change

that occurs in the spectral transmission as the concentration or thick- ness is varied (see Sec. V, 1), it is obvious that the difficulty has been

not merely in finding suitable materials but in adjusting the concentrations of these materials so as to secure the most suitable trans- mission and resulting energy match.

In the present investigation the problem has been not only that of selecting materials and adjusting them so as to secure the best spectral

energy match, but also that of securing exact color matches. This

has been done, furthermore, not merely with one filter but with a

large number of them, as already outlined in Section I. It is hoped

that this work will satisfy the demand felt in many lines of research

for reproducible white-light filters which have satisfactory spectral

characteristics and which may be readily prepared and conveniently

used in the laboratory.

III. SPECTRAL ENERGY DISTRIBUTIONS

Before describing the materials and methods of computation which

have been used in the development of the present filters, it is neces- sary to define the various energy distributions which enter into these

computations. These include sunlight and daylight energies, various

black-body distributions, and that of the acetylene flame.

1. NOON SUNLIGHT AT WASHINGTON

Sunlight distribution is of importance to photographic sensitometry,

not because it may approximately represent a colorimetric or subjective white, but because it represents, perhaps better than any other

single energy distribution, the average conditions 6 under which the

great majority of photographic negative materials are exposed.

(a) ABBOT'S DATA

Extensive data are available on the spectral energy distribution

of noon sunlight at Washington. Such data representative of average

atmospheric conditions at the two extremes, the summer and winter

solstices, were furnished the National Bureau of Standards by Doctor

Abbot, in a letter dated November 7, 1917. These data have served

as a basis in the development of Abbot-Priest sunlight as already

described. The two sets of data—average noon sunlight at Washington on June 21 and on December 21—are given in Table 1 and illus6 Varying from the incandescent sources often used in indoor motion-picture photography through sunlight to daylight or blue sky,
REPRODUCIBLE WHITE LIGHT FILTERS 15

trated in Figure 5. The difference between the two sets is of the same

order of magnitude as the variation at noon due to changes in atmospheric transmission at a given time of year, and the data do not, of

course, at all represent early morning and late afternoon sunlight,

which is relatively yellow.

An arithmetical mean of these two sets of noon sunlight energy

data, given in Table 1 and illustrated in Figure 5 and elsewhere, was

150 i iI I

I I I I

i

I I I I M I I n i i n i i i i i i i i i i i i i i i i n

125

t 100

c 75

50

2 5

3hl

Sunlight December 21

q ht f""Of

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/x) Figure 5.

Relative energy distribution of noon sunlight at Washington as given by Abbot's data of 1917 (Table 1 of the present -payer) chosen by the writers as the ideal energy distribution, the duplication

of which has been attempted in the first series of filters described

herein. It is hereinafter often referred to simply as "mean sun" or "mean sunlight"; and whenever these expressions are used in this

paper this particular energy distribution (or color) is meant. The

average noon sunlight given in the report (4, p. 563) of the Colorimetry Committee of the Optical Society of America is essentially

the same as herein adopted, except below 420nu*, where an error has

apparently entered in the average data used in that report. The data

as used by the writers extend from 3601^ to 720ntyt.

13416°—31 2
16 BUREAU OF STANDARDS

Table 1.—Sunlight energy distribution

Relative energy distribution of sunlight

. ..... Wave length in millimicrons Outside

earth's at- mosphere ^

At Washington, D. C,

at noon, average at- mospheric trans- mission Mean sun 2 ^ n *n 2 r\n t ci H tx atmosphere

reduced to

inn nn nr o60m^x June 21 1 Dec 21 1 Arithmetical mean of data for June 21 and

Dec. 21

Reduced

to 100.00 at 560m/*

59.

1

24.0 9.4 16.70 15.97 55. 34

64.

5

30.0 12.8 21.40 20.47 60. 39 67.6 35.0 17.4 26.20 25.06 63.30

390.. 69.7 99 ^ 31 45 Qn no 65.26

79.

1

56.0 38.5 47.25 45. 19 74. 06

410 . ... 93.0 69.3 50.6 59. 95 57.34 87. 08

420 - 101.

5

78.4 59.3 68. 85 65.85 95. 04

430 102.8 81.5 63.2 72.35 69.20 96. 25 440 108.9 SQ 7 79 Q 81. 30 77 7fi 4 4 . 40 101.97 117.0 99.1 82.5 90.80 86.85 109. 55 460.. . 120. 103.9 89.0 96.45 92.25 1 1 2. 36

121.

2

107.7 95.0 101. 35 96.94 113. 48 120.6 109.2 97.9 103.55 99.04 112. 92 490 119.4 iuy. o inn ^ 105 15 inn t;7 111.80

117.

7

110.2 102.6 106.40 101. 77 110. 21 116.0 109.2 102.3 105. 75 101. 15 108. 61 520 _ . . . 114.

3

108.7 102.9 105. 80 101.20 107. 02

112.

6

107.8 103.5 105. 65 101.05 105. 43 540 110.8 1U/ . o 103 7 105 50 100 91 103. 75

109.

1

107.3 105.4 106. 35 101. 72 102. 15 560 . 106.8 105.4 103.7 104.55 100.00 100. 00 570 - ... 104.6 103.4 102.3 102.85 98. 37 97. 94 580 . _ 102. 3 101.8 101.4 101. 60 97. 18 95. 79 590 100.0 inn n 100. u

inn n inn 1W. nnou yo. oo 93.63 97.7 98.8 100.2 99.50 95. 17 91. 48 610 . 95.2 97.5 99.6 98. 55 94. 26 89. 14

92. 4 95.7 99.1 97.40 93. 16 86. 52 630 . . . 89. 7 94.0 98.7 96. 35 92. 16 83. 99 640 86.8 92.1 98.2 95.15 91.01 81.27 650 83.9 90.2 97.3 93.75 89.67 78.56 660 81 2 88.5 96.6 92. 55 88.52 76 03 670 78! 4 85.9 94.7 90.30 86. 37 73. 41 680 75.8 83.6 93.5 88.55 84.70 70. 97 690 73.2 81.5 91.4 86.45 82. 69 68.54 700 70.5 79.1 89.3 84.20 80.54 66. 01 710 68.1 76.5 86.7 81.60 78. 05 63. 76 720. 65.8 74.3 84.8 79.55 76.09 61.61 730 63.6 72.0 82.6 77.30 73,94 59. 55 740 61.3 70.0 80.5 75.25 71.98 57.40

750 59.4 67.9 78.5 73.20 70. 01 55. 62

1 These values were submitted to the National Bureau of Standards by Dr. C. G. Abbot in letter of Nov. 7, 1917.

2 These values were computed from Abbot's data by the authors for the purposes of this investigation.

(b) A STANDARD FOR SENSITOMETRY

As previously noted, the American Committee on the Unit of Photographic Intensity originally recommended, and the Sixth

International Congress of Photography tentatively adopted in 1925,

5,000° K. as the primary standard of energy distribution for use in photographic sensitometry. The color of 5,000° K. is not greatly dif- ferent from that of average noon sunlight—it was, in fact, chosen for

this reason. In addition, it serves as a precise definition and is a good round number.
REPRODUCIBLE WHITE LIGHT FILTERS 17

As a standard for use in photographic sensitometry, however, the

authors believe their adopted mean sun is much preferable to 5,000° K.

The reasons are as follows

:

1. In photographic sensitometry it is not the color that is primarily important but the energy distribution, particularly in the

violet and near ultra-violet where all photographic materials are highly sensitive. In this important region the relative energy of

5,000° K. is not representative of sunlight, as is shown in Figure 6.

2. The energy distribution of mean sun can be duplicated much

more closely by means of incandescent lights and filters than can

that of 5,000° K. This is well illustrated by a comparison of charts

8 and 24 showing the energy distribution curves obtained with a 2,848° K. source and the respective filters for obtaining mean sun and

5,000° K. Not only are the deviations in the latter case much larger,

'150 1 i i i i i

i i

i i i i i i i i i i i i i i i i i i n i i i i i i i

550 600 650 700 750

Wave Length millimicrons (m/u.) Figure 6.

Relative energy distributions of 5,000° K. and mean sunlight

This shows that 5,000° K. is not a satisfactory representation of sunlight in the photographically important region below 420nnt. but the transmission is lower: and this difficulty of reproducing

5,000° K. from any practicable incandescent lamp color temperature,

considering both the spectral energy match and the light transmission,

is inherent in the differences in the energy distributions, not in the

filters employed.

It seems undesirable to choose any ideal standard of energy distri- bution for use in photographic sensitometry which is less representative of natural sunlight conditions than the adopted mean sun, and which is at the same time less satisfactorily reproducible in the

laboratory.

2. SUNLIGHT OUTSIDE THE EARTH'S ATMOSPHERE—A STANDARD

OF DAYLIGHT

Certain problems may arise in colorimetry where a daylight filter may be preferred to a sunlight filter. However, the spectral energy

distribution of daylight has apparently never been measured as such;
18 BUREAU OF STANDARDS

indeed, there may be question as to the proper definition of daylight,

which may show considerable variation in color, depending upon

atmospheric conditions, time of day, and other factors. Priest has previously shown (28; 43, p. 483) that the color temperature (6,000° to 7,000° K.) of the overcast sky is approximately

the same as the color temperature of the sun as it would appear outside the earth's atmosphere. Priest's later data (28) also show that,

for clear and partly cloudy skies, the color temperature of the combined light of sun and sky received on a horizontal surface exposed

to the whole sky lies generally within this same range, 6,000° to 7,000° K., for a wide range of solar altitudes. Tavlor's data ("The

Color of Daylight," Tab. I, 23d Ann. Con. I. E: S.; 1929) are in

substantial accord with Priest's. Kimball (29, p. 514) states (on

the basis of Priest's color-match data and other data on spectral

distribution): " Therefore, for the total daylight with either clear,

cloudy, or partly cloudy skies the energy distribution corresponds

closely to the distribution for black-bodv radiation at a temperature

from 5,500° to 6,500° K."

As will be shown later in this paper, we have found by actual trial that the color of the overcast sky may be nearly matched by the color

of the sun outside the earth's atmosphere. A filter was designed

(Sec. V, 6) to convert 2,848° K. to sunlight outside the atmosphere as

specified by Abbot's data of 1917, given in Table 1. At certain times on four different days near the autumnal equinox of 1928 and once

during the tests in December, 1928, a perfect color match was observed

between this source-and-filter combination and overcast south sky,

although on the average the two colors were not identical. Details

of these measurements are given in Section VIII, 4.

3. THE PLANCKIAN RADIATOR OR BLACK BODY

(a) AN EQUIVALENT OF INCANDESCENT ILLUMINANTS—COLOR TEMPERATURE

The determination of spectral energy distribution by direct radiometric methods is a matter of considerable experimental difficulty,

requiring elaborate apparatus and expert knowledge and technique.

The problem is considerably simplified provided one has available a standardized source of accurately known spectral energy distribution;

for in this case it is necessaiy merely to make comparison, wave length

by wave length, visually or otherwise, of the unknown source with

the standard, and various instrumental calibrations necessary in

direct radiometric measurements are eliminated. At best, however, it

is a laborious undertaking to make spectral energy determinations by

spectrophotometric or spectroradiometric methods.

However, a substitute method is available, involving much less apparatus, time, and skill, in which the spectral energy distribution

is derived by determining what is known as the color temperature of the source. The relative energy distribution from a complete

radiator or black body is a function only of the temperature of the

radiator. The theoretical equation derived by Planck expressing

this relation is of the form

;
REPRODUCIBLE WHITE LIGHT FILTERS 19

c2 where

.Ex is the energy per unit wave length at wave length X,

is the absolute temperature in ° K. (° C. + 273),

e is the base of natural logarithms, and

Ci and C2 are constants.

It has been found by experiment that the relative energy distribution from many common incandescent light sources in the visible

spectrum is practically the same as that from a Planckian radiator,

provided the temperature of the latter is taken to give an accurate

color match with the source in question. The color temperature of a

light source is defined (30, p. 1180) as "the temperature at which

a Planckian radiator would emit radiant energy competent to evoke a

color of the same quality as that evoked by the radiant energy from

the source in question." A light source accurately allocated on the

color temperature scale—that is, having a perfect color match with the

Planckian radiator at some temperature—may or may not have an

exact relative spectral energy match with this ideal radiator.

The practical advantage of the color-temperature method as a

substitute for spectral energy determinations lies in the fact, noted

above, that many common illummants, particularly incandescent

electric lamps, not only give a good color match with the Planckian

radiator but a very close approximation, at least in the visible

spectrum, to the relative energy distribution as well. This approximation is so good that a specification of color temperature serves in

practically all cases as an adequate specification of the relative

spectral energy values.

Of course, there are certain sources, such as the mercury arc, which can not be described in terms of color temperature, and others,

such as the Welsbach mantle, which can be placed only crudely on

the color temperature scale, not being a match with any black-body

color. However, practically all sources whose light is produced

solely by temperature may be quite accurately described in terms of color temperature. Passing over the varying red shades of the fur- nace, we may note as reference points the approximate color temperatures of various illuminants—the candle and kerosene flames at 1,900° K., the carbon lamp at 2,100° K., the acetylene flame 7 and vacuum tungsten lamp at 2,400° K., and the gas-filled tungsten

lamps from 2,600° to 3,100° K. In the case of flames, the values

vary for different portions of the flame and for different conditions of pressure, humidity, etc. With the incandescent electric lamps, the

values differ with the applied voltage and with the type of lamp,

the effective portion of the filament, etc. The melting point of tungsten is near 3,600° K., which is approximately the color temperature

of the low power carbon arc. For more complete discussions of color temperature the reader is referred to papers from the Nela

Research Laboratory (31) and from the National Bureau of Standards (30), and to references contained therein. An illustration

7 See Sec. Ill, 4 for discussion of the acetylene flame.
20 BUREAU OF STANDARDS

(fig. 7) from one of the bureau papers (Sci. Pap. No. 443, fig. 1, p. 225) will serve to illustrate the sort of deviations which may be

expected in the case of incandescent electric lamps between experimental determinations of spectral energy distribution and values

computed from Planck's equation.

Such actual experimental determinations of spectral energy

distribution would be all too meager for the needs of the present

paper. Furthermore, they are subject to experimental uncertainty,

especially at the shorter wave lengths where the energy values are

relatively small. It has, therefore, been not only preferable, but necessary, to use the theoretical values given by Planck's equation in

2.0

1.9

1.8

1.7 1.6 $1.5

£1.4

U

l.3

Id>l.2

5u

IS 1.0 0.9 0.8

0.7

0.6 0.5

0.4

0.3

0.2

0.1 0.0

400 20 40 60 500 600 20 40 60 60 700 20 40 60

820*r Vs

-

' 29SC •k

//

y f

/MM

r/1 /

/ /

SPECTRAL

DISTRIBUTION

of ENERGY

o = o B.S.Lamp 1717 al 118.0 volts Planckian Radiator

for Ct* 14350 micron degrees

;

2 85 w

20 r

40 20 40 60 80 5

20 40 60 60 620 40 60 80 720 40 60 WAVE LENGTH millimicrons Figure 7.

Relative energy distribution of standard lamp, B. S. 1717, at 118.0 volts, compared with Planckian radiator at 2,820°

and 2,850° K.

This illustration is reproduced from B. S. Sci. paper No. 443, by I. G. Priest. the computation of the filters to be described, with the exception of the acetylene-to-sunlight filter.

(b) ENERGY DISTRIBUTIONS, 2,000° TO 20,000° K. Relative spectral energy data computed from Planck's (or Wien's)

equation have been published by various investigators. The most

extensive of these data 8 comprise Bureau of Standards Miscellaneous

8 See also a paper by Skogland entitled, "Tables of Spectral Energy Distribution and Luminosity fcr Use in Computing Light Transmissions and Relative Brightnesses from Spectrophotometric Data.

-

' B. S. Misc. Pub. No. 86; 1929. These tables cover the range from 2,000° to 3,120° K., at 20° K. intervals

for the energy distributions and at 80° K. intervals for the luminosity data, differences for interpolation being also given. These tables are based on 14,330^° as the value of d.
REPRODUCIBLE WHITE LIGHT FILTERS 21

Publication No. 56 (hereinafter designated as M56), by Frehafer

and Snow, entitled " Tables and Graphs for Facilitating the Computation of Spectral Energy Distribution by Planck's Formula." By

means of these tables and graphs, Planckian energy distributions over

a wave length range from 400m^ to 720mu may be readily obtained for

all color temperatures from 1,000° to 28,000° K. A table is also given by means of which with further computation values may be

obtained at wave lengths beyond this range. The value of C2 used

in these computations was 14,350^°. The graphs are so arranged,

however, that values may readily be obtained for any reasonable

value of C2 . The accuracy claimed for these computations was in

general 0.15 per cent or better for the tables and 0.33 per cent or

better for the graphs.

Table 2.

Spectral energy distribution of a Planckian radiator or black body at various temperatures

Wave length in Relative energy distribution i at- millimicrons

2,000° K. 2,100° K. 2,200° K. 2,300° K. 2,360° K. 2,400° K. 2,450° K. 2,500° K.

350 0. 48 0. 69 0. 96 1. 31 1. 55 1. 73 1. 97 2.24 360 '. 74 1.04 1.41 1. 86 2. 19 2. 42 2. 73 3l 06 370 1 10 1. 51 2 00 2 60 3! 01 3. 31 3* 69 4! 11 380 L61 2*. 15 2. 79 3! 54 4! 06 4! 42 4! 90 5! 41

390. 2. 29 2.99 3.81 4.74 5.38 5. 81 6.40 7.00 400 3 19 4. 09 5 10 6 24 6. 99 7. 51 8. 18 8. 89

410 i. 37 5! 45 6! 71 8! 08 8! 94 9! 55 10! 34 ll! 15 420 5.88 7. 20 8.68 10. 26 11.29 12. 00 12. 91 13.83 430 7.78 9. 36 11.08 12.92 14.06 14. 85 15. 87 16.90

440 10.14 11.98 13.94 16.00 17.28 18. 16 19.26 20.38

450 13.01 15.12 17.32 19. 60 21.00 21.95 23. 13 24. 33

460 16.49 18.84 21.26 23. 75 25. 22 26. 25 27.50 28. 76 470 20. 63 23.22 25.82 28. 46 30. 03 31.07 32. 40 33. 71 480 25. 52 28. 31 31.03 33. 79 35. 37 36.45 37.80 39.12

490 31.24 34.11 36.92 39.67 41.32 42.39 43.77 45. 08

500 37. 85 40.76 43. 55 46.31 47.88 48.95 50. 20 51.45

510 45. 40 48. 25 50.95 53. 60 55. 08 56.03 57. 25 58.45

520 54.03 56.70 59.13 61.50 62.84 63.67 64.80 65. 80

530 63. 70 65.90 68.06 70.00 71.18 71.92 72.75 73.60

540 74.49 76. 35 77.94 79.35 80.23 80.72 81.35 82.00

550 86. 57 87. 60 88. 52 89. 30 89.74 90.04 90.45 90.75

560 100. 00 100. 00 100.00 100.00 100. 00 100.00 100. 00 100.00

570 114.5 113.3 112.3 111.3 110.68 110.4 109.9 109.5 580 130.5 127.8 125.4 123.2 122. 06 121.2 120.3 119.5

590 147.7 143.2 139.3 135.8 133.78 132.6 131.1 129.7

600 166.3 159.7 153.9 148.8 146. 07 144.2 142.1 140.2

610 186.3 177.2 169.5 162.5 158. 77 156.4 153.6 151.0

620 207.7 195.8 185.5 176.6 171.96 168.9 165.4 162.1 630 230.2 215.0 202.4 191.3 185. 35 181.7 177.3 173.2

640 254.3 235.5 220.0 206.4 199. 24 194.7 189.5 184.6

650 279.4 256.8 238.2 221.9 213. 36 208.1 201.9 196.2

660

670 306.0 279.5 256.9 237.7 227.83 221.8 214.6 207.9

334.0 302.0 276.0 253.9 242. 64 235.5 227.4 219.7

680 362.9 325.5 295.7 270.2 257. 36 249.3 239.7 231.2 690 393.1 350.0 316.2 287.1 272. 47 263.1 252.3 242.7

700 424.7 376.5 336.8 304.5 287. 56 277.2 265.2 254.4

710 456.8 402.0 357. 6 321.0 302.64 291.1 277.9 266.2 720 490.1 428.0 378.6 338.0 317. 87 305.2 290.7 277.4

(See footnote at end of table.)
22 BUREAU OF STANDARDS

Table 2.

Spectral energy distribution of a Planckian radiator or black body at various temperatures—Continued

Relative energy distribution 1 at—

millimicrons

2 600° K. 2 700° K. 2 800° K. 2 848° K. 2 900° K. q nnn° tr 3 100° K

350 2.84 3. 52 4. 32 4.75 5.23 6.22 7.36 8.58

360. 3. 81 4.67 5.64 6. 16 6.71 7. 92 9.22 10. 64

370.. 5.03 6.07 7. 22 7. 82 8.50 9.88 11.39 13.00

6. 52 7. 76 9. 11 9. 79 10. 58 12. 15 13.86 15. 66 390 8.33 9.74 11.30 12.09 12.96 14.74 16.65 18.60 400 10.44 12. 10 13. 82 14. 71 15.68 17.66 19.74 21.87 410 12.91 14. 76 16. 71 17. 69 18. 75 20. 87 23. 10 25.40 420 15.78 17. 82 19.94 21.00 22. 15 24.44 26. 81 29.20 430 - . . 19. 03 21. 24 23. 54 24.68 25. 91 28. 33 30.80 33.30 440 22.71 25.09 27. 52 28.73 30. 01 32. 52 35.05 37.60

450 26.83 29. 33 31.86 33. 11 34. 43 36.99 39.56 42. 17 460 31.36 33. 97 36. 55 37. 81 39. 16 41.78 44.35 46.89 470 36. 38 38. 99 41.60 42.87 44.24 46. 81 49.35 51.80 480 . . 41.80 44. 45 47. 03 48. 27 49. 59 52. 08 54.50 56.85 490 47.69 50.25 52. 71 53.92 55. 15 57.54 59.85 62. 15 500 53.99 56.50 58. 78 59.90 61. 10 63.22 65. 35 67.40

510 60. 75 63.00 65. 08 66. 08 67. 15 71.00 72. 80 520 67. 88 69. 85 71.64 72. 51 73. 40 75! 08 76.70 78.20 530 75. 37 76. 90 78. 45 79. 16 79. 80 81. 23 82. 45 83. 70 540 83. 26 84. 35 85. 45 85. 97 86. 45 87.43 88.30 89. 15 550 91.49 92. 10 92.68 92. 97 93.25 93. 71 94. 15 94.60

560 - 100. 00 100.00 100.00 100.00 100.00 100.00 100. 00 100.00 570 108.8 108.1 107.4 107. 20 106.8 106.3 105.7 105.3 580 . .-. 117.

9

116.

4

115.

1

114. 52 113. 8 112. 7 111. 6 110.

6

590 127.1 124.8 122.7 121.80 120.7 119.0 117.3 115.8 600 136.6 133.3 130.4 129.07 127.7 125.3 123.0 120.9 610 146.3 141.9 138.1 136. 42 134.6 131.5 128.6 125.8 620 156.1 150.6 145.8 143.64 141.4 137.5 133.8 130.6 630 165.9 159.3 153.5 150.88 148.2 143.4 139.0 135.1 640 — -- 175.

9

168. 161.1 158. 06 154.9 149.3 144. 3 139.

8

650 185 9 176 6 168.6 165.08 161.4 155.0 144 660 195^7 185^3 176.1 172. 12 167.8 160.5 153.9 148! 1 670 205.8 193. 6 183.2 178. 87 174.0 166.0 158. 7 152.

1

680 . 215.6 202.2 190.5 185. 52 180.2 171.2 163.2 155.9 690 225.6 210.5 197.5 192. 13 186.1 176.3 167.4 159.4 700 235.3 218.6 204.4 198. 32 192.1 181.2 171.5 162.8 710 244.9 226.6 211.

3

204. 49 197.6 185.8 175.3 166.0 720 254.4 234.5 217.7 210. 57 203.0 190.1 178.8 168.9

Relative energy distribution 1 at— Wave length in millimicrons

3 250° K. o,ouu iv. 3,400° K. 3,500° K. 3,600° K. 3,700° K. 3 750° K. 3 800° K.

350 9.26 9. 93 11. 40 12. 97 14.64 16. 43 17. 36 18. 32

360 11. 40 12 18 13.83 15. 58 17.44 19. 41 20. 42 21.45 370 13. 86 14. 72 16. 55 18. 49 20. 52 22. 63 23. 73 24.87 380 16. 60 17. 55 19. 56 21.66 23. 82 26. 13 27. 28 28.42 390 19. 63 20. 66 22.83 25.06 27.42 29. 77 31. 00 32. 23 400 22. 97 24. 06 26. 36 28.73 31. 17 33. 65 34. 90 36. 17 410 2Q. 57 27. 73 30. 14 32. 60 35. 10 37. 67 38. 98 40.29 420 30.42 31.65 34. 15 36. 67 39.23 41.83 43. 14 44. 45 430 34. 56 35. 80 38. 32 40.90 43. 50 46. 08 47. 41 48. 72 440 38.88 40.19 42. 73 45. 30 47. 84 50.35 51.72 53.00 450 43. 43 44.73 47.28 49.79 52. 30 54.85 56. 08 57.40 460 48. 15 49. 37 51.80 54. 38 56. 80 59.20 60. 45 61.60 470 53. 03 54. 25 56. 65 59.06 61.30 63. 65 64.80 65. 85 480 58. 07 59.20 61. 45 63.70 65. 85 68. 05 69. 14 70.15 490 63. 21 64. 30 66. 40 68. 47 70. 45 72.40 73.34 74.30 500 68. 41 69. 35 71.25 73. 14 74. 95 76.70 77. 57 78.40 510 73. 67 74. 50 76. 15 77.82 79. 30 80.85 81.63 82. 30 520 79. 01 79. 65 81.05 82. 39 83. 65 84. 90 85. 56 86. 15 530 84.29 84.80 85. 90 86.94 87. 95 88. 90 89.40 89.85 540 89. 56, 89.90 90. 70 91.36 92.10 92. 70 93. 10 93.35

(See footnote at end of table.)
REPRODUCIBLE WHITE LIGHT FILTERS 23

Table 2.

Spectral energy distribution of a Planckian radiator or black body at various temperatures—Continued

Wave length in millimicrons

Relative energy distribution 1 at

3,250° K. 300° X. 3,400° K. 3,500° K. 3,600° K. ,700° K. 3,750° K.

550

5 GO 570 580

500

600 610

620

630

640

650 660

670

6S0

690 700 710 720

94. 84

100. 00

105.1 110.2

115.^: 119.9

124.6

129.0 133.3 137.6 141.6

145.4

149.1 152.6 155.8

158. 8

161.8

164.4

95.00

100. 00

104.8

109.7 114.4

118.9

123.4

127.5 131.5 135.6

139.3 142.8

146.2 149.4

152.3

155. 157.7 160.0

95. 35

100. 00

104.4 108.8

113.0

117.1 121.0

124.6

128.2

131.8 135.0

137.9

140.8 143.4

145.9 148.2

150.2 152.2

95. 69

100! 00

104.0 108.0 111.8

115.4

118.9

122.

1

125.2 128.2

130.9

133.5 135.9

138.

1

140.1 142.0

143.6

145.1

96. 10

100. 00

103.6 107.2

110.6 113.8

116.9

119. 7 122.4 125.0

127.3

129.5 131.4

133. 2 134.8

136.3 137.6

138.8

96. 40

100. 00 103.2

106.5

109.5 112.4

115.1 117.5 119.9

122.1 124.0

125.7

127.3 128.8

130.1 131.2

132.2 133.0

96. 55

100. 00

103.

1

106.2 109.0

111.7

114.3

116.5

118.7 120.7 122.4

124.0

125.4

126.7 127.9 128.8

129.7

130.4

Wave length in millimicrons

Relative energy distribution 1 at900° E. 4,000° K. 4,500° K. 5,000° K. 5,500° K. 6,000° K. 6,500° K. 7,000° K.

20.29 22. 42 34. 32 48. 17 63.48 79. 83 96.73 113.9

23. 64 25.84 38. 37 52. 55 67. 85 83. 86 100.1 116.

5

27. 14 29. 56 42. 50 56. 84 71.95 87.51 103. 1 118.5

30. 88 33.41 46. 69 61.02 75. 85 90. 82 105.7 120.

1

34. 76 37. 36 50. 86 65. 04 79. 43 93. 74 107.7 121.2

38. 72 41. 39 54. 95 68. 91 82. 72 96.37 109.4 121.9

42. 90 45. 54 58. 97 72. 54 85. 73 98. 58 110.6 122.2

47. 05 49. 70 62.91 76. 00 88. 50 100.5 111.5 122.

1

51.30 53. 92 66. 76 79.23 90. 90 102.0 112.2 121.8

55. 55 58. 13 70.48 82. 17 93. 05 103.3 112.5 121.2

59. 85 62. 26 74. 00 84. 90 94. 93 104. 1 112.4 120.1

63. 90 66.31 77. 35 87. 42 96.51 104.8 112.2 119.0

68. 10 70.32 80.47 89.71 97. 86 105. 1 111.7 117.6

72. 20 74. 20 83. 42 91.72 98. 87 105. 5 111.0 116.1 76.15 77.93 86. 22 93. 49 99. 70 105. 3 110.0 114.3

80. 05 81.63 88.78 95.02 100.4 105.0 109.0 112.6

83. 70 85.11 91. 13 96.41 100.7 104.5 107.7 110.6

87. 25 88. 42 93. 36 97. 50 100.9 103.9 106.3 108.7

90. 70 91.55 95. 25 98.41 100.9 103. 1 104.8 106.5

93. 90 94. 50 97. 06 99. 07 100.7 102.2 103.4 104.5 97.00 97. 33 98. 58 99. 65 100.4 101.

1

101.7 102.3

100. 00 100. 00 100. 00 100. 00 100. 00 100. 00 100. 00 100. 00

102.6 102.4 101.1 100. 2 99. 33 98. 77 98. 20 97. 75 105.2 104.7 102.2 100.3 98. 73 97.51 96.41 95. 60 107.6 106.8 103.0 100.2 97.89 96.03 94. 51 93. 28 109.8 108.7 103.6 100.1 96. 90 94. 62 92. 58 91.05

111.8 110.4 104.1 99. 57 95. 97 93. 09 90. 75 88. 84

113.6 111.9 104.6 99. 13 94. 89 91.63 88. 80 86. 73

115.3 113.2 104.7 98. 50 93. 68 90.02 86. 94 84. 50

116.8 114.5 104.8 97. 92 92.49 88. 40 84. 96 82. 37

118.1 115.5 104.8 97. 16 91. 26 86. 71 83.05 80.21 119.2 116.3 104.6 96.24 89. 98 85.09 81.20 78. 15 120.2 117.1 104.3 95. 35 88. 63 83. 43 79. 33 76. 05

121.1 117.7 104.0 94. 39 87.24 81.83 77.44 74. 04

121.8 118.2 103.5 93.44 85. 88 80. 17 75. 59 72.11 122.4 118.5 103.0 92.33 84.46 78. 47 73. 80 70.16

122.8 118.7 102.5 91.24 82. 98 76. 82 72. 07 68. 30 123.1 118.7 101.7 90.07 81. 54 75. 20 70.29 66.43

350,

360.

370.

3.S0. 390.

400. 410.

420.

430.

440.

450. 460.

470.

4S0.

490.

500.

510. 520.

530.

540.

550. 500.

570.

580.

590.

600.

610.

G20. 630.

640.

650.

6 GO. 670. 680.

690.

700. 710

720.

(See footnote at end of table.)
24 BUREAU OF STANDARDS

Table 2.

Spectral energy distribution of a Planckian radiator or black body at various temperatures—Continued

Relative energy distribution 1 at—

millimicrons

8,000° K. 9,000° K. 10,000° K. 12,000 °K. 14,000° K. 16,000° K. 18,000° K. 20,000° K.

350 148.1 180.9 211.5 265. 6 310.3 347.0 377.6 403.0 360 148.4 178.5 206.2 254. 5 293. 6 325. 7 352.0 373. 7 370 148.2 175.7 200.7 243.6 278. 1 305.7 328.2 347. 1 380 147. 6 172 5 195 233. 263. 1 287 306 3 322 5 390 146." 4 169! 189! 1 22Z 6 248! 8 269! 4 2se! 3 300! b 400 145.0 165.3 182.9 212.4 235.1 253.4 267.6 278.9 410 143.

1

161.2 177.0 202. 7 222.5 237.9 250.4 260.0 420 140.9 157.2 170.9 193.6 210.4 224.0 234.3 242. 5 430 138. 5 152. 9 165.

1

184 6 199 1 210 5 219 4 226 2 440 136'. 148! 6 159! 2 176! 188! 4 198! 3 205! 6 21l! 3 450 133.4 144.3 153.4 167.9 178.4 186.6 192.8 197.5 460 130.5 140.0 147.7 160.0 169.0 175.8 181.1 185.0 470 127.5 135.7 142.3 152.7 160.0 165.8 170. 1 173.2 480 124. 5 131. 4 136. 8 145. 6 151. 7 156. 3 159 8 162 5 490 12L5 127! 2 13L7 138! 7 143'. 7 147'. 6 150!4 152! 5 500 118.3 123.0 126.7 132.3 136.3 139.3 141.7 143.2 510 115.3 119.0 121.8 126.2 129.3 131.6 133. 4 134.6 520 112.

1

115.0 117.0 120.4 122.7 124.5 125.8 126.6 530 109. • 111. 1 112. 5 1 14. 9 1 16 5 117 8 118 7 119 2 540 106! 107.' 2 108.1 109! 6 110! 7 11L5 112! 1 112.3 550 103.0 103.6 104.0 104.7 105.

1

105.5 105.8 105.9 560 100. 00 100. 00 100.00 100.00 100.00 100.00 100.00 100.00 570 97. 05 96. 53 95. 98 95. 46 95. 10 94.81 94. 60 94. 37 580 94 14 93. 14 92. 31 91. 19 90. 49 90. 05 89. 55 89 22 590 9L36 89! 92 88! 75 87! 16 86! 15 85! 46 84.' 91 84! 38 600 88. 50 86. 76 85. 21 83. 34 82. 05 81. 14 80. 42 79.83 610 85. 87 83. 66 82. 02 79. 73 78. 17 77. 16 76. 30 75.64 620 . . ... 83. 24 80. 71 78. 91 76. 20 74. 51 73. 29 72. 40 71. 67 630 80. 68 77.90 75. 78 72. 97 71.05 69. 74 68. 82 67. 95 640 78. 18 75. 21 72.93 69. 78 67. 74 66. 44 65. 32 64. 49 650 75. 73 72.54 70. 10 66. 83 64.69 63.23 62.09 61. 21 660 73. 36 69. 99 67.51 64.02 61.78 60.26 59. 10 58. 18 670 71. 12 67. 55 64. 90 61.33 59. 01 57. 42 56.25 55.29 680 68. 83 65. 20 62. 49 58. 77 56. 43 54. 79 53. 61 52. 61 690 66. 73 62.93 60.12 56.38 53. 90 52.28 51.08 50.05 700 64.68 60.75 57. 87 54.09 51. 61 49.94 48.66 47.68 710 62. 67 58. 65 55. 76 51.89 49. 35 47. 70 46. 41 45.40 720 60.70 56. 62 53. 70 49. 77 47..24 45. 55 44. 32 43.27

1 C2=14,350m°. All values from 350mju to 390rriAi, inclusive, have been computed from Table 1 of B. S. Misc. Pub. No. 56, Frehafer and Snow, 1925; the remaining values, 400m,ti to 720m/n, have been taken from the

tables of M56 or read from the graphs of M56 when not tabulated, except at 2,360° and 2,848° K., which came from previous computation in the colorimetry section. Note, however, that the tabulated values

of M56 are given consistently to four significant figures, whereas values less than 10 derived from the graphs

of M56 can be read only to the second decimal. Since further figures were of no practical importance in this investigation, all values less than 10, however derived, were taken only to the second decimal as abulated herein. The authors of the present paper have used the tables of M56 so

far as they were applicable,9 have read values from the graphs at in- termediate temperatures,9 and by means of the auxiliary table noted

above have extended values of relative energy to 3501^, the region

350mju to 390mfx being of importance photographically. These values

of relative energy are given in Table 2, and serve not only as a record

of the actual values used in the present investigation, but as a con- venient set of reference tables more extensive than those of M56 from

which they were derived.

9 Slight qualifications to these statements may be noted in the footnote to Table 2.
REPRODUCIBLE WHITE LIGHT FILTERS 25

The following method was used to obtain values of relative energy

from 350m^ to 390m/x. A large-scale plot of Table 1 of M56 over the

E

range applicable was made, plotting values of against the products

Xd as therein tabulated. Values of \6 were then computed for the

values of temperature, 6, given in Table 2 and for values of X equal to

350m.fi, 360mM, 370m/x, 380mM, 390mM, 400mM, 410mM, 420miu, 450mM, E

and 5601^. For any value of 6, thereupon, values of -^r were read

from the large-scale plot for the values of \d computed, and then

tabulated at the respective wave lengths. This gave a series of

relative energy values for a given temperature, 6, which were then

reduced to 100.00 at 560m^i by means of the proper factor. Values thus obtained at 400m/i, 410m,u, 420m/x, and 450m/i were

then compared with the values at those wave lengths previously obtained from the tables and graphs of M56. Let Ex be the values

obtained by the authors, as indicated above, from Table 1 of M56,

and let E2 be the values taken from Table 2b of M56 or read from

the graphs of M56 which are based directly upon Table 2b. Then

El —E

w

2 X100 represents the percentage deviation of the two sets of

energy values. Where the values of E2 were taken from the tables

of M56, the deviations were nowhere greater than 0.17 per cent; and

where values of E2 were obtained from the curves of M56, the deviations were nowhere greater than 0.29 per cent. These figures afford an interesting comparison with the accuracy claimed by the authors

of M56, viz, 0.15 and 0.33 per cent, respectively, for the tables and

graphs. In table 2 of the present paper the values given at 400m/*,

410m)Li, 420m/>i, and 450mju are those obtained from Table 2b and the

graphs of M56, not those derived from Table 1 of M56 for comparison

purposes.

(c) USE OF HIGH COLOR TEMPERATURES

It has been demonstrated by means of Priest's rotatory-dispersion

colorimetric photometer (30) that the various phases of sunlight and

daylight ranging from the yellow of morning and afternoon sun to the blue of a clear sky may be approximately matched in color by

light having a Planckian spectral distribution. The colors of the

stars may also be described in terms of the color temperature scale. Furthermore, the true subjective white may be found to lie at some

place along this high color temperature scale. It has, therefore, seemed desirable to design filters for duplicating

black-body colors, beginning at the color temperatures above which

bare Mazda C. lamps can not be operated and extending up to 10,000°

or 20,000° K., the color of blue sky. The energy distributions re- ferred to in the last section have served as ideals in the design of

such filters.

4. THE ACETYLENE FLAME

The acetylene flame has been used extensively in photographic

sensitometry and a filter has, therefore, been designed (Sec. V, 6) for

converting the spectral energy and color of this light to those of mean

sunlight. For this purpose the relative spectral energy data of Coblentz (27) have been used. These data were not given regularly at
26 BUREAU OF STANDARDS

every lOm/z. They were accordingly plotted and a curve drawn carefully through these experimental values. Values at every 10m/* were

read from this curve and then adjusted by an appropriate factor giving

100.00 at 560myu. In Table 3 are given the values of relative energy

as thus derived from Coblentz's data, and the same values reduced

to 100.00 at 560m/z.

The color temperature of the acetylene flame with an E. K. standard burner is commonly taken as 2,360° K., as determined by Hyde,

Forsythe, and Cady (32). The value computed from Coblentz's en- ergy data is, however, 2,413° K.10 Some preliminary tests of the

color temperature of this flame by the authors also gave a value over

2,400° K. This question is discussed further in Section VIII, 6. Table 3.

Spectral energy distribution of an acetylene flame

Wave length in millimicrons

380. 390.

410.

420.

430. 440.

450.

460.

470.

480.

490.

500. 510.

520.

530.

540.

550. 560.

570.

580.

590.

Relative energy

distribution

Coblentz's data

3.9

4.4

i 5.0 5.7

6.5 7.4

i 8.5

i 10.0

i 11.8 13.8

16.2

18.5

i 20.9 24 1 127.5 31.0

i 34.6

i 38.9

i 42.9 47.5

i 52.2

57.3

Reduced

to 100.00

at 5Wm.fi

10. 26

11. 66

13.29

15. 15

17. 25

19.81 23.31

27.51

32. 17

37. 76

43. 12 48.72

56. 18

64. 10

72. 26

80. 65

90. 68 100.00

110. 72

121. 68

133. 57

Wave length in millimicrons

Relative energy

distribution

Coblentz's data

600 610

f 2" 630

04"

550 660 670

6 n0 690

700 710 720 425

475

525 575

625

i 62.5

67.8

i 73.3 79.1

i 85.0

i 91.2

" 97.6

104.2

i 110.9 117.4

i 124.

1

130.7

i 137. 5

i 7.0

i 15.0

i 29.2

i 49.8

i 76.1

i 107. 5

i These values are taken from Table 1 of B. S. Sci. Paper Xo. 362, W. W. Coblentz, "Distribution

of Energy in the Spectrum of an Acetylene Flame," 1920. The remaining values were derived by the authors from a large-scale plot of Coblentz's data. IV. COMPONENT MATERIALS OF FILTERS

The several desirable characteristics of white-light filters have been

indicated in Section II, 2, and it was there noted that certain solutions

of inorganic salts apparently had none of the serious faults to which

the other materials were subject and which would prevent their

adoption and use as general working standards. Obviously, filters for converting lower to higher color temperatures should have little absorption in the violet with increasing absorption toward the red

end of the spectrum. Such filters will, of course, have a bluish

color. Several blue solutions are available, such as aqueous nickel

sulphate and ammonia, aqueous copper sulphate and ammonia, and

i° As computed by Davis' method, see footnote to Table 13.
REPRODUCIBLE WHITE LIGHT FILTERS 27

others, but they only crudely approximate the desired spectral absorption. An important step toward the solution of the problem was made by Pfund (23) in the discovery that the spectral deficiencies in

the absorption of an ammoniacal copper-sulphate solution could be

corrected to a large extent by an aqueous solution of cobalt sulphate

and copper sulphate. Considerable interesting information regarding the characteristics of various solutions and glass cells tested for photometric use is given in papers (33, 34) by Ives and Kingsbury.

1. CHOICE OF MATERIALS

(a) AQUEOUS AMMONIA SOLUTION OF COPPER SULPHATE

At the start of the investigation considerable time was spent designing filters, by trial and error and by computation from spectrophotometric data, with the component materials used in Pfund's filter. It was found that the transmittancy curve of the ammoniacal coppersulphate solution changed considerably with the ammonia content

for a given concentration of copper sulphate. There were shifts in

the wave lengths of the maximum and minimum transmittancies with

changes in ammonia content, accompanied by increased values of

transmittancy at some wave lengths and decreased values at others.

From the standpoint of the resulting energy distribution a content of

about 100 cc of ammonia water (28 per cent NH3 ) per liter of solution was determined as the most suitable.

When the copper sulphate is varied, keeping the ammonia-water

content constant at 100 cc per liter of solution, Beer's law was apparently obeyed over a range from 2 to 5 g of copper sulphate, but failed to some extent at 1 g. This question of obedience to Beer's law is, of course, important when a series of filters is designed by computation from standard spectrophotometric data obtained at some one

concentration, if it has been chosen to vary concentration rather than

thickness. (The data on Beer's law were rather meager, and the

measurements were not continued because of the decision to use

pyridine instead of ammonia.)

Ammonia water has two well-known characteristics which tend to make its use for filters unsatisfactory, and which led to the search for a substitute. These undesirable features are: (1) Its corrosive action on glass. It attacks the glass of cells and bottles. In addition to

this undesirable property, particularly with respect to its effect on

the optical glass plates of the filter cells, it was thought that this action might be accompanied by changes in the absorption of the

solution. (2) Its tendency to lose ammonia. Ammonia water must

be tested for strength before using. Furthermore, if the container is inadequately stoppered or sealed, there is danger of ammonia gas

escaping from the solution, with consequent change in absorption as noted above. Ives and Kingsbury (33, p. 799) speak of the change

of color caused by the "unpreventable evaporation" of ammonia.

(b) AQUEOUS PYRIDINE SOLUTON OF COPPER SULPHATE WITH MANNITE

SOLUTION A As a substitute for ammonia, pyridine (C5H5N) was tried. This

gives a blue solution with copper sulphate resembling that obtained

with ammonia. Spectrophotometric tests of the solutions showed
28 BUREAU OF STANDARDS

absorption changes with variation of pyridine content, as was the case with ammonia.

However, a very serious defect of the copper-sulphate-pyridine

solution soon developed. It was found to be rather unstable. This

instability varied considerably with the pyridine content, the solu- tions being most stable with a large proportion (20 per cent total volume) of pyridine and poorest with small proportions. The best

proportion from the standpoint of the final energy distribution was

judged to be about 30 cc of pyridine per liter, but with this amount

the solution showed signs of precipitation after standing a few days

in very warm weather (35° to 40° C).

Nevertheless, these preliminary tests showed the use of pyridine

to be so promising that some method was sought whereby the coppersulphate-pyridine solution could be made stable. During a series of

experiments having this end in view, it was found that the addition

of glycerine to the solution retarded the precipitation enormously.

When precipitation with glycerine present did start, however, a scum appeared on the surface of the solution and on the sides of the

vessel. (In these experiments just barely enough pyridine was used

to convert the copper sulphate to the soluble copper-pyridine com- pound so that precipitation started very shortly after the mixture was made. The temperature has a very marked influence on the

rate of precipitation.) This increased stability with glycerine was so striking that a search was made for other materials of similar structure

to glycerin, with the result that mannite (<i-manitol) was suggested.

This material proved to be very satisfactory, and solutions over four

years old show no precipitation. A very slight change in absorption

does take place upon long standing, however, and this feature is discussed in Section VII, 3. The quantity of mannite used is an amount equal to the weight of copper sulphate used. Apparently no

change in the transmittancy of the copper-sulphate-pyridine solution

is caused by the addition of mannite.

The copper-sulphate-pyridine-mannite solutions" showed the same

changes in absorption with changes in the pyridine content as were

exhibited by the simple copper-sulphate-pyridine solutions, analogous

to the changes in the ammonia solutions as already noted. The data

illustrated in Figure 8 show the effects of varying the pyridine content over the range from 10 to 100 cc per liter of solution with a constant

quantity (3 g) of copper sulphate. Further investigation (Sec. VIII,

3) confirmed the previous conclusion that 30 cc of pyridine per liter gave a more suitable type of absorption than larger or smaller proportions. An aqueous solution containing, unless otherwise stated,

30 cc of pyridine per liter and having varying amounts of copper

sulphate (and mannite) has been designated as " solution A" and is usually thus referred to throughout the rest of the paper.

Having determined the most desirable amount of pyridine to use, a study of Beer's law was made over the range of concentrations of copper sulphate (and mannite) necessary for the niters. A slight

failure was found which is described in detail in Section VII, 6. The method of making the Beer's law corrections in the computations

is illustrated in Section V, 3 and in Table 4. The comparative advantages and disadvantages in the use of

aqueous ammonia or pyridine solutions of copper sulphate may be

summarized as follows:
REPRODUCIBLE WHITE LIGHT FILTERS 29

1. In the most suitable proportions as determined spectrophotometrically, 30 cc of pyridine and 100 cc of ammonia per liter, the

ammonia solutions will attack glass while the pyridine solutions will not.

• 2. Ammonia gas readily escapes from ammonia water. This

difficulty is not encountered with pyridine, the boiling point of which

^. . . . <

fj °'6oj-)/>QUK>(fJO?Cf(/

is 115.5° C. In view of the change in absorption that takes place

with variation in either the ammonia or pyridine content this

advantage of the pyridine over the ammonia is important.

3. At the most favorable content of either ammonia or pyridine

there is no particular advantage of one over the other so far as spectral

transmission is concerned, except that in the violet and ultra-violet

the ammonia solutions have the higher transmittancy. If niters were

prepared using ammonia instead of pyridine, the agreement between
30 BUREAU OF STANDARDS

the computed and ideal energy distributions would be slightly better

at these short wave lengths in the majority of cases than was obtained

with the pyridine filters as herein described.

4. So far as the evidence goes, the obedience to Beer's law is closer

for the ammonia solutions than for the pyridine solutions. This

might have saved time and labor in the measurements and computations, but would not result in any greater accuracy in the final filters. 5. By spectrophotometric test, the ammonia solutions, if evaporation is prevented, appear to be somewhat more permanent than the

pyridine solutions. Here, as elsewhere, the data for the ammonia

solutions is less extensive than for the pyridine solutions, because of the decision to use pyridine in preference to ammonia. Whether the

ammonia solutions would remain as permanent under actual working

conditions is questionable.

It is believed the importance of the first two points noted above,

wherein the pyridine solutions are superior to ammonia solutions, is sufficiently great to outweigh the other points wherein the ammonia

solutions seem superior. Although the relative permanence could

not be known at the time a decision had to be made, the authors feel justified in their search for and use of pyridine, in preference to ammonia with copper sulphate for securing the principal (blue)

component of the filters.

(c) AQUEOUS ACIDIFIED SOLUTION OF COPPER SULPHATE—SOLUTION Bi With either solution A or the copper-sulphate-ammonia solution

the resulting energy distributions obtained with the most favorable

concentrations will show a considerable excess in the blue-green and

far red, and it has already been noted that Pfund found that a satisfactory correcting solution could be obtained from a mixture of copper

and cobalt sulphates in aqueous solution.

Copper sulphate (CuS04 .5H2 0) is a stable compound, readily

obtained in relatively pure form. Aqueous solutions containing 10

cc of sulphuric acid showed no failure of Beer's law over a range of concentrations from 5 to 40 g per liter. The standard medium

concentration used for the principal spectrophotometric measurements was 20 g per liter. While the sulphuric acid, which is a part of solutions Bi, B2 , and B, does not cause any change in absorption it serves the useful purposes of clarifying the solutions and preventing the

formation of basic copper compounds. An aqueous solution containing 10 cc of sulphuric acid (specific gravity 1.84) per liter of solution

and having varying amounts of copper sulphate will be designated as "solution Bl"

(d) AQUEOUS ACIDIFIED SOLUTION OF COBALT SULPHATE

Extensive spectrophotometric data were taken with cobalt sulphate

(CoS04 .7H20). The standard concentration, chosen to be in approximately the middle of the desired range, was 10 g of cobalt sulphate per

liter of solution, containing also 10 cc of concentrated sulphuric acid. It was found, however, that the cobalt sulphate had lost some of its water of crystallization and that the standard solution did not, therefore, contain exactly 10 g of CoS04 .7H20. Chemical analysis of this

solution showed that it had a concentration equivalent to 10.3 g of

CoS04 .7H20.
13416°

REPRODUCIBLE WHITE LIGHT FILTERS 31

(e) AQUEOUS ACIDIFIED SOLUTION OF COBALT AMMONIUM SULPHATE —

SOLUTION B 3 To avoid the uncertainty which would thus be present in the use of cobalt sulphate due to the uncertainty of the water content, the double

salt, cobalt ammonium sulphate (CoSO^NH^SO^GH^O), was

suggested. This material was also used in the preparation of photometric filters by Ives and Kingsbury (33) for the same reason. Spectrophotometric measurements proved the practical equivalence

of solutions containing 14.481 g of cobalt ammonium sulphate and the

standardized solution containing 10.3 g of cobalt sulphate, both

alone and in mixture with copper sulphate (See Sees. VII, 2 and VII,

3). Beer's law is obeyed in both cases over the range of concentrations used. An aqueous solution containing 10 cc of sulphuric acid

per liter and having varying amounts of cobalt ammonium sulphate

will be referred to as " solution B2 ."

(f) MIXTURE OF SOLUTIONS Bi AND B2 —SOLUTION B

It is, of course, desirable to mix the acidified cobalt ammonium

sulphate and copper sulphate solutions, as this reduces the number

of filter compartments from three to two. Tests on the effect of mixing these two ingredients to form a single solution showed, as expected,

that no reaction took place, and that the observed spectral transmittancies agreed perfectly with computed values based on the two

standard concentrations. An aqueous solution containing copper

sulphate and cobalt ammonium sulphate in varying amounts, together

with 10 cc of sulphuric acid per liter is referred to throughout the rest

of the paper as " solution B."

(g) CELL FOR HOLDING FILTER SOLUTIONS

The A and B solutions are contained in a 2-compartment cell, such, for example, as is illustrated in Figure 9. 11 The essential

specification of this cell is that it consist of three borosilicate crown

glass plates (each 2.5 mm thick, refractive index, D line, = 1.51),

separated by two glass frames, each 10.00 mm thick, for containing

the respective A and B solutions. Borosilicate crown glass was

selected for the windows because it is the hardest and most transparent

of the optical glasses and probably the most resistant to chemical

action.

(h) DEFINITION OF DAVIS-GIBSON FILTERS

Summarizing the selection of materials, a Davis-Gibson filter may

be denned as a 2-compartment cell with three borosilicate crown

glass windows, the two compartments being filled, respectively, with

solutions A and B, having compositions as specified below.

Solution A

Copper sulphate (CuS04 .5H2 0) g__ c Mannite (C6 H8 (OH) 6 ) g__ c Pyridine (C5 H5 N) cc_. 30.0

Water (distilled) to make cc__ 1, 000

The value of c, the weight in grams of copper sulphate (and mannite)

per liter of solution, varies with the purpose of the filter, as illustrated

in the various charts and tables. When c has the value of 3.000 g

This design is similar to that used by Ives and Kingsbury (33)

.
32 BUREAU OF STANDARDS

(the concentration at which the principal spectrophotometric determinations were made) the solution is referred to as solution A'.

Solution B

Copper sulphate (CuS04 .5H2 0) g__ ct Cobalt ammonium sulphate (CoS04.(NH4) 2 S04.6H2 0) _ __g__ C2 Sulphuric acid (specific gravity, 1.84) cc__ 10.0 Water (distilled) to make cc__ 1, 000

The values of cx and c2 depend, again, on the purpose of the filter. When Ci = 20.000 g and c2 = 14.481 g, the mixture is referred to as solution B'. Separate solutions of copper sulphate and cobalt

ammonium sulphate having the weights just given, and containing

10 cc each of sulphuric acid per liter, are referred to, respectively, as solution B/ and solution B2 '. Extensive spectrophotometric meas- urements were made on solutions B', B/, and B2 '.

2. PURITY OF CHEMICALS USED

It was decided early in the investigation to use a grade of chemicals

known as "C. P.," because such chemicals are readily available.

The term "C. P.," although originally intended to mean " chemically

pure," is not at present generally used to designate the purest material

available on the market. The terms "analytical reagent quality"

or "reagent quality" are often used to designate a higher grade.

"C. P." chemicals are, however, superior to "technical" or "commercial" grades. The impurities present in different lots of a given

chemical of the "C. P." grade are generally of the same nature, but

may differ in amount. It was believed to be the better plan to include

the effects of these impurities in the spectrophotometric data, pro- vided such effects were small or negligible, rather than to use specially

purified chemicals which would not be generally obtainable. Our

experience throughout the investigation has confirmed this opinion.

The nature and extent of the most important impurities are indicated

below, and their spectrophotometric effects are discussed and illus- trated in Section VII.

The analyses given in this section serve to define the materials used

in deriving the standard data given in the paper. Questions of reproducibility are discussed further in Section VII, 8. Certain pre- cautions to use in the selection of the materials are considered in Section IX on the preparation of the filters. To prevent possible contamination from cork stoppers, glass-stoppered bottles were used to hold all solutions.

(a) COPPER SULPHATE

This well-known chemical (CuS04 .5H20), is readily obtainable

in high purity. The copper sulphate used in deriving the standard

data for the filters was of C. P. quality obtained from Mallinckrodt

Chemical Works, St. Louis, Mo. Chemical analysis gave the following impurities (memorandum, March 25, 1926, from chemistry division) :
REPRODUCIBLE WHITE LIGHT FILTERS 33

Per cent Water insoluble material 0. 000

Iron and aluminum (as R2 3 ) . 007

Zinc (as Zn) .021

Calcium .000

Nickel (as Ni) .029

Magnesium and alkali salts .04

These small quantities of impurities have no practical effect on the

spectral transmission of the filters. The pentahydrate of copper sulphate is ordinarily considered as a stable salt, and for many purposes this is true. It should be noted,

however, that the water of crystallization is subject to minor changes

under ordinary conditions of keeping. The crystals tend to follow

atmospheric changes, gaining or losing water depending on the variation of the aqueous vapor pressure of the air. It may also happen

that the salt is not fully air dried, following crystallization before

bottling, in which case a given weight of the copper sulphate will contain less than the theoretical copper content. These variations

are usually small, seldom amounting to more than ± 1 per cent,

because of the tenacity with which copper sulphate clings to its water

of crystallization. The following chemical analysis (Chem. Lab.

No. 551/31079-84, January 3, 1929) made later on the same stock as analyzed above for impurities, illustrates this point:

Copper

content Percentage

of theoretical

Opened bottle from which sample had been taken over a period of more than

6 months ..- ... Per cent

25. 70

25. 24

100.9 Unopened bottle, same stock, on hand about 2 years... . 99.1 However, copper sulphate which has been kept for long periods of time, particularly in very dry atmosphere, may lose a larger per- centage of its water content. When such a change has taken place

to a considerable extent, a whitish powder (CuS04 . 3H20) will be

localized on or may cover the surface of the crystals. Such material

should not be used in preparing light-filter solutions. It may, of course, be made satisfactory for this purpose by recrystallizing or by

rinsing with distilled water and drying to equilibrium.

This uncertainty in the copper content of copper sulphate, amounting, in general, to perhaps ±1 per cent of the theoretical content, is apparently of much more importance in the use of the niters than is that resulting from any impurities which are likely to be present.

It is, however, of practical importance only in the case of the A

solution. (The magnitude of a 1 per cent variation in the copper

content of the A and B solutions, both as regards the spectral absorbancy of the solutions and the color of the filters, is apparent from

Table 4 and fig. 30.)

The results of the determination of the copper content (Chem.

Lab. No. 551/31079-84, above), made on the same lot of copper

sulphate as had been used in preparing the standard solutions,

rendered somewhat uncertain the exact copper content of solutions

for which the standard spectrophotometric data given in Tables 4

and 5 should apply. Accordingly, analyses were made of the A',
34 BUREAU OF STANDARDS

B/, and B' solutions which had been given the most weight in adopting the standard data. The following results were obtained (Chem.

Lab. Nos. 551/31079-84, January 3, 1929, and 551/31429-32, January

2, 1929):

A' (No. 62-15). Bi' (No. 62-18) B' (No. 65-2)..

Copper

content

Grams per

liter2.99

19.78

20. 10 The value 99.7 per cent, falling between the limits obtained on the

salt, may be taken as the copper content for which the data of Table 4, solution A', apply, as stated in the footnote to that table. Solutions B/ (No. 62-18) and B' (No. 65-2) were both supposed to have 20.000 g of copper sulphate per liter. It is shown later on

(Sec. VII, 2, (e) and Table 21) that, at 667.8nuz in the red where the

copper sulphate absorbs strongly, there w

T as a good check in the

absorbancy of the two solutions, allowing for the small absorbancy of the cobalt ammonium sulphate in solution B'. This check makes it impossible to choose either of the above analyses in preference to the

other; and their mean value of 99.7 per cent (the mean of 98.9 and

100.5, this being the same value as obtained with the A' solution,

No. 62-15) may be taken as the copper content for which the data of Table 5, Solution B/, apply.

(b) COBALT AMMONIUM SULPHATE

This is a double salt (CoS04 (NH4 ) 2 S04.6H2 0) of cobalt sulphate

and ammonium sulphate. Nickel is practically always present in the

salts of cobalt and it is extremely difficult to remove completely.

Nickel sulphate has absorption bands in the violet and far red and is, therefore, very undesirable to have present in any considerable quantity. Because of the cost and difficulty of obtaining cobalt ammonium sulphate free from nickel, which would make it prohibitive for general use in the filters, we chose as a standard a nickel content of 0.075 per cent (ratio of nickel to cobalt = 1 :200). At this proportion

the absorption of light due to nickel is practically negligible, but, such

as it is, is included in the spectrophotometric data. A ratio of nickel

to cobalt not exceeding 1:100 is satisfactory for use in the niters. The cobalt ammonium sulphate used in this work was prepared by

crystallization from cobalt sulphate and ammonium sulphate. The

following extracts from Doctor Wichers's memorandum (chemistry

division, March 17, 1927) on tests of these salts will be of interest:

All samples of cobalt sulphate and cobalt ammonium sulphate tested were

found to contain nickel. Two samples of cobalt sulphate made by American

manufacturers were found to have 0.10 per cent and 0.09 per cent Ni, respectively. This is a ratio of Ni to Co of about 1:200. A sample of cobalt ammonium sulphate of foreign make held 1.37 per cent Ni (ratio of Ni to Co of 1: 11). A sample of American-made cobalt ammonium sulphate held 0.20 per cent nickel. These results indicate that nickel may be present in excessive amounts in these

salts. Experience shows also that the amount of nickel indicated in the analysis on the label is often in error.
REPRODUCIBLE WHITE LIGHT FILTERS 35

There seems to be no reason why a sample of cobalt ammonium sulphate can

not be made as free of nickel (ratio of Ni to Co) as the corresponding cobalt

sulphate. The sample of cobalt ammonium sulphate made from the cobalt sulphate containing 0.10 per cent Ni had a nickel content of about 0.05 per cent

Ni. The ratio of nickel to cobalt here is 1 : 300. The nickel was determined in a

sample that had been twice crystallized by you and twice recrystallized in this laboratory. This indicates that a salt contaminated with nickel can not be

easily improved by successive recrystallizations as the double sulphate.

Cobalt ammonium sulphate crystallizes as CoS04 .(NH4) 2 S04.6H2 and is stable under ordinary atmospheric conditions. Exposed in a balance case for a period of two months it snowed practically no change in weight. As was to be expected, the weight changed slightly with changes in humidity. Weights

taken at different times and under different conditions covered a maximum

range of 0.0016 g on a 7.5 g sample. The final weight was 0.0004 g less than the

initial. The theoretical cobalt content in cobalt ammonium sulphate is 14.91 per cent. The cobalt content of the foreign sample was 14.58 per cent; that supplied by an

American maker was 14.86 per cent, while the material made by you held 14.60 per cent cobalt. In every case the cobalt is determined by electrolysis so that the

figure reported is really the combined cobalt and nickel content. Your sample was recrystallized twice in this laboratory to see if a salt with the theoretical cobalt content could be easily produced. The crystals after being separated

from the mother liquor were air dried. Upon analysis they were found to contain

the theoretical cobalt content. The values found in two determinations were

14.92 and 14.88 per cent Co. The following is the method regularly used in the chemical laboratories of the National Bureau of Standards for determinations of the

nickel content of cobalt ammonium sulphate (memorandum of

January 6, 1928):

Dissolve 1.00 g of cobalt ammonium sulphate in 300 cc of water. Add 1 g of tartaric acid and heat the solution to about 80° C. Then add 100 cc of a 1 per

cent alcoholic solution of dimethylglyoxime, and ammonium hydroxide solution

until the solution is distinctly alkaline to litmus paper (an excess is to be avoided). Cover the beaker and place on the steam bath for 3 hours and then keep in a warm

place (about 40° C.) for 36 to 48 hours longer. Filter off the precipitate. If

little or no precipitate formed allow the filtrate to stand an additional 24 hours to be sure that all the nickel has been precipitated. Add any precipitate which

forms to the first precipitate. Dissolve the combined precipitates in 10 cc of hot 1:1 nitric and dilute to 100 cc. Heat to about 80° C. Then add 10 cc of dimethylglyoxime solution and ammonium hydroxide solution until the solution

is alkaline to litmus paper, but avoiding an excess. Digest on the steam bath to coagulate the precipitate and filter through a tared, asbestos-covered Gooch

crucible. Wash the precipitate thoroughly with hot water and dry at 110° to 120° C. for 45 minutes. Multiply the weight of the precipitate by 0.2031 to get the weight of nickel (Ni). It was stated in Section IV, 1, (d) that the cobalt-sulphate solution

(No. 62-17) was found by chemical analysis to have 10.3 g of

CoS04 .7H2 instead of the 10.000 g which it was supposed to have.

This value was the mean of three analyses made by the chemistry

division of the bureau, the individual values being as follows : (1) 10.30

g per liter, November 19, 1926; (2) 10.39 g per liter, December

7, 1926; (3) 10.25 g per liter, December 9, 1926. The second

determination was possibly somewhat too high and less accurate than

the other two. At any rate the value of 10.3 g of cobalt sulphate per

liter (equivalent to 14.481 g of cobalt ammonium sulphate per liter)

may be accepted as accurate to a fraction of 1 per cent, and it is this

value for which the standard spectrophotometric data of Table 6

apply.
36 BUREAU OF STANDARDS

(c) PYRIDINE

This is a stable colorless liquid organic compound (C5H5N). It has

a strong unpleasant odor which gives one the false impression of

high volatility. Its boiling point is 115.5° C. at 760 mm; its specific

gravity is 0.979 ± 0.001 at 25° C. The change in specific gravity is 0.001 per °C. The pyridine used throughout the present investigation was J. T. Baker Chemical Co.'s C. P. grade.

Pyridine, like most organic compounds, is very difficult to obtain

in a pure state. The material which is available commercially in

the CP. grade always contains a small quantity of the higher homologues, the principal one being a-picoline. A small amount of water

is also usually present. Complete chemical analysis was not made,

because a satisfactory procedure to accomplish this result is not

available. Since cn-picoline is considered to be the major impurity

likely to be present in pyridine, and because of the difficulty of separating the higher homologues, we have expressed the total effect of these

impurities in terms of the equivalent a-picoline. Samples of commercial pyridine and a-picoline were purified both by fractional distilla- tion and fractional crystallization 12 of the mercuric chloride compounds

of the hydrochlorides, and A solutions prepared containing various

amounts of the picoline. The spectrophotometric analysis of these

solutions is described in Section VII, 9 (b). The amount of picoline

present in our standard pyridine was shown by the spectrophotometric analysis to be 2.4 per cent by weight. This analysis, combined

with titration tests and specific gravity determinations gave 1.2 per

cent water and 96.4 per cent pyridine, by weight.

Pyridine is ordinarily tested for strength by titration methods

(as described below), and values thus obtained show the true per- centage of pyridine only in the absence of the higher homologues

or other impurities which also react with the sulphuric acid. With

the presence of the higher homologues the combined strength, in

terms of pyridine, is obtained.

Our standard pyridine, as analyzed above and as used in deriving

the standard spectrophotometric data of Table 4, was thus titrated

for strength with half normal sulphuric acid, using methyl orange for an indicator. This work, based on a series of runs by two observers

gave a value of 98.4 per cent, which is equivalent to the analysis

given above in terms of pyridine, picoline, and water. Since the end

point is not particularly sharp, the following procedure was followed:

The pH value of the end point (3.2 ±0.1) was determined by elec- trometric titration with the hydrogen electrode. In the colorimetric

titrations a buffered solution, pH 3.2, was used as a comparison standard

for the end-point color. Making use of this end point a sample of pyridine which had been fractionated several times and was without

doubt practically pure, gave a value of 100 per cent strength.

We have examined, altogether, 17 samples of pyridine obtained

from various sources, and, with the exception of 2 samples which

were colored (yellow), all were suitable for use in the filter solutions.

The titration tests gave values for these 17 samples ranging from 97

to 100 per cent strength. As shown in Section VIII, 2 (c) a departure

12 We are indebted to I. M. Jacobsohn, of the chemistry division, for the fractional crystallization of pyridine and picoline.
REPRODUCIBLE WHITE LIGHT FILTERS 37

of a least 6 per cent from our standard of 98.4 per cent strength is necessary to affect the color of the filters as much as is caused by a

1 per cent error in the copper sulphate. It thus appears that for

general use it is unnecessary to test pyridine by titrating for strength.

Independent tests on Davis-Gibson filters made at the National

Physical Laboratory of Great Britain have led to the same conclusion.

(See quotation at end of Sec. VII, 8.)

The presence of the homologues might be indicated by treatment

with potassium permanganate. However, the reaction is slow at room temperatures, and other reducing agents which may be present

in pyridine react with the permanganate more rapidly than do the

homologues. Although this makes the test of little value as a means

of detecting the higher homologues, it is nevertheless useful in detecting the presence of other undesirable impurities. We recommend

that the pyridine used in the filter solutions pass the following simple

test: To 5 cc of pyridine add 2 drops of iV/10 KMn04

. This should

remain red for at least an hour.

Pyridine should be kept in glass-stoppered bottles because it reacts with cork, becoming yellow. Pyridine so contaminated will not pass the permanganate test. We would strongly recommend

that in purchasing pyridine, glass-stoppered bottles be specified.

(d) MANNITE

This material (C6H8 (OH)6 ), also referred to as d-mannitol is one

of the sugar alcohols. It is commercially obtainable as fine white

needles which melt at 166° C. The mannite used in this work was

prepared from crude manna and purified by recrystallization from

water solution by the polarimetry section of the bureau. This

mannite was compared by C. F. Snyder, polarimetry section, with a sample from the Eastman Kodak Co. in the following test: Optical rotation.—Ten grams of <i-mannitol and 10 g of borax were

dissolved in water and the volume made up to 100 ml at 20° C.

The solutions were filtered and observed on a Bates type saccharimeter

using white-light illumination. From the values so obtained the

specific rotations for sodium light were calculated. The same value was obtained for both samples, viz, +20.9° (with borax) for the D

line at 20° C. (The specific rotation of mannite is greatly increased by

borax. In simple aqueous solution the specific rotation is very small.)

Melting point.—Determinations on the two samples showed for the

Eastman sample, M. P. = + 166° C, and for the Bureau of Standards

sample, M. P.= +165°C.

The use of mannite in the filter is that of a stabilizer. Any rotatory

power possessed by the solution containing mannite need have no

influence on the color and the spectral energy distribution of the lamp

and filter combination. Rotatory effects could be important only

if the filter were placed between a polarizer and analyzer, and such

an arrangement need never be used. In cases where the intensity of

light is modified by polarization methods, any rotatory powers that

the filter might have could be nullified by placing the filter elsewhere

than between the polarizing elements.

Furthermore, even these precautions are entirely unnecessary as is shown by the following test, made by F. P. Phelps of the polarimetry
38 BUREAU OF STANDARDS

section of the bureau, on the A solution (containing mannite) of the

2,360° K-to-mean-sun filter: The rotation was measured in a 20-mm tube. The estimated experimental error is about ±0.005°.

1. The observed rotation for the band (blue) transmitted by the tube

of solution when illuminated by white light from a 100-watt

tungsten stereoptican lamp was +0. 001°

2. The observed rotation for A = 546 was +. 005°

3. The observed rotation for A = 578 was +. 002°

4. When a red screen was interposed under the conditions of (1), so much light was absorbed that no reading could be made.

It will be seen that while all the readings obtained are positive, their magnitude

is well within the experimental error. The rotation, therefore, if any, is so small

as to be entirely negligible.

(e) AMMONIA

Ammonium hydroxide (ammonia water) is easily obtained in high

purity. The strength of the usual C. P. grade is about 28 per cent

NH3. It is well known that ammonia water is unstable at ordinary

temperatures, decomposing into water and ammonia gas, and it is thus necessary to determine the NH3 content before using. In our work Baker and Adamson ammonia water of C. P. quality was used.

The NH3 content was tested by standard titration methods immediately before using, and the quantity used in preparing the solutions was adjusted in all cases so as to obtain the equivalent of 28 per cent

ammonia water.V. METHODS OF COMPUTATION

1. SPECTROPHOTOMETRIC TERMINOLOGY

Before describing the various computations required in the design

of the filters, it is necessary to define precisely the various spectrophotometric terms which are used. Throughout the paper, both before and following the present section, these terms are used in strict accord with these definitions.

Let radiant energy, homogeneous with respect to wave length, be

incident perpendicularly upon the first surface of a homogeneous,

isotropic, nondiffusing medium or series of such media in contact

with one another, having plane, nondiffusing, parallel surfaces, and

after passage through the medium or media emerge perpendicularly

from the last surface. If Ex be the energy thus incident upon the first surface and E2 the energy thus leaving the last surface in the same

direction as Eu then transmission, T, is defined as:

Thus, we may refer to the transmission of a plate of glass, or a cell with glass end plates containing a solution, or of the two-compartment

filters herein described, in all cases referring to the ratio of the energies transmitted by and incident (normally) upon the material or group of materials under consideration.

Density, D, is defined as the negative logarithm of the transmission,

thus:
REPRODUCIBLE WHITE LIGHT FILTERS 39

Consider further a substance in homogeneous solution in a solvent

contained in a cell with plane, parallel sides, perpendicular to the

direction of propagation of energy. Let Tsoi be the transmission of a

cell containing the solution, and let Tsov be the transmission of the same or a similar cell filled with the solvent. Then transmittancy,

T, is defined as

:

I" TsqiI Tsov Throughout the present paper Tsov refers to a cell containing distilled water only.

Absorbancy, A, is defined as the negative logarithm of the trans- mittancy, thus:

A= -log10T

Let b and c refer to the thickness and concentration, respectively,

of a solution. The quantity k, equal to A/bc, is known as the specific absorptive index. Its magnitude depends, of course, upon the

particular units chosen for thickness and concentration. The relation, A = k&c, is known as Beer's law. For a given concentration,

A = k'&. This relation is known as Lambert's law and may be derived

from theoretical considerations. The analogous relation, with thick- ness constant, viz, A = k"c, may or may not be valid, depending upon

the particular solution. Failure of Beer's law always refers to a failure

of this last relation.

The terms transmission, transmittancy, and density, as defined

above, are in accordance with the terminology of the O. S. A. committee report on spectrophotometry (35, p. 177). The primary

meaning of density as used in photographic sensitometry has more

nearly the meaning of absorbancy than of density as defined in this

paper. In the present case, it is thought best to use the terminology

developed for spectrophotometric work. The quantity k ( = A/bc) was defined in the O. S. A. committee report as the specific trans- missive index, but it seems more appropriate to name it the specific

absorptive index, as has been done by Peters and Phelps (36, p. 265)

and Brode (37, p. 503).

The term absorbancy was used by Peters and Phelps to represent

the quantity 1 — T instead of — logi T as defined above; howevei,

the need for a word or symbol to designate 1 — T is not so pronounced.

In discussions of the absorptive properties of solutions, on the other

hand, the negative logarithm of the transmittancy is one of the most

commonly used quantities. Hitherto, the terms density and extinction coefficient have often been used in this connection, but there are objections to both expressions. Density is widely used in photographic work where it usually has reference to transmittancy that is more or less diffused, in addition to its use in the consideration of

rectilinear transmission as defined above. Extinction coefficient is an unsatisfactory expression, for, when used in this sense, it is not

a coefficient and in its derivation does not accord with the usual

meaning of extinction. The use of the word absorbancy eliminates

the need for these expressions and with its symbol A makes unnecessary the continual repetition of the cumbersome expressions — logio

transmittancy and — log10T.
40 BUREAU OF STANDARDS

2. STANDARD SPECTROPHOTOMETRIC DATA

With solutions of medium concentration selected as noted in

Section IV, 1, and by spectrophotometry methods described in detail

in Section VII, standard absorbancy values were adopted for the

component solutions and standard density values for the double cell

filled with water. These fundamental values are given in Tables 4,

5, 6, and 7, and illustrated in Figure 10; in the tables are also given

values of absorbancy on a 1 g basis as used in the computations,

values of the Beer's law correction factor for the A solutions, and

values of transmittancy and transmission derived, respectively, from

the absorbancy and density values. Values of absorbancy for any

concentration may be computed by multiplying the 1 g absorbancy

values by the concentration in grams, provided that any Beer's law

failure is allowed for in such computation. As noted elsewhere, a
REPRODUCIBLE WHITE LIGHT FILTERS 41

Beer's law correction was necessary only with the A solutions and the

method of applying it is described in the next section.

The advantage of using the negative logarithms of spectral trans- mission and transmittancy in computations involving the combination

of two or more materials may be noted here. Let Dw represent the

density of a double cell, such as used in the present filters, when

filled with distilled water, and DF the density of the same cell when

filled with the A and B solutions. Furthermore, let AA , ABi, AB2 , and AB refer, respectively, to the absorbancies of the A, B1? B2 , and

B solutions. Then, at any wave length,

DF =DW +AA +AB

or DF = Dw + AA + ABl + AB2 For the computations in which we are interested these are simpler

formulas than the corresponding ones, TF = Tw X TA x Tb

and

TF = Tw X X TBl x Tb2 Table 4.

Standard spectrophotometry data for solution A'

Unfiltered, thickness 10.00 mm, temperature 25° C, having the following composition: Copper sulphate (CuS04 . 5H2 0)i g__ 3. 000 Mannite (C 6 H8 (OH)«) g_. 3.000 Pyridine (C5H5N) 2 cc. 30.0 Water (distilled) to make cc. 1, 000

Wave length in millimicrons

350. 360.

370- 380.

390- 400.

410.

420_

430- 440.

450-

460.

470.

480.

500. 510.

520.

530-

540.

550. 560.

570. 580.

590.

Absorb- ancy

A.

(c* = 3)

0.470

.360

.267 .190

.127 .0835

.053

.033 .0215

.0165

.017 .023

.0355 .0555

.085

.123 .175 .240

.311

.387

.464 .538

.603

.655

0. 156667

. 120000

. 089000

. 063333

.042333

. 027833

. 017667

. 011000

. 007167

. 005500

. 005667

. 007667

. 011833

. 018500

.041000

.058333

. 080000

. 103667

. 129000

. 154667

. 179333

. 201000

. 218333

. 231000

Beer's law cor- rection

factor 3 tB

-0. 120 .120

.118

.114 .105

.050 .032

.020

.013 .010

.010

.010

.010

.010

.010

.010 .010

.010

.010

.010

.010

.010

.010 .010

Transmit- tancy *

.437

.541 .646

.746

.825

.885

.927 .952

.962

.948 .922

.880 .822

.753

.668

.575 .489

.410

.344 .290 .249

.221 .203

Wave length in millimicrons

600. 610.

620.

630. 640.

650.

660.

670.

680. 690.

700. 710.

720.

730.

740.

750.

Absorb- ancy A.

c,=3

Hg 404.7. Hg 435.8. Hg 491.6. He 501.6. Hg 546.1- Hg 578.0. He 587.6. He 667.8.

0. 717 .726

.725

.714

.637 .603

.570 .536

.501

.468

.435

.405

.377

.351 .068

.018

.091

.131 .434 .646

.685

.610

0. 239000

. 242000

. 241667

. 238000

. 232000

. 222667

. 212333

. 201000

. 190000

. 178667

. 167000

. 156000

. 145000

Beer's law cor- rection

factor 3 B —0. 010

.010 .010

.010 .010

.010

.010

.010

.010

.010

.010

.010 .010

.065

.016

.010 .010

.010

.010

.010 .010

Transmit- tancy i

0. 192

.188

.188 .193

.201

.215

.231 .249

.269

.291

.316

.340

.367 .394 .420

.446 .855

.959

.811 .740

.368

.226

.207 .245

1 Chemical analysis showed the copper sulphate to have 99.7 per cent of the theoretical copper content.

2 Titration showed the pyridine to have an effective strength of 98.4 per cent, this including 2H per cent

picoline impurity.

3 The Beer's law correction factor is applied as follows (see Sec. V, 3): A=*° c[l+B(c-Cs)] Note that B is a negative quantity.

4 These values of transmittancy are derived from the adopted values of absorbancy. A„= —logioT,.
42 BUREAU OF STANDARDS

Table 5.

Standard spectrophotometry data for solution B/

Unfiltered, thickness 10.00 mm, temperature 25° C, having the following composition: Copper sulphate (CuS04. 5H2O) 1 g__ 20. 000 Sulphuric acid (specific gravity 1.835) ec.. 10. Water (distilled) to make cc. 1, 000 Wave length in millimicrons Absorb- ancy A,

Transmit- tancy 2 t;

350 0090 0. 000450 0. 979

360 .0063 . 000315 .986 370 .0046 . 000230 .989

380 . 0035 . 000175 .992 390 .0028 . 000140 .994 400 .0023 . 000115 .995

410 .0019 . 000095 .996 420 .0016 . 000080 .996 430 .0014 . 000070 .997

| 440 .0012 . 000060 .997 450 . 0011 . 000055 .997 460 !oon ! 000055 !997 !

. 0012 (\CV\C\fi.(\

. UUUUOU QQ7 . yy/ 1 480 .0014 . 000070 .997 490 .0018 . 000090 .996

500. .0026 . 000130 .994 510 .0038 . 000190 .991 520 .0055 . 000275 .987 530 .0079 . 000395 .982 540 .0111 . 000555 .975

550 .0155 .000775 .965 560 .0216 . 001080 .951 570 .0292 . 001460 .935

580 .0390 . 001950 .914 590 .0518 . 002590 .888

Wave length in millimicrons

600

610

620

630 640

650 660

670 680 690

700 710 720

730

740

750 Hg 404.7 Hg 435.8 Hg 491.6 He 501.6. Hg 546.1 Hg 578.0 He 587.6. He 667.8.

Absorb- ancy A,

.0885

.1125

.143 .180

.224

.274 .332

.392 .459

.527

.592

.656 .715

.817

.0021 .0013

.0019

.0028

.0135 .0368

.0487

.319

A.

20

0. 003400

. 004425

. 005625

.007150

.009000

.011200

. 013700

. 016600

. 019600

. 022950

. 026350

.029600

.032800

Transmittal! cy I

0. 855

.816 .772

.719

.661 .597 .532

.466

.406 .348

.297 .256

.221

. 193

.171 .152

.995 .997

.996

.919

1 Analysis showed the copper sulphate to have 99.7 per cent of the theoretical copper content.

2 These values of transmittancy are derived from the. adopted values of absorbancy. A,=—logioT,. Table 6.

Standard spectrophotometry data for solution B'2 Unfiltered, thickness 10.00 mm, temperature 25° C, having the following composition: 1 Cobalt ammonium sulphate (C0SO4. (NH4 )4 SO4.6H2O) 2 g__ 14.481 Sulphuric acid (specific gravity 1.835) cc. 10. Water (distilled) to make cc._ 1000 Wave length in milli- microns Absorb- ancy Aa A,

14. 481

Transmit- tancy 3 T»

350 0. 0038 0. 0002624 0. 991 360 .0040 . 0002762 .991 370 .0050 . 0003453 .989 380 .0065 . 0004489 .985 390 .0088 . 0006077 .980 400 .0125 . 0008632 .972 410 .0168 . 0011601 .962 420 .0224 . 0015469 .950 430 .0340 . 0023479 .925 440 .0522 . 0036047 .887 450 .0773 . 0053380 .837 460 .1031 .0071197 .789 470 .1213 . 0083765 .756

480 .1349 . 0093157 .733 490 .1472 . 0101650 .713 500 —

.1635 . 0112907 .686 510 .1742 . 0120296 .670 520 .1689 . 0116636 .678 530 .1452 . 0100269 .716 540 . .1113 . 0076859 .774

550 .0775 . 0053518 .837 560 .0496 . 0034252 .892 570 .0308 . 0021269 .932 580 .0207 . 0014295 .953 590 .0158 .0010911 .964

Wave length in milli- microns

600 610

620

630

640

650

660

670 680

690

700 710

720. __ _ 730

740

750 Hg 404.

7

Hg.435.8 Hg 491.6. He 501.6. Hg 546.1. Hg 578.0. He 587.6. He 667.8.

Absorb- ancy As

0. 0137

.0124 .0115

.0112 .0110

.0105

.0097

.0087

.0076

.0066

.00.54 .0046

.0038 .0032

.0030

.0028

.0144 .0437

. 1497 .1661

.0901 .0219

.0167

A,

14. 481

0. 0009461 .0008563

. 0007941

. 0007734

. 0007596

. 0007251

. 0005248

.0004558

. 0003729

. 0003177

.0002624

Transmit- tancy 3 T8

.972

.974 .975 .975

.976

.978 .980

.983 .985

991 993

993

.967

.904

,708 ,682

,813

,951 ,962

1 These data apply accurately also throughout the visible spectrum to a similar solution made up with

10.3 g. of cobalt sulphate (C0SO4 .7H2 0).

2 Chemical analysis showed a cobalt (plus nickel) content of 100.0 per cent of the theoretical, the ratio

of nickel to cobalt (metals) being 1 to 200.

3 These values of transmittancy are derived from the adopted values of absorbancy. A,= —logioT,.
REPRODUCIBLE WHITE LIGHT FILTERS 43

Table 7.

Standard spectrophotometric data on double cell filled with water

Three glass plates, each 2.5 mm thick (borosilicate crown, refractive index, D line = 1.51), two 1.000 cm

thicknesses of distilled water; two air-glass surfaces, four water-glass surfaces.

Wave length in millimicrons

350

360 370

380

390 400

410 420

430

440

450

460

470

4S0

400 500

510

520

530 540

550

500 570 580

590

Density

Dw

0.088

.070

.059

.053

.051 .050

.0495

.049 .0485 .048

.0475

.047 .0465

.046 .0455 .045

.0445

.044

.044 .044

.044 .044

.044

.044 .0445

Transmission

0.817

.851 .873 .885

.892

.897

.898 .900

.901 .902

.903

.904

.904

.904 .904

.904

.904

.904

Wave length in millimicrons

600

610

620 630

640 650

660

670

680

690

700 710

720

730

740 750 Hg 404.7 Hg 435.8 Hg 491.6 He 501.6 Hg 546.1 Hg 578.0 He 587.6 He 667.8

Density

Dw

0. 045 0.902

.045 .902

.045 .902

. 0455 . 901

. WX) . yuu

.046 .900

.046 .900

. 046 . 900

. 046 . 900

. 0465 . 898

047 !049 .893

.054 .883

.060 .871 .064 .863

.065 .861

.0498 .892

.0482 .895

.0454 .901

.0449 .902

.0440 .904

.0440 .904

.0444 .903

.0460 .900

1 Values of transmission are derived from the adopted values of density . Dw =~logio Tw . 2 These values for the Hg and He wave lengths are not fundamental experimental values as in the case of the solutions, tables 4 to 6, but are interpolated from the adopted density values above from 350rrnt to

750m/i. 3. BEER'S LAW CORRECTION FOR SOLUTION A

Let A be the absorbancy, at any wave length, of a solution having

a concentration of c grams per liter, and As the standard absorbancy

at the same wave length, for a concentration c s ( = 3.000 g) as given

in Table 4, thicknesses in both cases being equal to 10 mm. If there

is no Beer's law failure,

If B represents the Beer's law correction factors given in Table 4,

then B may be defined as „/(;•) *•

(c-c s )As Rearranging this equation gives

A=^s c[l + B(c-cs )] In the case of the A solutions, B is a negative quantity, as may be

seen by reference to the experimental data of Table 17 (see Sec. VII,

6), which results in the correction being negative when c>cs and

positive when c < cs

.
44 BUREAU OF STANDARDS

A

This formula was used in all computations of A, —s being comc 3 puted and tabulated once for all as in Table 4. The fact that B is constant from 450m/x to 720m// simplified the work of correction.

4. DESIGN OF FILTERS OF SERIES I—2,300°-4,000° K. TO MEAN NOON

SUNLIGHT

(a) TO OBTAIN SPECTRAL ENERGY MATCH

Obviously the first step in the design of a filter to convert one

spectral energy distribution, E, to another, E', is the computation of the relative spectral transmission of the ideal filter. This is obtained

by computing, wave length by wave length, values of E'/E. These

values of E /E have a value of unity at 560mju since the relative

energies are all given with reference to 100 at this wave length. They

may be reduced to a practical working basis by multiplying them by

an appropriate factor, known by trial and experience to bring the

relative transmissions, on the average, close to the spectral trans- mission of the final filter. The next problem is to find what concentrations of the component

solutions, A, Bl7 and B2 (considering also, of course, the data for the

double cell filled with water) will best approximate the desired

transmission or density.

The method devised for comparing the density of the ideal filter with the combined absorbancies of the three solutions at the trial concentrations plus the density of the cell filled with water is, briefly, as follows: A table of spectral absorbancies was prepared for each of the three solutions at a series of concentrations for the 17 spectral

positions (every 20mju) from 400mju to 720mju, inclusive. The concentrations of solution A were in 0.1 g steps, and of solutions Bx and

B2 in 1.0 g steps. The absorbancies for each concentration were

placed in horizontal rows, the values in the rows being arranged in order of wave lengths. A table of the spectral density values of the

various ideal filters was also prepared.

These tables were mounted on rollers and placed in a box with

a cover having slots so placed that one fine of figures across each

table is exposed to view. This is illustrated in Figure 11. The top

roller contains the table of the spectral densities of the various ideal

filters, the second line of figures, not mounted on a roller, are density

values of the cell filled with water. The next three rollers contain

the tables of the spectral absorbancies for solutions A, Bi, and B2 , respectively. The rollers may be turned by the handles outside of the box to expose to view any of the rows (horizontal) of figures.

The cover of the box is ruled into 18 columns (vertical). In the

first column, on the rollers, are the concentrations of the solutions in grams per liter and the name of the ideal filter. In the next 17

columns are the densities and absorbancies corresponding to the 17 wave lengths indicated at the tops of the columns. The operation

of the device is as follows:

The three lower rollers are turned so as to expose the absorbancies

corresponding to the concentrations of the solutions desired for the

first trial. Next is found the difference between the density of the

ideal filter and the sum of the absorbancies of the three solutions

plus the density of the cell at each wave length. This requires but a
Miscellaneous Publications, Bureau of Standards, No. 114

REPRODUCIBLE WHITE LIGHT FILTERS 45

few minutes with an adding or computing machine. The density

differences are plotted on coordinate paper as a function of wave

length. An ideal solution of the problem would be obtained if the

density differences were equal in amount at all wave lengths.

Having thus plotted the density differences for the first trial, it

is a relatively simple matter to determine what changes to make to improve the results, provided the general shape of the curve of each

of the ingredients is kept in mind. These adjustments are made and

the operation repeated until no further improvement can apparently

be made. The length of time required to obtain satisfactory results by this method depends largely upon experience, from two to four

hours work being usually necessary. Other methods were considered before the one just described was

adopted. For example, at each wave length there are three variables,

and it would, therefore, be possible to choose three wave lengths

fairly well distributed over the desired spectral range and make the

actual and ideal values of density come into perfect agreement at these three wave lengths. However, the values at the remaining wave lengths are beyond control; that is, the actual density curve makes excursions from the ideal at various points and the magnitude

of these excursions will depend upon the three wave lengths chosen for

the computation.

The method of least squares is also applicable to the problem, but

is quite complicated because the importance at a given wave length

of obtaining a close approximation varies with the color temperature

of the source. On this account a different set of weights would have

to be employed for each source. It is not feasible to select one set of

weights for each range of color temperature, because the continuity

of the filters would be spoiled and interpolation for intermediate

values would be difficult. The least square method appeared to be

an unnecessary refinement.

The method finally adopted for solving the problem has advantages

over either of these methods, chiefly because of its simplicity and

flexibility. It allows some choice in the selection of concentrations

so that a certain amount of control is possible over the light trans- mission of the filter. If the filter solutions had spectral properties

such that a perfect match with the ideal filter could be obtained, no

choice in concentrations would exist. However, this is not the case, since the actual densities depart from the ideal in several parts of the

spectrum even at the most favorable concentrations. With low concentrations a good match in the central part of the visible spectrum

is obtained with considerable departures at the ends; with higher

concentrations the ends are improved considerably with increased

departures in the central part of the spectrum.

Four filters, viz, 2,360°, 2,600°, 2,800°, and 3,000° K. to mean sun, were first designed by the adopted method. The concentrations of the three solutions as abscissas and the color temperature as ordinates were plotted and curves drawn. These curves gave trial data for intermediate filters, final adjustment being made as before. The

range of the filters was extended to 3,500° K. and later to 4,000° K.

by the same process. All of the data so obtained were again plotted

(concentrations against color temperature) and smooth average

curves drawn. From these curves the concentrations were read, at every 100° between the limits of 2,300° and 4,000° K., which were
46 BUREAU OF STANDARDS

used for deriving the data for the trial computations of the colorimetric characteristics, as outlined in the next section.

(b) TO OBTAIN COLOR MATCH

It is possible for two spectral energy distributions to be widely

different and still give the same color. If a color match between the

artificial and adopted sunlight were the only concern, solution Bu

for example, could be dispensed with. However, it is desirable for

both photographic sensitometry and colorimetry that the spectral

energy match be made as good as possible. This has been accomplished as outlined in the preceding section.

If it were possible to obtain, with the materials available, a perfect

energy match with the ideal, a perfect color match would thereupon

be automatically obtained. However, as previously noted, the energy

distributions resulting from the process described in the preceding

section depart from the ideal in several parts of the spectrum. From

the magnitude and distribution of these deviations large color differ- ences between the filtered energy and the ideal energy are not to be expected, and perfect color matches have been obtainable in all cases by making minor adjustments of the concentrations of one or more of the filter ingredients. Such adjustments do not materially

change the energy matches previously obtained.

The method used for computing the color characteristics of any

energy distribution follows the general description of such procedure

as given in the Optical Society of America colorimetry report (4). The method makes use of the three so-called elementary excitations

(red, green, and blue), based on the data of Abney and of Koenig

and Dieterici as reduced by Weaver (4). These are given in Table

6 of the colorimetry report, but have been extrapolated and republished in the Optical Society of America spectrophotometry report

(35), Table 5, page 230. These extrapolated excitations have been

used in the present computations.

In addition to the choice of the red, green, and blue excitations, a

spectral distribution defined as " neutral' 5 (white) must be adopted.

For this purpose the energy distribution of mean noon sunlight as given in Table 1 has been used. 13 The methods of computation were

as follows

:

" For the purposes of this paper, the computation of trilinear coordinates throughout the paper has been

based on the arbitrary convention of making the center of the Maxwell triangle (r=g=b = 1A) represent the spectral distribution which we have chosen for mean sun; that is, mean sun has been taken as the "neutral stimulus." The decision as to what stimulus should be given this central position is conditioned upon

the specific purposes which the system of colorimetric coordinates is to serve. The question as to what stimulus should be assigned this position for colorimetric purposes, in general,

is at present being discussed by a committee of the International Commission on Illumination, but no final recommendation has been made; and the question as to whether "noon sun" or "overcast sky" or "sun

outside the earth's atmosphere" or a "black body" at some specified temperature should be formally chosen

for this purpose is still in abeyance. The choice of spectral distribution which we have made to represent "mean sun " has been based primarily on the requirements which a standard for photographic sensitometry must meet. For some years pre- viously a slightly different spectral distribution of energy proposed by Priest (3) has served as a standard for "average noon sunlight at Washington" in the colorimetry section of the bureau and, to some extent, elsewhere. Priest sought to represent, for colorimetric purposes, the same data which we have sought to represent for the purposes of photographic sensitometry; the spectral distribution of energy he adopted is designated as "Abbot-Priest sun" throughout the present paper.

It may be noted that if "noon sun"should be formally chosen by the International Committee on Illumination to be placed at the center of the Maxwell triangle, Davis-Gibson mean sun would be equally as

eligible to represent "noon sun" for this purpose as a black body at 5,000°K., "O. S. A. noon sun," or Abbot-Priest sun. Naturally, for the purposes of this paper, it has been convenient to represent our ideal energy distribution (Davis-Gibson mean sun) by r=g=b=y3 ; that is, it has been convenient to make Davis-Gibson mean

sun the "neutral stimulus." Because of this difference in "neutral stimulus," the results of our computations of trilinear coordinates differ slightly from those which would be obtained on the basis of AbbotPriest sun, which is placed at the center of the Maxwell triangle in most other colorimetric computations at the Bureau of Standards.
REPRODUCIBLE WHITE LIGHT FILTERS 47

The Optical Society of America extrapolated excitations, as published, are weighted on the basis of an energy distribution of equal

value at all wave lengths. On the form 14 shown in Figure 12 these

excitation values have been multiplied, wave length by wave length,

by the mean-sun relative energy data and the resulting values further

Form to Facilitate the Computation of Dominant Wave Length (A), Excitation Purity (Pe ), and Sunliqht Transmission (Tn ). Excitations:- Optical Society of America extrapolated.

Neutral Standard:- Mean Noon Sunlight (as adopted by Davis and

Gibson from Abbots Data). 3 80

9 4

t230

4 4 50

6070

60

90 50

12340

5 560

780

90

6

(230

40

6 560

780

90

7 00

I20

el PET

w

S3

55

51

¥1

0ETIk

to

±2.1

s z£3

Source* and

Filter

^Combination

Identification Z3GO°K

to Mean

Sun

Remarks

Sums iooooi iooooi iooooi p-lpET

f?S~S7

7-IyET Z917SB

P+7

0.3333+ 0.33333 P+7+P 0.3333+ r+g+b |

/. 0000/

Figure 12.Computation of r, g, and b for the 2,360° K.-to-mean-sun

filter In column headed T are given values of the ratio

Tfj Iter E2,36 E"

E' ~ E'

, where, as a function of wave length, Tmter is the transmission of the filter, £2,360 is the energy distribution for 2,360° K., and E' is the energy distribution of mean sun, the "neutral" stimulus. The values of ~ are taken from the last column of Table 9, and are as given in chart 2 (with one less significant figure). (It should be noted that values of r, g, and b, computed on the basis of mean sun, E', as the "neutral" stimulus

will, in general, be slightly different from those computed on the basis of Abbot-Priest sun (see fig. 3)) adjusted, respectively, by three factors so chosen that the three sums

give the same total. These adjusted values are given in Figure 12

in the columns headed pE, yE, and (3E, totaling to 100,000 each.

14 To facilitate the computations a form designed by Priest with Abbot-Priest sunlight as the "neutral "

stimulus was revised by Davis using the adopted mean sunlight as the "neutral" stimulus. Part of this form thus revised is shown in Figure 12.
48 BUREAU OF STANDARDS

If the specification of the color of a glass were to be computed

on the basis of mean sun as the "neutral" stimulus, its spectral trans- mission would be entered in the column headed T. If the color

corresponding to an energy distribution (such as a lamp-and-filter

combination) is to be specified, the ratios of that energy distribution

to mean sun must be obtained, wave length by wave length. As an

example, this ratio in the case of the 2,360° K.-to-mean-sun filter is shown tabulated in Figure 12, under column T. These values are the same as the values of E"/E' given in chart 2, except that they are

carried to an additional decimal place.

The products pET, yET^jmA (3ET were then computed asjllustrated

and the respective sums p, y, (3 were then taken, p + Y + /3 obtained,

and the trilinear coordinates (r, g, b) computed, all as illustrated in

Figure 12. In the case of mean sun, r, g, and b are each, by definition,

exactly equal to one-third.

The first step in changing the energy distribution of any particular source-and-fiIter combination so that its color should agree with that

of the adopted mean sun was to compute the r, g, and b values for

this energy distribution as shown in Figure 12. In general, although

the spectral energy distribution of the source-and-filter combination

had been adjusted to as close an agreement with that of mean sun

as was possible by the method outlined in the preceding section, the

values of r, g, and b so obtained were not exactly 0.33333. The next

step thereupon was to make still further adjustment, in all cases

small, of one or more of the concentrations of the ingredients until

the values of r, g, and b for the source-and-filter combination differed

from 0.33333 by less than 0.00006.

These final adjustments were facilitated by plotting the r-b co- ordinates to a large scale near the "neutral" point. It was soon found that changes in the concentration of any one of the three

components always shifted the point on the diagram along a certain

direction and by an amount practically proportional to the change

in concentration. After a little experience it was usually possible to obtain the final desired values with three or four trials. In addition to choosing concentrations which would give correct values of r, g, and b, a further restriction was made, viz, that when

these concentrations, for the respective A, Bx and B2 components,

were plotted against color temperature they must fall upon smooth

curves, so that values of concentrations for any intermediate temperature read from such curves would fit within the series. As finally adopted, values of concentrations were obtained which

gave (1) correct values of r, g, and b, viz, 0.3333, (2) smooth curves when plotted against color temperatures, and (3) energy distributions

as illustrated in the charts. This was done at every 100° K. from

2,300° to 4,000° K., besides 2,360° and 2,848° K. These values of

concentration are given in Table 8, the values at intermediate color

temperatures being obtained from the concentration curves by

interpolation.

A detailed example of the computation of E" and E"IE' is given

in Table 9, which is self-explanatory. The 2,360° K-to-mean-sun

filter (chart 2) was chosen for this purpose. The data shown are

for the respective concentrations as finally determined, giving values

(fig. 12) of r, g, 6 = 0.3333. The same laborious computation had to be carried through at each set of trial concentrations in order to get
REPRODUCIBLE WHITE LIGHT FILTERS 49

values of E"IE' for the trial computations of r, g, and b. A procedure

similar to that herein described was carried out for all of the 38

filters illustrated in the charts.

Table 8.

Concentrations and light transmissions of filters of Series I, 2,300°-

4,000° K. to mean noon sunlight 1

Grams of copper

sulphate and

grams of mannite

per liter in solu- tion A

Grams of cobalt ammonium

sulphate per liter in solu- tion B

Grams of copper

sulphate

per liter in solu- tion B

Light

trans- mission

3. 901 28.300 28. 250 0. 1224

3. 835 27. 80 27. 90 . 1266

3. 770 27. 31 27. 54 . 1309

3. 707 26. 827 27. 180 . 1352

3. 644 26. 35 26. 83 . 1397

3. 582 25. 870 26. 480 .1442

3. 522 25. 40 26. 13 . 1487

3. 463 24. 94 25. 78 . 153

3. 405 24. 49 25. 43 . 158

3. 348 24. 04 25. 08 . 163 3.292 23. 590 24. 730 .168

3. 237 23. 14 24. 38 . 173

3. 183 22. 70 24. 04 . 177

3. 128 22. 27 23. 69 . 183

3. 076 21. 83 23. 34 . 188

3. 024 21.400 23.000 .193

2. 973 20. 97 22. 66 . 198

2. 923 20. 56 22. 32 . 203

2. 874 20. 14 21. 98 . 209

2. 825 19. 73 21. 64 .214

2. 777 19. 325 21.300 .220

2. 730 18. 92 20. 97 . 225

2. 684 18. 54 20. 64 . 231

2. 639 18. 16 20. 33 . 236

2. 595 17. 78 20. 02 . 242

2.551 17. 400 19. 720 .248

2. 507 17. 02 19. 44 . 253

2. 464 16. 66 19. 15 . 259

2. 421 16. 30 18. 86 . 265

2. 379 15. 94 18. 58 .271

2. 337 15. 580 18. 300 . 277

2. 296 15! 23 18! 03 !283

2. 256 14.89 17. 77 .289 2.217 14. 56 17. 51 .295 2.178 14.23 17.25 .301 2.140 13. 900 17.000 .307

2. 102 13. 58 16. 74 .313

2.065 13.27 16.49 .320

2. 028 12. 97 16.24 .326

1.992 12. 66 16.00 .332

1. 956 12. 360 15. 760 .338

1. 921 12.06 15. 51 .335

1.886 11. 76 15.27 .351

1. 851 11.47 15.04 .357

1. 817 11. 18 14. 80 .364

Color temperature (°K.)

of source

3,200

3,220

3,240

3,260

3,280

3,300

3,320

3,340

3,360

3,380

3,400

3,420

3,440 3,460

3,480

3,500

3,520

3,540

3,560

3,580 3,600

3,620

3,640 3,660 3,680

3,700

3,720 3,740

3,760

3,780

3,800

3,820

3,840

3,860 3,880

3,900

3,920

3,940 3,960

3,980. 4,000.

2,848.

Grams of copper

sulphate and

grams of mannite

per liter in solu- tion A

1.784

1.751 1.719

1.687

1. 655

1. 624

1.594

1. 564

1.534

1. 504 1.475

1.446

1.417

1. 388

1. 360

1. 332

1. 305

1. 278 1.251

1. 225

1. 199

1. 173

1. 148

1. 123 1.098

1.074

1. 050

1.026

1.002 .978

.955

.932 .910

.845 .824

.803 .782

;761

;741 2.445

Grams of cobalt ammonium

sulphate

per liter in solu- tion B

10. 890

10. 60

10. 32

10. 03

9. 75

9. 470

9. 19

8. 92

8.64

8. 37

8. 100

7. 83 7.57

7. 31 7.05

6. 790

6. 53 6.29

6. 05 5.81

5. 570

5. 33 5.09

4. 85 4.61

4. 380

4. 15

3. 92

3. 70 3.48 3.260

3.04

2. 82

2. 60 2.39 2.175

1.96

1.75

1.54 1.34 1.130

16. 520

Grams of copper

sulphate

per liter in solu- tion B

1 Spectrophotometric illustrations are given in charts 1 to 20; methods of computation are described in Section V, 4; details of preparation are given in Section IX.
50 BUREAU OF STANDARDS

llll iiiii §§§ii iiiii

O HH rArArArlrA rA ' ' ' *

-11.

12

-11.76 -10.

56

-7. 54

-12.

87

—

13. 35

-8. 49

+1. 10

+3. 44 +

1.79 +

1.

62

+3. 00

+5. 97

+6. 54

+2.

08

-2. 88-6. 17

-4.

30

-.28

E' (from

Table

1) i&^gg SSSSfc fcSSSsJ jdti&g sssssi ssssg ssss|

k

SS^SS Sgggfi S^SSS g^sss

(N^oo^'oi oicst^o-; a6c6ci*6tA. mooooo rn<N co-*iot^oo $>oo oo Oooo©

Mi

SSSSSg ggsss g22g£

ci^ao^^ c^oo^oi d^-ccoo loddf-^ rH« CO^^CO OOOOOS 00,000

Y{

iisii mil mm mil

O • 'tAtA HHi-iHH rJ r4 <N <N <N (rf r-i rH r-i CN

ii

iiiii mil iiiii mil . O tAtAtA rA rA rA rA rA tH r4 r-i rA rA

O+

22222 22222 22222 22222 m m m m

sasis ssggi ssisg sassa

Be, mil iiiii iiiii iiiii

PHI 11111 1111! Illil

< 1111! Iiiii IIIII IIIII

< IIIII IIIII Illil iiiii

< iiiii iiiii inn mil

^ |8£g§ 8233S §8£gg 828S3

1I1SS IsSli Iiiii iiiii

COCOCO'cOCO MMMMCO CO CO CO CO CO CO CO CO CO* CO
REPRODUCIBLE WHITE LIGHT FILTERS 51

mil urn mil in

ssss?; sssss sss? § +++ I I I

' ++ +++++ + l I +

£g£23 -S223 S§8

ggSSSS

So3»3 SSSSS 88338 §

gggSS SSSSSS 1

££8'~3 838 8

SSSSS ggSttSi SSE I

mil 11111 mi! 11

i ,_; ^- ,_;

,

urn mil iiiii 11

mil iiiii iiiii 11

IIIII IIIII IIIII 1!

IIIII fill! IIIII 1!

!82gg §2332 §82S8 82

islSi iiiii iiiii ii WMMMM COCOCOCOCO MMMWM CO CO

iiiii IIIII iiiii III If C* 04 04 ~H 1-i fH-rt'r-irtrt r-i r-i nH -h" HHr

Hill iiiii Hill HI

r4<N(N<N(N esesiNesoi oicioicici <n<n<n

22222 22222 22222 222 gj|

iiiii iiiii iiil Hi 1!

1

li

a

%mm mm mm n

>4

pa

III

IS

1

si
52 BUREAU OF STANDARDS

The complete computation of a filter is a long process and the

chances for error are, of course, correspondingly large. To avoid any

reasonable possibility of such errors, even though computing machines

were used in all computations, complete independent recomputations were made on each filter, starting with the fundamental spectrophotometric data.
REPRODUCIBLE WHITE LIGHT FILTERS 53

The filters of Series I are described and illustrated in charts 1 to

20, inclusive, and in Table 8 and Figure 13.

(c) TO COMPUTE LIGHT TRANSMISSION OF FILTERS

It is customary in photographic sensitometry to specify the intensity of the light source in terms of visual candlepower. If the light

used is obtained from a source-and-filter combination, it is essential

(in accord with this custom) to know the effective candlepower of

this source-and-filter combination. If the transmission of the filter for the light given by the source is known, the candlepower of the

combination may be obtained by multiplying the candlepower of

the source by the transmission of the filter. The transmissions of

the filters are also of interest in their applications to colorimetry.

The light transmission of the filters may, of course, be obtained

directly by photometric measurements, as, for example, by the

nicker method. However, in the case of colored filters the results may be influenced by the photometric field size and brightness and

may also vary from one observer to another because of the different

values of relative visibility possessed even by observers of normal

color sense. This difficulty may be avoided by computing the light trans- mission from the spectral transmission, using standard visibility data.

The values of light transmission so obtained should, for the present

type of filters, be practically identical with the values that would

be obtained if the light transmissions had been directly measured

under standard photometric conditions by a large number of observers

and averaged.

The light transmissions for the present filters have been computed

by the following formula

;

S720(EeVT)

40 > I (EeV)

Z-J400

The summations were carried out using values at every lOnuz from

400m/* to 720nux. In the formula 6 refers to the color temperature

of the light source, this varying from 2,300° to 4,000° K. Te is the light transmission of the filter for a 6° K. source, T is the spectral

transmission tabulated on the charts, Ee is the relative energy for

0° K. as given in Table 2, and V is the relative visibility function as given in Table 10. These values of relative visibility are those

recommended by Gibson and Tyndall (11, 12) and adopted by the

International Commission on Illumination (10) and by the American

Engineering Standards Committee and by the Illuminating Engineering Society (38).

The visibility data in Table 10 are given from 400m/* to 760m/t

while the spectral range used to compute the light transmission of the

filters does not go above 720mju. It was found by trial that excluding

values of T, Ee , and V above 720nnz caused an error so small (not more than 0.00003 in the light transmission of any of the filters) that

they could be safely omitted.
54 BUREAU OF STANDARDS

Values of light transmission for the various filters of Series I thus

computed are given on the charts and in Table 8 and illustrated in Figure 13. Computations were carried out at the same color temperatures as used for the colorimetric computations. The intermediate

values of Table 8 were obtained by plotting the computed values

against color temperature and reading the desired values from this curve.Table 10.

Standard values of the relative visibility of radiani energy

Wave lengtn m millimicrons Relative

visibility 1 \v ave lengtn in millimicrons Relative

visibilityi

400 0.0004 .0012

.0040

.0116

.023 .038

.060

.091 .139 .208

.323

.503 .710 .862

.954

.995

.995^ .952 .870

.757

600 0. 631 .503

.381 .265

. 175

. 107 .061 .032

.017 .0082

.0041

.0021 .00105 .00052

.00025 .00012

.00006

410 610

420 620 430 630

440 - 640 450 650 460 660

470 670 -

480 680 490 690

500 700 510 710

520 720 530 730

540 740... 550 750 560 - 760 .... 570

580

590

i These values were recommended by Gibson and Tyndall (B. S. Sci. Paper No. 475, p. 174; 1923; Trans.

I. E. S., 19, pp. 192-193; 1924; Proc, International Commission on Illumination, sixth meeting, 1924, p. 235). They have been adopted as standard by the International Commission on Illumination (Proc,

p. 67, sixth meeting) and by the American Engineering Standards Committee and the Illuminating Engineering Society (Trans. I. E. S , 20, pp. 629, 632; 1925).

5. DESIGN OF FILTERS OF SERIES 11—2,848° TO 3,500°-10,000° K. Because of the general interest in high color temperatures in the

fields of colorimetry and astronomy and the limited means available

for the reproduction of high color temperatures in the laboratory, it has seemed desirable to develop this second series of filters, in this case starting at a fixed lower color temperature of 2,848° K. By

means of this series of filters, radiant energy evoking the color of a Planckian radiator at any temperature between 3,500° and 10,000° K.

may be readily obtained in the laboratory. The particular starting

point, 2,848° K. was chosen for the filters of Series II for the reason that

it happens to be the color temperature of a certain primary standard

lamp and thus came to be a common reference point. This particular

color temperature is a very suitable one at which to operate gas-filled

lamps.

Before proceeding with the computations for the filters of Series

II, values of r, g, and b had to be computed for all the black-body

temperatures between 3,500° and 10,000° K. for which filters were

to be specifically designed. This was done and values of r, g, and b obtained as given in Table 11 and illustrated in Figure 14. (A similar

illustration for color temperatures from 2,000° to 4,000° K. is given

in the next section.)
REPRODUCIBLE WHITE LIGHT FILTERS 55

After designing these niters for what seemed to be the best energy

match, the colorimetric computations were carried out, striving now

to make the values of r, g, and b not equal to 0.33333, as was the case u—

Ns

«n, Dec 21 — a-

^1

Sun ®?v

, Sun June . ^ >

,rc k • 7

.26 .30 .32 .34 .36 38 .40 .42 .44 .46 .48 50 b Figure 14.

Section of the r-b diagram showing colorimetric -positions of various phases of sunlight relative to the black-body locus

(See fig. 16 for temperatures between 2,000° K. and 4,000° K.) Values of r, g, and 6 are given in Table 11, for every 20° K. up to 8,000° K. and for every 100° K. above that. All

value's of r, g, and b are computed on the basis of mean noon sunlight as the "neutral"

stimulus. * with the filters of Series I, but equal to the respective values given

in Table 11 for the particular color temperatures involved.

Table 11.Trilinear coordinates (r, g, b) of colors evoked by black-body radiant energy as a function of color temperature

Trilinear coordinates 2

0. 64249

.6407

6352

6333

6315

,6297 ,6279 .6262

, 62435

.6226 .6208 .6192

,6173

0. 32033

.3214

.3224 .3235 .3245

.3255

.3264

.3274 .3283 .3292

. 33010

.3309

.3318

.3327

.3336

0. 03718

.0379

.0388 .0397 .0404

.0413 .0422

.0430 .0438

.0447

. 04555

.0464

.0473 .0482

.0491

Color temperature

750. 760. 770.

7b0.

S00. 810.

820.

830. 840.

850. 860.

870. SS0- 890.

Trilinear coordinates 2

r 9 6

0. 6157 0. 3343 0.0499

.6139 .3352 .0508 .6123 .3359 .0518 .6106 .3367 .0527 .6089 .3374 .0536

. 60725 . 33815 .05460

.6056 .3388 .0555 .6039 .3396 .0564

.6023 .3403 .0573 .6007 .3409 .0583 .5991 .3416 .0593

.5974 .3422 .0603 .5958 .3428 .0613 .5943 .3434 .0622 .5927 .3441 .0632

1 C2= 14,350 micron-degrees. See Table 2.

2 "Neutral" stimulus: Mean sun as adopted in this investigation. Excitations: O. S. A. extrapolated

(J. O. S. A.andR. S.I., 10, p. 230; 1925). Computed values are given to five significant figures; other values,

to four significant figures, were read from large scale graph plotted from computed values.
56 BUREAU OF STANDARDS

Table 11.Trilinear coordinates (r, g, b) of colors evoked by black-body radiant energy as a function of color temperature—Continued

Color temperature

'K

1,900.

1,910.

1,920- 1,930.

1,940-

1,950.

1,960-

1,970. 1,980- 1,990.

2,000. 2,020.

2,040. 2,060.

2,080.

2,100. 2,120. 2,140-

2,160.

2,180-

2,200. 2,220.

2,240.

2,260-

2,280.

2,300 _ 2,320.

2,340. 2,360.

2,380.

2,400. 2,420-

2,440. 2,460-

2,480.

2,500.

2,520. 2,540-

2,560. 2,580.

2,600. 2,620 _ 2,640- 2,660.

2,680-

2,700. 2,720.

2,740.

2,760. 2,780.

2,800.

2,820. 2,840.

2,860. 2,880.

2,900.

2,920.

2,940.

2,960.

2,980.

3,000. 3,020.

3,040.

3,060.

Trilinear coordinates 2

0. 59108

.5895

.5879

.5864 .5848

.5833

.5818 .5802

.5787 .5772

. 57571 .5727 .5697

.5668

. 56101 .5582

.5554

.5526 .5499

. 54716

.5445 .5418

.5391 .5365

.5339

.5314

.5288

. 52631 .5238

. 52133

.5189

.5164

.5141 .5117

. 50936

.5070 .5047

.5024 .5001

.49788 .4956

.4934 .4912 .4890

.4848 .4827

.4806

.4786

. 47657

.4745

.4725 .4705

.4685

4646 4628

4609

4590 45716

4553 4534

4516

0. 34470

.3452

.3458

.3464 .3469

.3475

.3480

.3486

.3491 .3495 .34998

.3509

.3518

. 3526 .3534

.35416 .3549

.3555 .3562

.3568

.35727

.3578

.3583 .3588 .3592

.3596

.3599

.3603

. 36063

.36117

.3614 .3616

.3618 .3620

. 36217 .3622

.3623

.3624 .3625

. 36259

.3627

.3627

.3627

.3627

. 36260

.3625 .3624

.3623 .3622

. 36218 .3620

.3619 .3617 .3616

.36143 .3612

.3610

.3608

.3606

3599

3597 3594

0. 06422

.0652

.0662

.0672

.0682

.0692 .0702

.0712

.0722

.0733

. 07431

.0764 .0785

.0806

.0827

. 08483

.0870

.0891 .0912

.0934

. 09556

.0978

.1000

.1021

. 1033. .1065

.1087 .1109

. 11306 .1153

. 11750

.1198

.1220 .1242

.1264 .12847

.1308

.1330 .1352

.1374

. 13953

.1417

.1439

.1461 .1483

. 15050 .1527

.1549

.1571 .1592

. 16125 .1635 .1656

. 1678

. 17198 .1741 .1762

.1783 .1804

. 18240

.1845

. 1866 .1887

.1907

Color temperature :

Trilinear coordinates 2

9 5

0. 4480 9. 3592 0. 1927

. 4463 . 3589 . 1947

. 4446 . 3587 . 1967 .4429 .3584 . 1987 .4411 .3581 .2007

. 4394 . 3578 . 2027

. 4377 . 3575 . 2047

. 4360 . 3572 . 2067 .4344 .3569 .2086

.4328 .3566 .2106

. 4312 . 3563 . 2125

. 4297 . 3560 . 2144

. 4281 . 3557 . 2163 .4265 .3553 .2182 .4249 .3550 .2201

. 4234 . 3546 . 2220

. 4219 . 3543 . 2239

. 4203 . 3540 . 2258 .4188 . 3537 .2276 .4173 .3533 .2294

. 41592 . 35290 . 23118

. 4145 . 3525 . 2331

. 4130 . 3522 . 2349 .4116 .3518 .2367

.4101 .3514 .2385

. 4087 . 3510 . 2403

. 4074 . 3506 . 2421

. 4060 . 3502 . 2438 .4046 .3498 .2456

.4033 .3495 .2473

. 4019 . 3491 . 2490

. 4006 . 3488 . 2507

. 3993 . 3484 . 2524 .3980 .3480 .2541 .3967 .3476 .2558

. 3954 . 3472 . 2575

. 3941 . 3468 . 2591

. 3928 . 3464 . 2608 .3916 .3460 .2624

.3903 .3456 .2641

. 3891 . 3452 . 2657

. 3879 . 3448 . 2673

. 3867 . 3444 . 2689 .3855 .3440 . 2704 .3843 .3437 .2720

. 38313 . 34332 . 27356

. 3819 . 3429 . 2752

. 3808 . 3425 . 2767 .3797 .3421 .2782 .3785 .3417 .2787

. 3774 . 3413 . 2812

.3763 . 3409 . 2827 .3752 . 3405 . 2842

. 3741 . 2857 .3730 .3398 .2872

.3719 .3394 .2887

.3708 .3390 .2901 .3698 .3386 .2916 .3687 .3382 .2930 .3677 .3378 .2945 .3667 .3374 .2959

.3656 .3370 .2973

.3645 .3367 .2987

.3635 .3363 .3001 .3625 .3359 .3015

2 See footnotes 1 and 2, p. 55.
REPRODUCIBLE WHITE LIGHT FILTERS 57

Table 11.

Trilinear coordinates (r, g, b) of colors evoked by black, body radiant energy as a function of color temperature—Continued

Color temperature

Trilinear coordinates 2

0. 3615 .3605 .3596

.3586

.3576

. 35675

.3557

.3548

. 3539 .3530

.3521 .3512

.3503 .3494

.3485

.3476

.3468

.3459

.3451 .3443 .3434

.3426

.3418 .3410

.3401 .3393

.3386

.3378

.3370

.3362

. 33543 .3346

.3339

.3331 .3324 .3316

.3309

.3301 .3294

.3287 .3280

.3273 .3265

.3258

.3252 .3245

.3238

.3231 .3224

.3218

.3211 .3204

.3198 .3192

.3185

. 31788

.3173

.3167 .3160

.3154

.3148 .3142

.3136

.3130

.3124

0. 3355

.3351 .3347

.3343

. 33352 .3332

.3329

.3325 .3321

.3317

.3313

.3309 .3305 .3302

.3298 .3294

.3290 .3286

.3282

.3278 .3274

.3271 .3268 .3264 .3260

.3257 .3253

.3249

.3246

. 32419 .3238

.3235

.3231 .3228 .3224 .3220

.3217

.3213 .3210

.3207

.3203 .3200

.3197

.3193

.3189 .3186

.3183

.3179 .3176

.3173

.3169 .3166

.3163 .3160

. 31566 .3153 .3150

.3147

.3143 .3140

.3137

.3134

.3131 .3128

3070

,3084

,3110

,3123

,3136

,3149 ,3162

,3175

,3188

,3201 ,3214

.3226

,3238

.3251 ,3263 ,3275

,3287

,3299

,3311 .3323

.3335

,34038

,3416

,3427

,3438

,3449 .3460

.3471 ,3482

.3493

.3503 .3514

.3525

.3535 .3546

.3556

.3567

.3577

.3587

.3597

.3607 .3617

.3627 .3636

.3645

.3655

. 36647

.3674

.3684

.3693 .3702 .3712

.3721 .3730

.3739 .3748

Color temperature

6,820.

6,840. 6,860.

6,900.

6,920.

6,940.

6,960. 6,980.

Trilinear coordinates 2

r g b

0. 3118 0. 3125 0. 3757

'. 3112 '. 3122 1 3766

'. 3106 . 3119 . 3775 13100 .3116 !3784

.3094 .3112 .3793 .3088 . 3109 . 3802 !3083 .3107 . 3811 !3078 .3103 . 3819 .3072 .3100 .3828 .3066 .3097 .3837

. 3061 . 3094 . 3845

. 3056 ! 3092 . 3853 3050 ." 3089 . 3862 .3044 .3086 .3870

. 3039 .3083 .3878

. 30332 . 30801 . 38867

.3028 .3077 .3895

3023 . 3074 .3903

.3018 .3072 .3911 .3013 .3069 .3919

. 3008 . 3066 .3927

. 3002 . 3063 .3935

.2997 3060 . 3942 .2992 .3057 .3950

.2987 .3054 .3958

.2982 . 3052 .3966

1 2977 '. 3049 . 3973

. 2972 . 3046 . 3981 .2968 .3043 .3988

.2963 .3041 .3996

. 2958 . 3038 . 4003

. 2953 . 3035 . 4011

'. 2949 '. 3032 . 4018 .2944 !3030 .4026 .2939 .3027 .4033

. 2935 . 3024 . 4040

. 2930 . 3022 . 4047

'. 2926 '. 3019 ! 4054

!2921 ! 3017 !4062 .2917 .3014 .4069

. 29117 . 30124 . 40759

! 2908 . 3009

' 4083

! 2903 '. 3007 . 4089 !2899 !3004 !4096 .2895 .3002 .4103

. 2890 . 2999 . 4110

. 2886 . 2997 . 4117

. 2882 . 2994

* 4123 !2877 !2992 !4130 .2873 .2990 .4137

. 2869 . 2988 .4143

. 2865 . 2985 *4150

. 2861 '. 2982 . 4157 !2857 !2980 !4163 .2853 .2978 .4169 .2849 .2975 .4176

.2844 .2972 .4183 .2840 .2970 .4189 .2836 .2968 .4195 .2832 .2966 .4202 .2828 .2964 .4208

.2824 .2961 .4215

.2820 .2959 .4221

.2817 .2957 .4227

.2813 .2955 .4233 See footnotes 1 and 2, p. 55.-
58 BUREAU OF STANDARDS

Table 11.

Trilinear coordinates (r, g, b) of colors evoked by black-body radiant energy as a function of color temperature—Continued

Color temperature 1

Trilinear coordinates 2

r g b

ft 9Rft8fi ft 90^90 fl 49^04. u. tzoy-t

. 2805 . 2950 4245

. 2801 2948 4252

.' 2797 '. 2945 !4258 .2793 .2943 .4264 2790 2941

. _

. 4z/0

2786 2939 . 4275

. 2782 2937 4281

! 2779 !2934 !4287 .2775 .2932 .4293

2771 2930 4900

2768 2928 A1(\A . 1o\ri

. 2764 2926 4310

!2761 ^2923 !4315 .2758 .2921 .4321

2754 2919 4327

97 ^fl .2917 . 4332

. 2747 . 2915 4338 !2744 !2913 '. 4343 .2740 .2911 .4349

2737 2909 4354

. 2733 . 2907 4360

. 2730 2904 4365

.' 2727 !2902 !4371 .2723 .2900 .4376

. 2720 2898 4382

. 2717 . 2896 4387

2713 2894 4393 !2710 !2892 !4398

.2707 .2890 .4403

2704 OQCQ . ZooS . 4408

2701 . Z580 .4412

2698 2884 4417 !2695 !2882 !4422

.2692 .2880 .4427

2689 9C7S . Zo/O . rHiOZ

. 2686 . 2876 . 4438

OAS'}

. 2874 . 4443 .2680 .2872 .4448 .2677 .2870 .4453

0R7A . 2869 . 4458

OA71 . ZD/ I . 2867 . 4463

. 2668 . 2865 . 4468

.2665 .2863 .4472

.2662 .2861 .4477

2659 . zooy A/1C9 . 445Z

. 2656 . 2858 . 4487

. 2654 . 2856 . 4491

.2651 .2854 .4495

.2648 .2852 .4499

. 26453 . 28508 . 4l>039

. 2631 . 2842 . 4527

. 2618 . 2833 . 4549

. 2605 . 2825 . 4571 !2592 '. 2817 !4592 .2580 .2809 .4612

.2568 .2800 .4632

.2557 .2793 .4651 .2545 .2787 .4669 .2534 .2779 .4688

. 25225 . 27710 . 47065 .2513 .2764 .4723 .2502 .2758 .4740

.2493 .2751 1 .4757 .2483 .2744 .4774

Color temperature 1

°K.

9,500

9,600 9,700

9,800

9,900 10,000

10,100 10,200

10,300

10,400

10,500

10,600

10,700

10,800

10,900

1],000 11,100

11,200

11,300 11,400

11,500

11,600

11,700 11,800 11,900

12,000

12,100

12,200 12,300

12,400

12,500 12,600

12,700

12,800

12,900 13,000

13,100

13,200

13.300 13,400

13,500

13,600

13,700 13,800

13,900

14,000

14,100

14,200 14,300 14,400

14,500

14,600

14,700 14,800 14,900

15,000

15,100 15,200 15,300

15,400

15,500

15,600 15,700

15,800

15,900

Trilinear coordinates 2

x y 5

0. 24 m 0. 2/37 0. 4791

. 2463 . 2730 . 4807

. 24.54 . 2724 . 4823

.2445 .2718 .4848

.2436 .2712 .4852

. 242/7 . 27060 . 48663

. 2420 . 2700 . 4880

.2412 . 2695 . 4894 .2404 .2690 .4907 .2397 .2684 .4920

. 2389 . 2679 . 4932

. 2381 . 2674 . 4945

. 2374 . 2669 . 4957

.2367 .2664 .4969 .2360 .2659 .4981

. 2353 . 2654 . 4993

. 2347 . 2649 . 5005

. 2340 . 2645 . 5016 .2333 .2640 .5027 .2327 .2636 .5038

. 23zl . 2631 . 5049

. 2314 . 2627 . 5059

. 2308 . 2623 . 5069

. 2302 .2618 .5079 .2297 .2614 .5089

. 22916 . 26097 . 50987

. 2286 . 2606 . 5107

. 2281 . 2602 . 5116 .2276 .2599 .5125 .2271 . 2595 .5134

. 2266 . 2591 . 5142

. 2262 . 2588 . 5151

. 22.57 . 2584 . 5159

. 2252 .2581 .5167

.2248 . 2577 .5175

. 2243 . 2574 . 5183

. 2238 . 2571 . 5191

. 2234 . 2567 . 5198 .2229 .2564 .5206 .2225 .2561 .5213

. 2221 . 2558 . 5221

. 2217 . 2555 . 5228

. 2213 . 2552 . 5235 .2209 .2549 .5242 .2205 .2546 .5249

. 22010 . 25432 . 52559

. 2197 . 2540 . 5262

. 2193 . 2537 . 5269 .2190 .2535 .5275 .2186 .2532 .5282

. 2182 . 2529 . 5288

. 2179 . 2527 . 5294

. 2175 . 2524 . 5300 2172 . 2522 . 5306

.' 2169 '. 2519 '. 5312 .2165 .2517 .5318

.2162 .2514 .5323

.2159 .2512 .5329 .2156 . 2509 .5334 .2153 .2507 .5340

.2150 .2505 .5345

.2147 .2503 • .5350

.2144 .2500 .5356

.2141 .2498 .5361 .2138 .2496 .5366 See footnotes 1 and 2, p. 55.
REPRODUCIBLE WHITE LIGHT FILTERS 59

Table 11.

Trilinear coordinates (r, g, b) of colors evoked by black-body radiant energy as a function of color temperature—Continued

Color temperature

Trilinear coordinates 2

0. 21355

.2133

.2130

.2127 .2125 .2122

.2120

.2117 .2115 .2112

.2110

.2107

. 2105

.2103 .2100

.2094 .2092 .2090

. 20873 .2085

0. 24938 .2492

.2490

,2484 ,2482

,2480

,2478

,2476

,2474

,2472

,2471 ,2469 2467

2465

2464 2462

2460

,24571 ,2455

0. 53707 .5376

.5381 .5385 .5390

.5395

.5399

.5404

.5408

.5412

.5417

.5121 .5425

.5429

.5433

.5437

.5441 .5445

.5448 .5452 .54556

Color temperature 1

°K

18,200

18,300

18,400 18,500

18,600 18,700

18,800

18,900 19,000

19,100 19,200

19,300

19,400

19,500

19,600

19,700 19,800 19,900 20,000

2,848

Trilinear coordinates 2

r g 6

0. 2083 0.2454 0. 5463 .2081 .2452 .5466 .2079 . 2451 . 5470 .2077 . 2449 . 5473

. 2075 . 2448 . 5477 .2073 !2446 .5480 .2072 .2445 .5483 .2070 .2443 .5487

. 2068 .2442 .5490

.2066 .2441 .5493 .2064 .2439 .5496 .2062 .2438 .5499

.2061 .2436 .5503 .2059 .2435 .5506

.2057 .2434 .5509 .2056 .2432 .5512

.2054 .2431 .5515 .2052 .2430 .5518 .20507 .24283 . 55210 .47172 . 36185 . 16643 U See footnotes 1 and 2, p. 55. The methods of computation were the same as outlined in Section

V, 4. The range of concentrations of the ingredients is about the same as was used in the mean-sun series, only the relative proportions

are different. As just noted, each filter when combined with a blackbody energy distribution at 2,848° K. gives by computation, an

accurate color match with the respective high color temperature.

A maximum deviation of but 0.00006 between the actual and desired

values of r, g, and b was obtained. However, the energy distributions

obtained with this series do not, in general, match the ideal as well as

in Series I, due to the high relative energies in the violet and ultraviolet in black-body color temperatures above 4,000° K. (See

chart 8 for 2,848° to sunlight, Series I, and chart 24, 2,848° to 5,000°,

Series II.) The deviations in the energy distributions may be noted on the charts. The light transmissions, computed in all cases for 2,848° K., vary from 0.497 for the 3,500° K. filter to 0.0989 for the

10,000° K. filter. Computations were thus carried out with filters designed (with

2,848° K. source) to give 3,500°, 4,000°, 4,500°, 5,000°, 5,500°,

6,000°, 6,500°, 7,000°, 8,000°, 9,000°, and 10,000° K. These computations gave values of concentration and transmission which, when

plotted against color temperature, gave smooth, regular curves from

which intermediate values were obtained as was done for Series I. Likewise, as before, complete independent recomputations were made

on all the filters. The filters of Series II are described and illustrated in charts 21 to

31, inclusive, and in Table 12 and Figure 15. 13416°—31 5
BUREAU OF STANDARDS
REPRODUCIBLE WHITE LIGHT FILTERS 61

Table 12.

Concentrations and light transmissions of filters of Series II, 2,848° K.

to 8, 500°-10,000° K. 1 Grams of copper

sulphale and

grams of mannite

per liter in solu- tion A

1. 113

1. 249

1. 378

1. 501

1. 616

1. 726 1.832

1. 932

2. 028

2. 120 2.208

2. 293

2. 374

2. 452

2.527

2. 599

2. 668

2. 735 2.799

2. 860

2. 920

2. 977

3. 033

3. 086

3. 138

3. 188 3.236 3.282

3. 327

3. 370

3. 412

3. 453

3. 493

Grams of cobalt ammoni- um sul- phate

per liter in solu- tion B

9. 780

10. 96

12. 08

13. 14

14. 16

15. 133

16. 07

16. 97

17. 84

18. 67

19. 470

20. 24

20. 99

21. 71

22. 41

23. 080

23. 71

24. 32

24. 91

25. 47

26. 020

26. 54

27. 05

27. 55

28. 02

28. 480

28. 93

29. 36

29. 78

30. 19

30. 580

30. 97 31.34

Grams of copper

sulphate

per liter in solu- tion B

7. 197

8. 07

8. 91

9. 72

10. 49

11. 220

11.93

12. 61 13.26

13. 88

14. 490

15.05

15. 59

16. 11

16. 61

17. 090

17.55

17. 99

18. 41

18. 82

19. 215

19. 60

19. 97

20. 32

20. 67

21. 000

21.32

21.64

21.94

22.24

22. 520

22. 79

23. 06

Light

trans- mission

of filter for 2,848° K.

0. 497

.465

.435

.408 .383

.361 .341

.324 .309

.295 .282

.270

.259 .249

.240

.231 .223

.216 .209

.203

.197 .192

.186 .182

.177 .173

.169

.165

.161

.158

.155 .152

1.487

Color temperature (°K.)

of filter with

2,848° K.

6, 800.

6, 900.

7, 000.

7, 100.

7, 200.

7, 300.

7, 400.

7, 500.

7, 600.

7, 700.

7, 800.

7, 900.

8, 000.

8, 100-

8, 200.

8, 300.

8, 400.

8, 500.

8, 600.

8, 700.

8, 800.

8, 900.

9, 000-

9, 100.

9, 200.

9, 300.

9, 400.

9, 500.

9, 600.

9, 700.

9, 800.

9, 900.

10,000

Grams of copper

sulphate and

grams of mannite

per liter in solu- tion A

3. 532

3. 569

3. 605

3. 640

3. 675

3. 708 3.740

3.772

3. 802

3. 832

3. 861

3. 889

3. 916

3. 943

3. 969

3. 994 4.018 4.042

4.066

4.089

4.111

4. 133

4. 154

4. 175

4. 195 4.215

4.234

4. 253 4.272 4.290

4.308 4.325 4.342

Grams of cobalt ammoni- um sul- phate

per liter in solu- tion B

31.70

32. 06

32. 400

32. 73 33.06

33. 38

33. 68

33. 98

34.27 34.56

34. 83

35. 10

35. 355

35. 60

35.84

36. 08

36. 30

36. 52

36. 74

36. 95

37. 15

37. 35

37. 550

37.74

37. 93

38. 12

38. 31 38.49

38. 67 38.84

39. 02

39. 19

39. 353

Grams of copper

sulphate

per liter in solu- tion B

1 Spectrophotometry illustrations are given in charts 21 to 31; methods of computation are described in Section V, 5; details of preparation are given in Section IX.

6. DESIGN OF MISCELLANEOUS FILTERS

(a) ACETYLENE TO MEAN NOON SUNLIGHT

The acetylene flame has long been used as a light source for the

sensitometiy of photographic negative materials. The color of the

light is whiter than that of other gas flames, or oil and spirit lamps,

and approaches in quality the vacuum incandescent tungsten electric lamp. Since the flame is not homogeneous in color and intensity, it has been the custom to use a screen with an aperture close to the flame

to limit the area used. While the color and intensity of the flame are

slightly affected by atmospheric conditions (humidity and pressure)

it is generally considered to be a reliable working standard of light

quality.

# Its popularity is due chiefly to the simple and relatively

inexpensive equipment necessary to operate it under standard conditions. The burner designed by Mees and Sheppard (39) and sold by

the Eastman Kodak Co. is a standardized product and has practically

displaced all other types of acetylene burners for photographic sensitometry.
62 BUREAU OF STANDARDS

The color temperature of the Eastman Kodak Co.'s burner is generally taken to be 2,360° K., although the energy distribution measured radiometrically by Coblentz gives a higher value as noted else- where. (Sees. Ill, 4 and VIII, 6.) Figure 16 is a section of the r-b diagram showing the locus 15 of points representing colors of the black body from 2,000° to 4,000° K.,

together with the position of the acetylene burner computed from

Coblentz's data (r = 0. 51778, # = 0.35719, 6 = 0.12503). (A similar

illustration for color temperatures from 4,000° to 10,000° K. is given

in the preceding section.)

A filter was designed to convert the color-quality of the light from

the acetylene lamp to that of mean sun, with the idea that it could

k

\ .4 * o\

\•v6—

V —

i_ -•+ \k V U \

r"'V

> bA

. .08 40 .12 .14 JB .18 .20 .22 24 .26 28

h Figure 16.

Section of the r-b diagram showing the black-body

locus and the position of the acetylene flame (as computed

from Coblentz's energy data. Table 8)

(See fig. 14 for temperatures between 4,000° and 10,000° K.) Values of r, g, and b are given for every 20° K. in Table 11. All values of r, g, and b are computed on the basis of mean noon sunlight as the "neutral" stimulus. be used for determining the color temperatures of incandescent lamps

as outlined in Section VI, 2, (d), as well as for photographic sensitometry. The spectral energy distribution used in the computation of this filter was that of Coblentz as given in Table 3, reduced to 100.00

at 560nux. The acetylene-to-mean-sun filter, by computation, gives a better energy match with the ideal than does that of the 2,360°

K.-to-mean-sun filter, as may be seen by comparing charts 36 and 2. The use of this filter in color temperature determinations is accompanied, however, by some uncertainty, because of the imperfect color

« The expression " colorimetric locus" will hereafter' be used to designate the locus of points representing

colors on the trilinear diagram. Likewise, individual points on the diagram will be referred to as "colori- metric positions."
REPRODUCIBLE WHITE LIGHT FILTERS 63

match which may result when the acetylene flame and this filter is compared with, for example, a 2,360° K. source and its filter, and

because of the uncertainty of the color temperature of the acetylene

flame. This is further discussed in Section VIII, 6, where some tests with the filter are described. The use of the filter with the acetylene

flame in photographic sensitometry should be entirely satisfactory.

(b) 2,848° K. TO NOON SUNLIGHT AT SUMMER AND WINTER SOLSTICES By methods exactly analogous to those used in designing the filters of Series I and already described in detail, filters were designed for

converting 2,848° K. to noon sunlight at Washington on June 21

and December 21, respectively, using Abbot's data as given in Table 1. As is the case with all the filters, an accurate color match was obtained between the ideal energy distribution and the source-andfilter combination. The filters are described in charts 34 and 35.

The trilinear coordinates for these sunlight distributions are illustrated

in Figure 14. They are as follows: June 21, r = 0.32184, # = 0.32688,

6 = 0.35128; December 21, r = 0.34569, # = 0.34027, 6 = 0.31404.

(c) 2,848° K. TO ABBOT-PRIEST SUNLIGHT

A description of the energy distribution known as Abbot-Priest

sun, obtained with a 2,848° K. source and quartz-nicol rotatorydispersion filter, is given in Section II and Figure 3. This representation of sunlight is quite satisfactory for colorimetric purposes aud has

been used in considerable work, both computation and direct observation, in the colorimetry section of the bureau. Because of the low

light transmission of the rotatory-dispersion filter and relatively

small field sizes available with it, and because the liquid filters may be

used conveniently in many situations where the use of the rotatorydispersion filter is not feasible, it was suggested that it would be

desirable to duplicate the color of Abbot-Priest sunlight by means of a Davis-Gibson filter with its relatively high light transmission.

The filter so designed is described and illustrated in chart 33.

Its light transmission for 2,848° K. is 0.251. A comparison of this

filter and its resulting energy distribution with those for 2,848° K.

to mean sun and 2,848° to 5,000° K. (charts 8 and 24) illustrates the effect of the type of energy distribution which it is sought to duplicate upon the concentrations of the ingredients, the energy

distribution of the source-and-filter combination, and the light trans- mission of the filter. Methods of computation leading to the best proportions of ingredients to give a precise color match and a reasonably good energy match

were similar to those used for the filters of Series I and II, with the

exception that, in the color computation, the form containing excitations for Abbot-Priest sunlight was used. The trilinear coordinates

of this energy distribution (on the basis of mean sun as " neutral' 1 stimulus) are illustrated in Figure 14. They are as follows : r = 0.33472,

# = 0.33052, 6 = 0.33476.

Another filter for securing the color of Abbot-Priest sunlight is also available. This is the one described in the next section and

illustrated in chart 32 and Figure 17. The locus obtained with this

filter by varying the color temperature of the light source passes

through Abbot-Priest sunlight at 2,567° K. For this color temperature of source, the light transmission is 0.175.
64 BUREAU OF STANDARDS

(d) 2,848° K. TO SUNLIGHT OUTSIDE EARTH'S ATMOSPHERE—VARIATION WITH

COLOR TEMPERATURE OF SOURCE

As representative of daylight energy conditions—sun plus sky, or overcast sky—the energy distribution of sunlight outside the earth's

atmosphere is in our opinion more nearly correct than is that of any

black body between the temperatures of 6,000° and 7,000° K.,

although it might, perhaps, be expected that such true absorption as there may be by the water vapor of the atmosphere would tend to make the color of daylight slightly less red than that of the outside

sun. While the color of daylight may, perhaps, be represented to a

close approximation on the color temperature scale, its energy dis- tribution when thus specified undoubtedly suffers in accuracy just as sunlight does when represented by 5,000° K.

Figure 17.

Colorimetric locus obtained by varying the color temperature of source, as indicated, with the filter for converting 2,848° K. to sunlight

outside the earth's atmosphere

This locus passes practically through the point representing the color of Abbot-Priest sunlight when the light source is at 2,567° K. The colors of daylight and noon sunlight may be

closely matched with this filter and the variable source, as illustrated in Section VIII, 4 and

Figure 32. The trilinear coordinates ("neutral" stimulus, mean sun), light transmissions, and

color temperatures for this filter at every 20° K. of source are given in Table 13. With a view to its possible use as a reproducible standard of artificial daylight, a filter was designed for converting, 2,848° K. to sunlight

outside the earth's atmosphere, as represented by Abbot's data

(Table 1) of 1917. The methods used were similar to those described

above for the filters of Series II. A description of this filter is given

in chart 32. Its light transmission for 2,848° K. is 0.180. The trilinear coordinates for this energy distribution are: r =

0.29883, = 0.31261, 6 = 0.38856. Its location on the r-b diagram is illustrated in Figure 14. Such tests as have been made with this and other filters (Sec. VIII,

4) indicate that a close color match may be obtained by means of this

filter not only with daylight, but also with noon sunlight at any time
REPRODUCIBLE WHITE LIGHT FILTERS 65

of the year, merely by varying the color temperature of the source. Accordingly, values of r, g, and b were computed for this filter with

the source at every 100° of color temperature from 2,300° to 3,000°

K. These computed values are given in Table 13. Intermediate

values at every 20° K. were obtained by interpolation from a large

scale plot. The locus of colors thus obtained with this filter by varying the color temperature of the source is shown in Figure 17. The light transmissions of this filter for a black-body source varying over the range from 2,300° to 3,000° K. are also given in Table

13. They were computed at every 100° K. and values obtained at intermediate temperatures by reading from a large scale graph.

These computations were similar to those described in Section V, 4, (c). Methods of using this filter for the study of daylight and noon

sunlight are outlined in Section VI, 2, (6). Table 13.

Data resulting from varying the color temperature of the source in the case of the filter designed to convert 2,84-8° K. to sun outside atmosphere—trilinear coordinates, light transmissions of filter, and color temperatures, as a function of

color temperature of source.

Colorimetric positions are illustrated in Figure 17. The filter is described in chart 32.

Color temperature

(° K.) of source

Trilinear coordinates 1 Light

trans- mission

"Color

tempera- ture" by

Davis's method 2

Color temperature

(° K.) of source

Trilinear coordinates 1 Light

trans- mission

"Color

tempera- ture" by

T 9 b r 9 b Davis's meth- od 2 °K. °K. 2,300.— 0. 37655 0. 34684 0. 27660 0. 1700 4, 217 2,700. ... 0. 31689 0. 32199 0. 36112 0. 1776 5,812 2,320— .3731 .3457 .2812 .1704 4, 285 2,720...

.

.3143 .3207 .3650 .1779 5,905

2,340.... .3698 .3445 .2857 .1708 4, 353 2,740.. -. .3118 .3194 .3678 .1783 6, 001 2,360— .3665 .3433 .2902 .1712 4, 423 2,760— .3093 .3182 .3726 .1786 6, 099 2,380— .3632 .3421 .2947 .1716 4, 493 2,780— .3069 .3169 .3762 .1789 6,199

2,400 . 36006 . 34088 . 29907 .1721 4,564 2,800 .30452 . 31565 . 37983 .1792 6, 304 2,420—. .3569 .3397 .3034 .1725 4, 638 2,820—. .3021 .3144 .3835 .1795 6,409 2,440—. .3538 .3384 .3078 .1729 4,713 2,840 .2998 .3131 .3872 .1799 6, 517 2,460—. .3507 .3372 .3120 .1733 4, 789 2,860 .2974 .3119 .3908 .1802 6, 631

2,480.... .3477 .3359 .3163 .1736 4,867 2,880 .2951 .3106 .3943 .1805 6,746

2,500 .34470 . 33471 . 32059 .1740 4,945 2,900 .29290 . 30937 . 39773 .1808 6,864 2,520— .3417 .3334 .3248 .1744 5,025 2,920 .2907 .3081 .4012 .1811 6,987 2,540— .3387 .3322 .3290 .1748 5,106 2,940 .2885 .3069 .4046 .1814 7,112 2,560—. .3358 .3309 .3332 .1751 5, 188 2,960 .2863 .3057 .4079 .1817 7,240 2,580—. .3320 .3296 .3374 .1755 5,271 2,980—. .2842 .3045 .4113 .1820 7,369

2,600 . 33029 . 32835 . 34136 .1759 5,357 3,000 .28218 . 30327 . 41455 .1823 7, 497

2,620 .3275 .3271 .3454 .1762 5,445

2,640 .3248 .3258 .3494 .1766 5,535 2,848 .... .29883 .31261 . 38855 .1800 6, 565

2,660 .3221 .3245 .3533 .1769 5, 626 2,680— .3195 .3232 .3573 .1773 5,718

1 "Neutral" stimulus: Mean sun as adopted in this investigation. Excitations: Optical Society of America extrapolated (J. Opt. Soc. Am. and Rev. Sci. Inst., 10, p. 230; 1925). Computed values are given

to five significant figures; other values, to four significant figures, were read from large-scale graph plotted from computed values.

2 There is as yet no satisfactory, established method for computing the "color temperature"—that is, the

nearest black-body color—for colors not lying precisely on the black-body locus. The values of color temperature given in Tables 13, 14, and 15 were obtained by an empiric method of computation developed by

Davis. This method is, briefly, as follows: Values of rLT , gLs , and bLb are computed for the energy dis- tribution in question, where r, g, and b are the trilinear coordinates and LT , Le , and Lb are the luminosity

coefficients (=0.45014, 0.54417, and 0.00569, respectively, as derived by Davis from least-squares adjustment

of the O. S. A. excitations (extrapolated) to the visibility data of Table 10). These values are then multiplied by a factor, w, of such magnitude that cor_Lr +co0Z,g+w6Z,b=O.33333. These values are then compared, respectively, with similar values for the black-body energy distributions, which have been computed at closely adjacent intervals over the range from 1,600° to 20,000° K. Three "component temperatures" will thus be obtained and their arithmetical mean is taken as the color temperature. For the sake

of relative values and maintaining regularity of differences in the tabulated values, these color temperatures have been computed with a greater precision than their accuracy can be guaranteed. It is impossible at

this time to indicate definitely the accuracy of the values. The justification for the method is: (1) That it seems to give reasonable values, (2) that it is computationally precise, and (3) that no other wholly satis- factory method is available. The method just outlined has other features than here noted. These are reserved for discussion in a separate paper, along with a more detailed description and illustration of the above procedure, including the table of values of urLi, ugL£ , ubLb, summing to 0.33333, for the black-body

series.
66 BUREAU OF STANDARDS

(e) 2,450° TO 3,500° K. AND TO 6,500° K.—VARIATION WITH COLOR TEMPERATURE

OF SOURCE

The filters of Series II, with the source at 2,848° K. provide a means

for reproducing any desired black-body color from 3,500° to 10,000°

K. with high precision. It would be possible with any of these

niters, by varying the color temperature of the source, to obtain a

locus of colors slightly inclined to the black-body locus, and crossing

it at the color temperature for w

Thich the filter was designed. Such

a filter would thus enable various high color temperatures to be approximately obtained by merely varying the color temperature of the

source.

.52

50 1/ \ llll

.to Black Body locus. Locus Obtained u/itn Filter by Varying

'OOO'K <^ / Source as Indicated. V —

-— 2450'K to 3500'K

Color Temperature o

100*1 \—x

,0

) \2 \

2: 00-K

24 V

.0 245 - K

2 SC

2 60( Ak i f,+

700'

28 v' yN_ & „0°" •* \l C 1 „<» *

31 OO-K MSO?'To >

3i oe

.12 14 .»* .16 .20 .22 24 .26 .28 .30 .32 .34 .36 Figure 18.

Colorimetric locus obtained by varying the color temperature

of source, as indicated, with the filter for converting 2,450° to 3,500° K., compared to the black-body locus The trilinear coordinates ("neutral" stimulus, mean sun), light transmissions, and color temperatures for this filter at every 20° K. of source are given in Table 14. This color temperature of source (2,848° K.) is eminently suitable

for the niters of Series II and for the various miscellaneous filters where it is used. The higher the color temperature of source the

better spectral energy match can be secured. However, the ordinary

gas-filled lamps do not operate normally at much higher color temperatures than 2,848° K. so that to raise this value would ordinarily

shorten the life of the lamps to an undesirable degree. It is believed

the value of 2,848° K. is a very satisfactory compromise for the

purpose.

When, however, it is desired to design a filter by which a range of color temperatures may be secured by varying the color temperature

of the source, the point of exact color match with the black-body
REPRODUCIBLE WHITE LIGHT FILTERS 67

locus should be chosen near the middle of the desired range, not near

its upper limit as would be the case with the 2,848° K. filters, so that

the deviations resulting from the crossing of the two loci may be

kept as small as possible.

The practicable working range of a gas-filled lamp may be considered

as from 2,000° to 3,100° K., although there are certain objections to operation near either extreme. The mid point of this range of colors

is approximately at 2,450° K., as estimated on the assumption that

the sensation scale along the black-body locus of colors is proportional

to the spectral centroid (30, p. 1191). It was desired to have one

filter by which the complete range of color temperatures conceivably

called white could be covered. It was found that by means of a

filter converting 2,450° to 6,500° K., a range of colors closely approxi1 1 1 1 1 1 1 I 1 1 1 -20 00 -k \

Black Body Locus. —

Locus Obtained u/it/i Filter by Varying \ f Source as Indicated. _ h

2450'K to 6500'K

Color Temperature o V

Z3Q0 N \

/ S

1 .26 .28 .30 .32 .34 .36 .38 .40 .42 .44 .46 .48 .50 .52 .54 .56 b Figure 19.

Colorimetric locus obtained by varying the color temperature

of source, as indicated, with the filter for converting 2,4-50° to 6,500° K., compared to the black-body locus The trilinear coordinates ("neutral" stimulus, mean sun), light transmissions, and color temperatures for this filter at every 20° K. of source are given in Table 15. mating those of a black body from about 4,000° to 19,000° K. could

be obtained by varying the source from 2,000° to 3,100° K.

A filter was accordingly designed converting 2,450° to 6,500° K.

with an accurate color match. Methods of design were similar to those previously described. A second filter was also designed,

converting 2,450° to 3,500° K. so that by means of these two filters and the source alone a range of colors closely approximating blackbody colors from 2,000° K. or below to about 19,000° K. could be

obtained.

These niters are described and illustrated in charts 37 and 38. In Tables 14 and 15 are given the values of r, g: and b, the light transmissions, and the color temperatures (via Davis's method) attainable

when the color temperature of the source is varied. Comparisons

with the black-body locus are shown in Figures 18 and 19.
68 BUREAU OF STANDARDS

Deviations of the colors so obtained from the nearest black-body

colors are in all cases considered to be less than one satron (Sec. VIII,

2), so that these filters may be considered as affording a means of practically duplicating all the black-body colors from 2,000° K. or below up

to about 19,000° K.

Table 14.Data resulting from varying the color temperature of the source in the case of the filter designed to convert 2,4-50° to 3,500° K.—trilinear coordinates,

light transmissions of filter, and color temperatures, as a function of color temperature of source

Colorimetric positions are illustrated in Figure 18. The filter is described in chart 37

Color temperature (°K.) of sourceTrilinear coordinates 1 Light

trans- mission

" Color temperature" bv

Davis's method2

Color temperature (°K.)

of source

Trilinear coordinates 1 Light

trans- mission

" Color temper- ture" Dsvis s TnptVirw12 T 9 b r 9 b

°K. °K.

2,000 0. 49206 0. 36195 0. 14599 0. 2850 2, 649 2,600 0. 39459 0. 34733 0. 25808 0. 2982 3, 816

2,020 .4883 . 3619 . 1498 . 2855 2, 685 2,620 . 3919 . 3465 . 2616 . 2986 3, 859

2,040 .4845 .3619 . 1536 . 2861 2, 721 2,640 . 3892 . 3457 "2651 . 2989 3, 903

2,060 . 4807 . 3618 . 1575 . 2866 2, 757 2,660 . 3865 . 3449 . 2686 . 2993 3, 947

2,080 .4770 .3617 . 1614 .2871 2,793 2,680 . 3839 .3441 » 2720 . 2996 3, 992

2,100 . 47334 . 36148 . 16519 . 2876 2, 829 2,700 . 38138 . 34325 . 27538 . 3000 4, 038

2,120 .4697 .3612 . 1690 . 2881 2, 865 2,720 .3790 .3424 . 2787 . 3003 4,082

2,140 . oouy 1729 . 2886 2, 901 2,740 .0(0* . 3416 . 2821 .3007 4, 126 2,160 .4626 .3606 ! 1767 . 2891 2, 939 2,760 .3739 . 3407 . 2854 . 3010 4, 171 2,180 .4592 .3603 . 1805 .2895 2, 976 2,780 .3715 .3398 .2887 .3013 4,216

2,200 . 45576 . 35991 . 18433 .2900 3, 012 2,800. ... . 36907 . 33895 .29198 .3016 4,262

2,220 .4523 . 3595 . 1882 .2905 3, 051 2,820 .3666 .3381 . 2953 .3019 4, 309

2,240 .4489 .3590 . 1920 .2909 3, 088 2,840 .3643 . 3372 . 2985 .3023 4, 356

2,260 .4456 .3586 . 1958 .2914 3, 127 2,860 .3619 .3363 .3018 .3026 4,404

2,280 .4423 .3581 . 1996 .2919 3, 165 2,880 .3596 .3354 . 3050 .3029 4,453

2,300 . 43904 . 35757 . 20339 .2923 3,202 2,900 . 35735 . 33451 . 30814 .3032 4, 502

2,320 .4358 .3570 .2071 .2927 3,242 2,920 .3551 .3336 .3113 . 3035 4, 551

2,340 .4327 .3564 . 2109 .2932 3, 281 2,940 .3529 .3327 .3144 . 3038 4, 600

2,360 .4295 3558 .2146 .2936 3, 320 2,960 .3507 .3318 .3175 .3041 4,650

2,380 .4264 .3552 .2183 .2940 3, 359 2,980 .3485 .3309 .3205 .3044 4,700

2,400 . 42337 . 35460 . 22203 .2944 3, 398 3,000 . 34642 . 33005 . 32352 .3047 4,751

2,420 .4204 .3539 . 2257 .2948 3, 439 3,020 .3443 .3291 .3265 .3050 4, 803 2,440 .4174 .3533 .2294 .2952 3, 480 3,040 .3422 .3282 .3295 .3053 4,855

2,460 .4144 .3526 . 2330 .2955 3, 521 3,060 .3402 .3273 . 3325 .3056 4, 908

2,480 .4115 .3519 .2366 .2959 3, 563 3,080 .3381 .3264 . 3355 .3058 4,961

2,500 . 40861 . 35117 . 24022 .2963 3, 605 3,100..- . 33607 . 32555 . 33839 .3061 5, 015

2,520 .4057 . 3504 .2438 .2967 3, 647

2,540 .4029 .3497 .2474 .2971 3, 688

2,560 .4001 .3489 . 2510 .2975 3, 730 2,580... .3973 .3481 .2545 .2978 3, 773

1 "Neutral" stimulus: Mean sun as adopted in this investigation. Excitations: Optical Society of America extrapolated (J. Opt. Soc. Am. and Rev. Sci. Inst., 10, p. 230; 1925). Computed values are given

to five significant figures; other values, to four significant figures, were read from large-scale graph plotted from computed values.

2 There is as yet no satisfactory, established method for computing the "color temperature"—that is, the nearest black-body color—for colors not lying precisely on the black-body locus. The values of color temperature given in Tables 13, 14, and 15 were obtained by an empiric method of computation developed by Davis. This method is, briefly, as follows: Values of rJU, gLe , and bLb are computed for the energy

distribution in question, where r, g, and b are the trilinear coordinates and Lr, Le , and Lb are the luminosity

coefficients (=0.45014, 0.54417, and 0.00569, respectively, as derived by Davis from least-squares adjustment

of the 0. S. A.. excitations (extrapolated) to the visibility data of Table 10) . These values are then multiplied by a factor, w, of such magnitude that corLr +a>gLg

-1rccbLb= 0.33333. These values are then compared,

respectively, with similar values for the black-body energy distributions, which have been computed at closely adjacent intervals over the range from 1,600° to 2*0,000° K. Three "component temperatures"

will thus be obtained and their arithmetical mean is taken as the color temperature. For the sake of rela- tive values, and maintaining regularity of differences in the tabulated values, these color temperatures have been computed with a greater precision than their accuracy can be guaranteed. It is impossible at this time to indicate definitely the accuracy of the values. The justification for the method is: (1) That

it seems to give reasonable values, (2) that it is computationally precise, and (3) that no other wholly satis- factory method is available. The method just outlined has other features than here noted. These are re- served for discussion in a separate paper, along with a more detailed description and illustration of the above procedure, including the table of values of urJU, wgLB, ubLb, summing to 0.33333, for the black-body

series.
REPRODUCIBLE WHITE LIGHT FILTERS 69

Table 15.

Data resulting from varying the color temperature of the source in the case of the filter designed to convert 2,450° to 6,500° K.—trilinear coordinates,

light transmissions of filter, and color temperatures as a function of color temperature of source

Colorimetric positions are illustrated in Figure 19. The filter is described in chart 38

Color

Trilinear coordinates 1 Light

trans- mission

Color temperature" by

Davis's method 2

Color temperature (°K.)

of source

Trilinear coordinates 1

.L/lglJl trans- mission

Color temperature" by

Davis's method 2

temperature

[ (°K.) of source

r 9 b T Q

° K. °K.

2,000 0. 37898 0. 33964 0. 28138 0. 0849 4 031 2 600 0. 26875 0. 28851 0. 44274 0. 0937 7, 773

Z,UZU _ - - - .3744 .3381 .2875 . 0852 4' 116 2 620 .2660 .2868 .4472 ' 0939 7' 976

2,040 .3698 .3365 .2937 !0856 i 201 2^640 —

-

.2632 .2852 .4516 !0942 S, 185

2,060 .3653 .3349 .2998 .0859 4, 289 2,660 .2605 .2836 .4559 .0944 8, 404

2 080 .3608 .3333 .3059 . 0863 4 378 2 680 .2579 .2819 .4602 . 0947 8, 632

o inn ^,1UU . 35644 . 33165 . 31191 . 0866 4 468 2,700 . 25533 . 28033 . 46435 . 0949 8 869

2 120 .3521 .3300 .3179 . 0869 4* 560 2720 .2528 .2787 .4685 . 0951 9' 123 2440 1 "I .3479 .3283 .3237 !0873 i 656 2^740. — .2503 .2771 .4725 !0953 9, 387

2,160 — .3438 .3266 .3295 .0876 4, 752 2,760 .2479 .2756 .4766 .0956 9, 665

2 180 .3398 .3248 .3354 . 0879 4 853 2 780 .2455 .2740 .4805 . 0958 9, 957

Z,ZIAJ . 33578 . 32311 . 34112 . 08S2 4 956 2 800 . 24310 . 27246 . 48444 . 0960 10 252

2 220 .3318 .3214 .3468 . 0885 5' 060 2,820 —- .2408 .2709 .4884 . 0962 10, 600 2i240"" .3279 .3196 .3525 !0888 5' 168 2,840 —- .2385 .2693 .4922 !0965 10^956

2,260 .3241 .3179 .3580 .0891 5, 279 2,860 —- .2362 .2678 .4959 .0967 11, 320

2 280 .3203 .3161 .3635 . 0894 5 393 .2,880 .2340 .2663 .4997 ; 0969 11, 696

2 300 . 31670 . 31436 . 36894 . 0897 5 511 2 900 . 23178 . 26487 . 50335 . 0971 12 090

2 320 .3131 .3126 .3743 . 0900 5 632 2 920 .2297 .2634 .5070 . 0973 12' 517

2^340 —. .3096 .3108 .3796 !o903 i 757 2^940 .— .2276 . 2619 .5105 \ 0975 12, 979

2,360 .3062 .3091 .3848 .0906 5, 884 2,960 .2255 .2605 .5140 .0977 13, 482

2,380 — .3027 .3073 .3899 .0908 6, 014 2,980 .2235 .2591 .5174 .0979 14, 030

2,400 — . 29933 . 30560 . 39507 .0911 6, 143 3,000 — . 22149 . 25769 . 52082 .0981 14, 631

2,420 .2961 .3039 .4001 .0914 6,286 3,020 .2195 .2563 .5242 .0983 15, 299

2,440 .2929 .3022 .4050 .0916 6, 430 3,040- — .2176 .2549 .5275 .0985 16, 032

2,460 .2897 .3004 .4099 .0919 6, 578 3,060

3,080 .2157 .2535 .5308 .0987 16, 844

2,480 — .2865 . 2987 .4148 .0922 6, 728 .2138 .2522 .5340 .0989 17, 737

2,500 — . 28343 . 29700 . 41957 .0924 6,884 3,100.— . 21194 . 25082 . 53724 .0991 18, 667

2,520 .2804 .2952 .4244 .0927 7, 047

2,540 — .2774 .2935 .4290 .0929 7, 217

2,560- — .2744 .2918 .4337 .0932 7, 394

2,580 — .2716 .2902 .4383 .0934 7,580

1 "Neutral" stimulus: Mean sun as adopted in this investigation. Excitations: Optical Society of America extrapolated (J. Opt. Soc. Am. and Rev. Sci. Inst., 10, p. 230; 1925). Computed values are given

to five significant figures; other values, to four significant figures, were read from large scale graph plotted from computed values.

2 There is as yet no satisfactory, established method for computing the "color temperature"—that is, the

nearest black-body color—for colors not lying precisely on the black-body locus. The values of color temperature given in Tables 13, 14, and 15 were obtained by an empiric method of computation developed by

Davis. This method is, briefly, as follows: Values of tLt , gLg , and bLb are computed for the energy dis- tribution in question, where r, g, and b are the trilinear coordinates and LT , Lg , and Lb are the luminosity

coefficients ( = 0.45014, 0.54417, and 0.00569, respectively, as derived by Davis from least-squares adjustment

of the O. S. A. excitations (extrapolated) to the visibility data of Table 10). These values are then multiplied by a factor, co, of such magnitude that wrir+co(7JL5 +wf'I'5=0.33333. These values are then compared,

respectively, with similar values for the black-body energy distributions, which have been computed at

closely adjacent intervals over the range from 1,600° to 20,000° K. Three "component temperatures" will thus be obtained and their arithmetical mean is taken as the color temperature. For the sake of relative values and maintaining regularity of differences in the tabulated values, these color temperatures have been computed with a greater precision than their accuracy can be guaranteed. It is impossible at this time to

indicate definitely the accuracy of the values. The justification for the method is: (1) That it seems to give reasonable values, (2) that it is computationally precise, and (3) that no other wholly satisfactory method is available. The method just outlined has other features than here noted. These are reserved for discussion

in a separate paper, along with a more detailed description and illustration of the above procedure, including

the table of values of urLr , ugLg , ubL b , summing to 0.33333, for the black-body series. VI. USE OF THE FILTERS

Certain of the uses to which these filters may be put have already

been noted in connection with the discussions of the reasons for designing the various filters. Such uses are more specifically outlined in the present section and, in addition, other classes of work for
70 BUREAU OF STANDARDS

which the filters are readily adaptable are given. The use of the

standard component solutions (Tables 4, 5, and 6), in checking or

calibrating spectrophotometric apparatus is discussed later in Section

VII, 10

1. IN PHOTOGRAPHIC SENSITOMETRY

(a) INTERNATIONAL UNIT OF INTENSITY FOR SENSITOMETRY

The need for an accurately reproducible filter for obtaining a

spectral distribution of energy suitable for photographic sensitometry

was, as already stated, one of the principal objectives of the present

investigation. Likewise, in the fields of colorimetry and photometry,

filters have been needed for converting low color temperatures to high color temperatures and for calibrating incandescent lamps for color temperature and intensity. In designing filters for use in these

fields, a relatively large number have been developed giving colors

and energy distributions approximating those of sunlight and daylight, and, therefore, suitable for use in photographic sensitometry.

The action of the Sixth International Congress of Photography in 1925 and the report of the American Committee on the Photographic

Standard of Intensity were noted in the introduction. The subject was considered again at the Seventh International Congress of Photography meeting in London in July, 1928. At that time the

American committee made another report (40) to the congress

repeating its recommendation for a single high color temperature

standard of light quality for use in sensitometry (as contrasted to the

dual standard adopted in 1925), and urging the use of the DavisGibson filters, particularly the 2,360° K.-to-mean-sun filter, as a means of securing the desired sunlight distribution in the laboratory.

A paper by the authors, entitled, " Artificial Sunlight for Photographic

Sensitometry/' was also read at the congress. (Proceedings, pp. 611—

170.)

The action of the Seventh International Congress has been described

elsewhere (41). It is sufficient here to quote the resolution adopted,

this quotation being taken from the published Proceedings, page 173 : This meeting of the Seventh International Congress recommends to the

national committees that the photographic unit of intensity for the sensitometry

of negative materials shall be denned as the intensity of a filtered source of radiation having a luminous intensity of 1 international candle, and produced by

a gray body at a colour temperature of 2,360° K., together with a selectively

absorbing filter made up as follows: Two solutions compounded according to the

following formula, the complete filter to consist of 1 cm* layer of each solution contained in a double cell made by using three pieces of borosilicate crown glass

(refractive index, D=1.51), each 2.5 mm thick. Solution A

Copper sulphate (CuS04 .5H2 0) g_.f3. 707

Mannite (C6 H8 (OH) 6 ) g__f3. 707

Pyridine (C5 H5 N) cc__**30. Water (distilled) to make cc__ 1, 000

Solution B

Cobalt ammonium sulphate (CoS04 .(NH4 ) 2 S04 .6H2 0) __g__f26. 827

Copper sulphate (CuS04 .5H2 0) g--f27. 180

Sulphuric acid (specific gravity 1.835) cc__ 10. Water (distilled) to make cc__ 1, 000

* Tolerance in thickness shall be ±0. 05 mm.

t For practical purposes an accuracy to tbe second place of decimals is probably sufficient. **Editors' Note.—It is obvious that this 30 cc refers to pure pyridine, and that the appropriate volume

correction should be applied if pyridine of lesser purity is used.
REPRODUCIBLE WHITE LIGHT FILTERS 71

It is recommended that the foregoing resolution shall come into force as a

decision of this Seventh International Congress if and when ratified by the

national committees represented at this congress. (The editors' note given just above, as published in the Proceedings,

p. 173, was not a part of the resolution adopted and is in error, since the pyridine used in designing the niters was of 98.4 per cent

strength, as explained on p. 165 of the authors' paper published in

the Proceedings.)

The filter herein specified is the Davis-Gibson filter described in

chart 2 of the present paper. In accordance with the last sentence

of the resolution, the matter was considered 16 by the Optical Society

of America, through its council, meeting in February, 1929, and the

recommendation of the Seventh International Congress adopted:

In ratifying this proposal the Optical Society of America understands that the

intent of this recommendation is as follows:

1. The intention is to specify two things (a) the unit in which the intensities of

light sources are to be expressed, and (6) the quality of light to be used.

2. The unit is to be the international candle, implying further that the intensities measured and stated will be luminous intensities as in visual photometry.

3. The quality of light to be used for sensitometry of negative materials is to be that which results from passing the radiation from a grey body at 2,360° K.

normally through the filter described.

4. The grey body and the selectively absorbing filter, together shall be considered as the effective source in specifying the intensity (candlepower)

.

5. The procedure recommended for determining the intensity of the combined

effective source is to multiply the intensity of the primary source (grey body) by

the appropriate transmission factor of the filter which is 0.135. This factor has been computed from the spectral transmission of the filter via the relative energy

distribution of 2,360° K. and the relative visibility function adopted by the

sixth session of the International Commission on Illumination at Geneva, 1924.

6. This resolution does not state or imply the value of illumination to be used

at the test plane during the sensitometric exposure, nor does it place any limitations on the intensity of the light source to be used.

(b) COMMENTS ON USE OF THE FILTERS

The essential details regarding the preparation of the filters are given in Section IX.

It is the custom to use low-intensity light sources (1 to 20 c. p.) for photographic sensitometry. Several lamps should be available,

calibrated by one of the standardizing laboratories at 2,360° K.

Lamps of small candlepower should be calibrated in terms of amperes

(with potentiometer control) rather than volts. The filaments

should be of such shape and dimensions as to approximate as nearly

as possible a point source. The cylindrical or T type of bulb is to be preferred in gas-filled lamps, because the blackening which takes

place with use in such lamps occurs mostly at the top where it does

not interfere with the light taken in a horizontal direction.

These niters were designed to be used at a temperature of 25° C.

where the highest accuracy is desired. However, a departure of

5° or 6° either way is of little or no importance in sensitometry.

The effect of temperature is discussed in Section VII, 4. To avoid

excess absorption of heat from the lamp by the filter, particularly

when large lamps are used, the filter may be placed in the sensitometer

with the light shutter between it and the light source. Thus the

16 The official proceedings had not been published at the time the matter was considered by the optical

society and the clarifying word "filtered" in the third line of the resolution was not present in the copy

of the resolution at hand.
72 BUREAU OF STANDARDS

filter would be exposed to the radiant energy from the lamp only

during the sensitometric exposure. It is also recommended that the

side of the filter holding the B solution be placed facing the light source, thereby protecting the A solution, the most important one, from undue heating by the lamp.

2. IN COLORIMETRY

(a) NEUTRAL STIMULI

If the Planckian radiant energy at some temperature between

3,500° and 10,000° K. is eventually adopted as a standard neutral

stimulus or white light, a filter converting 2,848° K. to this color is available from the niters of Series II, Table 12 and charts 21 to 31.

If mean noon sunlight or sunlight outside the atmosphere, the latter representing average daylight, be adopted as standard white, the

filters of Series I or the sun-outside-earth's-atmosphere filter (chart

32) are available. A filter (chart 33) duplicating the color of AbbotPriest sunlight is also available.

As an aid in determining the true subjective white, the filter described in chart 38 and Table 15, has been devised, by which any

color temperature from approximately 4,000° to 19,000° K. may be

obtained by varying the color temperature of the source from 2,000°

to 3,100° K. The use of this filter for this purpose is planned by

Priest.

(b) METHODS FOR MEASURING THE COLOR OF NATURAL SUNLIGHT AND

DAYLIGHT

A knowledge of the color of sunlight and daylight is of great importance both in photographic sensitometry and in colorimetry, as has already been pointed out. Such knowledge should include in- formation regarding: (1) The variations that may take place both in

daylight and sunlight with altitude of sun and with atmospheric

conditions, and (2) statistical averages of such data, extensive and

reliable enough to serve in the establishment of colorimetric standards

of noon sunlight and daylight.

Such information as we have at present comes mostly from the

extensive radiometric measurements by Abbot and his coworkers (42) on the spectral energy distribution of sunlight and the transmission

of the atmosphere and on various isolated measurements of the color

of sunlight and daylight made by Priest (28, 30, 43) with the rotatorydispersion colorimetric photometer and the leucoscope. The data

thus obtained give us an idea of the magnitude of the variations that

take place in the color of sunlight and daylight, and serve to locate

approximately the statistical averages of these variable colors. Perhaps the chief reason for the lack of further data on these important subjects is the extreme difficulty of making reliable radiometric measurements and the lack of reliable color standards with

which the colors of sunlight and daylight could be compared and

measured. The rotatory dispersion colorimetric photometer has so

far been the only one devised which can be made up from specification with known spectral characteristics and which is flexible enough

to cover approximately the range of colors presented by natural sunlight, daylight, and skylight. This instrument has, however, not

been readily available for such purposes.
REPRODUCIBLE WHITE LIGHT FILTERS 73

Some of the filters already described afford excellent means for the

precise measurement of the colors (r, g, and b coordinates) of all phases of sunlight and daylight. Inasmuch as the light transmissions of all these niters have been computed, it is also possible to measure illumination at the same time that the color measurements

are made.

The following apparatus, in part or in full, in addition to the filters,

is required

:

1. A lamp calibrated in terms of color temperature (and intensity,

if measurements of illumination are to be made). Such calibration

need be only at 2,848° K. for a certain limited group of measurements

as explained below. For the complete study of all phases of sunlight and daylight the lamp must be calibrated in terms of voltage

or current over the color temperature range from 2,000° to 3,100° K.

Such calibration may be made by means of the filters, as explained

in Sections VI, 2 (d) and VIII, 5. The calibration of this working

standard lamp, which should be a 500 or 1,000 watt Mazda-C lamp,

should be checked from time to time to insure the desired accuracy.

For this purpose, a calibrated primary standard Mazda-B lamp

should be used, the checking being carried out via the filters in the same manner as the original calibration.

2. A good comparator or photometer by which the light from the sun or sky may be compared with that from the source-and-filter

combination. A Martens photometer is especially good for this purpose. Both the natural and artificial sunlight or daylight should

be diffused from identical pieces of magnesium carbonate or oxide or other suitable white diffusing material and the diffused light taken

into the photometer. The usual precautions regarding stray light

should be taken. If, in calibrating the working standard, the primary

standard lamp is on that side of the photometer later taken by the

natural sunlight or daylight, the method becomes a substitution

method.

3. Auxiliary filters for compensating small color differences. While

perfect color matches may occasionally be observed between the

natural and artificial sunlight or daylight, there will, in general, be

slight color differences remaining when the two fields are matched as

closely as possible either by change of filter or by adjustment of the

color temperature of the source. Such residual color differences

may be easily eliminated by the use of Lovibond 17 glasses of low

denomination. Where the color temperature of the source is varied,

red (magenta) glasses only are necessary; otherwise, yellow or blue

17 The Lovibond glasses consist, as indicated above, in a set of red, yellow, and blue glasses. The glasses

of any one of the three series give colors of approximately constant hue, but of varying saturation and

brilliance. Each of the red, yellow, and blue series of glasses is graded by the makers from 0.01 to 20.0. The 0.01 glass has very little absorption and its color is barely perceptible, while the 20.0 glass absorbs

strongly in certain regions of the spectrum and gives a color of high saturation. The range from 0.01 to

20.0 is subdivided into a large number of steps, there being 155 glasses in each series (465 in all in a complete

set of the three series) arranged and numbered in order of increasing saturation. By combining glasses of the same or different series, nearly all colors can be matched, especially if independent means is available

for varying the brilliance. The glasses are prepared and sold by The Tintometer (Ltd.), Salisbury, England, and may be also obtained through importers. A descriptive advertising circular is issued by the makers. The glasses of a given series are supposed to be additive; that is, 10.0+1.2=11.0+0.2=4.4+6.8, etc. There

are certain inaccuracies in the scales, however, which have caused annoyance and error in their use and

which have led to insistent demands that the bureau standardize the Lovibond glasses. The work which

the bureau has done in this respect is partially described in the following publications: Gibson, Harris, and Priest, A Spectrophotometric Analysis of the Lovibond Glasses, B. S. Sci. Paper No 547; 1927. Judd

and Walker, A Study of 129 Lovibond Red Glasses with Respect to the Reliability of their Nominal

Grades, Oil and Fat Industries, 5, pp. 16-26; January, 1928. Priest, Judd, Gibson, and Walker, Calibration of Sixty-five 35-Yellow Glasses, B. S. Jour. Research, 1 (RP58), pp. 793-819; April, 1929.
74 BUREAU OF STANDARDS

glasses may also be required. This is indicated more specifically

below. The colorimetric loci of the fractional Lovibond glasses are

illustrated in Figure 20, wherein are shown also the black-body locus

and the locus of the 2,848° K.-to-sun-outside-atmosphere filter obtained by varying the color temperature of the source. The coor- dinates r and g are used in this figure, as least perceptible color

differences in any direction are more nearly proportional to distances

2300'

Lovibond . /.O/Yellow

ptyoon Sun, Dec. 2

1

2500°

.19 .30 .31

g Mean Noon Sun

600°

Noon Sun, June 21 J00°

/ 6 Lovibond 1.0 Blue

300°

28

//t8°

<p Sun Outside Atmosphere

f Locus Obtained by Varying

Color Temperature of Source,

as Indicated, with 2848 °K to ^Sun Outside Atmosphere Filter

I I L

.32 .33 .34 .3S .36 .37 9

Figure 20.

Section of the r-g diagram showing colorimetric positions of Lovibond glasses of low denomination which may

be used with certain of the filters in determining the color of sunlight and daylight (See Sec. VI, 2, (b) for details) The Lovibond glasses shown (B. S. 9940) have the following maker's numerals: Red 1.0, 0.70, 0.54, 0.40, 0.32, 0.26, 0.19, 0.09; Yellow 1.0, 0.70, 0.60, 0.50, 0.40, 0.30, 0.20, 0.10; Blue 1.0, 0.50, 0.30, 0.20, 0.10. on this scale than on the r-b scale. A comparison of this figure with

Figure 17 will illustrate the differences in plotting in the two cases. The effect of temperature change on the niters should not be ignored. This is discussed in Sections VII, 4 and VIII, 2, (d). If a 500 or 1,000 watt lamp is operated in close proximity to the filter, the

temperature of the filter will rapidly increase. This may, in general,

be prevented by turning off the lamp or interposing a shutter except

during the actual observations and by placing a water cell between
REPRODUCIBLE WHITE LIGHT FILTERS 75

the lamp and filter. The color of the latter may be ignored if made of borosilicate crown glass as specified for use in the filter, as illustrated

in Section VIII, 2, (d). The following combinations of source and filter may be used in the

study of the color of sunlight and daylight.

(1) Source Calibrated at 2,848° K. only.—The following filters are

available

:

(a) 2,848° K. to sun outside earth's atmosphere, chart 32;

(b) 2,848° K. to mean noon sun at Washington, chart 8; (c) 2,848° K.

to noon sun at Washington on June 21, chart 34; (d) 2,848° K. to noon sun at Washington on December 21, chart 35; and (e) 2,848° K.

to Abbot-Priest sun, chart 33 —all as based on Abbot's data of 1917.

On the basis of direct examination of daylight and noon sunlight made

with filters (a) and (b) in September, 1928, and with filters (a) and (d)

in December, 1928, the locus of colors represented by the first four

filters is slightly greenish relative to the natural sunlight and daylight.

Exact color matches may be obtained, however, by inserting Lovibond

glasses of low denomination in combination with the source and filter

red alone, or red with blue, or red with yellow, depending on atmospheric conditions. This method was used in securing the data illus- trated in Figure 32. The Lovibond glasses required varied from

about 0.10 to 1.0 in each of the three colors. They were of known

spectral transmission and thus of known r, g, and b, so that the

colorimetric values may be derived from the measurements.

The same sort of measurements could be made, using filters of

Series II; for example, 2,848° to 5,000°, 5,500°, 6,000°, or 6,500° K.

On the basis of the measurements referred to above, the black-body

locus is slightly purplish relative to the color of natural daylight

and noon sunlight so that combinations of yellow and blue glasses

should give the desired color match.

(2) Source calibrated over range from 2,300° to 3,000° K.—The most

objectionable feature of the previous method is the necessity for the

use of so many Lovibond glasses. If, however, the color temperature

of the source may be varied, exact matches may be made with the

red glasses only. If the color temperature of the source is varied,

in the case of the filter for converting 2,848° K. to sun outside atmosphere (chart 32) the resulting locus of colors falls very close to that

of the natural daylight and noon sunlight, crossing apparently at a

slight angle, so that when perfect matches are not obtainable by

varying only the source they may be obtained by inserting a red

glass, usually of very low denomination, on one side or the other

of the photometer. The values of r, g, b, the approximate color

temperatures, and the fight transmissions of this filter with varying

source are all given in Table 13. On the basis of the observations made in September and December, 1928 (fig. 32), this method is better than that of (1) above, or

(3) below for the study of daylight and noon sunlight, although

method (1) was used in securing the data illustrated. The only

Lovibond glasses required are red glasses having maker's numerals

of about 0.1, 0.2, 0.4, unless glasses less than 0.1 are desired for dis- tinguishing extremely minute color differences. It is an interesting

coincidence that the locus of colors obtainable with this filter passes

almost exactly through the point representing the color of Abbot-Priest

sun, so that, as previously noted, the latter color may be secured

13416°—31 6
76 BUREAU OF STANDARDS

not only with a 2,848° K. source and the filter described in chart 33,

but also with a 2,567° K. source and the filter described in chart 32.

(3) Source calibrated over range from 2,000° (or below) to 3,100°

K.—Bv means of the two filters designed as outlined in Section

V, 6 (e)—viz, 2,450° to 3,500° K. and 2,450° to 6,500° K.—and by

practically the same method as outlined under (2) just above, the

colors of sunlight at practically any altitude, of daylight, and of blue sky may be determined. In addition to the Lovibond red glasses,

the use of one or two blue glasses would eliminate the necessity of operating the lamp at the highest temperatures in measuring the color

of blue sky. Data on these filters are given in charts 37 and 38 and

in Tables 14 and 15.

(c) STANDARDS FOR GRADING DAYLIGHT GLASSES AND ARTIFICIAL WHITELIGHT ILLUMINANTS

That there is need for a ready means of checking the color of so- called "daylight" illuminants has been pointed out in a paper already

referred to (17). The manufacturer of such lighting units or of daylight glass should also have available a means of calibrating or grading

the material which he is selling. Such a means is available in the

filters of the present paper.

He can take his choice of mean noon sunlight (the filters of Series

I, charts 1 to 20), average daylight or overcast sky (chart 32), AbbotPriest sunlight (chart 33), or any one or more of the color temperatures from 3,500° K. (yellowish) to 10,000° K. (bluish) (the filters of

Series II, charts 21 to 31) including 6,500° K., which is close to the

color of average daylight. Furthermore, filter No. 38, which by varying the color temperature of the source will give him approximately

any color temperature from 4,000° to 19,000° K., will enable him to grade any piece of daylight glass or any artificial daylight illuminant

at will. For precise work, a good comparator, such as the Martens photometer, should be used. Simple visual comparison without any in- strument will, however, immediately detect any serious deviations

from the standard which it is desired to maintain and which is given

by any one of the filters that may be selected for that purpose.

(d) CALIBRATION OF INCANDESCENT ELECTRIC LIGHTS IN TERMS OF COLOR

TEMPERATURE

By designing the filters of Series I to give an accurate color match

with mean sun, and, therefore, with each other, they may be used to calibrate an incandescent electric light in terms of color temperature

over the range of the filters, and also, as indicated in the next section, to compare the intensities of two lights of different color temperatures

under conditions of perfect color match, thus eliminating some of the

uncertainties and difficulties of heterochromatic photometry.

The calibration of a lamp in terms of color temperature is relatively

simple provided one has available a good photometer and a lamp

already standardized for color temperature over the range to be used.

The standardization of a lamp for color temperature has, however,

been difficult or impossible for any but a few specially equipped

standardizing laboratories. The only instrument for such purposes

which is self-contained for its calibration—that is, not dependent for

its calibration upon some other calibrated apparatus, other than
REPRODUCIBLE WHITE LIGHT FILTERS 77

upon a standardized lamp—is Priest's rotatory dispersion colorimetric photometer (30). Blue glass or gelatine wedge photometers,

even if available, are of no fundamental assistance, as they must be

calibrated. "Black bodies" are difficult to operate and can not

readily be used at the ordinary operating temperatures of the gasfilled tungsten lamp. The usual procedure, if one has wished to secure a lamp standardized for color temperature, has been to obtain

such a lamp from the National Bureau of Standards or from some other

standardizing laboratory.

The present series of filters will enable one to standardize his own

lamp from 2,300° K. up to as high a temperature as is advisable and

furthermore to check such cah oration at any time desired. Aside

from the filters the sole requisites for such standardization are a

source whose color temperature is known at any single temperature

within the range of the niters, together with a good photometer. It had been hoped that the acetylene name might be used as a reproducible source of accurately known color temperature and thus free the worker from any dependence upon the standardizing laboratory.

Certain questions have arisen, however, in connection with the acetylene flame (discussed in Section VIII, 6) and, unless approximate

values only are desired, it is doubtless preferable to secure a lamp from

a standardizing laboratory calibrated at some one temperature and

use this as the basic fundamental standard. Color temperatures ol 2,360° and 2,848° K. are commonly used reference points, 2,360° K.

being especially favorable as the vacuum tungsten lamp has great

constancy both in color and intensity at this color temperature. The

following procedure is recommended for the standardization of a lamp

over a range of temperatures by means of the filters of Series I

:

1. All lamps to be used should be seasoned; that is, operated at normal voltage until they give a constant current at this constant

voltage.

2. Upon receipt of the fundamental (primary) standard lamp

(standardized at some one color temperature by a standardizing

laboratory) secondary working standards should be calibrated in

terms of it (without the use of any filters) and thereafter used for the

basic color temperature. The primary lamp should never be used

except to check the secondary standards.

3. Mean-sun filters should be prepared (a) for the basic temperature

of the primary and secondary standard lamps and (6) for as many

other color temperatures as are necessary in order to cover the desired

range at sufficiently close intervals.

4. It is best to use a substitution method. The working standard

and its filter should illuminate one side of the photometric field and

a comparison lamp and its filter should illuminate the other. The

voltage or current on the comparison lamp should then be varied

until an exact color match is obtained in the two parts of the photometric field. The brightnesses in the two halves of the field should be

continually matched during that procedure.

5. The particular color temperature chosen at which to operate the

comparison lamp is perhaps unimportant. If it is a vacuum tungsten

lamp, 2,360° K. is a suitable temperature. If it is a gas-filled lamp,

it should probably be operated at some color temperature near the

middle of the range being covered.
78 BUREAU OF STANDARDS

6. The working standard and filter should then be replaced by the

lamp being standardized and its voltage or current determined with

the various filters prepared for the desired color temperatures.

7. The number of readings taken for the single determinations and

the amount of repetition and checking necessary will depend on the

observer's skill, his sensibility to color differences, the consistency of the results, and the accuracy desired. A graph of the relation between

color temperature and voltage or current of the lamp should have a

perfectly regular (smooth) curvature.

8. This lamp being once standardized may be treated as a working

standard and other lamps calibrated in terms of it, or several lamps

may be standardized at the same time with the same filters. The question of precision is touched upon in the following section,

and a test of the reliability of the filters for color temperature measure- ments is described in Section VIII, 5.

3. IN PHOTOMETRY—COMPARISON OF INTENSITIES OF INCANDESCENT ELECTRIC LIGHTS OF DIFFERENT COLOR TEMPERATURES

The difficulties and uncertainties of heterochromatic photometry

are well known to those engaged in the testing and standardizing of lamps. Even when the uncertainties of attempting to make direct

photometric matches of two differently colored lights are eliminated

by flicker photometry under standard" conditions there still remains

the important difference that may result when two observers, both

of normal color sense but having different visibility functions, make

respective determinations. A difference of several per cent may be

thus obtained when the candlepower of a gas-filled tungsten lamp is measured relative to that of a vacuum tungsten lamp.

The use of the filters would not be convenient where the intensities of lamps of unknown color temperature are desired. But a calibration

of intensity for any given lamp over a range of color temperatures

may be readily obtained, at the same time as the color temperature

calibration described in the previous section if desired, and always

with perfect color match in the two parts of the photometric

field. Also, of course, any number of lamps, at one or several known

color temperatures, may be measured relative to a standard lamp

operating at some other temperature, all color temperatures being

2,300° K. or above. The procedure would be very similar to that

outlined for the color temperature standardization. The substitution method is even more important in this case. The light trans- missions of the filters for their respective sources are given in Table

8 and in the charts.

It may be noted that in the methods of use of the filters as outlined above, both for the determination of color temperature and

intensity, various uncertainties entering into the determination of the color and transmission of a filter, and discussed in Sections VII

and VIII—viz, possible errors in the spectrophotometric data, possible lack of reproducibility of the chemicals and glass plates, temperature effects, etc.—have a tendency to cancel out, their importance

decreasing as the differences in the color temperatures of the sources

decrease. These filters should, therefore, have, along with their general

applicability for all color temperatures above 2,300° K., a reliability

equal to that obtained with the various special bluish or yellowish

filters which have been used to bridge certain steps in the color
RE PRODUCIBLE WHITE LIGHT FILTERS 79

temperature scale of incandescent sources, even when these latter

are carefully calibrated.

The precision of measurement is probably about the same whether

niters are used or not, for it has been shown that (30, p. 1191) the

precision of determination of the spectral centroid is independent of

the color temperature, and also that (44) when the color temperature

of a light is changed (for example, by altering the voltage on a lamp),

the variation in the spectral centroid of the light transmitted through

a white-light filter is slightly greater than the variation in the spectral

centroid of the light alone. A real cause for decreased precision

might, however, be found in the lowered brightness resulting from

the use of filters on both sides of the photometer. With most incandescent lights, however, intensities are sufficiently great to make

this objection of little importance.

An extension of the present series of filters to make them applicable

to sources at color temperatures less than 2,300° K. is impracticable.

The lower the color temperature of the source, the less must be the

transmission of the filter required to produce artificial sunlight,

which has been our objective in designing these filters. This low

transmission, combined with the generally lowr intensities of sources

of low color temperatures, would decrease the brightness undesirably.

If there were a demand for filters applicable for use below 2,300° K.,

a new series could be designed, choosing, not sunlight, but light

from the Planckian radiator at some relatively low temperature (say

2,848° K.) as the light to be delivered by the source-and-filter combination. With this objective, practicable filters could be computed

for use in combination with sources at temperatures from below

2,000° to above 2,300° K.

(Approximate values of color temperature from 2,000° to 2,300°

K. may be obtained by use of the 2,450° to 3,500° K. filter. For

example, from Table 14 it may be noted that color temperatures of

2,649°, 2,829°, and 3,012° K. result when this filter is used with

source at 2,000°, 2,100°, and 2,200° K., respectively. A lamp may be

calibrated for 2,649°, 2,829°, and 3,012° K. by means of the filters of Series I as illustrated above. To calibrate a lamp for color temperatures of 2,000°, 2,100°, 2,200° K., and other values between

2,000° and 2,300° K., it is merely necessary to vary the voltage on

this lamp until its color viewed through the 2,450°-to-3,500° K.

filter is matched with that of a bare gas-filled tungsten lamp cali- brated at the appropriate color temperatures by means of the filters of Series I. The values thus obtained may be somewrhat uncertain

because of the lack of exact match between the black-body color and

that of the source-and-filter combination at these low color temperatures.) (Fig. 18.)

4. IN ASTRONOMY—DETERMINING THE COLOR TEMPERATURES OF

THE STARS

The two niters described in Sections V, 6 (e), together with a cali- brated light source, whereby color temperatures from 2,000° K. or below up to 19,000° K. may be obtained, should furnish a simple means of accurately determining the color temperatures of the stars. Aside from the usual astronomical equipment, the essential apparatus

would consist of a suitable comparator, the two filters, and a lamp

calibrated over a color temperature range from 2,000° to 3,100° K.

with its accompanying voltmeter or ammeter.
80 BUREAU OF STANDARDS

VII. SPECTROPHOTOMETRIC DATA

1. APPARATUS AND METHODS

(a) VISUAL METHOD

Except for some of the preliminary measurements not reported

quantitatively in the paper, all spectropho tometrie measurements

were made in the colorimetry section of the bureau. The larger

part of the measurements were made visually with the KoenigMartens spectrophotomoter and auxiliary equipment. A description

of this apparatus and method of use is given in a recent publication

(45). In the present measurements the instrument has been used

in two ways:

1. For the measurements leading to the standardization of the

absorbancy and transmittancy values for the standard component

solutions (medium concentrations) and the density and transmission

values for the double cell filled with distilled water, both the homogeneous (Hg and He) light sources and the heterogeneous (incandescent Mazda C) light source were used, the former, in general, at the

eight wave lengths noted below, with 0.5 mm slits, and the latter over the range from 420mju or 450m/i to 650m/z or 710m;u, usually

at every 10m^ and with 0.2mm slits (except at the ends of this range

where wider slits are usually necessary).

2. For the studies of permanence, reproducibility, temperature

effect, Beer's law, variation of pyridine content, and various miscellaneous measurements these eight wave lengths—viz, Hg 404.7, Hg

435.8, Hg 491.6, He 501.6, Hg 546.1, Hg 578.0, He 587.6, and He

667.8—in whole or in part, plus various intermediate wave lengths

with the incandescent light if necessary, have furnished sufficient data for the information desired. The obvious advantage in their use is that differences in values obtained in a study of the various

effects can not be ascribed to wave-length or slit-width errors. Furthermore, they cover the most important part of the wave-length

range used in the present investigation. Various details concerning

the measurements are noted later in the sections devoted to the

respective subjects.

(b) AUXILIARY METHODS

Supplementing the visual method in the determination of the spec- trophotometrie data on the standard component solutions, thermoelectric, photoelectric, and photographic methods were used. The

thermoelectric and photoelectric methods have been described else- where (46). The former was used over the range from 610m/x or 650mju to 750mju or 780m,u, at every lOm^ or 20mju, depending upon

the type of transmittancy, thus overlapping the visual method at several wave lengths. The photoelectric method was used over the

range from 390mAt to 510m.fjL or 550m^, likewise at every 10m/z or

20mju, thus overlapping the visual method for an extensive range in the blue and green. The photographic (Hilger sector photometer) method was used

essentially as previously described (47, 48), except that for part of the measurements a ribbon-filament Mazda C lamp was used as source instead of the underwater spark. The primary purpose in the use of this method was to extend the data from the visible to 350m/*. It was used not only for the standard component solutions,

but also in the study of Beer's law.
REPRODUCIBLE WHITE LIGHT FILTERS 81

(c) MISCELLANEOUS

The thicknesses of cell used in the spectrophotometric determinations were 10, 20, 40, and 100 mm, these being accurate to one-tenth

of 1 per cent or better. The particular thickness used in any case depended on the solution and the purpose of the measurements.

Sometimes but one thickness, sometimes three or four thicknesses were used during a set of measurements on any given solution. The

particular ones used will not, in general, be further stated. Regardless of the thickness used, values were always computed to 10 mm,

inasmuch as this is the standard thickness used in the filters described

herein.

The temperature of the solutions was in all cases kept at 25° ± 1° C, unless otherwise noted. The apparatus for all four methods of

measurement includes temperature control for solutions. This has

been previously described (45, 47).

The concentrations given throughout this section are accurate to 0.001 g, and the volumes to 0.1 cc (except in the case of 1,000 cc or

1 liter, where 0.1 cc is not significant), whether indicated by the proper

number of significant figures or not.

2. STANDARD FILTER COMPONENTS

In Tables 4, 5, 6, and 7 (Sec. V, 2) are given values of the density

of the double cell with water, and of the absorbancy of the standard

solutions A', B/, and B2

' as adopted for the present investigation.

A precise description of the cell and of the solutions is given in Section

IV, 1. An outline of the experimental procedure, by means of which these

data were obtained, follows. The final use of the data was, of course, kept in mind throughout the measurements. While it was convenient

and desirable in view of the various computations, to consider the

fundamental spectrophotometric data in terms of absorbancy (the

slide rule used in the visual measurements on the Koenig-Martens

spectrophotometer contained a — logio scale, so that values of absorbancy were obtained directly from the readings of the spectrophotometer and not via a previous transmittancy computation), it is ultimately

the transmittancy values, entering into the final transmission of the

cell and its contents, comprising any given filter, which is of importance;

for it is the product of transmission of the filter by the energy of the

light source that gives the energy distribution of the source-andfilter combination. A difference or error in absorbancy of 0.00434 means a difference or error in transmittancy amounting to 1 per cent

of the transmittancy, regardless of the values of absorbancy and

transmittancy. Consequently it was never attempted to reach a

constant percentage accuracy in the absorbancy data; that is, if A A

represents the uncertaintv in the values of A, it was never attempted

A A

to make —^- constant regardless of A. It was attempted, rather,

to maintain A A constant, less than 0.00434 so far as possible, so that

-y- should be constant and not greater than 1 per cent. A discussion

of the reliability of the various data is given later. In view of the uncertainty in the absorbancy caused by filtering

the A solution through filter paper, it was, as elsewhere noted, decided
82 BUREAU OF STANDARDS

to base all standard adopted data on unfiltered solutions. This point

is of little importance in the case of the B solution, but is not negligible

with the A solution. All the spectrophotometric data given in this paper, unless otherwise specifically noted, were obtained with unfiltered solutions. In using the solutions during the measurements

care was taken not to shake or tip the bottles unduly, and the solutions were transferred from bottle to cell usually by means of a

pipette. The reason for this is that all chemicals contain a minute

quantity of trash or insoluble material which settles to the bottom

upon standing.

(a) TWO-COMPARTMENT GLASS CELL WITH DISTILLED WATER

Measurements were made by the visual method from 460ni/i to 720m/i in addition to the Hg wave lengths 404.7ni/z. 43o.SniAi. 491.6mm

546.1ni/z, and 578.0m/*. Photoelectric measurements were made from

400in/i to oSOm^i and thermoelectric measurements from 640ni/i to TSOmju. In addition, measurements were made by R. Stair, of the

radiometry section of the bureau, by the thermoelectric method, at the Hg wave lengths 436ni/i, 405ni/x, 365m^, 334ni/z, 313ni/z, 305ni/i,

and 2S0m^.

The adopted values of density and transmission are given in

Table 7 (see also fig. 10), those below 400ni/x being based primarily on Mr. Stan's data. The uncertainty in the absolute values of trans- mission might be - 1 per cent, considering the spectrophotometric

data alone, although the relative accuracy is better than this. However, the value of transmission computed from the refractive indices

of the glass and water, assuming no absorption, is equal to 0.904. and

a value of 0.906 was reported by the optical instrument section for the "white light " transmission. The values of transmission in Table

7 are, therefore, considered accurate to =0.5 per cent or better, as the spectral transmission over the range of highest luminosity varies

from 0.900 to 0.904. The water absorption band at 750rn/x is plainly shown. The slight

absorption shown by the remaining data in the visible, and rapidly

increasing in the ultra-violet, is ascribed mostlv to the glass.

(b) SOLUTION A' An outline of the experimental work leading to the adoption of

this combination of materials for the principal (blue) solution has

been given in Section IV. 1. Having decided on the most suitable

amounts of copper sulphate and pyridine, viz. 3.000 g and 30.0 cc, respectively, but without mannite as yet. spectrophotometric measurements were made by the visual, thermoelectric, and photo-electric

methods, covering the range from 390ni/i to 750m^. Values of

absorbancy for 10 mm thickness at every 10m/x from 390m/i to 750niM were adopted from these measurements (May, 1926). They refer to a single solution. Xo. 61—11, the measurements being made within a few days after preparation. After the hot summer weather had

shown the copper-sulphate-pyridine solutions to be rather unstable

at temperatures between 35" and 40 z C. and mannite had been added

as an essential constituent of these blue solutions, measurements of

absorbancy were repeated, similar to but more extensive than those

of May. 1926. New values of absorbancy were thereupon adopted
REPRODUCIBLE WHITE LIGHT FILTERS 83

(October, 1926). They refer to solution No. 62-2, the data being

taken from 3 to 32 days after preparation of solution. Certain small

but fairly definite discrepancies which were more or less a function

of time became apparent during these measurements. These were

at first considered as a turbidity effect, but later work has proved

them mostly a result of the very slow change in absorbancy which

takes place with time. (This characteristic of the A solution is discussed in Section VII, 3.)

Another solution was thereupon prepared, No. 62-15. The eight

reference wave lengths previously noted—viz, Hg 404. 7m^, Hg

435.8mM, Hg 491. 6mM, He 501. 6mM, Hg 546. lm^, Hg 578.0m/z, He

587.6mju, and He 667.8m/z—were the only ones used in the study of

this new solution. Measurements were made on the day of preparation and on the first, second, sixth, eighth, twenty-third, and twentyfourth days after preparation. The changes that took place from the

sixth to the twenty-fourth day were all less than 1 per cent of the

transmittancy.

Based principally on the average of these measurements from the

sixth to the twenty-fourth on solution No. 62-15, and guided also by miscellaneous measurements at these wave lengths on other

A' solutions, values of absorbancy were adopted (Nov. 19, 1926)

which are the values listed at the bottom of Table 4. A summary

of all the experimental data obtained with the eight Hg and He wave

lengths on the various A' solutions is given in Table 18. The data of October, 1926, were then adjusted so that the data of November 19, 1926, would lie precisely upon the curve drawn through

the adjusted values. These adjusted values of absorbancy (adopted

Nov. 30, 1926) are the ones given in Table 4 from 400m,u to 750m^

(see also fig. 10). The finally adopted spectral absorbancy values of

November 30, 1926 (with mannite) differed from the original data of

May, 1926 (without mannite), in no case by more than 0.005. There

is no reason to think that mannite influences the transmittancy of

the copper-sulphate-pyridine solutions except to make them much

more stable. During February, 1927, measurements were made photographically (solution No. 65-5, seven days after preparation), overlapping

the adopted data of November, 1926, from 400m^ to 510hiai and

extending into the ultra-violet beyond 3501^. The agreement with

the adopted values above 400m//. was satisfactory. The values given

in Table 4 from SdOmfj, to 390m/i were obtained from these measure- ments on solution No. 65-5, taking a graphic average of the data for

10, 20, and 40 mm thickness, all reduced to 10 mm as usual. These

data were closely checked by later measurements on solution No.

65-8-C, 20 days after preparation, made during the study of Beer's law from 350m,u to 400mju.

(c) SOLUTION Bi' As previously stated, 20.000 g of copper sulphate per liter of acidified aqueous solution was chosen as standard. A filtered solution was prepared, No. 62-1, and spectrophotometrie measurements

made by visual, thermoelectric, and photoelectric methods during the

first month after preparation. Values of absorbancy from 390mix to 750mju were thereupon adopted (September, 1926). These values were checked closely by the measurements made in the test of Beer's
84 BUREAU OF STANDARDS

law (solutions Nos. 62-7-A to E, filtered, October, 1926), but were

revised somewhat in the red on the basis of later measurements.

After the decision had been made not to use filtered solutions, an

unfiltered solution, No. 62-5, was prepared. A single set of meas- urements at the Hg wave lengths (see Table 19 for summary of

measurements made with Hg and He wave lengths on the B'x solutions), made on the second day after preparation and using 100 mm

cells, showed a very distinct decrease in transmittancy compared

with the values previously obtained with filtered solutions, although

this amounted to less than 1 per cent when reduced to a 10-mm basis. This discrepancy was considered a turbidity effect and it was decided

to study the change with time in a manner similar to that described

for the A' solution, No. 62-15.

Accordingly, a new solution was prepared, No. 62-18, unfiltered,

and measurements made on the day of preparation and on the first, fourth, twelfth, and fourteenth days after preparation, using the

eight Hg and He wave lengths. It was found (Table 19) that the

measurements made during the first four days after preparation

checked those made on No. 62-5, unfiltered, made the second day

after preparation, and that the measurements made on the twelfth

and fourteenth days checked closely with the measurements made

on the filtered solutions Nos. 62-1 and 62-7-C. Apparently, then,

the insoluble matter causing the turbidity settles to the bottom of the bottle after the first few days and is removed entirely by filtering.

Measurements on solution B' (below) show that no further change

of any importance takes place. With Bx solutions, therefore, there

is no practical difference between filtered and unfiltered solutions,

provided the latter are not used for a week or so after preparation

and care is taken not to disturb the insoluble material at the bottom

of the bottle. Based partially on the measurements with solution No. 62-18,

particularly the data for 12 and 14 days after preparation, and also on the other measurements noted, absorbancy data for the Hg and He

wave lengths were adopted (Nov. 13, 1926) and are given at the

bottom of Table 5. Particular attention was paid to the He 667.8

measurements, seven different determinations being finally averaged

to give the value adopted.

The previous data of September, 1926, were now adjusted to fit these data of November 13, 1926, in a manner similar to that done

with solution A'. The only important change was in the red. The

change required to make the previous data consistent with the He

667.8 values varied from less than 1 per cent of the transmittancy

below 630m//, to about 3 per cent at 750m//. The finally adopted data

(Nov. 15, 1926) from 400m//. to 750m/* are given in Table 5 (see also

fig. 10).

Photographic measurements in the ultra-violet to approximately

320m//, were made on 40-mm solutions (No. 65-3, unfiltered) about two

weeks after preparation. The absorbancy was too low to obtain any

data above 370m// by this method, but it was not difficult to connect

the visible and photographic data by a reasonable curve. The values

of absorbancy are so small in this region that they might be considerably in error without affecting the transmittancy by more than a fraction of a per cent. The values from 350m// to 390m// in Table 5 were thereupon adopted.
REPRODUCIBLE WHITE LIGHT FILTERS 85

(d) SOLUTION B'2 It has already been noted that cobalt sulphate was at first used

instead of the cobalt ammonium sulphate later adopted because of its greater stability. After preliminary measurements had indicated that

10.000 g of cobalt sulphate per liter was a suitable concentration, an

unfiltered solution was prepared, No. 61-10, containing, however,

but 4 cc of sulphuric acid. (The exact amount of sulphuric acid is apparently of little, if any, importance.) Measurements were made

visually, thermoelectrically and photoelectrically, covering the wavelength range from 390m^ to 750m/z. Values of absorbancy were

thereupon adopted (May, 1926). Later, after a study of Beer's law

had been made, with filtered solutions, Nos. 62-8-A to E containing

10 cc of sulphuric acid, and no deviation from Beer's law detected,

the values were revised slightly (October, 1926).

In the latter part of October, 1926, a new solution was prepared,

No. 62-17, unfiltered, containing 10 cc of acid, and measurements

were made on the day of preparation and on the first, fourth, thir- teenth, and fifteenth days after preparation, using the eight Hg and

He reference wave lengths. No change of absorbancy was detected over this interval of tune. Based principally on these measurements on solution No. 62-17,

but guided also by the other previous measurements, the absorbancy

values given at the bottom of Table 6 were adopted (November 13,

1926). In a manner similar to that for solutions A' and B/, the

revised data of October, 1926, were adjusted to fit these adopted data

of November 13, 1926, and the data thus finally adopted (November

17, 1926) are given in Table 6, 400m,u to 750 m,u(see also fig. 10). The

absorbancy values finally adopted November 17, 1926, differ from those

originally obtained in May, 1926, in no case by more than 0.0028.

The data summarized in Table 20 indicate that a turbidity effect,

such as was found with copper sulphate, was not present, and it is apparently of no practical importance whether a B2 solution is clarified by filtering or by decanting.

The loss of water of crystallization of cobalt sulphate (Sec. IV, 1) led to the analysis of solution No. 62-17, which was found to contain an amount of cobalt equivalent to 10.3 g of cobalt sulphate instead of 10.0 g as supposed. The change to cobalt ammonium sulphate was

thereupon made. A single set of absorbancy measurements on a

cobalt ammonium sulphate solution (No. 62-20) equivalent in

cobalt content to No. 62-17, at the eight Hg and He wave

lengths, agreed with the adopted data of November 13, 1926,

to 0.0022 or better. Further test as to the agreement between

absorbancy data for cobalt sulphate and cobalt ammonium sulphate was afforded by Beer's law measurements on cobalt ammonium

sulphate. The measurements made (at 435. 8111,11, 491.6m/x, 501.6 m/z,

546.1m^t, and 578-Om^) on the solution having the standard concentration (No. 65-1-C) agreed with the adopted values to 0.0015 or

better. A more important check, however, was obtained on the

mixture of cobalt ammonium sulphate and copper sulphate, as indicated in the next section.

Photographic measurements were made from 450miu tobelow350m/x on cobalt ammonium sulphate as with copper sulphate. The absorbancy is very small below 400mM. Measurements from one to two

weeks after preparation were made on solution No. 65-4, 14.481 g,
86 BUREAU OF STANDARDS

and on No. 65-9, approximately 144.81 g, using 40-mm cells in both

cases. With the latter solution Beer's law was assumed for computing to 14.481 g. Perfect agreement with the adopted data of Table 6

from 400m/x to 450mju was not obtained. The differences, however, were not outside the uncertainties of the photographic data. The

data were adjusted to fit the adopted values from 400m/z to 450mju

and are given as adopted in Table 6. Inasmuch as the absorbancy

is itself less than 0.01 below 400mfx, there is no chance for any important error to be present in the adopted data.

Comparative measurements showed a difference in absorbancy

between cobalt sulphate and cobalt ammonium sulphate below 400m/z.

Cobalt sulphate has a minimum absorbancy at approximately 370m/i.

For cobalt ammonium sulphate this minimum is shifted to about

355mjLt, and the values of absorbancy are, in general, lower. As

shown elsewhere, however, in the visible spectrum, the differences in transmittancy between cobalt sulphate and the equivalent amount of cobalt ammonium sulphate, at the concentrations used and for 10

mm of solution, must be considerably less than 1 per cent, if any

difference exists at all.

(e) SOLUTION B' Solution No. 65-2, unfiltered, was prepared containing 20.000 g

of copper sulphate and 14.481 g of cobalt ammonium sulphate per

liter of acidified aqueous solution. Measurements were made on this solution eight and nine days after preparation, using 10, 20, 40, and

100 mm of solution at the eight Hg and He wave lengths. The

values were reduced to 10 mm thickness as usual and averaged. The

maximum difference between these average observed absorbancies

and those computed, adding the absorbancies tabulated for solutions

B/ and B2

', Tables 5 and 6, was but 0.0012, or less than one-third

of 1 per cent of the transmittancy. (Table 21.)

In greater concentrations this strict additivity of the components

of the B solution might not hold accurately, but over the range of concentrations used in the various filters, considering also various

other spectrophotometrie checks (Sec. VIII), it is certain that any

error involved in using this additivity is negligible.

As implied in the preceding section, this set of measurements on

Solution B' also served as a check on the equivalence of cobalt

ammonium sulphate and cobalt sulphate in the visible spectrum, for the latter had been used in adopting the values of Table 6 and the

former in the preparation of solution No. 65-2.

3. PERMANENCE OF SOLUTIONS

There was no indication of change taking place in the separate

copper sulphate and cobalt sulphate solutions, except the very slight

turbidity effect noticed with the former, as already described. A

much more important question is whether or not the mixture of the

copper sulphate and the cobalt ammonium sulphate is unchanging

in its transmittancy. Accordingly, when solution B' above described

(No. 65-2) was 300 days old, a set of measurements was made,

similar to those made 9 days after preparation. The maximum

deviation between the two sets of absorbancy values was 0.0028

(Table 21), about two-thirds of 1 per cent of the transmittancy, and

no change at all was indicated outside of possible experimental error.
REPRODUCIBLE WHITE LIGHT FILTERS 87

However, small but definite changes took place in the A' solution

with time. The absorbancy measurements on solution No. 62-15,

using the eight Hg and He wave lengths, have already been noted.

These were made on the day of preparation and on the first, second,

sixth, eighth, twenty-third, and twenty-fourth days after preparation.

Measurements were again made on this solution 384 and 385 days

b

4^ 4« +' -v.* Q f i i' i i ^

after preparation. The data for 0, 1, and 2 days were averaged

together, likewise the data for 6 and 8 days, 23 and 24 days, and

384 and 385 days; so that, in effect, we have data on this solution

representative of the first, seventh, twenty-fourth, and three hundred

and eighty-fourth day after preparation. These data, given in

Table 18 and plotted in Figure 21, show the differences in absorbancy

between the data representative of the time intervals as noted and
88 BUREAU OF STANDARDS

the standard values of Table 4. The change throughout the brighter

part of the spectrum is small, but there is a definite decrease in

transmittancy in the red and an increase in the violet, resulting

in the solution becoming slightly bluer after long standing. However,

over a period from a week to a month, probably two months, the

values do not deviate from the adopted values by more than 1 per

cent of the transmittancy. The resulting effect of this and other

factors on the color of completed filters is discussed further in

Section VIII.

4. TEMPERATURE EFFECTS

As noted elsewhere, the standard temperature adopted for these

solutions was 25° C. and all measurements of transmittancy or absorbancy reported in this paper refer to this temperature unless otherwise

specifically noted. It is well known that aqueous inorganic salt solutions may show a relatively rapid change in transmittancy with

temperature, whereas aqueous dye solutions are often practically

unaffected.

It might, perhaps, have been assumed that any variation of absorbancy thus caused would be proportional to the concentration

of the solution for a given temperature range. It seemed best,

however, to test this.

(a) SOLUTION A Three solutions were prepared (Nos. 65-1 7-A, 65-1 8-A, and

65-1 9-A), having concentrations of copper sulphate (and mannite)

equal to 3.707, 2.445, and 0.741 g per liter, respectively. The

pyridine content of each solution was 30 cc per liter, as usual. These

concentrations are those used in the 2,360°, 2,848°, and 4,000° K.-tosunlight filters, covering nearly the complete range of concentrations

used.

For each concentration, measurements of absorbancy were made

at 25° C, at 40° C, and again at 25° C. These measurements were made in December, 1927, from one to two weeks after preparation

of the solutions. The eight Hg and He wave lengths were used in addition to 460rmx, 520m^, 560m,u, 610m/*, 640mju, 690m/*, and 720m/*.

The 3.707 g solution was measured at 10 mm thickness, the 2.445 g at 20 mm, and the 0.741 g at 40 mm.

Values of absorbancy were then reduced to 10 mm thickness for

all three solutions. The two sets of 25° C. values were averaged

and taken to represent 25° C, as no permanent change was detected.

The differences in absorbancies—that is, A25o —

A

40o—at the various wave lengths were then taken for each solution, and it was found

that at practically every wave length this difference was proportional to the concentration of the solution within the experimental

uncertainty.

These values of A25° — A40 o were thereupon divided by the concentration and are plotted in Figure 22. The general lack of any consistent differences in the values computed from the different concentrations shows that any attempt to correct these values on the basis of the Beer's law data already obtained would have been an unwarranted refinement. (The values at 404.7m/*, where the Beer's law

correction is several times as large as at the other wave lengths,

probably illustrate the effect of ignoring such a correction.)
REPRODUCIBLE WHITE LIGHT FILTERS 89

There are given also in Figure 22 the mean values of A25° — A40° per gram and a curve plotted through the adopted values. These

adopted values are given in Table 16, and, with those obtained for

solution B, were used in computations of the temperature effect on

the completed filters. (Sec. VIII.)

(b) SOLUTION B

The transmittancies of the components of the B solution were known to decrease with increasing temperature. As in the study of permanence, however, the effect should be tested on the B mixture,

and not computed from data on the two components, even if such

were available in suitable form. Accordingly three B solutions were
90 BUREAU OF STANDARDS

prepared, having concentrations corresponding to the filters 2,360°,

2,848°, and 4,000° K. to mean sun, viz:

Number

Concentration (in grams per liter)

Copper

sulphate Cobalt ammonium

sulphate

65-17-B..

65-18-B..

65-19-B..

27. 180

i 16. 520

6. 250

26. 827

i 19. 820 L 130

i This solution was made up with the weights inadvertently reversed from the intended values. The

mistake was no'u discovered until after the measurements were completed, but no error or disadvantage

results from using the data, the values of absorbancy being applied to the weights as actually used. The measurements were made in December, 1927, from two to three weeks after preparation of the solutions. Thicknesses of solu- tion of 10, 20, and 40 mm, respectively, were used, and the other

experimental conditions were the same as for the A solutions above

described.

Previous measurements (49) on a copper-sulphate solution at 57 g

per liter have shown that the change with temperature was inappreciable at the shorter wave lengths, increasing as the absorbancy

increased toward the longer wave lengths. It was also known (50,

plate 30) that the principal temperature change with cobalt sulphate was in the green with little change in the red where the absorbancy

is small. Furthermore, the weights of the two components are not

greatly different for two of the three solutions.

Accordingly, the values of A25°-A4oo were reduced to a 10 g. basis

(in a manner analogous to the A solutions, which were reduced to a

1 g basis) assuming the change in absorbancy below 600nnx to be

caused by the cobalt ammonium sulphate alone and that above

QOOm/jL by the copper sulphate alone. These values are plotted in

Figure 23 for two of the three solutions. Values for solution No.

65-1 9-B are not given. The concentrations were so low that observational uncertainty masked the real effect over most of the range. These data were accordingly not used.

The curve drawn more or less through the average of the values

for the two solutions was adopted as representative of the temperature effect on solution B per 10 g of each component. The separate

values derived and estimated from this curve for each component are
REPRODUCIBLE WHITE LIGHT FILTERS 91

given in Table 16 and plotted in Figure 23. Remembering that

0.00434 is equivalent to a 1 per cent change in transmittancy, such

uncertainty as is present in the data is probably negligible. 18 All the

values of Table 16 are considered uncertain in the fourth decimal.

is An apparent discrepancy may be noted (fig. 23) between the present data for copper sulphate and

the data referred to in a previous publication (49), particularly at wave lengths greater than 650nni. This

difference is probably not outside of the possible combined experimental error in the two cases, but may

also be caused by the different experimental conditions, viz, 57 g per liter, no acid, no mixture with cobalt ammonium sulphate in the previous determination, as contrasted with the respective differing conditions

of the present determination. 13416°—31 7
92 BUREAU OF STANDARDS

Table 16

Changes in absorbancy 1 with temperature for component filter solutions

Wave length in millimicrons

Values of-^^-for concentration A solu- tion per gram

Bi solu- tion per

10 g

B2 solu- tion per

10 g

380— —0. 0267 yj. UUUU —U. UUU8 390 -. 0212 .0000 -. 0009 400 —. 0162 . 0000 — . 0010 410 — . 0116 . 0000 — . 0011 420 —. 0078 . 0000 —. 0012 —. 0049 . 0000 — . 0014

440 -.0032 .0000 -. 0015

450. -. 0021 .0000 -.0017

460 —. 0013 . 0000 —. 0018

470 . -.0006 .0000 -.0019

480— .0000 .0000 -.0021

490... +. 0008 .0000 -.0023

500 +. 0020 .0000 -.0027

510 +. 0038 .0000 -.0039

520 +. 0062 .0000 -. 0053 530 +• 0087 -.0001 -.0062

540 +• 0107 -.0002 -.0062

550 +. 0123 —. 0004 -. 0060

Wave length in millimicrons

560 570

580

590

600 610

620 630

640 650 660

070

680 690

700

710

720

Values of-concentration A solu- tion per gram

+0. 0129 +.0132

+.0127 +.0110 +. 0084 +. 0057 +. 0026 -.0007

-.0037 -.0066

-.0090

-.0109

-.0124

-. 0138 -.0148

-.0157

-. 0164

Bi solu- tion per

10 g

-0. 0006 -.0009 -.0012

-.0016

-.0021 -.0028

-.0036 -.0045 -.0056

-.0068

-.0081 -.0095

-.0110 - 0125 -.0141 -.0158 -.0175

i All the values of this table are uncertain in the fourth decimal place. See Section VII, 4, and Figures

22 and 23 for details of derivation.

5. VARIATION OF PYRIDINE CONTENT IN SOLUTION A

To study and illustrate the effect of varying the amount of pyridine

used, five solutions were prepared, Nos. 62-26-A to E, all containing

3 g of copper sulphate and mannite, and containing, respectively,

10, 20, 30, 50, and 100 cc of pyridine per liter of solution. Measurements were made in November, 1927, from 4 to 12 days after preparation of the solutions. Solution thicknesses of 10 and 100 mm w^ere used. The eight Hg and He wave lengths were used, in addition to 460mM, 620miu, and 720m/x in some cases. The values obtained are illustrated in Figure 24 compared with

the standard data for solution A' as given in Table 4. The effect of increasing the pyridine content is to increase the absorbancy over most of the spectral range and to shift the wave length of maximum

absorbancy toward shorter wave lengths. The effect in the blue and

violet is small, but may, of course, increase in the ultra-violet.

The results of this test have been further illustrated in Figure 8

(Sec. IV, 1) where the absorbancy is plotted as a function of the

pyridine content at the various wave lengths. The smooth curves

therein drawn pass through the standard values for 30 cc; and the

effect of any variation of pyridine strength may thus be obtained at these wave lengths from this figure.

Similar results had been found by preliminary measurements made

in 1926. Computations based on the later data illustrated in Figures

8 and 24 show that little if any improvement in the energy distribution of the completed filters could have been obtained with a pyridine content other than the standard concentration chosen. This is illustrated in Section VIII, 3.
REPRODUCIBLE WHITE LIGHT FILTERS 93

6. BEER'S LAW

(a) SOLUTION A Measurable deviations from Beer 's law were found for solution A.

The principal measurements were made on solutions Nos. 62-1 3-A

to E, containing 1, 2, 3, 4, and 5 g, respectively, of copper sulphate

and mannite per liter of solution, each solution containing also 30 cc

of pyridine. Measurements were made nine days after preparation

of the solutions, at 404.7m)U, 435.8m/*, 546.1m/*, 578.0mAc, and 680m/i

for each solution. Thicknesses of 10, 20, and 40 mm were used, the

thickness used depending on the concentration. Reduction of the

data to 10 mm and 3 g resulted in the values given in Table 17,
94 BUREAU OF STANDARDS

columns 2 to 6. Each value of there tabulated represents the

absorbancy measured, divided by the thickness (in centimeters), and

further divided by one-third of the concentration (in grams of copper

sulphate and mannite per liter). They are thus proportional to the

specific absorptive indices, and their variation with concentration

illustrates the failure of Beer's law. Within experimental uncertainty and to an accuracy sufficient for the purpose, this change in the absorbancies of Table 17 may be taken as constant from one concentration to another at any wave length. An average change per

gram is accordingly tabulated in cloumn 7 of Table 17, these values

being computed by averaging the differences between the 3 g values

of column 4 and the values of columns 3 and 5 and one-half the

differences between column 4 and columns 2 and 6. Table 17.

Beer's law data for solution A

Thickness 10.00 mm, temperature 25° C. Other experimental data are illustrated in Figure 25. The

Beer's law correction factor is defined as (see Sees. V, 3 and VII, 6):

wherein A„ and c 8 refer to the 3-g solution.

Wave length in millimicrons

Values of A ) for solutions Nos. 62-13-A to E

having the following concentrations in grams of copper sulphate and mannite per liter (pyridine content 30.0 cc throughout)

Mean

value of

c—c>

(from pre- ceding columns)

(C-C.) A,

equals B

(from col- umns 7 and 4) 1.000 2.000 3.000 4.000 5.000

404. 7

435. 8

546. 1 578. 680

0.088

.023

.448 .658

.577

0.080

.022

.438

.652

.573

0.076 .C22

.435 .647

.576

0.068 .020

.427

.640

.568

0.073

.025 .423

.634 .570

-0.0049 -.0002

-.0059 -.0060

-.0021

-0. 065 -.009

-.014

-.009

-.004

This average change per gram is seen to approximate an average

change in transmittancy of about 1 per cent. For purposes of

applying the Beer's law correction, it is more convenient, however,

to express the change as a fractional change in the absorbancy. This

is done in column 8 of Table 17, these values resulting from dividing

the values of column 7 by those of column 4. They are plotted in Figure 25. This fractional change in absorbancy per gram difference

from the standard has been defined as B (Sec. V, 3) in the equations

A=£ C [l+5(c-c)5]

and

(c-cs ) As These measurements were made on unfiltered solutions, the usual

condition unless otherwise noted. Similar measurements on a

series of filtered solutions gave a similar set of values of B, these being
REPRODUCIBLE WHITE LIGHT FILTERS 95

also shown in Figure 25. Measurements were also made with 2 and

4 g unfiltered solutions, with mannite constant at 3 g, and likewise

with mannite constant at 10 g. These results are all plotted in

Figure 25. Only at 404.7m/* was it felt that the value of B was

certainly different from — 0.010. Accordingly, this value was adopted

for the wave-length range from 450m^ to 720miu. In order to confirm the results obtained at 404.7m/x and to extend

the correction into the ultra-violet, photographic measurements were made. Six solutions were prepared, Nos. 65-8-(/<>)A and 65-8-A

to E, containing, respectively, % g and 1 to 5 g of copper sulphate

and mannite per liter of solution. Each solution contained 30 cc of

pyridine per liter. Measurements were made three weeks after

preparation of the solutions. The results were derived in a manner

similar to the values of Table 17 and gave the values of B shown in

Figure 25. In the same figure are shown also the adopted values

of B—from 720nuz to 400mya on the basis of the previous visual measurements described above, and from 390m/x to 350nnx on the basis

of the photographic measurements. These adopted values are those

given in Table 4. Measurements of the effect were again made

visually at 404.7nnx, 435. 8m/x, and 491. 6m^ with the six solutions

used in the photographic measurements, four weeks after preparation.

These values are also given in Figure 25. It might reasonably be questioned, after examination of Figure 25,

whether the adopted values of the Beer's law correction factor represent the experimental data as satisfactorily as some alternative curve which might be plotted. As a matter of fact, if the true Beer's law correction factors were of primary importance, more

data would have to be taken to determine them with high accuracy.

Here, as elsewhere throughout the investigation, the influence of

the data on the complete filters was effective in determining the extent of the data.

For example, could any other set of Beer's law correction factors

be adopted on the basis of the experimental data of Figure 25 which

would change the computed spectral transmission of any of the

filters (and, therefore, the resulting energy distribution) by as much

as 1 per cent? That there could be no such set of values may be

shown as follows:

If A A represent the error in A caused by an error AB in B, then

from the first equation above, we have

AA=A2?[c(c -

(f-)A-]

For any given values of AB, cs , and As , it is obvious that AA =

when c = or when c = cs . For values of c greater than cs , AA continuously increases as c increases. By differentiating AA with

respect to c and equating to zero, we obtain c = cs /2, so that for

values of c between and cs , AA has a maximum absolute value

when c = cs /2, or in the present case when c = 1.5 g of copper sulphate.

We wish, however, to know what values of AB will cause AA

to equal 0.00434, thereby producing a 1 per cent error in the trans- mittancy. The equation connecting AA and AB may be rewritten

^fAA

AB =

\c-cs ) A,
96 BUREAU OF STANDARDS

as is shown in Figure 25. It follows, for any given values of AA, c„ and As , that AB is infinite when c = c s or when c = 0, that for values

of c greater than c S} AB continuously decreases as c increases, and

that for c less than c S) AB has a minimum absolute value at c = c s /2.

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o X c

O ">< f

CM — ~->- C> r r r r

vl 'i

p S3 & - Z ~

°

*" "5 ~ .£ §

c OT £ Z o -^.^

o ai a tx— —

- — - — -

~ —

*"

3 a - c S

c: if -r ~ -

•p c~ ooB

= = = s g 3 3r* s ofi

•2 g^oy; •_ — — HIP

^g § ?^ |

o 2 >• 5 - £ So o 2 2 S ® 3 a

*tj

<D S 8 2^3

1 §£S a2

£

p > i. -= -

05 5 ^i: £ c

^ +j +j O c3 03 ft

Values of AB have been computed for values of AA = 0.00434,

for values of As as given in Table 4, with c s = 3, and with a value

of c = 1.50 (or 3.62, which gives practically the same absolute values

of AB). These values of AB, which are required to produce a 1
REPRODUCIBLE WHITE LIGHT FILTERS 97

per cent error in the transmittancy, are illustrated in Figure 25

where values of B ± AB are plotted as a function of wave length.

It wull be noted that these dashed curves in practically all cases

lie much beyond the range of the experimental data, and it has just

been shown that for any values of c less than 3.62, other than 1.50,

the deviations would be even greater, becoming infinite at c = and 3

.

For values of c greater than 3.62 the values of AB which will cause a value of AA = 0.00434 decrease, becoming one-half the values illus- trated at c = 4.10, and one-third the values at c = 4.50, which is a

greater concentration than used in any of the filters. Even for this extreme case, any curve which might reasonably be put through

the plotted data of Figure 25 would still be within the 1 per cent

limits, except possibly above 600m/*.

Values of AB computed as indicated above from the equation given

in Figure 25, with c, c s , and AA constant, are, of course, inversely

proportional to the standard absorbancy As , which explains the

general shape of the B ± AB curves. The irregularity near 400m/x

would be eliminated by adopting slightly different values of B in

this region.

(b) SOLUTION B No failure of Beer's law was detected for the components of the

B solution. Measurements were made on copper sulphate, cobalt

sulphate, and cobalt ammonium sulphate, as follows: Five copper

sulphate solutions were prepared, Nos. 62-7-A to E, filtered, containing, respectively, 5, 10, 20, 30, and 40 g of copper sulphate per

liter of solution. Each solution contained 10 cc of sulphuric acid.

Thicknesses of 40 and 100 mm were used. Measurements were made at 435.8m/*, 546. Inn*, 578. 0m/*, and 640m/*. The values of absorbancy for each solution were then computed to 10 mm thickness

and 20 g of copper sulphate. When this was done the maximum

total variation for any of the five solutions at any of the four wave

lengths was 0.004, with no indication of any consistent variation

with concentration.

In a similar manner five solutions of cobalt sulphate were prepared,

Nos. 62-8-A to E, filtered, containing, respectively, 2.5, 5, 10, 15,

and 20 g of cobalt sulphate per liter of solution. Each solution

contained 10 cc of sulphuric acid. Thicknesses of 40 and 100 mm

were used. Measurements were made at 435. 8m/*, 491.6rn/*, 546.1m/*,

and 578m/*. Upon reduction of the absorbancies to 10 mm and 10 g, the maximum total variation for the five solutions at any of the four wave lengths was but 0.002. Three solutions of cobalt ammonium sulphate were measured,

Nos. 65-1-A, C, and F, unnltered, containing, respectively, 4.827,

14.481, and 28.962 g of cobalt ammonium sulphate per liter of solution. Each solution contained 10 cc of sulphuric acid. Thicknesses

of 40 and 100 mm were used. Measurements were made at 435. 8miu,

491.6m/*, 501.6m/*, 546.1mju, and 578.0m/*. Upon reduction of the

absorbancies to 10 mm and 14.481 g, the maximum total variation

for any solution at any of the five wave lengths was 0.0014. They are undoubtedly merely a result of experimental uncertainty. Further- more, both copper sulphate and cobalt sulphate in aqueous solution

have previously been found to obey Beer's law over a larger range

of concentrations (51, p. 31; 52, p. 775).
98 BUREAU OF STANDARDS

There still might remain the question of whether the mixture of

copper sulphate and cobalt ammonium sulphate would obey Beer's

law, as mixed in the present filters to form solution B. This was not

tested directly, but two of the experimental checks elsewhere described show that any failure is quite negligible. These checks are:

1. The measurements on the completed 4,000° K.-to-mean-sun

filter. (Sec. VIII, 1.) The concentrations of both components of

solution B are in this case near their lower limits. The very good

check obtained between the computed and directly measured values

leaves no reasonable question of the practical obedience to Beer's law at the low concentrations.

2. The test of reproducibility (Sec. VII, 8) on solution B of the

2,360° K.-to-mean-sun filter. In this case the concentrations of

both components are near their higher limits. The values thus

obtained, compared with the standard computed values for the 2,360°

K. B solutions, show that there can be no important deviation of Beer's law at the higher concentrations.

7. RELIABILITY OF THE SPECTROPHOTOMETRIC MEASUREMENTS

The data given in the present section are intended primarily to

illustrate the reliability of the adopted absorbancy values given

in Tables 4, 5, and 6 as pertaining to solutions prepared from the

chemicals described in Section IV, 2. They do not, except where

clearly indicated, illustrate the reproducibility obtainable from differ- ent lots of chemicals, which subject is considered in Section VII, 8, following.

(a) AT THE Hg AND He WAVE LENGTHS

The most fundamental and reliable spectrophotometric data on the

solutions are the absorbancy values given for the standard solutions,

Nos. A',B/, B2

', and B', at the eight reference Hg and He wave lengths,

viz, 404.7mM, 435.8mju, 491.6mju, 501.6m/x, 546. lm^, 578.01^, 587.6m/x,

and 667.8nijU. The use of these light sources eliminates the possibility

of errors arising from incorrect wave-length calibrations and the use

of finite slit widths with a continuous spectrum source. The data

with the homogeneous light sources at these eight wave lengths are, in general, quite extensive, including usually measurements on several

different solutions and at various thicknesses and intervals after preparation.

A complete summary of the experimental values obtained on the

standard solutions is given in Tables 18 to 21. As a function of wave length is tabulated first the adopted absorbancy values and

then for the various solutions, as indicated, values of A - As where

As refers to the adopted absorbancy values and A the experimental

values. Algebraic means are also tabulated as may be noted. In

compiling Tables 18 to 21, no data which were taken at any time

during the investigation have been omitted, whether obtained before or after the standard values were adopted, except that in Table 18

only those data are shown which were obtained on solutions prepared

from commercial pyridine. Where values are tabulated representative of two or more determinations they are an arithmetical mean,

unweighted, all values being reduced to 10 mm thickness before

being averaged.
REPRODUCIBLE WHITE LIGHT FILTERS 99

The data for solutions B/ and B2

' show, in general, higher precision

and better agreement with the adopted values than do the data for solution A', particularly if the data for the first four days on solution

B/ be excluded, as they were when the standard values were adopted.

This difference between the A' solution and the components of the

B' solution results, at least principally, from two causes:

1. In the concentrations as used and for these eight wave lengths

the absorbancies for solution A' reach a higher value than for either

B/ or B2 '. Above 546.1m/x, thicknesses of 10 and 20 mm were the

only ones used for solution A', whereas for solution B/ and B2

', thicknesses from 10 to 100 mm could be used, except only at 667.8mn with B/. Increased thickness makes for more precise determination

when the values are reduced to a 10 mm basis.

2. Solutions B/ and B2

' show no important change with time as does A'. (Sec. VII, 3.) The absorbancies for solution B', the sum of those adopted for B/ and B2

', should, therefore, have a reliability fully the equal of, probably better than, solution A'. It is rather difficult to summarize

the data further than is done in Tables 18 to 21. It seems, however,

correct to say—considering the respective algebraic means of the

deviations of the complete data from the adopted data, for a time

interval from a week to perhaps two months after preparation, for the chemicals as analyzed and used, and possibly excepting the

data at 404.7m/z and 667.8mju—that the adopted absorbancies at these eight wave lengths for solutions A', B/, B2

', and B' in 10 mm

thicknesses are accurate to 0.001 or better, or one-quarter per cent of the transmittancy. (The accuracy of the photometric scale of the

Koenig-Martens spectrophotometer is, of course, involved in such a conclusion. As shown in a recent paper by McNicholas (45), by

tests with accurately calibrated rotating sectors, part of which tests were carried out during the period of the present measurements, no

error in the photometric scale of this particular instrument has ever been found, provided that measurements are made under such experimental conditions that the angle of photometric match is not close

to the 0° or 90° points. The accuracy of the present data is further

certified by the fact that the adopted data are in all cases based on measurements with two or more thicknesses of solution.)
100 BUREAU OF STANDARDS

1

3 "o>

so ^

OH

GO

5 ?42g %

8

%++++++

\

ssssllsl

f++ r+++

r

issslsll

c' r r+ r++ r §88

+

1

8888

t' i" f r

-h <M r-1 O —< I 8 = 5giEi;

I I 1+ I iiiiiiii

;83

sslllsii

? r r f r r r+ 11 ; ;§! ? r i iff

88

r r

5 88;

C|55555 O O O O O I ?

'+ f

'

» O o o o 5 5 c:

i o o o o o o o :++++++ f

lO lO oooooooo oooooooo

OOOOWH^OLOO

c5

ij" lO H rt' C 00 N 1> ocooio^nooo tj< io !0 'O lO o

fcuo be bo 05 bJDbco ©
REPRODUCIBLE WHITE LIGHT FILTERS 101

'able 19.

Experimental data on solution B\ , standard acidified aqueous solution

of copper sulphate, at the eight Hg and He wave lengths

[Thickness 10.00 mm., temperature, 25° C]

Wave length

in millimi- crons

Eg 404.7 Hg 435.8 Hg 491.6. He 501.6. Hg 546.1 Hg 578.0. He 587.6. He 667.8

Standard

absorb- ancy As

0. 0021 .0013 .0019

.0028

.0135 .0368

.0487

.319

Absorbancy as measured, A, minus adopted standard absorbaney, A«

Solution number

62-18 2 62-18 62-1 62-1 62-5 2 62-7-C 62-7-C

Determinations

1

I 1 Days after preparation

0, 1, 4 12, 14 27 35 Solution filtered? No No Yes Yes No Yes Yes

+0. 0031 +.0030

+. 0030 +. 0023 +. 0021 +. 0019 +. 0018 +. 001

-0. 0004 -.0001 +. 0003 +. 0004 -.0009

-. 0011 +. 0002 .000

+0.0012 +. 0008

+.0003 +. 0004

+0. 0005 +. 0002

.0001 .0000

+0. 0031 +. 0030

+.0030 +.0030

0.0000

+. 0001 +.0002

+0. 0001 .0000 -. 0005 -. 0006 -. 0005 -. 0002 -.0013 -.001

Algebraic means 3

All data

+0. 0015 +. 0013 +. 0015 +. 0012 +. 0005 +. 0004 +. 0006 +. 0003

Exclud- ing values

for <5

days

0. 0000

.0000

.0000

[+. 0001 -.0006

-. 0006 -. 0001 -. 0002

1 A- A, =0.00434 is equivalent to a 1 per cent difference in transmittancy.

2 These data illustrate turbidity effect of first few days with unfiltered solutions.

3 Weighted according to the number of determinations. Table 20.

Experimental data on solution B2

r standard acidified aqueous solution

of cobalt sulphate or cobalt ammonium sulphate, at the eight Hg and He wave

lengths

[Thickness. 10.00 mm, temperature 25° C]

Wave length

in millimicrons

Standard absorb- ancy A8

Absorbancy as measured, A, minus adopted standard absorbancy, A, Material Cobalt sulphate Cobalt ammonium

sulphate

Solution number

62-17 62-17 61-10 62-8-C 62-8-C 62-20 65-1-C Determinations

• > •

1 1 Days after preparation

0, 1, 4 13, 15 (?) 3 38 4 16 Filtered? No No No Yes Yes No No

Algebraic mean, 2 all data

Hg 404.7 Hg 435.8 Hg 491.6 He 501.6 Hg 546.1 Hg 578.0 He 587.6 He 667.8

0. 0144

.0437

. 1497 .1661

.0901

.0219 .0167

+0. 0006 +. 0007 -. 0001 +. 0009 +. 0010 +.0005

+. 0004 +. 0005

+0. 0004 +. 0002 +. 0006 +. 0002 -. 0003 -. 0001 +. 0001 .0000

+0. 0003 +. 0010 -0. 0004 -. 0007 +. 0019 +. 0005 -. 0010 -. 0004

+0. 0034 +. 0023 +. 0001 +. 0025 +. 0013 +. 0013 +. 0023 +. 0021

+0. 0022 +. 0017 +. 0008 -. 0008 +. 0002 +. 0007 +. 0007 +. 0008

0. 0000 +. 0015 +. 0001 +. 0007 -. 0001

+0. 0009 +. 0008 +. 0003 +. 0006 +. 0008 +.0004 +. 0006 +.0006

1 A-A 8=0.00434 is equivalent to a 1 per cent difference in transmittancy.

2 Weighted according to the number of determinations.
102 BUREAU OF STANDARDS

Table 21.

Experimental data on solution B', standard acidified aqueous solution

of copper sulphate and cobalt ammonium sulphate, unfiltered, at the eight Hg

and He wave lengths

[Thickness 10.00 mm, temperature 25° C]

Wave length in millimicrons

Standard absorbancy A„

For

solution

B'i

For

solution For

solution B'=

B'l+B'a

Absorbancy as measured, A, minus

adopted standard

absorbancy, Aa1

Solution number

65-2 65-2

Determinations

3, 4 3, 4 Days after preparation

8, 9 300, 301

-0. 0010 -.0010

+. 0012 -.0007 +. 0010 -.0007

-.0008 +.001

-0. 0038 -. 0028 +. 0011 +. 0005 +. 0001 -.0009

-.0022

.000

Hg 404.7 0.0021 Hg 435.8 I .0013 Hg 491.6 [ .0019 He 501.6 ! .0028 Hg 546.1 i .0135 Hg 578.0 1 .0368 He 587.6 I .0487 He 667.8 I .319

0.0144

.0437

.1497

.1661 .0901

.0219 .0167

0. 0165 .0450

.1516 .1689

. 1036 .0587

.0654

.328

1 A-As=0.00434 is equivalent to a 1 per cent difference in transmittancy.

(b) FROM 350mM TO 720mM The discussion in Section VII, 1 and 2, regarding the methods of measurement and the extent of the data, should be recalled in this connection. It will be remembered that the data were in all cases adjusted so that the adopted values at the eight Hg and He wave

lengths fall precisely upon a smooth curve drawn through the values

adopted at every lOnux.

In the neighborhood of these eight reference wave lengths, therefore, the accuracy of the data at every lOmju should be practically

equal to that for these eight wave lengths. It may well be true, however, at some of the intermediate wave lengths, particularly where

the intervals are the largest, that the reliability of the data is slightly

decreased. It is estimated that the accuracy of the standard absorbancies in these cases should hold to 0.002 or better, or to one-half of

1 per cent of the transmittancies. Outside of the range from 404.7nnx

to 667. Smu, the uncertainty may become greater.

8. SPECTROPHOTOMETRIC REPRODUCIBILITY OF THE CHEMICALS

The spectrophotometric accuracy of the data obtained with the

chemicals as analyzed and used in this investigation is further dis- cussed in Section VIII, 1, where spectrophotometric data are given

for various completed filters. A question of equal importance is whether or not the data would be duplicated when materials are
REPRODUCIBLE WHITE LIGHT FILTERS 103

obtained from various sources, with or without careful analysis.

This has been investigated with the cooperation of Messrs. Jones and

Richardson, of the research laboratory of the Eastman Kodak Co.,

in connection with special studies of the 2,360° K.-to-mean-sun

filter recommended as a standard for photographic sensitometry in a

resolution adopted in 1928 by the Seventh International Congress of Photography. (Sec. VI, 1.) Three sets of solutions were prepared at the Eastman Kodak Co.

and submitted to the bureau for spectrophotometric measurement.

The materials were obtained from the sources listed in Table 25, and

will be considered more in detail later in the section. The first two

sets consisted of five A and five B solutions, numbered 1 A to 5A and

IB to 5B, having compositions as specified in chart 2 for the 2,360°

K.-to-mean-sun filter. So far as this first set of A solutions was concerned, the purposes of the test were partially defeated because the

authors neglected to inform those preparing the solutions that they

should not be filtered. Solutions Nos. 1A to 5A and IB to 5B were

filtered. As shown in Section VII, 7, it is a matter of practical

indifference whether the B solutions are filtered or not; but this is not the case with the A solutions, so that the spectrophotometric

measurements with the first set of A solutions may show among other

things the effect of filtering the solutions.

Later on another set of A solutions, Nos. 1A-2 to 5A-2 was pre- pared at the Eastman Kodak Co. These A-2 solutions were not

filtered; they were prepared from the same lots of copper sulphate as the previous A solutions; they were prepared from new lots of pyridine

obtained from the same sources as before; the pyridine was carefully

tested for strength, and the solutions prepared using the equivalent of

30.0 cc per liter of 98.4 per cent strength pyridine in each solution.

The spectrophotometric measurements on all three sets of solutions were made at the bureau. Values of absorbancy were measured at certain selected wave lengths, at 25° C, and at two or more thick- nesses (except with the B solutions, where measurements were made

at but one thickness in the regions of minimum absorbancy).

The results are shown in Tables 22, 23, and 24, wherein are given:

(1) Standard values of absorbancy, that is, those obtained by computation using the standard spectrophotometric data of Tables 4 to 6 in the same manner as was done for the A and B components of all of the filters, (2) values of absorbancy obtained experimentally with the

respective solutions, (3) differences between the standard and these

observed absorbancies, (4) the percentage differences between the

standard and observed transmittancies, and (5) the percentage ratios

of the observed to the standard absorbancies, in those cases where the

absorbancies are great enough to make these ratios of possible

significance from an analytical standpoint.
104 BUREAU OF STANDARDS

Table 22.

Test of spectrophotometry reproducibility of chemicals for solution A,

filtered, 2,360° K. to mean sun; thickness, 10.00 mm, temperature, 25°C., age,

1 month. Pyridine not tested before preparation of solutions

[Solutions prepared at Eastman Kodak Co.; spectrophotometric measurements at National Bureau of Standards]

Spectrophotometric data 1 Wave length in millimicrons Standard

Experimental values for solutions Nos. Mean 1A

values 1A 2A 3A 4A 5A

to 5A

Hg 404.7 0. 0802 0. 0836 +.0034 +.8

0. 0786 -.0016 -.40. 0825 +.0023 +.5

0. 0760 -.0042 -1.0

0. 0956 +.0154 +3.5 +0.7 Hg 435.8 .0220 .0256 +. 0036 +.8

.0218 —. 0002 .0

.0218 —. 0002

.0

.0193 —. 0027 -.6

.0300 +. 0080 +1.8 +.4 Hg 491 6 . 1117 . 1132 +. 0015 +.3

.0990 -.0127 —2.9

. 1110 -.0007 —. 2

.1038 -.0079 —1.8. 1026 -.0091

—2.

1

-1.3 Be 501.6 . 1607 .1576 -. 0031 .1504 -. 0103 -2.4

.1598 -.0009 -.2.1541 -.0066 -1.5. 1561 -.0046 -1.1 -1.2 Hg 546.1 . 5325 .5182 -. 0143 -3.3

97.3

.5109 -.0216 -5.0

95.9

.5254 -.0071 -1.6

98.7

.5236 -.0089 -2.0

98.3

.5193 -. 0132 -3.0

97.5 —3. Hg 578.0.. .7926 .7784 — 0142 -3^3

98.2

.7569 — 0357 -8^2

95.5

.7827 — 0099 -2.3

98.8

.7844 -.0082 -1.9

99.0

.7690 —. 0236 -5^4

97.0 —4.2 He 587.6 .8405 .8292 -.0113 -2.6

98.7

.8072 -.0333

—7. 7 96.0

.8288 -.0117 -2.7

98.6

.8380 -.0025 -.6

99.7

.8232 -. 0173 -4.0

97.9 -3.5

Inc. 620 .8896 .8804 -.0092 -2.1

99.0

.8579 -.0317 -7.3

96.4

.8823 -.0073 -1.7

99.2

.8897 +.0001

.0 100.0

.8738 -.0158 -3.6

98.2 -2.9

Inc. 667.8 .7485 .7303 -. 0182 -4.2

97.6

.7264 -. 0221 -5.1

97.0

.7441 -.0044 -1.0

99.4

.7474 -.0011 -.3

99.9

.7394 -.0091

-2. 1 98.8 -2.5

Average values of A/As fner cenf* = 98.2 96.2 98.9 99.4 97.9

1 At each wave length the values given represent the following quantities:

First line, values of absorbancy, As for standard values, A for experimental values. Second lines, values of A —

A

s . Third line, values of (A—

A

s )/0.00434, representing the percentage difference in the standard and ob- served transmittancies. Fourth line, values of A/As (per cent), at wave lengths indicated.
REPRODUCIBLE WHITE LIGHT FILTERS 105

Table 23.

Test of spectrophotometric reproducibility of chemicals for solution A,

unfiltered, 2,360° K. to mean sun; thickness, 10.00 mm, temperature, 25° C,

age, 1 to 2 weeks. Pyridine tested for strength and amount used equivalent to standard

[Solutions prepared at Eastman Kodak Co.; spectrophotometric measurements at National Bureau of Standards]

Spectrophotometric data

»

Wave length in

values

Erperimental values for solutions Xos.

3A-2 Mean

1A-2 to 5A-2 5A-2 Hg m.

B.Z 491.

He 501.

Hg 575.0.

He 587.6.

Inc. 620.

Inc. 667.S.

0. 0S02

.0220

1117

.5325

7^2-i

.8405

Average values of A.'A, (per cent) =

0.0599 —.0097

_!_2. 2.0286 +.0066 +L5

.1105 -.0012 i

.1678

4-. 0071 +L6

.5307 -.0018 -.4

. :8>: -.0050 -1.2

99.4

.8434 -K0029

1(XX3 _ 9 99.9

.7495 +. 0010 +.2

100.1

a 0B5G + 0054 -L2

-4-. 0060 +1-4

.1130 -. 0013 +.3

.1594 -.0013 -.3

.5260 -.0065 -1.5

95. S

-2.2

98.8

.8318 -.0087 -2.0

99.0

-2.0

99.0

.7433 -.0052 -1.2

99.3

0. 1154 —.0352

+8.1

.0464 - 1244

.1213

-J-.0096 +2.2

. 1725 +. 0118 +2.7

.5306 -.0019 -.4

99.6

.7833 -.0093 -2.1

98.8

.8362 -.0043 -1.0

99.5

.8854 -.0042 -1.0

99.5

----- +.9

100.5

0.0501 -.

.0

.0195 -.0025

.1049 -.0068 -1.6

.1494 -. 0113 -2 6.5268 -.0057 -1.3

98.9

.7839 -.0087 -2.0

98.9

.8445 —.0040 +.9

100.5

.8905

100.1.7454 -.0031 -.7

99.6

0. 1055 —.0253

f5.8 +.0076 +1.8

.1046 -.0071 -1.6

.1584 -.0023 -.5

.5138 -.0187 -4.3

96.5.7752 -. 0174 -4.0

97.8

.8310 -.0096 -2.2

98.9

.5772 -. 0124 -2.9

.7454

-.0031

-.7

9.6

-3. 5

+L!

-2.3

-.7

-1.2

99.9 99.0 98.3

explanation of values, see footnote to Table 22.
106 BUREAU OF STANDARDS

Table 24.

Test of speetrophotometric reproducibility of chemicals for solution B,

filtered, 2,360° K. to mean sun; thickness, 10.00 mm, temperature, 2S°C, age,

1 month

[Solutions prepared at Eastman Kodak Co.; speetrophotometric measurements at National Bureau of Standards]

Speetrophotometric data Wave length in millimicrons Standard

values

Experimental values for solutions Nos. 3B IS 5B Mean IB

to 5B

0828

.2799

.3115

. 1852

4.500

Average values : - - (per cent)

:

Cobalt (mean of 491.6.

501.6, and 546.1 m^) =

Copper ( weighting

667.8/820=2 1) =

0.027S -.0018 -.4

.0771 -.0057 -1.3

.2546 -.0153 -3.5

94. 5.2906 -.0209 -4.8

93.3.1776 -.0076 -1.8

95.9

0O3T

-.0019 -.4

.1751 -.0009 +.2

100.5

.4414 -.0086 -2.0

98. 1

0.0288 -.0008 -.2

.0788 -.0040 -.9

.2703 -.0096 -2.2

96.6

0109 5

1 .5.1801 0051 2

.2.0878

.0948 -.0023 -.5

.1764 -.0022 +.5

10L3

.4465 -.003/

94.6 9*. 8

0.0308 +. 0012 +.3

.0814 -.0014 -.3

.2811 +v 0012 +.3

100.4.3128 +. 0013 +.3

100.4.1916 -.0064 +L5

103.5.0920

-~ 0014 +.3

.0964 -.0007 -.2

.1763 -.0021 +.s

101.2.4492 -.0008 -.2

99.8

-f. 0010 +.2

.2770 -.0029 -.7

99.0

.3072 -.0043 -1.0

98.6

.1821 -.0031 9&3

.0898 -.0008 -.2

.0995 -.0024 +.6

.1782 -.0040 +.9

102.3.4513 +. 0013 +.3

100.3

0.0397 +. 0101 +2.3

.0841 -. 0013 + 3

.2706 -.0093 -2.1

96.7

.3012 -.0103 -24

96.7

0022

-.0013 -.3

.0984 +. 0013 +.3

.1772 -.0030 +.7

10L7

.4521 -. 0021 +.5

100.5

101.4

100.3

98.6

101.0

97.4

100.9

1 For explanation of values, see footnote to Table 22. Item (4) above illustrates the percentage effect of the deviations on the energy distribution of the source-and-filter combination : item

(5) affords a speetrophotometric analysis of the relative copper or cobalt content of the solutions, as qualified below.

A summary of the analyses of these three sets of solutions is given

in Table 25. Explanations for the larger discrepancies in Tables 22

to 24 between the standard speetrophotometric data and the values

obtained with these solutions become immediately obvious. The

following cases should be noted in particular:

1. Solution 2A. —The pyridine was yeUow and of but 92 per cent strength in the solution. As noted elsewhere, it requires about 6 per
REPRODUCIBLE WHITE LIGHT FILTERS 107

S3

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108 BUREAU OF STANDARDS

cent error in pyridine strength to equal 1 per cent error in copper

strength. Since this solution is about 4 per cent weak (in copper

content or its equivalent) the pyridine weakness is not the sole explanation of the discrepancy. However, as noted above, this and

the other A solutions (1A to 5A) were filtered. In view of the fact

that the filter paper is liable to remove some of the copper (Sees.

VII, 2, (6) and IX), no further explanation of the copper weakness

in these A solutions should be necessary. It is to be emphasized

that these data of Table 22 can not be used to show lack of reproducibility in the Davis-Gibson filters. 2. Solution 3A-2.—The pyridine used was yellow. Spectrophoto- metric analysis of this pyridine (Sec. VII, 9 (d) ) showed that this cause would explain about half of the relatively large deviations in the

transmittancies at 404.7m/x and 435.8m/i. The reason for the lack of such deviation in solution 2A is not clear.

3. Solutions 1A-2, 2A-2, and 3A-2.—The remaining discrepancy

at 404.7m/z with solution 3A-2 and other positive deviations at this wave length would be adequately explained by assuming small

amounts of picoline impurity in the pyridine. (Sec. VII, 9 (6).) This,

however, would not explain the positive deviation at 404.7mju in the

case of the 5A-2 solution as the same pyridine was used here as in

the 4A-2 solution.

4. Solutions 5A, 5A-2, and 5B.—The deviations at 404.7mM and

to a less extent at 435.8m^i indicate quite certainly a common source

of trouble. An analysis of the copper sulphate, common to all three

solutions, showed a relatively high proportion of nickel and iron

impurities. No study of nickel sulphate in pyridine solution has been

made, nor of iron solutions with or without pyridine. The analyzed

impurity of nickel would cause barely + 1 per cent error at 404.7m/z

in the B solution, compared with the observed +2.3 per cent error. (The cobalt component of solution 5B is common to IB and 2B,

which shows that nickel impurity in the cobalt can not contribute

appreciably to this 2.3 per cent.) While no certain conclusion can

thus be drawn, it is believed that the discrepancies at 404.7m/x,

allowing for the usual spectrophotometrie uncertainty, could be

adequately accounted for by the impurities of this sample of copper

sulphate. As noted in Table 25 its general appearance was quite

inferior, and the chemical and spectrophotometric analysis (solution

5A-2 and possibly 5A) indicate copper weakness of 1 or 2 per cent. It

is reasonably certain that solutions 5A and 5A-2 were not weak in

pyridine because the same supplies were used as in solutions 4A and

4A-2, respectively, which showed much smaller deviations.

5. Solutions IB, 2B, and 5B.—The same lot of cobalt ammonium

sulphate was used in preparing these solutions. The salt was dis- tinctly wet, adhering to the sides of the bottle. Analysis of the salt showed 98.6 per cent of the theoretical cobalt content. However,

chemical analysis of the three solutions (Table 25) gave cobalt contents

which account for most of the discrepancies, the chemical and spec- trophotometric analyses agreeing to about 1 per cent in two of the

three cases.

If the data be rejected for those solutions in which yellow pyridine,

wet cobalt ammonium sulphate, or inferior looking copper sulphate were used (in addition to the filtered A solutions), the remaining

solutions show a very satisfactory degree of reproducibility. It is
REPRODUCIBLE WHITE LIGHT FILTERS 109

true that the pyridine used in solutions 1A-2 to 5A-2 was tested and

adjusted for strength, but it is also true that the total variation in

strength in these and the pyridines used in solutions 1A to 5A, as tested in the undiluted stock, varied only from 97.0 to 99.0 per cent. The total variation in all the samples of pyridine examined during

this investigation, 17 in number, varied only from 97.0 to 100.0 per

cent or ±1.5 per cent approximately from the 98.4 per cent taken as standard. As stated above, 6 per cent variation in pyridine is practically equivalent to a 1 per cent variation in copper content; and

1.5 per cent variation in pyridine causes less than 1 per cent variation

in transmittancy at the wave length (587.6m/*, approximately) where

this variation is the greatest.

Excluding the solutions just noted leaves Nos. 1A-2, 2A-2, 4A-2,

3B, and 4B as representative of what may be expected when materials

are used without chemical or spectrophotometric analysis, but when

certain precautions are used in the specifications and choice of the

materials and when the A solution is not filtered. The maximum

deviations in transmittancy of the observed from the standard computed values are from 1 to V/2 per cent for the B solutions and from

2 to 2){ per cent for the A solutions. Furthermore, little increase in

these maximum deviations are noticeable if data from the rejected

solutions be included at those wave lengths where the objections stated

do, not apply—for example, (1) with solutions 1A to 4A from 404.7m/*

to 501.6m/*, where loss of copper strength by filtering would be barely

noticeable; (2) with solution 3A-2 at the longer wave lengths, where

the yellow pyridine has practically no absorption when diluted to 30

cc per liter; (3) with solutions IB and 2B at 404.7m/* and 435.8m/*

and above 546.1m/*, where variations of a few per cent in cobalt content would be of no importance.

It will be noticed that the region where the B solution is most apt

to be in error is in the green, including particularly the 491.6m/* and

501.6m/* lines, in which region the A-2 solution is on the average the

least in error. Therefore, the combination of the A and B solutions

(prepared from properly chosen materials as noted just above) will give for the complete filter a maximum deviation little if any greater

than is given by the A solution alone; for example, the combination

of either 3B or 4B with either 1A-2, 2A-2, or 4A-2 gives a maximum

deviation of 2% per cent, excepting only in one case (4A-2 + 4B at 501.6m/*) where a 3% per cent deviation is obtained.

These maximum deviations compare favorably with those obtained

when a completed 2,360° K. filter was examined spectrophotometri- cally (Sec. VIII, 1), the same materials having been used in preparing

the completed filter as were used in deriving the adopted data, and

are not certainly outside of the combined spectrophotometric uncertainties. Filters using lower concentrations would be subject to still less uncertainty. With due consideration for the source and appearance of the materials, therefore, as summarized in Section IX, it would seem that filters can readily be prepared whose resulting

energy distributions will not deviate from those herein given by more

than 2 or 3 per cent. Two other items of interest regarding the reproducibility of pyridine

may be noted.

1. A new supply of pyridine was obtained at the bureau in 1929

from Eastman Kodak Co., titrating 97.7 per cent strength. (It
110 BUREAU OF STANDARDS

will be recalled that the standard absorbancy data for solution A',

as illustrated in Tables 4 and 18, were obtained with pyridine from

Baker Chemical Co. which was of 98.4 per cent strength.) An A'

solution was prepared, No. 65-24, using the same copper sulphate

as used throughout the investigation and with the new pyridine—3 g

of copper sulphate, 3 g of mannite, and 30 cc of pyridine per liter of

solution. Spectrophotometric measurements on this solution made

three weeks after preparation gave, at every wave length tested

(Table 18, column 15), values agreeing with the adopted standard

values to better than 1 per cent of the transmittancy.

2. The following statement, to which reference has been made in Section IV, 2, appeared in a memorandum (15 Great Britain 1) submitted under date of June 13, 1929, by the subcommittee on

colorimetry of the National Illumination Committee of Great

Britain

:

Several of the Davis-Gibson filters have been made up at the National Physical Laboratory using c. p. pyridine obtained from commercial dealers without

taking any precautions about purity. The properties of these filters agreed

so closely with those published by Davis and Gibson that we may conclude, so

far as Great Britain is concerned, that pyridine can be obtained commercially

of sufficient purity to obviate the need for any elaborate precautions.

9. AUXILIARY DATA

(a) AQUEOUS PYRIDINE SOLUTION OF COBALT SULPHATE

A solution containing 6.000 g (supposedly) of CoS04 .7H2 0,

30.0 cc of pyridine, and distilled water to a total of 1 liter, was pre- pared and filtered, No. 61-12, and spectrophotometric measurements

made from one to three weeks after preparation. Visual, photoelectric, and thermoelectric methods were used, with thicknesses of

20, 40, and 100 mm. The adopted values are illustrated in Figure 26

in comparison with the standard IV solution. They are not tabulated

because of the uncertainty in the cobalt content. (Sec. IV.)

(b) AQUEOUS a-PICOLINE SOLUTION OF COPPER SULPHATE WITH MANNITE

It was noted in Section IV, 2 that the commonest impurity of

importance apt to be present in pyridine is a-picoline, the pyridine

used in the standard solutions having a picoline impurity of about

2}i per cent. The effect of the picoline was studied in two ways:

(1) As an impurity, 3 g copper-sulphate (with mannite) solutions

being prepared, containing, respectively, by volume, 0, 5, and 10 per

cent of picoline in the pyridine; (2) without pyridine, a 3 g copper

sulphate (with mannite) solution being prepared, containing 35 cc

of picoline (the molecular equivalent of 30 cc of pyridine) per liter of aqueous solution.

The results obtained are shown in Figure 27. The most striking

differences between the pyridine and the picoline solutions are the

general shift of the latter curve toward longer wave lengths and the

notable increase in its absorbancy at 404.7mju.

This marked change in absorbancy at 404.7m^t affords a sensitive

method of analysis of the picoline impurity in pyridine. It will be

noticed that in the green, yellow, and red the effect of the picoline is not very marked and, such as it is, is very similar to the change

produced by a simple weakening of the pyridine strength (fig. 24);
REPRODUCIBLE WHITE LIGHT FILTERS 111

but the change at 404.7mju produced by the picoline is in striking contrast to the very small change occurring at this wave length when

the pyridine content is varied.

On the basis of the measurements at 404.7nijU with the solutions

containing 0, 5, and 10 per cent of picoline impurity (fig. 27), the

standard solution had 2% per cent of such impurity, and within the

b

observational uncertainty this value is checked by consideration of

the data from 546.1m/x to 620m/x, inclusive. It may be noted that

the changes in absorbancy produced by the small amounts of picoline

are not as great proportionately as are indicated by the 100 per cent

picoline solution, even when reduced to a 30 cc basis. The data discussed in Section VII, 8 show that a-picoline impurity

in colorless commercial pyridine as great as 5 per cent has not been
112 BUREAU OF STANDARDS

found in this investigation. a-Picoline impurity between and 5 per

cent will cause deviations from the standard data of less than 3 per

cent of the transmittancy at 404. 7mpi and of less than 1 per cent at all wave lengths in the visible spectrum above 430m/x. Under most

circumstances its effect, therefore, may be ignored.

b

(2 °/~) fouv 9J0?9t/

(c) AQUEOUS SOLUTION OF NICKEL SULPHATE

Nickel sulphate has a well-known characteristic transmittancy

curve with maximum transmission in the green and ultra-violet and

maximum absorption in the extreme violet and red. As an important

impurity in cobalt sulphate and cobalt ammonium sulphate, it is of interest to determine its maximum selectivity in very weak solution;

that is, the ratio of its transmittancy in the violet to that in the green.
REPKODUCIBLE WHITE LIGHT FILTERS 113

Accordingly a solution, No. 61-1, containing 1.000 g of NiS04 .6H2 per liter was prepared and measurements made visually, photoelectrically, and thermoelectrically. The measurements were not

extensive, but sufficient data were obtained to indicate that the

ratio of minimum to maximum transmittancy was certainly not less than 0.950.

If nickel were present in cobalt ammonium sulphate in the proportion of nickel to cobalt of 1 to 100, the concentration of nickel sulphate

present in the standard B2

' solution would be approximately 0.1 g

per liter. The value of the ratio above indicated will in this case probably not be less than 0.995, amounting to one-half per cent

selectivity, and for the cobalt as analyzed and used about one- quarter per cent. This small amount is, of course, negligible, so that if the nickel to cobalt (metals) proportion varies from zero to 1 per cent, no important error is introduced.

(d) AQUEOUS SOLUTION OF PYRIDINE

Two samples of pyridine were examined spectrophotometrically.

One of these samples was from the stock used throughout most of the investigation, practically colorless and shown by analysis to be

of 98.4 per cent strength. (Sec. IV, 2.) In a 10 mm thickness of a

3 per cent aqueous solution (that is, 30 cc per liter, as used in the

filters), this shows a maximum variation in transmittancy between

404.7m.fjL and 700m/* of but 0.2 per cent. This. value was derived

from measurements made on the full-strength material in a 100 mm

thickness. Values for 10 mm are plotted in Figure 28. The other sample tested was the yellow pyridine used in solution

3A-2. (Sec. VII, 8.) It was examined in a 10 and 20 mm thickness,

full strength, and in a 100 mm thickness, 3 per cent strength. Values

for 3 per cent strength were computed from the full-strength measurements, assuming Beer's law, and showed good agreement with those

obtained directly on the 3 per cent strength solution. The average

values are plotted in Figure 28. A maximum selectivity of 3.3 per

cent may be noted.

It is obvious that if the strongly colored yellow pyridine be avoided

and only that used in preparing A solutions which is practically

colorless, showing no more than a trace of yellow, no error of any

importance can result from the color of the pyridine.

(e) AQUEOUS SOLUTION OF SULPHURIC ACID

Sulphuric acid is present in the B: , B2 , and B solutions in the proportion of 10 cc per liter. It was of interest to see if a sulphuricacid solution of this strength showed any selectivity. A solution was

prepared containing 100 cc per liter, or ten times the strength used

in the standard solutions. A single set of measurements was made,

at 404.7m/*, 435.8m/x, 546.1m/*, 578.0m/*, and 700m/*. Values of transmittancy thus obtained, in a 10 mm cell, differed from unity

nowhere more than 1 per cent. The selectivity of the dilute sulphuric

acid as used in the Bi, B2) and B solutions is therefore negligible.
114 BUREAU OF STANDARDS

10. USE OF SOLUTIONS AND DATA IN SPECTROPHOTOMETRIC

CALIBRATION

The discrepancies which may be obtained when the same or dupli- cate samples are measured spectrophotometrically on different apparatus has been well illustrated in a recent publication (53). To one who has available but a single instrument or method of measurement

it may not be apparent that, along with the excellent precision and

£1

O'£o/~)fiout?qjo?qu

the high degree of reproducibility which he may be obtaining, serious

errors of measurement are, perhaps, entering into his data.

Such errors may result from stray light or energy, from impure

spectra caused by the use of too wide slit widths, from inaccurate

wave-length calibration, and from temperature changes occurring in
REPRODUCIBLE WHITE LIGHT FILTERS 115

the sample during measurement. The magnitude of such errors depends greatly on the particular sample being measured. In addition

the photometric scale—nicol prisms, variable sector, moving wedge,

inverse-square law, etc.—may for one reason or another not be functioning in its nominal manner. It may be inaccurate throughout or

at certain places only. In measuring transmission samples the introduction of the sample may change the focus or optical distance or

displace the beam so that, in effect, a different source is being used

when the sample is in the beam than when it is removed. Unless

the source is of uniform brightness and of sufficient extent this may

cause considerable error. Reflection measurements made on different

apparatus will differ because of differing conditions of illumination

and observation, and permanent changes may. take place in the

sample during measurement if subject to excessive radiant energy.

Furthermore, practically every instrument or apparatus has troubles

or uncertainties peculiar to itself which require constant attention to avoid the introduction of error. General familiarity with the subject of spectrophotometry is, of course, of great help in securing reliable transmissive and reflective

data. The Optical Society of America Report on Spectrophotometry

(35) should assist in this respect. Detailed study of one's particular

apparatus should, of course, also be made. A third method often

used in attempting to verify the reliability of spectrophotometric

data is to send colored glass samples to the bureau for calibration.

The spectral transmissions reported by the bureau are then taken as standard values, and by measuring the samples on his own apparatus one can readily locate serious errors of measurement.

It is believed that the standard solutions used in the present investigation can be used in this manner. They have been very carefully

analyzed and measured as indicated elsewhere in the paper. The

series is incomplete for this purpose in that no solution is present

which absorbs strongly in the blue. With this exception, by varying

the thickness of the solutions, values of absorbancy or transmittancy

may be obtained of almost any magnitude at any wave-length region

in the visible spectrum.

The data of Tables 4,5, and 6 only should be used for this purpose.

These data do not involve any Beer's law failure. They are standard

at 25° C, and any deviation from this temperature may be corrected

for by the data of Table 16. The uncertainty of the data is indicated

in Section VII, 7. The data apply to 10 mm thicknesses, and the

uncertainty in the absorbancy values will increase with the thickness.

If one is unable to analyze his chemicals or solutions, the resulting

uncertainty must be allowed for. The questions of choice, analysis,

and reproducibility of materials is discussed in Sections IV, VII, 8, and IX.

VIII. EXAMINATION AND TRIAL OF COMPLETED FILTERS

In Section VII was given in considerable detail a description of

the methods used and the data outlined in the spectrophotometric

study of the filter components. This included absorbancy measure- ments on solutions A', B/, B2

', and B' and density measurements on

the double glass cell filled with water, a study of Beer's law over the

range of concentrations used, the measurement of time and tempera-
116 BUREAU OF STANDARDS

ture effects, and of the effect of variation of pyridine content (on the

A' solution), a discussion of the reliability of the spectrophotometrie data, and tests of the spectrophotometrie reproducibility of the chemicals. The information therein given has permanent value so far as these solutions are concerned not only as regards their use in filters, but also in connection with their use as standards in spectrophotometric calibration or chemical analysis.

In the present section the influence on the completed filters of the

time and temperature effects is computed along with certain other

effects which can be most advantageously considered in this manner,

and, in addition, various direct experimental checks on the completed

filters are described.

1. SPECTROPHOTOMETRIC MEASUREMENTS

Using the same chemicals and the same glass plates as were used

in the measurements described in Section VII, three different filters were prepared and tested, viz, 2,360° K. to mean sun, 2,848° K. to Abbot-Priest sun, and 4,000° K. to mean sun, the respective solutions

being approximately, 3, 3, and 7 weeks old at the time of the measurements. The concentrations thus used are fairly representative of the two extremes and the medium of the concentrations used in the

filters of Series I and II, and the first of these filters is, furthermore,

of special interest in its application.

Measurements were made solely on the Koenig-Martens spectrophotometer using the mercury and the incandescent light sources, with other experimental conditions as usual. It was not possible,

however, to maintain any temperature control over the filters as had

been done with the solutions, because the apparatus for temperature

control was not designed to accommodate specimens as large as the

filters. The temperature of the air near the filter deviated in extreme

cases by 5° or 6° from 25° C, but in such cases measurements were

repeated on days when the temperature of the air deviated from the

standard in the opposite direction, so that the temperature effect was

partially eliminated. No attempt was made to correct the data on

the basis of the temperature corrections established in Section VII, 4, as it was uncertain just how truly the temperature of the air near the

cell represented that of the filter solutions. Probably on the average

the filter temperatures were somewhat in excess of 25° C.

The results of this experimental check are given in Table 26. For

each of the three filters are given as a function of wave length, the

percentage differences between the transmissions as computed and

tabulated in the respective charts and those experimentally determined; that is, T — T

The value given may come from one or two determinations as is the case for the 4,000° K. filter or from one to five determinations as is the case for the other two niters. In averaging two or more determinations at any wave length, equal weights were used.

Deviations less than 1 per cent can not be considered of much significance unless possibly when carrying consistently over a certain
REPRODUCIBLE WHITE LIGHT FILTERS 117

wave-length range for all three niters. The following cases may be

noted particularly:

L At 404-7mn and 435.8m}±.—These values undoubtedly result from

lack of temperature control. For example, at 404.7m/* with the

2,360° K. filter, values of transmission were obtained varying from

0.661, air temperature 31° C, to 0.704, air temperature 23.5° C.

Furthermore, the differences at 420niyu (single determinations, at air temperatures from 24.5° to 28° C.) of opposite sign in two of the

three cases, although of less precision, make practically certain that no important error can be present in this region.

2. From 480mp, to 510m p., inclusive.—The temperature effect is very

small in this region. The deviations average about 0.8 per cent, all of the same sign without exception. This may be significant, but

can not be considered important.

3. From 610mp. to 660mp.—The deviations average about 1.3 per

cent, all of one sign without exception, those for the 2,360° K. filter running over 2 per cent. From 610m/x to 630111^ the temperature

effect is smaU, changing sign near 620nijii.

Table 26.Comparison of standard computed values of transmission (Ts ) as given in the charts with values experimentally determined (Texp .), using the same chemicals and glass plates in the latter case as were used in determining the standard

values

Wave length in milliniicrons

T — T

Values of - T

"r

' X100 for the following niters—

2.360° K. to mean

sun

2,848° K. to AbbotPriest sun

4,000° K. to mean

sun

Ale an

+1.6 +1.9 +0.9 +1.5

-1.0 +.8 -.2 -. 1 +1.0 +1.1 +.6 +.9 +.3 +.1 +.7 +.4

+1.1 —. 5 +.6 +.4

-.4 +.7 -.2 +.5 +.9 +.1 +.5

+.8 +1.2 +.1 +.7 +.8 +1.0 +.2 .+7

+1.3 +1.6 +.6 +1.2 +.8 +1.3 +.4 +.8 —. 5 +.3 +.5 +.1 +.6 +.3 +.3 +.5 +.7 +.1 +.4

-.6 +.3 +• 7 +.1 +.4 +.1 +.2

+.1 —. 5 —. 1

Wave length in millimicrons

570— Hg 578.0. 580—

590— 600—

610—

620—

640_

650.

670-

"00-

Values of T' JT"P ' X100 for the following filters2,360° K. to mean

sun

+.4

+.8

+1.2

+2.5 +1.8

+2.5

+1.8

+2.8

+2.5

+2.9 +1.8 +1.5 Mean.

2,848° K. to AbbotPriest sun-.4

-.4

-.9

-1.0 +1.7 +.6

+1.2 +.6 +.2

+.4

+1.2

4.000° K. to mean

sun

+.6

+.7 +.7

+.4

+.7

+.9 +.7

+.8 +.4

+1.2

+1.2 +.4 +.4

+.4

Mean

-.1

-. 1 -.2 +.2

+.5

+1.6

+1.1 +1.5

+1. 1 +1.1

+1.4

+1.1 +.9

+.4

+.3

+.6

4. The final mean value.—This average, +0.6 per cent, illustrates the tendency of the values to be positive. . The discrepancies illustrated in Table 26 may be considered from

two standpoints

(a) their reasonableness from the spectrophoto- metric standpoint, assuming no change in the chemicals and no error

in preparing the solutions, (?) their connection, if any, with the results obtained with solutions prepared from other chemicals.

(a) The uncertainty estimated in the standard data for solutions

A' and B' was in each case from % to % per cent of the transmittancy from 404.7 to 667.8m,u, and % per cent of the transmission
118 BUREAU OF STANDARDS

for the cell filled with water. In addition, must be included the uncertainty resulting from any error in the Beer's law correction. A

maximum uncertainty of approximately 1 per cent is therefore to be

expected in the standard computed values (Ts ) from which the per- centage deviations of Table 26 were derived.

Regarding the Beer's law correction factor, it has been shown in Section VII, 6 and Figure 25 that the only region in which any reasonable error could exert an appreciable effect on the computed values was in the red. Such possible errors can not be the principal explanation of the deviations shown in the red region of Table 26, for in that

case values for the 2,360° K. and the 4,000° K. filters would be of opposite sign.

The experimentally determined values (T(Xp ) can not be considered reliable in general to better than 1 per cent. With the incandescent light source but one or two determinations were made. As

many as five determinations were made in some cases with the mercury lines, but because of the temperature variations the final values

did not have the usual reliability.

The possible combined uncertainties of the computed and experimental values are, therefore, sufficient to explain most of the deviations in Table 26; that is, all those less than 2 per cent. In the red,

with the 2,360° K. filter, the computed values are probably more

reliable than the directly determined values; in fact, a 2 per cent

error in these latter would not be surprising. It must be remembered

that the transmission in this case is low, varying from about 0.08

at 610mju to about 0.04 at 700m//. The 2.5 per cent deviation at 610m^u equals

0.0834-0.0813

0.0834

X 100

and the 2.8 per cent deviation at 650m/x equals

0.0646-0.0628

0.0646

X100

While, therefore, it is not to be expected that the computed values

of spectral transmission given on the charts are without error, it would take further work to prove conclusively that they were in error by as much as 1 per cent, for the particular chemicals used in this investigation.

(6) The spectrophotometric uncertainty discussed above is some- what less than may be caused by variations in the materials when

obtained from various sources unless due care is taken in the selection

of these materials. This question of reproducibility has been dis- cussed in Section VII, 8. It is noted again merely to point out the

fact that the tendency for the computed values of transmission to be

high from 610niju to 680niju, as illustrated in Table 26, was not

found in the case of the 2,360° K. solutions measured in the reproducibility test. In that case the deviations tend to be in the opposite direction; so that the net result of all the testing indicates that

the values of spectral transmission tabulated on the charts should

be a very close representation of what may be expected in the use

of the filters.
REPRODUCIBLE WHITE LIGHT FILTERS 119

2. COLORIMETRIC COMPUTATIONS

The effect of the spectrophotometrie uncertainties, the time and temperature deviations, etc., on the energy distribution of a source-andfilter combination (E" on the charts) readily follows from the tables

and curves already given in the sections devoted to these subjects.

It is of importance also to consider the influence of these uncertainties

and deviations on the color of the light transmitted by the filter.

JO ZO .30 .4 .50 £0 .70 £0 30 LOO b

Figure 29.The complete r-b -projection of the trilinear diagram showing

the locus of spectrum colors Mean sun is the "neutral" stimulus and is represented at the point r=6=H. Purples (colors whose stimuli -are represented by white light minus homogeneous light of some wave

length) are represented in the region bounded by the hypotenuse and the radial lines labeled "410" and "670," which represent colors whose dominant wave lengths are found near the ends of the visible spectrum. Colors which may be evoked by a mixture of white light and

homogeneous light are represented in the area bounded by the radial lines marked "410" and "670," and the curve marked "spectrum, 100 per cent purity." The color of a light may be described in terms of the trilinear coordinates (r, g, b), as outlined in Section V, 4, (b). It will be

recalled that the values of r, g, and b sum to unity and that the color

of mean sun has, by definition, a value of one-third for each coordinate. In Figure 29 is illustrated the r-b diagram on which is also shown the location of the adopted mean sun (r = b = %) y the locus of

the spectrum from the red to the violet, and the regions of the spectral

colors and purples.
120 BUREAU OF STANDARDS

As previously stated, all the niters of Series I, low color temperatures to mean sun, were adjusted to give an accurate color match

with the adopted mean sun; that is, to have by computation values of

r

==g = b = 0.3333. The uncertainties and deviations in which we are interested may, therefore, be illustrated by plotting the resulting r

and b values on the r-b diagram. The differences from 0.3333 are so

small, however, that they can not well be shown to the scale of

Figure 29. The small rectangular section near the center of Figure

29 is therefore shown to a greatly enlarged scale in Figure 30. A question which immediately arises is whether the various small

differences illustrated in Figure 30 and discussed in detail in the fol-
REPRODUCIBLE "WHITE LIGHT FILTERS 121

lowing sections are visually perceptible. No final answer to this can

be given. Data on the " least perceptible difference " in color are very meager. Furthermore, this more or less vaguely defined quantity is so dependent upon the experimental conditions as to make any

statement regarding it rather meaningless unless these experimental

conditions are fully described.

One investigation has been carried out, however, which furnishes

data of particular interest in this connection. This w

rork was by

Priest and Brickwedde on "The minimum perceptible colorimetric

purity as a function of dominant wave length with sunlight as netural

standard" (54). The complete paper has not yet been published.

At each wave length they each made two determinations: (1) The

least purity perceptible with certainty (Pmax), (2) the greatest

imperceptible purity (Pmi n ). In the present discussion we use a new unit of saturation derived

from these data of Priest and Brickwedde and called the " satron. ? ' Since there is no previously published definition nor mention of this unit it is necessary to introduce some explanation of it. The idea of such a unit is predicated upon the concept of saturation as a psychological entity (4, p. 534) and the definition of purity as a ratio of

brightnesses as defined by Priest (55, p. 175). Saturation is regarded

as the minimum number of equally perceptible increments in purity

between white and the color in question. The so-called just perceptible or least perceptible increment in purity is taken as the most

convenient case of "equally perceptible." It is assumed that, for

colors of very low saturation, and of constant dominant wave length,

equal increments in purity are equally perceptible. It is, however,

known (from the work of Priest and Brickwedde and others) that,

for different wave lengths, equal increments in purity may be very

far from being equally perceptible. It is indeed on this account that we find the data of Priest and Brickwedde particularly instructive in

the discussion of the approximation to white which we now undertake. The term "satron" was tentatively proposed and defined by

Priest in the spring of 1927 for the purpose of facilitating local dis- cussion relating to colors of low saturation among those concerned

with this subject at the National Bureau of Standards. The definition

written by Priest at that time is as follows: For colors of low saturation ^j^-<20^ one satron is the increment in saturation due to an increment in purity equal to P

f

, where P' , a function of dominant wave length, is taken as 1 max ^—— from the mean values of Priest and

Brickwedde.

The effect of this definition is that a color having a saturation of one

satron would be hardly distinguishable from white.

The locus of such colors on the r-b diagram has been computed and

plotted for us by D. B. Judd as shown in Figure 30. Colors whose

trilinear coordinates locate points lying on this curve will be dis- tinguished from white only by keen observers making careful observations—and even then usually with some doubt and hesitancy.

Obviously, the colors corresponding to points lying within the area bounded by this curve will be distinguishable from white with still greater difficulty—and the more so the nearer they approach the

center (r=g = b = %). Roughly it may be said that the area bounded
122 BUREAU OF STANDARDS

by this curve represents colors which may for most practical purposes

be considered indistinguishable from white.

The experimental measurements on which its determination is based did not include the purples, so that the boundary on the purple

side has been more or less arbitrarily estimated. It should be further

noted that the white or "neutral" stimulus used in the experimental

measurements was Abbot-Priest sunlight (Sec. II, 1), and that the

present authors have assumed that the same relative values would

have been obtained if the adopted mean sun had been used as the

"neutral" stimulus. For the purposes of the present illustration this assumption is undoubtedly justified.

As already noted, the absolute values of the so-called least perceptible difference are subject to variations with experimental conditions, such as brightness, field size, observer, and observing technique. The relative values, and, therefore, the shape of the satron

locus as shown in Figure 30, would probably, however, not be greatly

affected, and it is a fortunate coincidence that the deviations to be

considered are of such a nature as to lie more or less along the major

rather than the minor axis of this region.

A record of the particular conditions under which Priest and

Brickwedde made their observations has been published (54). Moreover, Priest has subsequently made other independent experiments

which lead to the following previously unpublished conclusion which

he has -authorized us to quote:

For nearly white mixtures of Prussian blue with zinc oxide, comparing areas

of about 3 or 4 cm2 in immediate juxtaposition in diffuse daylight, under ordinary

conditions at a viewing distance of 30 to 40 cm, it has been found, by fair tests on 16 observers, that a saturation of one satron is clearly recognized by the

keenest observers, recognized with some doubt and hesitancy (but correctly) by

the great majority, and is quite imperceptible only to a very small minority.

The perceptibility appears to be of the same order as the perceptibility of a 1 per

cent difference in brightness under the best photometric conditions.

(a) SPECTROPHOTOMETRIC UNCERTAINTY

Consider first the spectrophotometric discrepancies given in

Table 26. It was of interest to compute the values of r, g, and 6 that result if the experimental data are assumed to be wholly correct and the values given in Table 26 to be due entirely to error in the

standard data. The values of r, g, and 6 thus obtained are as follows:

For the 2,360° K. filter r = 0.33236, g = 0.33407, 6 = 0.33357; for the

2,848° K. filter, r = 0.33384, # = 0.33360, 6 = 0.33256 (relative to Abbot-Priest sun as neutral); for the 4,000° K. filter, r = 0.33305,

# = 0.33346, 6 = 0.33349. The results are shown in Figure 30 by the

three points plotted as "Exp (2,360° K.)," "Exp (2,848° K.)," and

"Exp (4,000°K.)." (The 2,848° K. point is plotted as if Abbot-Priest sun were located at r = 6 = 0. 3333, since this filter was designed to give Abbot-Priest sun.)

If it be remembered that the standard values are, in fact, fully as

reliable as the experimentally determined values (Sec. VIII,

# 1)> it may be assumed that the spectrophotometric uncertainty in the

standard data would cause deviations of not more than half those

shown in Figure 30.
REPRODUCIBLE WHITE LIGHT FILTERS 123

(b) DEVIATIONS FROM THEORETICAL CONCENTRATIONS

In view of the small deviations of the actual content from the

theoretical content which may occur in the copper and cobalt salts used in this investigation (Table 25) it is of considerable importance

to know how such variations affect the color of the filters. For this

purpose the trilinear coordinates were computed for the colors evoked

by the 2,360° K.-to-mean-sun filter—(1) for a 1 per cent increase in

the copper content of solution A, keeping the concentrations of solution B unchanged, (2) for a 1 per cent increase in the copper content

of solution B, keeping solution A and the cobalt content of solution

B unchanged, (3) for a 1 per cent increase in the cobalt content of solution B, keeping the copper content of solutions A and B unchanged.

Values of r, g, and 6 were obtained in the three cases as follows:

(1) r = 0.33135, {7 = 0.33267, 6 = 0.33598;

(2) r = 0.33301, # = 0.33340, 6 = 0.33360;

(3) r = 0.33368, # = 0.33311, 6 = 0.33321.

These values are plotted in Figure 30, the points. They

indicate closely the directions of change caused by small variations of the concentrations in any of the filters. The magnitudes of such

variations will be approximately proportional to the concentrations.

For most of the filters (Tables 8 and 12) the concentrations are less than for the 2,360° K.-to-mean-sim filter, and the deviations caused

by a 1 per cent change in the copper and cobalt salts will be proportionately less. It is obvious that small changes or uncertainties in the concentrations are of possible importance only in the case of the copper sulphate

in solution A. Since the deviations from the theoretical copper

content are rarely more than a fraction of a per cent, the results

plotted in Figure 30 indicate that no perceptible differences in the

color of the filters may be expected to result from uncertainties in the

theoretical contents of the salts used.

(c) DEVIATION FROM THEORETICAL PYRIDINE CONTENT

It will be recalled that the pyridine used in deriving the standard

absorbancy data of Table 4 had a strength of 98.4 per cent and that

the total variation in the strengths of 17 samples of commercial

pyridine examined in this investigation varied from 97.0 to 100.0 per

cent or approximately 1% per cent from the standard. In view of the

questions that have been raised regarding the use of pyridine (41,

56) it is of special interest to compute the effect on the color of the

filters caused by deviations of the pyridine strength from the standard.

The effect of large deviations on the spectral absorbancy has been

illustrated in Figures 8 and 24. Values of r, g, and 6 were computed for both the 2,360° and 2,848°

K.-to-mean-sun filters with 28.5 cc of pyridine instead of the usual

30.0 cc and with the concentrations of the salts unchanged. Values

were obtained as follows: For the 2,360° K. filter, r = 0.33531,

g = 0.33484, 6 = 0.32984 ; for the 2,848° K. filter, r = 0.33463, g = 0.33431

6 = 0.33106. The results are shown in Figure 30 by the points labeled

"28.5 cc pyridine." It is apparent that the deviations caused by a given change in pyridine strength are approximately proportional to the copper concentration in solution A, these being 3.707 and 2.445 g

respectively. It is also undoubtedly true that such deviations would

13416°—31 9
124 BUREAU OF STANDARDS

be approximately proportional to the deviations in the pyridine

strength.

It should be remembered that the results shown in Figure 30 are

for a 5 per cent variation in pyridine strength, which is over three

times as great a variation from the standard as was found in any of the 17 samples of pyridine examined. It may be shown from Figure

30 that the 1% per cent variation from the standard, which is about

the maximum to be expected in commercial pyridine, is of no greater

importance colorimetrically than one-fourth of 1 per cent variation

in the copper content of the A solution, or, conversely, that it would

require more than a 6 per cent variation in pyridine strength to be

colorimetrically equivalent to a 1 per cent variation in the copper

content of solution A. (These values are derived as follows: The rel- ative distances from the point r = g = b = }i to the points showing the

5 per cent variation in pyridine and the 1 per cent variation in the copper of the A solution are, respectively, in the proportion of 5.65:4.65.

If the lines connecting these points and the point r = g = b = % be

extended to intersect the satron locus, we may estimate to what

fraction of a satron these deviations are proportional. The values are 5.65/12.40 = 0.455 for the pyridine, and 4.65/8.00 = 0.581 for the copper. From this we find that 5 per cent variation in pyridine is 0.455/0.581 =0.783 as perceptible as 1 per cent variation in copper in

the A solution, and that it, therefore, requires 5/0.783 = 6.4 times as great a percentage change in the strength of pyridine to produce a

perceptible color difference as is required in the copper sulphate in

solution A.)

This illustration should end all opposition to the use of commercial

pyridine arising from uncertainties regarding its strength. It may

again be noted that the commercial pyridine obtainable in Great

Britain has been found satisfactory for use in the filters. (See quotation at end of Section VII, 8.)

(d) DEVIATION FROM STANDARD TEMPERATURE OF 25° C. In Section VII, 4 are given data showing the change in absorbancy

for each of the three component solutions as the temperature is raised

from 25° to 40° C, it being further shown that this change in absorbancy is proportional to the concentration of the solution. On the

basis of these data of Table 16, the colors of the 2,360° and 2,848°

K.-to-mean-sun filters, in combination with their respective sources, have been computed at 40° C. and are plotted in Figure 30. The

values of r, g, and b thus obtained are: For the 2,360° K. filter, r = 0.33999, g = 0.34286, b = 0.31715 ; for the 2,848° K. filter, r = 0.33778,

# = 0.33986, 6 = 0.32236. Here again the deviations are approximately proportional to the copper concentration of solution A.

The values given at 35° and 30° C. are estimated by taking 2/3

and 1/3, respectively, of the distance along the straight line connecting

the 25° C. (standard white) point and the 40° C. point; the values at 20° and 15° C. are likewise estimated by reversing the sign of the

deviations shown at 30° and 35° C. No possible uncertainty in

such estimations can be of any practical importance in this connection.

The importance of the temperature deviations shown in Figure 30

and the tolerance to be allowed will, of course, depend on the particular use being made of the filters. It would seem that a ± 5° C. tolerance

from the standard 25° C. temperature would be perfectly safe in most
REPRODUCIBLE WHITE LIGHT FILTERS 125

colorimetric, photometric, and sensitometric work, and in many

cases still larger tolerances could be permitted. It should be noted,

however, that these solutions are powerful absorbers of radiant

energy, due to the action of both the water and the copper sulphate,

and the use of the filters in close proximity to lamps may be attended

by relatively rapid increase in temperature of the solutions.

Since most of the temperature change is in the A solution, it is best to place the filter so that the A solution is on the side opposite

the light source. A surer means of protection, where necessary,

would be to place an auxiliary water cell between the filter and the

source. A single or double water cell made with borosilicate crown

glass windows, such as used in the niters, should have no perceptible

effect on the color, as is illustrated in Figure 30, where the color of the double cell filled with water, computed from the standard spectral

transmission data of Table 7, is shown by the point labeled " water

cell." The effect of the water cell on the energy distribution is shown

by its spectral transmission. In cases where two beams are being

used, water cells may be placed in both beams, if considered desirable.

(e) CHANGE WITH TIME

It was shown in Section VII, 3 that the B' solution showed no

certain change over a year's time, but that solution A' showed small

but definite changes as illustrated in Figure 21. A 2,600° K.-to- mean-sun filter has concentrations close to those of the standard

solutions, and it has accordingly been chosen for illustrating the aging

effect. No tests have been made as to how this effect may vary with

the concentration. In the computations under consideration the

values shown in Figure 21 were multiplied by 3.024/3.000, the ratio of the copper content of the 2,600° K. A solution to that of the A'

solution.

The results are shown in Figure 30, the plotted values being labeled

"1 day," "7 days," and "384 days." The trilinear coordinates for these three cases are: For 1 day, r = 0.33449, # = 0.33322, 6 = 0.33228;

for 7 days, r = 0.33328, # = 0.33309, 6 = 0.33362; for 384 days,

r = 0.33060, # = 0.33266, 6 = 0.33675. Values for 24 days were not

computed, as they would be practically identical with the standard

values.

It is apparent that the color of any of the filters would not change

perceptibly from a few days to several weeks, perhaps two or three

months after preparation, but that solutions or filters a year or more

old might be perceptibly different in color (but only so under the best

colorimetric conditions) from newly prepared solutions or filters.

(f) CHANGE WITH C2 IN PLANCK'S EQUATION

In Planck's equation (Sec. Ill, 3) ^ = -C2 e — 1 the relative values of E\ will obviously depend upon the value of C2 (but not upon the value of C\) which is used. It has been stated

that a value of C2 = 14,350/*°, as used in B. S. Misc. Publication No.
126 BUREAU OF STANDARDS

56, enters into all the Planckian energy distributions of Table 2 as used throughout the present investigation. A value of 14,330 (1.433

cm°) has been used in the International Critical Tables. It was of interest to compute the effect on the color of the filters caused by this change in the value of C2

. Values of Ex from 2,000 to 3,120° K., based on values of C2 = 14,330,

have been recently published by Skogland (57), and values for 2,360°

and 2,848° K. were taken from his tables and carried through in the

regular computation for r, g, and 6, in the case of the respective filters of Series I. The following values were obtained: For the 2,360° K.

filter, r = 0.33278, = 0.33309, 6 = 0.33413; for the 2,848° K. filter, r = 0.33292, g = 0.33312, 6 = 0.33395. These values are indicated in Figure 30 by the designations "& (2,360°)" and "C2 (2,848°)."

It is more or less obvious, and has been pointed out in the appendix

to M56, that differences in C2 may be counterbalanced by differences

in 0. A change of 20 in C2 is approximately equivalent to 3° K. at 2,360° K. and 4° K. at 2,848° K. To get the same respective energy

distributions with C2 = 14,330 as were obtained with C2 = 14,350,

values of equal to 2.357° and 2,844°;K. must be used.

If the filters of Series I had been designed with energy distributions

derived on the basis of C2 = 14,330 instead of 14,350, slightly lower

concentrations of copper sulphate in the A solutions would have been

necessary to secure perfect color match with the adopted mean sun. From the equivalents of C2 given above and from Table 8, it may be

estimated that such perfect color match would have been obtained

by using approximately 0.01 g less of copper sulphate than has been

used in this series of filters. In the filters of Series II the change

required would be even less, inasmuch as both the source and the

ideal are computed from Planck's equation, and any change in C2 affects both distributions in a similar manner.

It is apparent from Figure 30 that the error introduced by using

C2 = 14,350 instead of 14,330 (assuming that the latter is really correct) in the design of the filters is of the same order of magnitude as the spectrophotometric uncertainty in the data. It is probably less than is caused by the differences between the energy distributions as given by Planck's equation and as emitted by incandescent lamps.

(Sec. Ill, 3.) It is much less than is caused by a 5° C. deviation from

the standard 25° C. temperature. In fact, probably no case will arise in the use of the filters where the uncertainty in the value of C2 will make the slightest practical difference.

(g) EXPERIMENTALLY DETERMINED ENERGY

It was shown in Section III, 3 (fig. 7) how the energy distribution

of a particular Mazda C lamp as determined experimentally differed

from the energy distribution as given by Planck's equation for the same color temperature (2,848° K.). The effect on the color computed in the case of a 2,848° K.-to-mean-sun filter, using the experimentally determined values as illustrated in Figure 7 instead of as given by Planck's equation for 2,848° K., is shown in Figure 30 by

the point plotted at "lamp 1717." The values of r, g, and 6 obtained

were, respectively, 0.32954, 0.32935, and 0.34111. The shift toward

the blue is caused by the excess of energy at the shorter wave lengths

in the experimentally determined values.
REPRODUCIBLE WHITE LIGHT FILTERS 127

While it was of interest to compute the magnitude of the difference

caused by the two energy distributions noted above, it is not believed

that any such difference can be assumed in the case of incandescent

tungsten lamps in general. In the first place, it is not felt that the

experimentally determined values in the blue and violet can be

entirely free from error caused by stray energy. Such stray energy

from longer wave lengths in the visible and infra-red would undoubtedly cause an increasing percentage error toward the shorter wave

lengths where the true spectral energy is relatively very low. Furthermore, it may be noted in this connection that Forsythe found (58)

that "tungsten radiates in such a manner that when its temperature

is set to give the same red-blue ratio as the black body for a particular

temperature the green part of the tungsten spectrum is relatively

brighter than that of the black body. This difference is, however,

small, being less than one-half per cent at a color temperature of

1,600° K. and increasing to about 1 per cent at a color temperature of

2,600° K." The wave lengths of the red, green, and blue light used

in these tests were about 660m^, 540mju, and 470m/x, respectively.

It is believed that, if the same comparison were made in the case of

the experimental energy distribution of Figure 7 and of a Planckian

radiator at 2,848° K., the experimental data would be low in the green

relative to the red-blue values instead of high.

It would appear from the above evidence that the differences

between the spectral energy distributions from a black body and from

a tungsten lamp at the same color temperature are of no practical

importance; and the reality of this small difference in the particular case illustrated in Figure 7 is subject to considerable question.

3. EFFECT OF PYRIDINE CONTENT ON THE RESULTING ENERGY

DISTRIBUTION

In Section VII, 5 was illustrated the effect of the variation of the

pyridine content on the spectral absorbancy of the A solution; and

in Section VIII, 2 (c) was illustrated the effect on the color of the

source-and-fllter combinations of small variations of pyridine content, such as might possibly occur as a result of the use of pyridine

of subnormal strength. It remains to demonstrate the statements

previously made that a pyridine content of approximately 30 cc per

liter, such as used in all the filters, gives the most suitable energy

distribution which it is possible to obtain.

For this purpose two filters have been computed for converting

2,360° K. to mean sun, following the methods described in Section

V, 4 (a) and Table 9, using in one case a pyridine content of 10 cc per liter and in the other a content of 100 cc per liter, and in both

cases adjusting the concentrations of all three components so as to obtain the best possible energy match with mean sun. The results are illustrated in Figure 31, where the superiority oi

the 30 cc content is clearly apparent. The concentrations used are

therein given. The effects are readily understandable if it be remembered (figs. 8 and 24) that increasing the pyridine content increases

the absorbancy over most of the spectral range and also shifts the

maximum absorbancy toward shorter wave lengths. At 10 cc of pyridine the maximum absorbancy is too close to that of the copper

sulphate (B0, at 100 cc it is too close to that of the cobalt (B2 ). In
128 BUREAU OF STANDARDS

both cases it is impossible to minimize the deviations from the ideal as satisfactorily as can be done with 30 cc of pyridine, where the

region of maximum absorbancy is properly spaced between those of the Bi and B2 components.

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It is not claimed that slight improvement might not possibly still be made at some other pyridine content—for example, at 29 or 31 cc per liter—but it is obvious that no such improvement would be of any

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REPRODUCIBLE WHITE LIGHT FILTERS 129

the exact value derived from the measurements and computations

that were made at the time the value of 30 cc was adopted.

-. PRELIMINARY TESTS OF THE COLOR OF NOON SUNLIGHT AND

DAYLIGHT AT WASHINGTON

The measurements described were undertaken both as a check on

the color of certain of the niters and as a test of the applicability of

the niters for precise colorimetric determinations of sunlight and

daylight. The general methods possible for this purpose have been

outlined in Section VI, 2 (6). The tests under consideration were made in a south room, the

window being raised during observations so that neither the skylight nor the sunlight passed through the glass of the window. A Martens

photometer was placed about 2 feet from the window. For the

overcast sky, the window was opened wide so that a considerable

aperture of the southern sky was used. The horizon was screened

off. For the noon-sun observations the window aperture was

covered with cardboard except for a small opening which transmitted

the sunlight beam to the photometer.

A standard lamp (B. S. 3256) was operated at 2,848° K. This is the same lamp as that noted in the next section, and the 2,848° K.

point was checked before beginning the tests in September. The

light from this lamp transmitted by the Davis-Gibson filter was

incident on a white diffusing surface, light from which entered one

field of the Martens photometer. Light from the sun or sky was

diffused into the other field of the photometer by another surface of

the same white material. The artificial fight was incident on the

diffusing white material at approximately the same angle as the

natural light.

The brightnesses in the two fields of the photometer were always

equated when the colors were being compared. In the September

measurements the Lovibond glasses necessary for exact color match

were inserted in the sunlight or skylight beam; in the December

measurements the glasses were inserted in the beam from the source and filter combination. The latter is a more accurate method,

although in the method proposed in Section VI, 2 (6), where the

light source is varied and only Lovibond red glasses of small denomination are necessary, the glass may be inserted in either beam as required

with practically equal accuracy.

The results of this preliminary testing are shown in Figure 32. In

addition to these experimental results, there are illustrated, in terms

of the r and g coordinates used in plotting, the following data:

(1) A portion of the black-body locus; (2) the sunlight locus as given by Abbot's 1917 data. Mean sun, lying on this locus, is taken

as neutral, r = g = b = 1/3 ; (3) the sunlight locus as derived from Abbot's

1923 data; (4) Abbot-Priest sunlight; (5) the locus obtained by varying the color temperature of the source from 2,300° to 3,000° K. in

the case of the filter designed to convert 2,848° K. to sun outside atmosphere (1917 data), chart 32; and (6) the satron locus as determined

by Priest and Brickwedde and reduced by Judd. (Sec. VIII, 2, and

fig. 30.) In the illustration the irregular shaped figure surrounding

the point representing the color of mean sunlight (r = g=l/3) represents this locus.
130 BUREAU OF STANDARDS

The experimental data plotted include (1) the individual observations on overcast southern sky in September, 1928; (2) the same for December, 1928; and (3) the means of the observations on direct

sunlight from 11.30 to 12.30 on several days in December, 1928.

The data on sunlight taken in September, 1928, are not plotted.

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Color Temperature of Source, as Indicated, with 284 8°K to Sun Outside Atmosphere Filter _

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9

Figuee 32.

Section of the r-g diagram showing colorimetric positions

of winter noon sunlight and of overcast sky as experimentally observed by one of the authors, using certain of the filters and Lovibond glasses

(see Sec. VIII, 4 for details) The colorimetric positions of sunlight designated as Abbot's 1917 data were computed from

the energy values of Table 1. The 1923 data for sunlight outside the earth's atmosphere were derived from Abbot's published data (Bib. ref., 42, Table 10, last column); the other

1923 data were derived by the authors as follows: By dividing the 1923 energy values for sun outside atmosphere by the corresponding 1917 values, wave length by wave length, correction factors were obtained which were applied to the other 1917 energy distributions

to derive the corresponding 1923 distributions. They were rendered questionable because of insufricient attention to a certain stray-light effect which was noticed at the start of the

December measurements and thereupon eliminated. The results obtained, so far as this error can be estimated for the September
REPRODUCIBLE WHITE LIGHT FILTERS 131

sunlight measurements, would be consistent with the results shown in

Figure 32. The error did not apply, at ieast to any important extent,

to the data on overcast sky taken in September, 1928.

These data are presented more to illustrate the applicability of the

filters for such measurements than as any important contribution

toward the color of sunlight and daylight. Nevertheless, they are considered fairly reliable as far as they go. It is interesting to note

that all the data obtained fall between the two sunlight loci as derived from Abbot's 1917 and 1923 data. The color temperature of

practically all the data on overcast sky comes apparently between

6,000° and 7,000° K., confirming these figures as found by other

observers. However, it may be noted that the colors observed are not exactly black-body colors, but are relatively slightly greenish.

Inasmuch as Abbot's sun outside atmosphere (1923 data) is itself slightly green relative to the black body and since such true absorption

as may take place in the atmosphere because of the water vapor would

accentuate this greenness, it is believed the results obtained are exactly as to be expected.

The data on noon sunlight obtained in December, 1928, average

nearer Abbot's 1923 data than the 1917 data, although lying between

them. They tend to confirm Abbot-Priest sunlight as an excellent

colorimetric representation of average noon sunlight, without con- c c'

o o

Lamp A Lamp B

Figure 33.

Schematic diagram showing 'photometer used to test the color match of the lamp-and-filter combinations

See text for explanation. tradicting in any way the authors' contention that their adopted mean-sun energy distribution is a more suitable choice as a standard

for photographic sensitometry than either 5,000° K. or Abbot-Priest

sun. The data further indicate the particular suitability of the

second method proposed in Section VI, 2 (6), for the study of daylight

and noon sunlight, viz, the use of the filter for converting 2,848° K.

to sun outside atmosphere (1917 data), chart 32, with a source cali- brated for color temperature from 2,300° to 3,000° K. and Lovibond

red glasses of low denomination (for example, maker's numerals 0.1,

0.2, and 0.4).

5. DETERMINATION OF COLOR TEMPERATURES OF INCANDESCENT

LIGHTS

The following test of the filters was made by K. P. Teele, of the

photometry section. The quoted parts are from his report, B. S. 156 (e)-l-28, dated October 1, 1927:

Lamp A (fig. 33) is a 500-watt gas-filled standard lamp, B.S. 3256,

belonging to the colorimetry section. It had been calibrated by

Gibson in May, 1925, color temperature vs. applied voltage, over the

range from 2,050° to 2,900° K.

Lamp B is a 500-watt 116-volt floodlighting lamp. * * * The procedure followed was to have an observer vary the voltage of lamp B

to match the color of lamp A at 2,360° K. The distance of A from P was
132 BUREAU OF STANDARDS

constant. The observer moved B to get a brightness match as well as color match between A and B. The voltage of B was recorded. The cell designated as 2,360° K. to mean sun was then placed at C and the

cell being observed placed at C. The cells placed at C were 2,500° K. to mean

sun, 2,700° K. to mean sun, and 2,848° K. to mean sun. The lamp A was then run at the desired temperature and the observer varied the voltage of B to obtain a color match. The voltage of B obtained with the cells in place and with the bare lamps should be the same if the cells are correct. * * * From 6 to 8 observers took 2 observations each, both with and

without the filters as described above, so that from 12 to 16 single

observations went into the following averages:

Xo cells 2,500° K.

cell at C

Xo cells 2.700° K.

cell at C

Xo cells 2,848° K.

cell at C

Average voltage.-- - - „

Average percentage deviation ...

71.

2

8 1.25 71.26 1.95 70.64 1.24 70.6s

1.5*

70.

8

4 1.3t 70.8i

1.1, The " average percentage deviation" is an indication of the precision of the determinations, in one case at the color of 2,360° K. and

in the other at the color of mean sun. The final averages are 1.29 and 1.57 per cent, respectively, and the difference, if significant, may

easily be accounted for by the fact that the determinations at the

color of mean sun were made at a brightness level of only 17 to 26

per cent (the transmission of the respective filters at C) of those at the color of 2,360° K.

It may or may not be significant that the average percentage

deviation stays practically constant at the three trials with the

2,360° K. color (no filters), while, with the mean-sun color, it consistently decreases as the field brightness increases. (The distance

AP being constant, the field brightness increases notably with increase

in temperatoe of lamp A for two reasons, viz: (1) Increase in the

candle-power of A and (2) increase in the transmission of the filter C.)

This suggests that the variation in precision may be due essentially to the variation in field brightness. Since, with lamp A at 2,848° K.

(relatively high brightness for the sunlight color), the average deviation is less than with the color match at 2,360° K., one is led to suspect that the deviation with lamp A at lower temperatures would

also have been less than with field color 2,360° K. if the field bright- ness in these cases had been as high as with lamp A at 2,848° K.

At any rate there would seem to be no loss of precision in determining

the color temperatures of lamps by means of the filters of Series I. So far as the accuracy of the filters for this purpose is concerned,

the test is obviously satisfactory. The filters were prepared by

Davis, using the same materials as used in deriving the standard

data; but, as noted elsewhere, any deviations from the standard data

caused by different chemicals would tend to cancel out in this use of

the filters. The color matches were reported as perfect in all cases.

6. TEST OF ACETYLENE-TO-MEAN-SUN FILTER

A similar test was carried out using the filter designed to convert

the acetylene flame, as represented by Coblentz's energy data (Table

3 and Sec. V, 6 (a)), to mean sun. Quoting from the same report as before

:
REPRODUCIBLE WHITE LIGHT FILTERS 133

The lamp A (fig. 33) was replaced by an acetylene burner and the acetylene

filter placed at C. The 2,360° K.-to-mean-sun filter was placed at C and the

voltage of lamp B varied for color match.

Eight observers took two readings each, so that 16 observations

enter into the final average. A value of 71.

3

4 volts was obtained,

compared with an average of 70.

9

2 obtained from the three previous

measurements with the bare lamps and 70. 9i with the filters. This

difference is equivalent to about 5° at 2,360° K. The average

"percentage deviation" was 2.18, larger than obtained with the

lamp and filters, and is readily accounted for by the fact that a perfect

color match was not obtained with the acetylene and its filter. The

conditions under which the acetylene was burned were as follows

:

Temneratures <f Wet bulb' 20- 2° C' temperatures

| Dry bulbj 23 2

o c

Barometer 759 mm.

Burner E. K. standard. Gas Generated from calcium carbide and water. The pressure of the acetylene was 9 cm of water and was measured by an open manometer furnished with the burner. Further examination of the filter by the authors, in the photometry

section, confirmed the fact that the acetylene-and-filter combination

did not give a perfect color match with mean sun as obtained with a standardized electric incandescent lamp operated at 2,360° K. in

combination with a 2,360° K.-to-mean-sun filter. Removing the

two filters showed a still larger difference in color. The acetylene was distinctly less saturated (whiter) in comparison. Next the cur- rent in the electric lamp was adjusted, without filters, in an attempt to obtain a color match. It was found, however, that a perfect color

match between the acetylene light and the unfiltered electric light

could not be obtained at any color temperature. The nearest approach to this was somewhat over 2,400° K. During these experiments two Eastman Kodak acetylene burners, two calibrated electric lamps, and two sets each of acetylene-to-mean-sun and 2,360° K.-

to-mean-sun filter solutions were used. The acetylene gas used in this experiment was made in the same way as described above.

The observations just described were not extensive enough to

justify giving any newT value for the color temperature of the acetylene

flame as used with the standard Eastman Kodak burner, nor are they extensive enough to justify saying that a value of 2,360° K.

might never be obtained. Nevertheless, in view of the fact that

Coblentz's energy data and the observations under consideration

both give color temperatures definitely greater than 2,360° K. (Sec.

Ill, 4), it is felt that this value is rendered somewhat questionable,

and that further careful measurement should be carried out. So far

as these observations go they show that, if one wishes to calibrate an

electric lamp for 2,360° K. by means of the acetylene flame, a much

more accurate value will be obtained when the filters are used than

when not. Presto-Lite acetylene was tried and found to be different from generated acetylene. The gas is absorbed by an earth impregnated with

acetone in the Presto-Lite tanks. The chemistry division of the

bureau has done considerable work on Presto-Lite, and finds that the

acetone is carried over with the acetylene by an amount that varies

from about 1.6 per cent when the tank is freshly charged to about
134 BUREAU OF STANDARDS

3.0 per cent when the tank is nearly empty. It would seem inadvisable to use Presto-Lite for this type of work as the variation in acetone present changes the color of the flame. However, the

Presto-Lite gas used gave a flame that within the uncertainties of the experiment corresponded to a color temperature of 2,360° K.

IX. PREPARATION OF DAVIS-GIBSON FILTERS

1. SELECTION OF CHEMICALS

In selecting chemicals to be used for preparing these filters care should be taken to obtain those of a quality at least equal to that

known to the trade as "c. p." (chemically pure), from manufacturing

chemists who can certify to their purity. A higher grade of chemicals

known as

1 'reagent" quality is recommended, as this would be some protection where no facilities exist for making chemical or spectrophotometric analysis of purchased materials.

Chemical analysis of the principal materials used in the design

of these filters has already been given in Section IV, 2, and further

noted in Section VII, 8. The following comments, however, ma}7 be

of some assistance in detecting unsuitable chemicals:

1. Copper sulphate (CuS04 -5H20)

Appearance.—Clean blue crystals of the pentahydrate free from white powder (CuS04

- 3H20) or from any signs of moisture, such as small crystals (not the powder)

adhering to the sides of the bottle. It should make a clear solution

in water.

2. Cobalt ammonium sulphate (CoS04 (NH4 ) 2S04 -6H20).

Appearance.—Well-defined crystals, which should be free from any signs of

moisture. It should make a clear solution in water.

If for any reason cobalt ammonium sulphate is not obtainable,

cobalt sulphate (CoS04 -7H20) may be used (see, however, sections

IV, 1 (d), and IV, 1 (e)) , provided the weights of cobalt ammonium

sulphate specified are adjusted to give the same cobalt content. This may be obtained by multiplying the weights given on the

charts by 0.7113.

3. Mannite (C6H8 (OH)6 ), d-mannitol.—Colorless needles which

melt at 166° C. (fine crystals, which, en masse, appear white).

4. Pyridine (C5H5N).—A colorless liquid organic compound having

a strong unpleasant odor. Specific gravity 0.979 at 25° C, boiling

point 115.5° C. Should be practically colorless, and when added to water the solution should be clear.

I 5. Sulphuric acid (H2S04 ). —Should be clear and practically colorless. Specific gravity 1.84.

2. PREPARATION OF A AND B SOLUTIONS

Solutions A and B are given to milligrams on the charts. This may

appear to be an unwarranted refinement. However, we have at- tempted to design these filters so that the color in each case, as ex- pressed in triliner coordinates, r, g, and b, would coincide with those

of the ideal to four decimal places by computation. In Tables 8 and

12, giving the concentrations of the ingredients for intermediate temperatures, the weights of cobalt ammonium sulphate and the copper

sulphate in solution B are given to centigrams only, as this is suffi- ciently accurate for all practical purposes. The weights of copper
REPRODUCIBLE WHITE LIGHT FILTERS 135

sulphate in solution A are given to milligrams because 1 mg in this case has even more significance than 10 mg of the ingredients in solution B.

1. Solution A.—Select several crystals of copper sulphate, enough

for both A and B solutions, and crush in a mortar (not pulverize)

until sufficiently fine to facilitate weighing. Weigh out in watch

crystal the quantity required for solution A, and dissolve in about

one-half the total quantity of distilled water in a beaker. Use a wash

bottle to get all of the weighed copper sulphate off the watch crystal

into the solution. With the same weights, weigh out the mannite

and add to the solution. After dissolving, add the pyridine and pour

into a volumetric flask. Use the wash bottle to rinse out the beaker

and add this rinse to the flask and make up the full volume with distilled water. Mix thoroughly and pour into a glass-stoppered bottle,

which should be labeled and dated. Most chemicals contain a small

quantity of minute insoluble material (trash) which will settle to the

bottom of the bottle if left undisturbed for a few days. This solution

should not be filtered. The clean solution can be removed with a pipette or by careful pouring if the bottle is nearly full. For work

where the highest precision is desired this solution should be made up

fresh every two or three months.

2. Solution B.—Follow the same procedure as given for solution A,

adding the sulphuric acid after the copper sulphate and cobalt sul- phate are dissolved. This solution apparently does not change with

time. It is difficult to state just how long the B solution will keep.

It is known to be good for over a year and probably is good for a much

longer period of time.

3. PREPARATION AND FILLING OF CELLS

The cell illustrated in Figure 9 for holding the filter solutions consists of three plates of optical glass, borosilicate crown, 19 76 mm square

by 2.5 mm thick, separated by two black glass 20 frames, fine ground

(not polished) to 10.00 mm thickness, with a central circular aperture for the solution 43 mm in diameter. A small hole, 5 mm in

diameter, is cut from one side of the black glass frame to the central

aperture. This hole is closed with a waxed cork or glass stopper and

may be used in filling the cell. The black glass separator frames when new should be thoroughly

scrubbed with soap and water, using a brush, to remove particles of glass mud and abrasives with which the ground glass surfaces are usually impregnated; particular attention should be paid to cleaning

the small hole through which the cell is filled. To avoid lint, the

frames should be air dried without wiping with a cloth. The polished

optical glass windows should be cleaned and wiped dry with a soft cloth or with lens tissue. When assembling the cell the parts should

be carefully brushed (with a camel hair brush) to remove any dust

particles and lint that may be adhering. In case the cell is to be filled without sealing the windows on with wax, it may be assembled wet

and each compartment rinsed out with the filter solution several times

before final filling.

19 Borosilicate crown glass was chosen as the most suitable of the optical glasses. It has very high and

almost nonselective transmission, and as previously noted, it is also the hardest of the optical glasses and

probably the most resistant to chemical action.

20 Black glass is recommended because it prevents the passage of light through all portions of tbe cell other than the central aperture, thereby ehminating a mixture of filtered and unfiltered light in the apparatus

In which it is used.
136 BUREAU OF STANDARDS

The cell may be held together, as shown in Figure 9, with rubber

bands or may be further sealed with paraffin or ceresin before filling.

White ceresin is preferable to paraffin because of its higher melting

point and hardness. The cell should be warmed (70° to 80° C.) and

then placed (with rubber bands and stoppers in position) alternately on each edge in a pan of melted wax about 1 cm deep and allowed to remain long enough in each position for a film of the wax, by capillary

action, to fill the seam up to the compartment for the filter solutions.

If the cell is to be used only a short time (a few hours or even days)

and the solution changed, it is advisable not to wax the edges as it will not leak if the small filling hole is tightly stoppered. In this case, the solution is easily removed and the cell washed out by separating

the parts. If the windows are on very tight, as is usually the case

after standing several days, the stoppers may be removed and reinserted. In this way the stopper serves as a pump forcing the solutions

between the windows and the black glass frame. When wax is used

to seal the cell it is, of course, necessary that the cell be warmed to take it apart. If a cell with larger aperture is desired, it is recommended that the outside dimension (not thickness) be also increased, be- cause the cell is likely to leak if insufficient distance (surface area)

between the outside and the central hole in the black glass is not

maintained.

It should be remembered that the data given for the filters are based on a temperature of 25° C. for the filter solutions. As noted

before a tolerance of ± 5° C. is permissible for most work (see Sees. VII, 4, VIII, 2, (d), VI, 1, (6), and VI, 2, (6)). X. CHARTS DESCRIBING FILTERS

Charts 1 to 20, inclusive, contain the filter formulas and the spectral

specifications of the mean-sun group of filters, referred to throughout

the paper as Series I. Chart 2 is the 2,360° K-to-mean-sun filter recommended for use in photographic sensitometry by the Seventh

(1928) International Congress of Photography. The data on these

charts are self-explanatory.

Charts 21 to 31, inclusive, Series II, comprise similar specifications

for filters designed to convert the fight from a 2,848° K. source to color temperatures between the limits 3,500° to 10,000° K.

Several other filters not in the above series are given in charts 32

to 38, inclusive, as listed in the accompanying index.

The weights of the ingredients and the light transmission for any

intermediate filter, not given on the charts, may be found in Table 8

for the filters of Series I and in Table 12 for those of Series II,
REPRODUCIBLE WHITE LIGHT FILTERS 137

Index to charts

*1. 2,300° K. to mean sunlight.

2. 2,360° K. to mean sunlight.

*3. 2,400° K. to mean sunlight.

4. 2,500° K. to mean sunlight.

*5. 2,600° K. to mean sunlight.

6. 2,700° K. to mean sunlight.

*7. 2,800° K. to mean sunlight.

8. 2,848° K. to mean sunlight.

*9. 2,900° K. to mean sunlight.

10. 3,000° K. to mean sunlight.

*11. 3,100° K. to mean sunlight.

12. 3,200° K. to mean sunlight.

*13. 3,300° K. to mean sunlight.

14. 3,400° K. to mean sunlight.

*15. 3,500° K. to mean sunlight.

16. 3,600° K. to mean sunlight.

*17. 3,700° K. to mean sunlight.

18. 3,800° K. to mean sunlight.

*19. 3,900° K. to mean sunlight.

20. 4,000° K. to mean sunlight.

21. 2,848° K. to 3,500° K.

22. 2,848° K. to 4,000° K.

*23. 2,848° K. to 4,500° K.

24. 2,848° K. to 5,000° K.

*25. 2,848° K. to 5,500° K.

26. 2,848° K. to 6,000° K.

*27. 2,848° K. to 6,500° K.

28. 2,848° K. to 7,000° K.

29. 2,848° K. to 8,000° K.

30. 2,848° K. to 9,000° K.

31. 2,848° K. to 10,000° K.

**32. 2,848° K. to sunlight outside

earth's atmosphere.

33. 2,848° K. to Abbot-Priest sun.

34. 2,848° K. to summer noon sunlight. 35. 2,848° K. to winter noon sunlight. 36. Acetylene to mean sunlight. **37. 2,450° K. to 3,500° K.

**38. 2,450° K. to 6,500° K.

* In order to save space these charts are net published herein but any of them

may be obtained from the authors on request. ** Colorimetric characteristics for these filters obtained by varying the color temperature of the source are described in Sections V, 6, (d) and V, 6, (e), Tables

13, 14 and 15.
138 BUREAU OF STANDARDS

2360°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 2

150

125

i ioo

cU

75

50

25

-fL J F

-<

? 1 c pt - >- J k—

"Si {

i

j-

-t

-A / A

• t

/

/

/ t

/ t 7

/ r / I /

/

1.00

.75

.50 E

.25

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/J.)

.00

X

(rry-i) T E"

* 9e' / L- 3 50 o~o 0.Z3O 2.5 60 .32 / 1.9 .3 0^

70 .12.0 $ n .7 4.6 80 .SI S" I is .57 9 90 .60S .799

6 7/ Tic

10 .7 19 11. .767 20 .737 S7.1 .T7I 30 .72S 70.3 1 .016 40 .6fZ Tl.Z 1 .09H- 450 .61 Z ff.6 1 .OZI 60 .SIO 93.9 1 .0 1 s 70 .1V3 99.9 1 .031 80 .131 IOS.0 1 .0 60 90 .376 1 07.1 I.06S 500 .3IS 1 03.9 1 .02.0 10 .Z5f 9V.3 .17Z 20 .219 9S.O .939 30 .117 96.7 .9S7 40 ./ rz 1 0.6 ,19 7 550 .167 1 03.Z I.OII 60 JUS 1 03.1 1.031 70 .13 I 6 1 oo.i I.OZI 80 .1 1 SI 9 6.9 .997 90 .10/3 93.1 .977 600 .0907 9 1.3 .9S9

10 .0S31 91.Z .96? 20 .on? 9Z..3 .99/ 30 .073 O 93.3 I.OIZ 40 .06f1 91.o 1.032. 650 .0616 9S.I 1 .06O

60 .06 OS 9S.I 1 .071

70 .ossr 93.9 1 .OS / 80 .osio 90.S 1.069 90 .01S7 ss.r 1 .03? 700 .0101 ?/.z 1 .OOF

1 .0366 76.1 .979 20 .03Z6 71.6 .990

T — Spectral Transmission of Filter at 25*0 V Relative Visibility Function E Relative Energy of 23 60°K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* oooo —Relative Energy of 2360 K and Filter Combination

Light Transmission of Filter

for 2360° K = 0.135 2

FILTER FORMULA

A

Copper Sulphate (CuS04-5H20) 3 .707 grams

Mannite (C 6 Ha (0H)6) 3.707 grams

Pyridine (C s HcN) 30.0 cc Water (distilled) to make 1000. cc B

Cobalt Ammonium Sulphate v (CoS04-(NH4),S04-6H20) Z6.3Z7 grams

Copper Sulphate (CuS04-5H2 0) 27.180 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l), each 2.5 mm thick.

Bureau of Standards. Davis-Gibson Filters

Adjusted- to make sum of E - E from 400

to 720 m/j. equal * practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination
REPRODUCIBLE WHITE LIGHT FILTERS 139

ZSOO°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 4

150

125

100

75

50

25 t 1

^

1.00

.75

.50 Ea

c .25

150 400 450 500 550 600 650

Wave Length —mil 700 750

imicrons (myu)

.00

(m/i) T E"

* / L 350 O.Z¥? 3.2

60 ,3¥Z S.1 .37 Z

70 .¥¥1 I 0.3 .50 1 80 .53? I6.S .6>S7 90 .6Z5 Z¥.f ,fZ¥

400 .6ft 39.6 .766

10 .730 ¥6.1 .SO¥ 20 .7S3 sr.i .915 30 .7*3 71 ./ I.0Z7 40 .7oH- n .z 1.0 ¥5

450 .6¥l tf.Z. 1.0 16 60 .S73 13.

¥

I.OIZ 70 .SZO 11.Z I.0Z3

80 .¥70 I OH.O 1 .051 90 .HI 6 106.3 1.057 500 .3S6 1 03.6 1 .01$

10 .211 11.0 .17*

20 .ZSf 16. 1 .1SO 30 .Z3¥- 17.6 .166 40 .Zl? 101 .0 1.001 550 .ZOI I03.Z I.OIS 60 .i rz 103.0 1.030 70 .I6Z 1 00.6 I.OZZ 80 .I¥37 17.3 I.OOI 90 .IZZl 1¥.l .1S9 600 .11 SI 1Z.0 .167

10 .1079 11.1 37S

20 .tool 1Z.7 MS

30 .oisz 93.3 I.OIZ 40 .ozi6 13.6 I.OZ1 650 .0?¥1 1¥.¥ I.OSZ 60 .0711 1¥.l I.063 70 .0791 1Z.I 1.067

80 .06*1 *9-.Z /.OS

3

90 .ObIS ft.

5

t.OZZ

700 .osss fo.o .993

10 .05Ol 7S.S .167 20 .0¥SO 70.6

T —Spectral Transmission of Filter at25*C V

-5-—Relative Visibility Function E

f—Relative Energy of Z500*K

E'— Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* pooo —Relative Energy of ZSOO'K and Filter Combination

Light Transmission of Filter

for 2S00°K =0.168

FILTER FORMULA

Copper Sulphate (CuS04«5He0) Mannite (C 6 Ha (0H)6) Pyr idine ( 3.212 grams 3.Z1Z grams 30.0 cc Water (distilled) to make I000. cc

B

Cobalt Ammonium Sulphate „ (CoS04-(NH4),S04-6H20) 23.510 grams

Copper Sulphate (CuS04

- 5

H

20) 24.730 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc

Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,* 1.5 . each 2.5 mm thick.

Adjusted to make sum of E - E

to 720 mjx equal practically from 400to zero

Bureau of standards. Davis -Gibson Filters

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination 13416°—31 10
140 BUREAU OF STANDARDS

Z700e K to MEAN NOON SUNLIGHT at WASHINGTON. CHAPT 6

150

125

100

75

50

25

;2

7-

it

4

1.00

.75

.50 E

.25

350 400 450 500 550 600 650

Wave Length —milli 700 750

ticrons (rryx)

.00

T E"

* 9e'

3 50

-=—

0.2 lr 2 H-.H ' —

60 .3 77 7.7 70 .H-7H- 1 2.6 .*>/7 80 r/ tr

1 9 .3 .772 90 ,6SZ .126 400 .7/2 37.

f

S3 6 10 .7S3 .fso

20 .77+ 6o.S .1/1 30 .767 71 .5 1. 033

40 .73¥ ?o.i t.O¥0 450 .68-0 97.S- I.OOf 60 .621 12.6 I.OOH 70 .S7Z 17.1 I.OIO 80 .SZ6 IOZ.7 1. 036

90 .¥ 76 / 0¥.1 1. 0*3

500 .*I7 1 03.¥ 1.0 1 6

10 .360 11.7 .its

20 .31? 17.¥ .163 30 .21

Z

1g.6 .17S

40 .273 1 oi.z 1.003 550 .zssr I oz.i 1.0 12 60 .233 1 oz.i- /.OZ¥

70 .211 1 oo.z 1 .01 Z 80 .no 17.2 I.OOI 90 .172 9H.H- .1S7 600 .Iff 12.6 .173

10 ./¥?/ 12.3 .171 20 1ZS .116 30 .1333 13.2 1.0 IZ 40 .1269 13.3 i.ozs

650 .1207 13.6 /.#¥¥ 60 .l/H-f 13.1 1 .OS/

70 J07O lo.i 1 .0S3 80 .011S SZ.3 1 .OH-Z 90 .0901 f*t.O 1 .0 1 6 700 .Of31 71.

S

.111

10 .0 7SI 7S.S .16S 20 .O690 71.0 .133

T —Spectral Transmission of Filter at 25*C V Relative Visibility Function E Relative Energy of 2.700" K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* oooo —Relative Energy of Z700°K and Filter Combination

Light Transmission of Filter

for Z700°H = 0.220

FILTER FORMULA

Copper Sulphate (Cu$rv5Ht0) Z.77 7 Z .777

Pyridine (C 5 H S N) " 30.0

Water (distilled) to make I000.

Mannite fC6 H6 (0H).) gramsgrams

cccc

B

Cobalt Ammonium Sulphate „ (CoS04-(NH4),S04-6H20) Copper Sulphate (CuS04

, 5H20) Sulphuric Acid (sp.gr. I.835) Water (distilled) to make

I1.3Z5 grams 21.300 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line.= l.5l), each 2.5 mm thick.

Bureau of Standards. ©avis-Gibson Filters

Adjusted- to make sum of E"- E' from 400

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination
REPRODUCIBLE WHITE LIGHT FILTERS 141

2848°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 8

150

125

t !00

75

50

25

1.00

.75

.50 i

.25

350 400 450 500 550 600 650

Wave Length —mill 700 750

microns (m/x)

.00

X

(m/i) F"

* u /E 350 0.30<? s.s -

60 9.Z .S79 7Q .SOO It.

5

.710 80 .S9I Z 1.5 ssg90 .671 30. 1 i.ooz 400 .7ZT 37. 8 .88/

10 .768 so.* .880 20 .7?? 6 1 .S .93 f

30 .7?3 71. 8 1.03 7 40 .7SS go.

6

1.03 6 450 .707 87.0 1 .001 60 .65* 91 .9 .996, 70 .609 97.1 I.OOI 60 .S6 7 1 Ol .6 1.0 Z 6 90 .Si 9 IOH.O I.03H 500 M63 IOZ.9 1 .01 1 10 M07 I0O.0 20 .3 6 ¥ 98.1 .970 30 .337 99.

Z

.98Z 40 .3 17 101 .3 I.OOf 550 .297 ioz.st 1 .008 60 .Z7¥ 101 .8 1.0 If

70 .ZSO 99.8 1 .01* 80 .zzr 97.1 .999 90 .zc9 9H-.6 .989 600 .1 9 + 9Z.9 .976

10 .1 83 9Z.6 .9?Z 20 J7¥ 9Z.9 ,99 7 30 .166 93.1 1.0 II 40 .isr 93. I.OZZ 650 .ISZ 93. Z 1.039 60 .1 HH6 9Z.S I.OHS

70 .136Z 90.S 1.01? 80 .IZ79- Z7.Z 1.037 90 .1 17S 23.

9

1.015 700 JOS* 79.9 .991

1 .0998 7S.8 .97Z 20 .09IS 71.

S

.9¥0

T —Spectral Transmission of Filter at 25°C V —Relative Visibility Function E Relative Energy of 2848aK

E'— Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE) pooo —Relative Energy of 2 848 K and Filter Combination

Light Transmission of Filter

for 2848°K = 0.262

FILTER FORMULA

A

Copper Sulphate (Cu504-5H 20) 2.445* grams

Mannite (C 6 H a (0H)6). 2.445" grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate , (CoS0v(NH4),S04-6H2 0) I6.SZO grams

Copper Sulphate (Cu50v5H2 0) I9.0ZO grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line.= l.5l). each 2.5 mm thick.

Adjusted to make sum of E - E from 400

to 720 mfj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Bureau of Standards. Davis-Gibson Filters
142 BUREAU OF STANDARDS

3000°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 10

150

125

3)« 100

75

50

25

/

-

,.1 E

<

- V in 1 f J?

/ VLi*

s/

J \ \ T

/ 7 / t

/ > \

-\ V-

) V

( t >

-

> /

1.00

.7 5

.50 E

.2 5

350 400 450 500 550 600 650

Wave Length —mill 700 750

microns (mjj.)

.00

A.

(rryu) T

—it * E EV

350 0.3^0 t> .7 60 .939 / / ,0 .696 70 .SZ8 16.6 .VIZ 80 .6/S Z3.8 .951 90 .69 1 3Z.S 1.079 400 .795 ¥1.9 .9Zg

1 .7$Z 5Z.O .90$ 20 .$02. 6Z.H .9*2 30 .79$ 7Z.I 1 .091 40 .779 20.3 /.03Z

450 .733 86.9 .995

' 60 f .686 91

M

.991 70 .6*7 96.5 .995

80 .607 1 .0/8 90 .S63 103.3 I .0Z7 500 .510 IOZ. 7 I.OIO

10 .*Sb 100.5 .993 20 Ml* 99. .97?

30 .386 99.$ .9$$

40 .36* ID 1.5 1 .006 550 .3*3 I0Z.3 1 .006 60 .3 IS IOI .5 1 .OI5

70 .Z9H 99.5 I.Oll 80 .Z70 97.1 .999 90 .z5o 9*.

8

.991 600 .Z33 93.

Z

.9 $0

10 .ZZZ 9Z.9 .986 20 .ZIZ. 93. .99 2

30 .Z03 93.0 I.009 40 .195 9Z.7 I.0 19 650 .12? 9Z.6 I.033 60 .179 91.7 1 .036 70 .170 89.8 I.039

80 .160 87. Z 1 .OZ9

90 .1*89 23. ¥ 1 .00$

700 .13 7 S 79.6 .929

1 .IZV9 757 .970

20 .1 120 71.5 .990

T — Spectral Transmission of Filter at 25°C V Relative Visibility Function E

f—Relative Energy of 3000°K

E*—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* 0000 Relative Energy of 3000°K and Filter Combination

Light Transmission of Filter

for 3O0O"K = 0.307

FILTER FORMULA

Copper Sulphate (CuS04-5H 20) 2 . 140

Mannite (C 6 H e (0h%) 2./+0

Pyridine (C 5 H 5 N) 30.0

Water (distilled) to make 1000.

B

Cobalt Ammonium Sulphate

,

(CoS04-(NH4) 2 S04 -6H2 0) Copper Sulphate (CuS04-5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

13.900 17.000

10.0 1000.

grams

grams

CCcc

grams

grams

cccc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line, 9 1.51), each 2.5 mm thick.

Bureau of Standards. Gibson Filters

Adjusted' to make sum of E - E from 400

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination
REPRODUCIBLE WHITE LIGHT FILTERS 143

3200°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 12

50

125

v 100

75

50

25

350

5t

1.00

.7 5

.50 £

c

<i

(_ I— .2 5

400 450 500 550 600 650 700 750

Wave Length —millimicrons (myu)

.00

, \ T E"

* / E 350 0.3*2 r.7 -

60 .¥75 13.

5

.29-* 70 .565 19.6 .956 80 .$¥7 Z7.0 1.077

90 .71b 35.5 1 .i?o

400 . 766 H¥.b .9??

1 . ?oo 59.1 .9¥¥ 20 .?l s 63.6 .96 b 30 .?i b 7Z.¥ t.O¥6 40 .79? 79.9 1 .0Z7

450 .765 F5.9 .9?9 60 .7Z6 90.7 .9?3

70 .69Z 9SM .9?*

80 .65? 99.b 1 .006

90 .61 ? I0Z.3 500 .57 O IOZH 1 .006

1 .SZI 1 0O.9 .99? 20 H79 99.? .9?6 30 MSI loo.H .99*

40 .HZ? ioi .5 1 .006 550 .¥OS IOI.9 1 .ooz 60 .379 I o/.o 1 .OIO 70 .3SH 99.1 1 .oo? 80 .3Z9 97. .9 9?

90 .3 Of 9S.0 .993 600 .Z9I 93.6 .9?¥

1 .Z7? 93.Z .9?? 20 .Z6? 93.

Z

1 .ooo 30 .ZS? 9Z.9 I .oo? 40 ,Z*9 9Z.6 1 .0/7 650 .Z¥0 9Z.Z 1 pz?

60 .23/ 91.1 / .OZ9 70 .2ZO £9.1 I.03Z 80 .Z09 ?6.7 1 .0Z3 90 J9b ?3.0 1 .OOH 700 J S3 79.5 .9*7

10 .I7Z 75.? .97Z 20 .1 60 71.? .9V-V

T —Spectral Transmission of Filter at 25°C V Relative Visibility Function E —Relative Energy of 3200°K

E*—Relative Energy of Mean Noon

Sunlight at Washinqton

E"(=TxE)* oooo —Relative Energy of 3200 K and Filter Combination

Light Transmission of Filter

for 3200°K 0.370

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) 1.784 grams

Mannite (C 6 H a (0H)6) 1.784 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate (CoS04-(NH4), S04 -6H2 0) Copper Sulphate (CuS04-5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

10.8*10 grams 14.570 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l). each 2.5 mm thick.

Adjusted- to make sum of E"~ E* from 4 00

to 720 rry^ equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Jureau of Standards. •Gibson filters
144 BUREAU OF STANDARDS

3400°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 14

50

125

v 100

75

50

25

t

H y

r- / Hfa n — , it L=»

c < \

/

S

\

-

V

V

c /

[> •

.00

.7 5

.50 E

.2 5

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/x)

.00

X

(m//) T

,, * E 350 0.H-Z9 11. 60 .sir 16.3 1 .Oi? 70 .603 ZZ.6 1 .106 80 .679 30. / 1 .101 90 .791 3?.H- / .Z76 400 .7?6 ¥7.0 1 .OHO

10 SI6 SS.S .973 20 S33 6H.5 .9fO

30 S33 7Z.9 1.09-6 40 .fzo 79.S 1 .OZZ 450 .79S Z5.Z .9?/ 60 .769 f9.% .973 70 .737 9V.7 .977 80 .70 Z 9S .7 .996 90 .67* 101 .sr I.OIO 500 .63 Z IOZ.I 1.003

10 .SS7 /&/.¥ 1 .OOZ 20 .SH7 1 00.7 .995

30 .sir l 01.0 I.OOO 40 .¥99 IO/.6 1 .007 550 .*70 1 01.5 <99S 60 .HH3 (OO.S 1.00s

70 Ml 7 9?.

7

1.003 80 .39Z 96.Z .996 90 .3 7/ 9S.O .99¥ 600 .3S3 93.7 .9FS

1 .3 VO 93.3 .990

20 .3Z9 93.0 .999 30 .3/9 9Z,f 1.007 40 .30 </ 9Z.3 /..£?/¥ 650 .300 9 l.f f.OZf

60 .Z90 90.6 / .OZ3

70 .Z7r / .OZ7

80 .Z6S F6.3 1 .0/9 90 .Z&l F3.0 1 .oo*t 700 .Z37 79.7 .990

1 .ZZ9 76.3 .9 77

20 .zio 7Z.S .9S3

T —Spectral Transmission of Filter at 25°C V Relative Visibility Function E Relative Energy of 3400°K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* 0000 —Relative Energy of 3400°K and Filter Combination

Light Transmission of Filter

for 3400°K = 0.437

FILTER FORMULA

Copper Sulphate (CuS04*5H20) 1.475 grams

Mannite (C 6 H a (0H)6 ) / .475 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate „ (CoS04-(NH4),S04 -6H2 0) Copper Sulphate (CuS04*5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make I

8.100 grams IZ.330 grams

10.0 cc 000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l), each 2.5 mm thick.

Adjusted to make sum of E

- to 720 mjx equal practical from 400

to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Bureau of Standards. Gibson Filters
REPRODUCIBLE WHITE LIGHT FILTERS 145

3600°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 16

150

125

t 100

c

Id 75

50

25

350

2! 2

z:

i id li t=S ^| ^ ^ 5

1.00

.7 5

.50 E

.25

400 450 500 550 600 650 700 750

Wave Length —millimicrons (myu)

.00

A. T E"

f" /

Ve'

350 0.9-76 13. 7 60 .563 1 9 2 / .205

70 A*/2 2S. 2 1 .2 62. 80 7 1 O 33 . 2. 1 ,323

90 .765 91 .2 1 .362

400 .2OS H1.Z 1 .029

1 .23Z S7.Z .992

20 .297 65.

1

.929 30 .299 nz..H- 1 .097 40 fit 72.9 1 .01 ¥

450 .223 29.9 .972 60 .2oo 29 .2 .7 6 7 70 .779 93 .7 .967 60 .756 77-7 .927 90 .7Z2 too.7 1.001 500 .693 I0I.9 l.ooi

1 .659 IOI.7 1 .005

20 .6/7 I0I.3 1 .OOI 30 .522 IOI.S 1 .009

40 .562. IOI.6 1 .0 7 550 .S3 6 I Of. I .999 60 .SO 9 79.7 .999 70 .923 92.

1

.992

80 .959 76. 9 .992 90 .937 9 9.9 .992 600 .920 73.7 .92S

1 .V07 93.3 .990 20 ,3<J6 73. .999 30 .3 26 92.

6

1.005

40 .37S 92.0 1.0/ 1 650 .3 6 6 9 1 .9 1 .0/9 60 .3S6 90.3 1 .020

70 .3 93 $2.5 1.029

80 .330 26.3 f.019 90 .3 IS 23.3 1.007

700 .300 20.3 .997

1 .226 77. 1 .922

20 .271 73.7 .962

T — Spectral Transmission of Filter at 25°C V —Relative Visibility Function E Relative Energy of 3600°K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)'--oooo —Relative Energy of 3600 K and Filter Combination

Light Transmission of Filter

for 3600°K = 0.506

FILTER FORMULA

Copper Sulphate (CuS04

- 5H 20) 1 .199 grams

Mannite (C 6 H 6 (0h%) 1.19*1 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate (Co504-(NH4), S04 -6H2 0) Copper Sulphate (CuS0y5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

S.570 grams fO.ZZO grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l), each 2.5 mm thick.

Adjusted- to maKe sum of E - E from 400

to 720 mju equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Davis- Gibson Fillers
146 BUREAU OF STANDARDS

3800°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 18

150

125

ii ioo

75

5G

25 -7/

z: 2±±

1.00

.75

.50 E

.2 5

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (nyt)

.00

X

(m^u) T E"

* E/E'

3 5 0.5"26 / 6. 6 60 .6 & 22. * / .*03

70 ,6?0 2 1. O 1 .HI ^ 80 f-r u n . f*U 3 6./ 1 MHZ

anS3 U 1 9 U *"0."3 1/1 UK?

400 SZ3 SI. 1 1 .13 1

1 .Z*6 5?. 5 1 .OZI 20 sst 6 5.6 .196 30 .?63 7Z.2 I.OHH

40 .?6 7? .3 1 .007

450 .?H>1 ?3.7 .96*

60 .?35 F?.* .15?

70 .?ZI iz.i .15?

80 so* 16.? .17?

90 .7?Z .91

Z

500 .75* 1 Ol .5 .117

1 .72./ /Of. 9 1 .007

20 .bfS 1 01.1 1 .006

30 .66-9 I I. 7 1 .007

40 .632. I Ol .3 1 .00*

550 .60S I 00. H .1?7

60 .577 11. / .111 70 .551 17.* .110 80 .5Z? 15.1 .1?7

90 .SOB 1+.6 .9?1 600 .HI 1 13.7 .IS*

1 .*71 13.

Z

.Ifl 20 M6? 1Z.1 .11? 30 .*5? 1Z.5 1 .00* 40 . H-H-i 11.% 1 .001 550 1 / .z 1.017

6C tzi 10 . 1 1 .01? 70 .HI 7 ZS.5 1 .OZH 80 .HO* ?6.6 1 .02Z

90 .3? 1 S *.0 1 .01 s

700 .37* ? 1 .3 1 .010

1 .351 7?.S 1 .005

20 .3*Z 75. 1 .1??

T — Spectral Transmission of Filter at 25°C V — Relative Visibility Function E Relative Energy of 3800°K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* 0000 —Relative Energy of 3S00°K and Filter Combination

Light Transmission of Filter

for 3800°K •= 0.577

FILTER FORMULA

Copper Sulphate (Cu504-5H20) 0.755 grams

Mannite (C 6 H a (0H)6) 0.155 grams

Pyridine (C 5 H 5 N) 30.0 cc

Water (distilled) to make I000. cc

5 Cobalt Ammonium Sulphate (Co504-(NH4) 2 S04 -6H2 0) Copper Sulphate (Cu504-5H2 0) Sulphuric Acid (sp.gr. I.835) Water (distilled) to make

3.260 grams 8.2.IO grams

I0.0 cc I000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line.= l.5l). each 2.5 mm thick.

Adjusted to make sum of E - E

to 720 m/t equal practically from 4 00

to zero

Factor to be usee' to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-fiiter combination

Bureau of Standards. Davis - GiDson Filters
REPRODUCIBLE WHITE LIGHT FILTERS 147

f000°K to MEAN NOON SUNLIGHT at WASHINGTON. CHART 20

150

125

100

75

50

25

: / - H U

-I v > / < t

/ c 7-= \

* J—

>— t = \

( t

<. j//

y

- \ <

1.0

.7 5

• 50

£

c .25

350 .400 450 500 550 600 650 700 750

Wave Length —millimicrons (myu)

.00

X

(m/z) T f" * /e

350 0.577 1 1.7 . 60 .653 25.7 / .608

70 .717 32.

7

1 .581 80 .770 31.2 1 .563

90 .810 76. 1 1 .532

400 .831 52.7 1 .1 70

1 .851 51 .6 1 .031

20 .87

1

65.1 1 .0 01 30 .877 72.Q 1 .0*0 40 .878 77.7 .911 450 .8 75 82 .5 .150

60 .869 87 .7 .151 70 . 860 12. 1 .950

80 • 8 79 15.1 .961 90 .833 18.1 .183

500 .813 10 I .1 .993

1 .7 ft 102. 1 I .009

20 .751 102.2 I .010 30 .730 1 01.8 1 .008 40 .70/ 100.1 1 .000

550 .673 11.7 .180

60 .6f5 18.2. .18-2. 70 • 62Q 16. 6 .182

80 .598 15.3 .181 90 .577 77.2 .185 600 .567 13. 7 .18

1

10 .553 13.

O

.187 20 .577 12.7 .115

30 .535 12.2 1.001 40 .52.6 11 .7 1 .007 650 .518 7/./ 1 .01 6 60 .501 10.1 1 .018

70 •tit 8 8.8 1 .028

80 .H86 8 7. 1 1 .021

90 .772 85.0 I ,028

700 .H58 SZ.7 I .02.7

1 • V77 80.2 I .028

20 .727 77.2 I .015

T — Spectral Transmission of Filter at 25°C V — Relative Visibility Function E

/—Relative Energy of 100C°K

E'—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* oooo —Relative Energy of 7000°K and Filter Combination

Light Transmission of Filter

for 7000°K 0.67?

FILTER FORMULA

Copper Sulphate (CuS04 -5H a0) 0.77 I Mannite (C 6 H e (0H)6) 0.771

Pyridine (C 5 H 5 N) 30.0

Water (distilled) to make 1000.

B

grams

grams

cccc

Cobalt Ammonium Sulphate

,

(CoS04-(NH4),S04 -6H2 0) Copper Sulphate (CuS04

, 5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

1.130 grams 6.250 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l). each 2.5 mm thick.

Adjusted- to make sum of E - E from 400

to 720 m/x equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. Davis -Gibson Filters
148 BUREAU OF STANDARDS

2848° K to 3500°K CHART 21

150

125

O)$ 100

75

50

25

/

J- E c / 1

]

> _ / \ — Af

1

—

1

K / V

/

•V-

/ -

J"

(

, <

1.00

.7 5

.50 E

.25

350 400 450 500 550 600 650

Wave Length —mill 700 750

imicrons (myu)

.00

(m/i) \ T E"

* 9e'

350 0.V92, 1 .6 .353

60 .577 7 .0 .117

70 .6 S3 1 O.O .51

1

80 .7 / 7 13.? .635

90 .76 8 1 ? .Z .7Z6

400 .SOI 23.2 .?06

1 S26 Z2.6 .27? 20 .S36 3H.1 .932 30 .231 10.

Z

.9?Z 40 .f 13 is.

7

1.009 450 .7S3 SO. 1.019 60 .719 S5.1 1.019 70 .72 / 60.5 I.0Z5

60 .69 9 65.6 1 .030

90 .66 V 70.1 1 .025" 500 .62.7 73.

S

LOOS

10 .SS9 76.3 .9 20

20 .561 71.7 .967

30 .516 21.7 .17*

40 .S3? to.

5

.99 1 550 .5ZS 96.1 LOOS

60 .511 1 00.7 1.007 70 .19? 1 01.1 1.001 60 .179 10 7.1 .99S

90 .16 2 1 10.

1

.92S

600 .197 1 13.0 .179

10 .137 1 1 6.7 .9*2. 20 .12.1 1 zo.7 .929 30 .1ZZ 1 Z1.7 .996

40 .116 1 Z2.6 1 .OOH

650 .HI 1 I3Z.9 I.O 15 60 .H06 136.7 1 .oz*t 70 .319 139.7 1 .0 Z2

80 .39/ 1 1Z. 1 1 .0 Z9

90 .3X1 1 13.H 1 .ozi

700 .372 1 11.1 1 .0 1 7

1 .36 / 1 117 1 .00?

20 .39 9 1 H 3. 9 .992

T —Spectral Transmission of Filter at 25*C V —Relative Visibility Function E —Relative Energy of 2848° K

E'—Relative Energy of 3500°K

E"(=TxE)* 0000 —Relative Energy of 2848*K and Filter Combination

Light Transmission of Filter

for 2848° K =0.4«*7

FILTER FORMULA

Copper Sulphate (CuS04-5H20) f . / / 3 grams

Mannite (C 6 H a (0H)6 ) I.I 13 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate,, (CoS04-(NH4),S04-6H2 0) 1.760 grams

Copper Sulphate (CuS0V5H20) 7.117 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc

Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line.= l.5l). each 2.5 mm thick.

Adjusted to maKe sum of E - E from 400

to 720 m^t equal practically to 2ero

Factor to be used to multiply the candle-power

of the light scurce to obtain the candle-power

of the source-and-fiiter combination

Bureau of Standards. Dav
REPRODUCIBLE WHITE LIGHT FILTERS 149

2848°K to 4000°K CHART 22

150

125

v 100

75

5C

25

• / E u 1 (J ) ( )

( / 7

— 7 \

A <

i f7 <- 7

1 77

-J

1 = 7 f

f

V

f -

7

-77 > 7 ,J

1.00

.75

.50 £

c .25

350 400 450 500 550 600

Wave Length 650 700 750 —millimicrons (myu)

.00

(mjj.) T E"

* / E 350 0.390 5.0 .22/ 60 .9 23 2 .0 .302 70 .571 H.9 .HOH 80 .650 1 7.0 .510 90 .7/7 2.3.2. .6ZI 400 .763 30. 1 .7Z6

1 .793 37.6 ,fZ5

20 S07 95.* .913 30 .799 5Z.Z .979 40 • 77/ 59.3 I.02O 450 .7Z6 69.

¥

1.039

60 .67 f 68 .6 1 .035

70 .639 73.3 1 .0*3

80 .60Z 77-9 1 .09-9 90 .563 2 1 .z 1 .0*Z 500 .51 9 2Z.H- 1 .010

1 .H-67 2Z.6 .971 20 . 9-33 2 9. 1 .9 SI 30 .9 1 6 fg. 1 .962. 40 .H06 9 3.3 .922 550 .399 9 2 .Z 1 .009 60 .379 1 1 .5 1 .015

70 .360 1 03.3 1 .009 80 .3*0 1 OH. 1 .99*

90 .3ZI 1 OH. 6 .979 600 .305 1 05.5 .971

1 .Z9S 1 07.7 .975

20 .Z27 1 10.3 .926 30 .Z2 I 13. .99?

40 .273 1 1 5.5 1.009 650 .Z62 1 12.9- 1 .OZS

60 • Z6Z. 1 20.1 1 .0*0

70 .zss IZZ.3 1 .095

80 .ZH2 IZ3. 1 1 .0H6

90 .Z3 f 1 ZZ.S 1 .036 700 .ZZ9 IZI.6 I.0Z6

1 .Zll 1 ZO.Z I.OIZ 20 .Z09 II 7.9 .99*

T— Spectral Transmission of Filter at 25°C V Relative Visibility Function E Relative Energy of 2848° K

E'—Relative Energy of 4000°K

E"(=TxE)* oooo —Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.361

FILTER FORMULA

Copper Sulphate (CuSOvSHjO) Mannite (C 6 H a (0H)6) Pyridine (C 5 H 5 N) Water (distilled) to makeB

Cobalt Ammonium Sulphate (CoS04-(NH4) 2 S04 -6H2 0) Copper Sulphate (CuS04-5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

/ .726 grams

/ .726 grams 30.0 cc 1000. cc

IS. 133 grams

/ 1.220 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l). each 2.5 mm thick.

of Standards. Gibson Filters

Adjusted to make sum of E - E from 400

to 720 mfj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination
150 B (JBJEAU OF STANDARDS

2848° K to 5000°K CHART 24

50

125

100

75

50

25

/

/' /

11 /

/

-x

f y

»< 1 I J

f

i <^ i }>

.

j

( r

c >< V

1 [ ) s / /

f s n /

-< >- / f

9

V f \ ? -

f

-< >- s -

r - /

1.00

.75

.50 £

.2 5

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m/x)

.00

X

(nryt) T e"

* / L. 350 0.Z96 5.1 .IZ3 60 .390 10./ J9Z

70 .986 16.0 .Z8I 80 .577 23.7 .389 90 .658 33.

¥

,SI3

400 .71

1

¥¥.! .6HO

1 .7SO 55.$ .769 20 .767 67,7 .890

30 .75 ¥ 78.1 .9^6 40 .71 ¥ 86.1 I.O¥8

450 .651 10.5 1.066

60 .586 93.0 I.06H 70 .535 96.3 1 .07¥

80 .¥89 99.Z 1 .0X2

90 .¥¥1 99.9 1.068

500 .3 8S 96.8 1.019

10 .333 9Z.¥ .9S9

20 .2*77 90.6 .9Z9

30 .ZSO 93.1 .9 ¥6

40 .£7/ 97 .7 .987

550 .Z60 IOI .7 1 .OZI 60 .Z¥6 I03.Z 1 .03Z

70 .ZZ8 IOZ.6 I.OZ¥

80 .Z09 100.5 1 . OOZ

90 . 1 9Z 9S.Z .9 SO

600 .1 78 96.7 .9 66

1 .169 96.9 .973

20 .1 6Z 97.9 .987 30 .156 99.9 1 .00¥ 40 .ISO 99 .9 1 .OZO 650 .l¥6Z IO 1 .H 1 .0¥3

60 .l¥l¥ IOZ.Z 1 .06Z

70 .I3SS 1 Ol .8 1.068

80 JZ9Z I oo.7 1.066

90 JZIS 92.1 I.OSO

700 .1 I¥¥ 9S.3 I.03Z

1 . 1073 9Z.Z 1.0 1 1 20 .IOOZ 8 8.6 .9?*

T — Spectral Transmission of Filter at 25*C V — Relative Visibility Function E Relative Energy of 2848* K

E'— Relative Energy of 5000" K

E"(=TxE)* oooo —Relative Energy of 2848*K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.231

FILTER FORMULA

Copper Sulphate (CuS04-5H20) Z.599 grams

Mannite (C 6 H 8 (0H)6) Z.599 grams

Pyridine (C 5 H*N) 30.0 CC Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate (CoS04-(NH4) 2 S04 -6H2 0) 23.080 grams

Copper Sulphate (CuS0y5H2 0) /7.090 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l), each 2.5 mm thick.

Bureau of Standards. Davis-Gibson Filters

Adjusted to maKe sum of E - E from 400

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination
REPRODUCIBLE WHITE LIGHT FILTERS 151

284 8° K to 6000°K CHART 26

150

125

t 100

75

5

25

/

__

-

>- -f r

>

i

- M r

/

s /

/ 1 >< / T NK x r X

.00

.7 5

.50 I

.25

350 400 450 500 550 600 650

Wave Length —mill 700 750

.00

imicrons (mjj.) X

(m/j.) T E"

* / c. 350 o.zss 6.9 .OS6 60 .3 ¥8 / Z..2. . 1 ¥5

70 .¥¥6 1 1 .8 .ZZ6

80 .5¥l 30. 1 .3 3 1 90 .6Z6 ¥3.0 .¥51 400 .685 57.3 .57¥

1 .7Z¥ 7Z.f .738 20 .7¥Z 8 8.6 .88

1

30 . 7Z5 IO 1 .7 .117 40 .677 1 1 0.5 1 .070 450 . 60¥ 1 13 S 1 .093 60 .530 1 1 H.O 1 .Off

70 .¥7¥ 1 1 S~.6 1 .Oil 80 .vzs- 1 1 6.6 I JOS'

90 .37¥ 1 l¥.b 1.081 500 .316 1 07.7 1 .OZ6

1 .Z65 19.6 .153

20 .Z3I 15.Z .116

30 .ZIS 16.7 .13?

40 .Z06 10 o.i .If7 550 .117 1 0¥.3 1 .03 1 60 ./8¥ 1 0¥.7 1 .0¥7

70 .168 1 OZ.3 1 .036

80 .151 If.H I.OIO 90 .1365- 1¥.5 .18¥ 600 .IZ¥&~ 11. 6 .16 8

1 .1 1 61 90.6 .971

20 .1110 10.6 .181

30 J05f 9 0.8 1 .00? 40 JO/Z 10.1 1 .OZ1 650 .0176 1 1 .6 1 .057 60 .0137 1 1.6 1 .077 70 .0881 10.3 1 .083 80 .0837 8 8.3 1 .079

90 .0776 SH-.Z 1 .055 700 .07ZO € 1 .Z 1 .03 5

1 .0667 77.5 1 .009 20 .06 11 73.5 .178

T — Spectral Transmission of Filter at 25°C V Relative Visibility Function E Relative Energy of 2848° K

E'— Relative Energy of 6000°K

E" (=T x E

)* 0000 Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.173

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) Mannite (C 6 H a (0H)6 ) Pyridine (C 5 H 5 N) Water (distilled) to make

B

Cobalt Ammonium Sulphate (Co504-(NH4); S04 -6H2 0) Copper Sulphate (CuS04'5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

3.188

3.188 30.0

1000.

28A80

ZS.000

10.0 1000.

grams grams

cccc

grams

grams

cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l), each 2.5 mm thick.

Adjusted to maKe sum of E - E from- 400

to_ 720 myu equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. Davis - Gi
152 BUREAU OF STANDARDS

2848° K to 7000°K CHART 28

150

125

« 100

'5

50

25

>

( /

L

n

i

i ( /

1 M

1 I 7

\ \

,—

/

V A <

\ /- N —

t ( j

5"!

1.00

.7 5

.50 En

c

<«L .25

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (m^i)

.00

A.

(rryz) T

1-" * E Ve'

350 0. 23 V 7 . 2 ,,0 69 60 .3 Z5 1 H . 1 .'21 70 . ttZ3. Z3 .3 .117

80 .5Z0 35 .1 .Z11

90 .60 7 SI .7 .HZ7

400 .667 61.1 .567

1 .706 $ € .O . 7ZO

20 .7Z5 I07.3 . S 72 30 . 70S IZZ.6 1 .007 40 . 6SZ I3Z.0 1 .021 450 .573 133.7 1 .1 13 60 .*1H 13 1.6 1.106

70 .^35 I3I.¥ 1 ./ 17 80 .3 ft 1 3 0.6 1 .IZS

90 .33Z 1 Z6.3 I.I05

500 . Z7S 1/6.1 1 .03 1

1 .ZZS 1 OH.

2

.1H-2 20 .113 IS.

5

.106

30 ./ 7 2 11.3 .13 2 40 .170 1 03.3 .122

550 . / bZ 106.3 1 .031 60 .ISO IOS.I 1 .051 70 .1356 IOZ.5 1 .OHS

80 .IZOS 17.3 1 .Ol 2 90 .10 73 1Z.Z .122 600 .017

1

2 2.3 .170

1 .oqol £6.7 .176 20 .Ofso 2 6. 1 .113

30 .0206 25.7 1 .015

40 .0765 25.3 1 .035 650 .0 73S 25.5 1 .066

60 .0 7OO 25. 1 .02 7 70 .0651 2-3 ./ 1 .013 80 .06 16 SO.

6

1 .022

90 .0565 76 .6 1 .06Z

700 .0519 7Z.b 1 .035

1 .01 76 62 .6 1 .OOH

20 .0H-3H 6H. 1 .1 70

T — Spectral Transmission of Filter at 25*C V —Relative Visibility Function E Relative Energy of 2848* K

E'—Relative Energy of 7000°K

E"(=TxE)* 0000-

2848*K and — Relative Energy ofFilter Combination

Light Transmission of Filter

for 2848° K : 0. 1407

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) 3. 60S grams

Mannite (C 6 H (OH)6) 3.605 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make I000. cc B

Cobalt Ammonium Sulphate ' (CoS04-(NH4),S04 -6H2 0) 32.400 grams

Copper Sulphate (Cu504-5H2 0) 23.600 grams

Sulphuric Acid (sp.gr. I.835) 1 0.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l), each 2.5 mm thick.

Adjusted to maKe sum of E - E from 40.0

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candie-power

of the source-and-filter combination

Bureau of Standards. Davis- Gibson Filters
REPRODUCIBLE WHITE LIGHT FILTERS 153

2848° K to 8000°K CHART 29

50

125

100

75

50

25

-4 5- / — — >- j-

? -f £ /

-i f 1 ) / i- / \

/

»v \

/ s \ r

/ V

\ \

K V

/

/

/ s

r I k-

> \

LOO

.7 5

.50 £

.2 5

350 400 450 500 550 600 650

Wave Length —mill 700 750imicrons (rryu)

.00

X T E"

* / c. 350 0.22/ S~.7 .059 60 .3 / Z 15.9 .1 07

70 .109 Z6. 1 .1 7f

80 .S06 m.o .Z78 90 .59 v 59. 1 .106 400 .651 79.5 .5*/f

1 .69 3 1 O 1.3 .70S 20 .712 1 Z3.5 .877

30 .61 1 1 10. f 1 .0 1 7 40 .63 1 1 50.1 1 .106 450 .55O ISO.

5

I .izf

60 .H6S 1 16. Z 1 .IZO

70 .907 1 99.3 1 .I3Z 80 .3S6 11 IS 1 .139 90 .30 1 135.1 1 J 15

500 .ZH7 1 ZZ1 1 .035

1 .1 99 IOF.7 .9 93

20 ./ 6g 1 OO.F .900 30 .155 1 01 .1 .727

40 .1177 101.9 .99 O 550 JIOZ 10 7.7 1 .OH5

60 .IZ9Z 106.7 1 .067

70 J 157 ioz.1 1 .056

80 .1019 16.1 1 .02.1 90 .0299 9 0.5 .190

600 .0FC7 ZB.O .972

1 .0799 ?3.£ .976 20 .06? f ZZ.9 .196 30 .0660 2Z.Z 1 .019 40 .06Z3 ?l .1 1 .Of! 650 .059 b SI .3 1 .079

60 .0566 SO.

5

1 .097

70 .053O 7?.

9

1 .IOZ

80 .0193 75.5 1 .097

90 .0199 7 1.3 1 .O6S 700 .OHO? 6 7. 1 1 .03 7

1 .037Z 6Z.1 1 .001

20 .0337 5f.7 .967

T — Spectral Transmission of Filter at 25°C V Relative Visibility Function E Relative Energy of 2848° K

E'—Relative Energy of 8000°K

E"(=TxE)* 0000 Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K =0.1211

FILTER FORMULA

Copper Sulphate (Cu504

, 5H 20) 3.116° • grams

3.SI6 grams 30.0 cc 1000. ccMannite (C 6 H a (0H)6) Pyridine (C 5 H 5 N) Water (distilled) to make

B

Cobalt Ammonium Sulphate (CoS04-(NH4),S04 -6H2 0) Copper Sulphate (CuSGySh^O)

Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

35.355 grams 25.800 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l), each 2.5 mm thick.

Adjusted to make sum of E - E from 400

to 720 m^u equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. Davis- Gibson Filters
154 BUREAU OF STANDARDS

2848°K W@0°K CHART 30

150

125

^>i ioo

c

Id 75

50

25

{

1 h

' - s

^-<~

t

/ t

; -/

/. / s

/ A

- / s

-

1.00

.7 5

.50 i

.2 5

350 400 450 500 550 600 650 700 750

Wave Length millimicrons (myu)

.00

X

(mjx) T E"

* / L- 350 0.2/3 1 . H .0 5

Z

60 .303 17;** .017

70 ,*00 2.1 .2 .16 6 80 . HI & H5. H ,Z63 90 .596 6 6.0 .31 / 400 . 6 15 SS. H .53 5

1 ,6SH / iz.S . 700

20 .703 137.5 . 875

30 .6 SO 1 56. H 1 .023 40 . 6ZO 1 66.

1

I .1 H

450 .53* 1 6H.S 1 .l*Z

60 .HH1 1 Sf.H 1 .I3Z

70 .3SS 155. 1 1 . 1 H3

80 .336 151. 2 1 .151 90 .2SH / H3.0 I.IZH 500 .229 IZ7.7 1 .039

1 ./ ?Z IJ 1.1 .970

20 . 1 sz 1 oz.s .21*

30 .131 O I0Z.6 .923

40 .I3Z6 10 6.3 .11 i 550 JZ5S ios.? 1 .oso

60 .11 SZ 10 7.* 1 .07*

70 .IOZ6 IOZ.S 1 .06Z

80 .OS17 IS.

7

I .ozf

90 .0 7 S6 S1 .2 .11Z

600 .07OO SH.3 .171

1 .06H3 SI . S .17 S

20 .060Z. SO .6 .11 s

30 .OS67 71 .7 I .02.3 40 .OS3H 7 ? .6 I .OHS

650 .osoi 7$ . V 1 .OS

o

60 .ohsz 77 .3 1 .1 OS

70 .OHSO 7S.O I.I io 80 .OH 16 7Z.O /./OH

90 .03 77 67.6 1 .073 700 .03 HZ 63.3 1 .OH 1

1 .03 to 51 .0 1 .006 20 .OZ7f 5H.f i .167

T — Spectral Transmission of Filter at 25°C V — Relative Visibility Function E Relative Energy of 2848° K

E'— Relative Energy of T000°K

E"(=TxE)* 0000 —Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.1082

FILTER FORMULA

Copper Sulphate (CuS04-5H 20) 4-/54 grams

Mannite (C 6 H a (0H)6) 4.154 grams

Pyridine (C S H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B Cobalt Ammonium Sulphate (CoS04-(NH4), S04 -6H2 0) Copper Sulphate (Cu50y5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

37.550 grams 27.290 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l). each 2.5 mm thick.

Adjusted to make sum of E - E from 400

to 720 myu equal practically to 2ero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. GiOson F, iters
REPRODUCIBLE WHITE LIGHT FILTERS 155

284 8° K to mpO^K CHART 31

150

125

01$ 100

75

50

25

s N

1 N

/ N

€

1

( /

/

• 4

v-\

\

\

?

\ \

\

\

c ) X n

/

/ v v- i

\ /

, \

) A

X 7

y

- /f

X

\

1.00

.7 5

.50 i

.2 5

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (myu)

.00

X

imp.) T E"

* / L 350 0.20* 1 0.1 .OHS

60 .297 1 f.f .01 1 70 .31 9 3 1 .6 .157 80 .91 1 H9.3 .253 90 .5?0 71.? .3?0 400 .63? 16.Z .526

1 .677 122.7 .613 20 .695T IH9.6 .275

30 .671 1 61.7 1 .02? 40 .609 I71.H I./Z7

450 .5ZI 1 76.7 1 .I5Z

60 ,H3S 16?.

5

1 .IHI 70 .373 163 J 1 .151 80 .3ZI IS?. 7 1 .1 60

90 .Z61 I9S.1 1 .131 500 .ZIH 13 1.7 1 .031

1 .169 1 IH.Z .937

20 . 1 HOI lOH.I .SIO

30 .1277 103.5 .9ZO

40 .IZI6 I0 7.Z .191 550 .1150 101.6 I.05H

60 .IOSZ I07.1 1 .071 70 .0933 1 02.5 1 .06?

80 .0?/ 1 95.2 1.031 90 .0707 Si.

2

.119 600 .0627 ?Z.9 .173

1 .0S7H SO ,2 .17? 20 .0S3S 7? .? .919 30 .0503 77.7 1.026 40 .0H72 76.5 1 .099 650 .OHSO 76.

Z

1 .0?6

60 .09Z5 75.0 1 J / I 70 .0316 7Z.6 1 .11 ?

80 .036S 69. H 1 .11 1 90 .0330 69.9 1 .o?o 700 .021S 60.S 1.096

1 .0269 56.3 1 .001 20 .0291 SZ. 1 .969

T — Spectral Transmission of Filter at 25"C V —Relative Visibility Function E —Relative Energy of 2848° K

E'—Relative Energy of 1 0.000°

K

E"(=TxE)* 0000 —Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.0181

FILTER FORMULA

Copper Sulphate (Cu504-5H 20) 4.342

Mannite (C 6 H e (0H)6 ) 4.342

Pyridine (C 5 H 5 N) 30.0

Water (distilled) to make I000.

B

Cobalt Ammonium Sulphate v (CoS04-(NH4),S04-6H20) 39.353

Copper Sulphate (CuS04-5H2 0) 28.420

Sulphuric Acid (sp.gr. I.835) 1 0.0 Water (distilled) to make I000.

grams

grams

cccc

grams

grams

cc

These data are for a' one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line.= l.5l), each 2.5 mm thick.

Adjusted to make sum of E - E from 4.00

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Bureau of Standards. 13416°—31 11
156 BUREAU OF STANDARDS

284 8° K to SUNLIGHT OUTSIDE EARTHS ATMOSPHERE. CHART 32

150

125

i 100

75

50

25

E 7 € / /

/ / /

:

1 / f /

/

/ -

/ f

K \ V

F t ? \ / /

\ /

V

( > \

- /

/

/ s -

.'

- /

1.0

.7 5

.50 e

.2 5

350 400 450 500 550 600 650 700 750

Wave Length — millimicrons (nryx)

.00

A. T E"

* e/e'

3 50 0. 2.5% oU . o~> 1

1 1 Q . A. 1 O

7 . T r 7

1 Q -3

1 7 .<S 80 ,0 TO a. 7 . *y 163

90 63 O 1 1 .1 . 611

400 .69 O ss.$ .7 S3

1 .73 1 7 1.0 .$16

20 .7SI $6.6 .111 30 .737 100.0 1 .039 40 .691 I01.6 1 .075

450 .62.7 1 H.l 1 .012

60 .SSf 1 IS.9 1 .03 \ 70 .S03 1 1 8.6 1 .OIS

80 .9S1 1 20.

S

1 .067

90 .H03 1 11.3 1 .067 500 .311 / 1 3. I 1 .OZ6

1 .290 I 0S.3 .969 20 .25\3 I 00.6 .910

30 .2.33 /Of .3 .96/ 40 .22/ IOH.Z 1 .oos

550 .ZO£ I 06.

1

1 .039 60 .111 IOS.Z 1 .osz

70 .1 73 IOI.1 1 .01

1

80 .15-5- 17.3 1 .0)6 90 .13$ 6 9Z.S .111 600 .IZ51 $1.3 .176

1 .IIS? $7.S .1fZ

20 .1016 $6.6 1.000 30 .103 1 $ss 1 .01$ 40 .0967 $¥.0 1 .031 650 .0113 $2.$ 1 .OSS

60 .ossi $0.$ 1 .063

70 .07$$ 77. ¥ 1 .OSS

80 .P7ZO 73. ST 1 .03s

90 .06H6 6 $.2 .196 700 .OSSO 63. Z .is?

1 .05Z0 Sf.V .917 20 .Olbl S3.

7

.$7Z

T-- Spectral Transmission of Filter at 25°C V Relative Visibility Function E --Relative Energy of 2848° K

E'— Relative Energy of Sunlight Outside

Earth's Atmosphere E" (=T x E

)* 0000 — Relative Energy of 2848'K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.180

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) Manmte (C 6 H 8 (0H)6) Pyridine (C 5 H 5 N)

3.12,5" grams 3.125" grams 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate (CoS04-(NH4),S04 -6H2 0) Z5A70 grams

Copper Sulphate (CuS04-5H2 0) Z5.O0O grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,= l.5l), each 2.5 mm thick.

Adjusted to make sum of E - E from 400

to 720 mjj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Bureau of Standards. Davis - Gibson Filters
REPRODUCIBLE WHITE LIGHT FILTERS 157

2848° K to ABBOT- PRIEST SUNLIGHT CHART 33

150

125

t 100

c

UJ 75

50

25

l_ /

-E Is -

( -

\ // V

/

\

i

s h

-i 4-

1.00

.7 5

.50 £

.2 5

350 400 450 500 550 600 650

Wave Length —mill 700 750

imicrons (mjx)

.00

X

(rn/z) T e"

* 350 0.306 5.5 • 60 .*to 1 9.9 • 70 .t17 /</. 8 . 80 .587 Z2.0 .*9Z

90 .667 3 0.8 .559

400 . 1ZH VO. 7 .679

1 .7 62 51.9 .786

20 . 78 / 6Z.6 .891 30 .772 7Z.8 .97Z

40 . 73 9 e/.i I.0Z5 450 .686 8 6.7 1 .OH-5 60 .628 9 0.6 1 .096

70 .581 95.1 /.063

80 .537 9 9.0 1 .073 90 .969 100.7 1 .065

500 . 93 3 9 9.0 1 .0Z6

10 .379 9 5.6 .976

20 . 339 93.9 .9 96

30 .3/6 95.6 .957

40 .301 96.6 .985

550 .Z85 101 .1 1 .008

60 .Z66 10 1 .9 1 .0/* 70 .299 99.9 1 .002 80 .ZZ3 9 7.5 .983 90 .zov 95.0 .967 600 J 89 93.3 .9 6 Z

1 .179 93.0 .97Z 20 .1 70 93. f .99 Y 30 .163 9 3.7 1 .017 40 .155 93.6 1 .03Z 650 .1989 93.8 1 .058

60 ./VI 9 93- 2 1 .0 71 70 ./338 91 .V 1 .078 80 .IZ5f 88 .8 1 .07

Z

90 .115 7 8't.9 1 .05*f 700 ./069 30.9 1 .036

1 .0985 76.9 1 .0 13 20 .0909 72.7 .98H

T — Spectral Transmission of Filter at Z5°C V — Relative Visibility Function E Relative Energy of 2848° K

E —Relative Energy of Abbot- Priest Sun E"(=TxE)* oooo Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter for 2848° K 0.25"/

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) 2. ¥77 grams

Mannite (C 6 H a (0H)6 ) 2. ¥77 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B

Cobalt Ammonium Sulphate (CoS04-(NH4),S04 -6H2 0) 18.955 grams

Copper Sulphate (CuSGy 5

H

2 0) / 9.020 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line.= l.5l). each 2.5 mm thick.

Adjusted to make sum of E - £ from 400

to 720 mfj. equal practically to zero

Factor to be used to multiply the candle-power

of the light source to oblam the candle-power

of the source-and -filler combination

Bureau of Standards. Davis- Gibson F.iter;
158 BUREAU OF STANDARDS

2848°Kro NOON SUNLIGHTat WASHINGTON june 21 CHART 34

150

125

o>u 100

c

UJ 75

50

25

/ /

/ —

—

t

—

i

-J /

/-

5-3 —

,

—t M / —

1

-7/

—

«

I ^7

. 7

4A 1 /

t \ i • *-< r-

: : : s

.00

.7 5

.50

.2 5

350 400 450 500 550 600 650 700 750

Wave Length — millimicrons (m^t)

.00

X

(m/j.) T E"

* Vt

J jU 0. Z i 5~ 8

60 ,3 gf —1 °' ° 9 H 70 .9 81 1 5. 7 5S 9 80 .5 7 ¥ 2 3.7 'TIT on 65

7

33 .5 .8 75

400 .7 / 5 99.

f

.837

10 .7SS 56.9 .858 20 .7 75 68 .8 .9Z5

30 .76 8 SO ,o 1 .03 5

40 .73? 89.

1

1 .OH7 450 .680 9 S.I / .on

60 .6ZI 99 .Z 1 .0O6 70 .S7Z 103.7 1 .0 15 80 .SZ7 1 O 7.5" 1 .037 90 .H78 1 08 .8 1 .OH9 500 .fzo 1 Ob.Z 1 .Ol 6

1 .36 V 1 Of.

7

.98 Z 20 .3Z3 98.9 .959 30 .Z99 99.8 .176 40 .ZS 1 I02.Z 1 .0 09

550 .Z6¥ 1 03.6 1 .017 60 .Z¥3 IOZ.8 1 .OZS

70 .ZZI 1 00.3 1 .ozz

80 .Zoo 9 7.0 1 .009

90 .1 8Z 93.8 .9 89

600 .168 9 / .5 .9 76

1 .157 90.7 .9 80

20 .I99Z 90.5 .99 7 30 ./V/7 90.3 1 .013 40 .13 99 89.7 1 .OZ7 650 JZfZ 81:* 1 .09-5 60 JZ/9- 88.3 1 .0 51

70 J/35 85.8 1 .05Z 80 .IC59 8Z.6 1 .0* Z

90 .0963 78.

Z

1 .01 1 700 .088/ 73.8- .983

1 .OSOH 69.* .957

20 .0730 65.0 .9ZZ

T-- Spectral Transmission of Filter at 25*C V — Relative Visibility Function E Relative Energy of 2848* K

E —Relative Energy of Noon Sunlight

at Washington JuNe 21 E"(=TxE)* oooo — Relative Energy of 2848*K and Filter Combination

Light Transmission of Filter

for 2848* K = 0.231

FILTER FORMULA

Copper Sulphate (CuS04 -5H t0) 2.67/ grams

Mannite (C 6 H a (0H)6 ) 2.67/ grams

Pyridine (C 5 H 5 N) 30.0 CC Water (distilled) to make 1000. CC

B

Cobalt Ammonium Sulphate (CoS04-(NH4) 2 S04 -6H2 0) / 9.430 grams

Copper Sulphate (CuS04-5H2 0) 21.000 grams

Sulphuric Acid (sp.gr. 1.835) 10.0 cc Water (distilled) to make 1000. cc

These data are for a one centimeter layer each of solutions A and B m a double cell with three plates of borosihcate crown glass

(refractive index. D line.= l.5l). each 2.5 mm thick.

Adjusted to mane sum of E - E from 400

to 720 mjx equal practically to zero

Factor to be used to multiply tne candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. Davis - Gi
REPRODUCIBLE WHITE LIGHT FILTERS 159

284SK to NOON SUNLIGHT at WASHINGTON dec.2I CHART 35

150

125

Z ioo

75

50

25

A

E / /

r r

j s- / \

1

1 /

y\ / ( s

I >- < r v- t t

r- k \

>—

( / f

v. s > r /

1.00

.7 5

.50 £

.2 5

350 400 450 500 550 600 650

Wave Length mill 700 750

imicrons (nry/)

.00

T E"

* 350 0.33 H 5 . 1 60 .9z9 8 ,S .93 6 70 .5Z3 13.1 1 .0 63 80 .6 1 2 1 9 .2 1 .1 ¥H

90 /r e C .boo 2 6 .7 1 .2 3 O 400 .7¥¥ 3 5". 1 .9¥6

1 .7£Z ¥¥.¥ .9IO 20 .803 SH.I .995

30 .SOI 63 .¥ 1 .090 40 .719 71 .8 1 .OZI 450 .1HO 78.6 .9 88

60 .695 8 ¥.3 .9$Z

70 .656 90.Z .985

80 .617 95.5 1 .012 90 .5 72 9 8.9 1 .OZI 500 .518 99.6 1 .006

1 .V6V <7 8.3 .996 20 MI9 97.

¥

.98 z 30 .38$ 98.5 .987 40 .36 ¥ IOO. 3 1 .003 550 .390 1 OI.Z .996 60 .31 ¥ IOO. 6 1 .OOb 70 .Z88 98.9 1 .003 80 .Zif¥ 96.9 .991 90 .Z¥¥ 95. 1 .987 600 .ZZ 7 9¥. 1 .979

1 .Zl 6 9¥.¥ .983 20 .Z07 95.1 .996 30 .198 9 5.9 1 .008 40 .19 O 9 6 .3 1 .OI7 650 .1 S3 9 7.0 1 .033 60 .1 7S 9 6.9 1 .0¥O 70 .166 95.5 1 .0¥5

80 .157 93.

¥

1 .03 h 90 .1 ¥61 90. 1 .ozz 700 .1360 86.5 1 .005

1 .IZ6¥ 8Z.9 .99 Z 20 .1167 79.0 .966

T —Spectral Transmission of Filter at 25°C V Relative Visibility Function E Relative Energy of 2848° K

E —Relative Energy of Noon Sunlight

at Washington Dec.2l E

' (=T x E

)* 0000 Relative Energy of 2848°K and Filter Combination

Light Transmission of Filter

for 2848° K = 0.302

FILTER FORMULA

Copper Sulphate (CuS04-5H20) Z./94 grams

Mannite (C 6 H a (0H)6) 2.. 194 grams

Pyridine (C 5 H 5 N) 30.0 cc

Water (distilled) to make 1000. cc. B

Cobalt Ammonium Sulphate v (CoS04-(NH4),S04 -6H2 0) 13.06

Copper Sulphate (CuS04-5H2 0) 16.890

Sulphuric Acid (sp.gr. 1.835) 10.0 Water (distilled) to make 1000.

grams grams

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of bonosilicate crown glass

(refractive index. D line,= l.5l), each 2.5 mm thick.

Adjusted to make sum of E - E from 400

to 720 mjjL equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and -filter combination

Bureau of Standards. Davis -Gibson Filters
160 BUREAU OF STANDARDS

ACETYLENE to MEAN NOON SUNLIGHT at WASHINGTON. CHAPT 36

150

125

100

75

50

25

r

7"

[

/ E

/

r J <

: r sI-,

i 1

1

h -i >-

1 r

K

r \

|

|/ r

1 V

4

r V u

< M-

r - x

1.00

.7 5

.50 E

.25

350 400 450 500 550 600 650

Wave Length —milli 700 750

microns (m/x)

.00

X T E"

* ODU —

0,3.5 60 . 3 9 3

"7 n .9 9 2. oUa n .007 <*- O. 8 /.Coo 90 . 6 X T 35 1 / / 6 r

400 .69X 9H-.0 lit

10 .73 6 53.9 .93 1 20 .761 63»9 ,155

30 .753" 71. 1 1.0X7 40 .7X1 7r.o 1 .003 450 .665 r*f.4r .113 60 .600. 9 0. H- fro

70 .550 96.6 .197 80 .501 1 03.X ',09-a. 90 .W7 1 OS. 1 1.095

500 .3 TH- iox.x l,QQ9

10 .3X5 99.6 .its 20 .zro 97.T ,166 30 .XSI 9SS .17? 40 .119 100.9 I.OOO 550 .3.0 S~ 10 3.0 i.o i a. 60 .1 T6 1 1.9 I.oih- 70 .1 69 99.0 1.007

80 .l*t+3 95.r .9T6 90 .ia.ro 13.3 .175- 600 .1 ISH 91.7 .16 9

1 .\Q6t 91 .T .973 20 .0999 .195 30 .0931 73.7 1.017 40 .0T7O 19.1 1 .039 650 .onz 99.9 1 .OS?

60 .0 761 19.5 1 .067

70 .0697 9X.H- 1 .069

80 .0633 Z9.3 1 .059

90 .0563 Z¥.l 1.017 700 .050 1 79.1 .1F3

1 .OHH6 79,1 .150

20 .0395 69.1 .10 r

T-- Spectral Transmission of Filter at Z5*C V —Relative Visibility Function E Relative Energy of Acetylene E

#—Relative Energy of Mean Noon

Sunlight at Washington

E"(=TxE)* OOOO —Relative Energy of Acetylene and Filter Combination

Light Transmission of Filter

for Acetylene = 0.I70

FILTER FORMULA

Copper Sulphate (CuSQ^-SHjO) 3.2 87 grams

Mannite (C 6 H8 (0H)6) 3.287 grams

Pyridine (C 5 rUN) 30.0 cc Water (distilled) to make 1 000. cc

B

Cobalt Ammonium Sulphate (CoS04-(Nrg, S04

- 6H20) Copper Sulphate (CuS04-5H2 0) Sulphuric Acid (sp.gr. I.835) Water (distilled) to make I

20.580 grams 26.480 grams

I0.0 cc 000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index. D line,*l.5l), each 2.5 mm thick.

Adjusted- to maKe sum of E - E from 400

to 720 m/x equal practically to zero

Factor to be used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination

Bureau of Standards. Davis- Gibson Filters
REPRODUCIBLE WHITE LIGHT FILTERS 161

2450°K to 3 50CVK CHART 37

I3U

125

is ioo

c

Id 7 5

50

25

1.00

.75

c

o

cO .50 i

c

flj LH

.25

1 6^ /

_|fY- I \

t -

/ jS — -\~

/ —

i Ft-

>*

1 | \A - \ 1

( i T J- - - — +rff

350 400 450 500 550 600 650 700 750

Wave Length —millimicrons (rry^) X

(m/x) T E"

* 9e' / L- 3:Z 0.3 9 7 2 .2 .161 60 . H 9 1 3 .

1

.ZHS

70 .533 6 .3 .3 9 1 80 .619 9 .7 .99 F 90 . 69 Z 1 9 .2 .5 65

400 .7 93 11 .5 .677

! .77

6

ZS.7 .788 20 .79Z 3Z.7 .89Z 30 .7?Z 39.7 .971 *C .7*9 V6.Z 1 .OZO

450 .698 SI.

7

1 .03 8 60 .69Z 56.5 1 .090

70 .59 8 6Z.O 1 .050

80 .557 67.5 1 .059

90 .519 7Z.O 1 .05

1

3:

:

.961 79. 1 1 .O 13

1 .9 II 75.3 .9 6S

20 .3 75 77.9 .9*5

30 .357 S3 .Z .957

40 .397 9o.3 .988

5 50 .335 97.

1

1 .019 60 .3 1 9 1 OZ.3 1 .OZ3

70 .300 1 OS. 6 1 .OI5

80 .z fo IC7.7 .997

90 .Z6 / I 09.6 .9 SO

600 .Z96 1 1 1.9 .969

1 .235 1 15.7 .9 73 20 .ZZ7 i zo.9 .9F6 30 .ZZO 1 Z5.0 .999 \ 40 .2 / 9 IZ9.5 1 .01 O 650 .zos 13 9.6 1 .OZ8

60 .ZOZ. 13 9.1 1 .OH-Z 70 .19 5 I9Z.Z 1 .OH6

80 .If 7 1 93.9 I .09Z

90 .17* 1 93. F 1 .CZ7

700 . 169 1 ¥3.7 1 .C 12.

1 .1 bC 1 HZ.b .993 20 .151 1 HO.b .969

T —Spectral Transmission of Filter at 25°C V Relative Visibility . Function E Relative Energy of 2450° K

E'—Relative Energy of 3500°K

E"(=TxE)* pooo Relative Energy of 2450°K and Filter Combination

Light Transmission of Filter

for 2450 ° K = 0.295

FILTER FORMULA

Copper Sulphate (CuS04-5H20) Z. 071 grams

Mannite (C 6 H e (0H)6 ) 2.0 7/ grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

5 Cobalt Ammonium Sulphate (Co504-(NH4), S04-6H2 0) Copper Sulphate (CuS04'5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

18.045 grams 14.000 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B in a double cell with three plates of borosilicate crown glass

(refractive index, D line,= l.5l). each 2.5 mm thick.

Bureau or Standards. . Davis - Gibson Filters

Adjusted to make sum of E" - E' from 400

to 720 mfj. equal practically to zero

Factor to be -used to multiply the candle-power

of the light source to obtain the candle-power

of the source-and-filter combination
162 BUREAU OF STANDARDS

2450'K to 6500°K CHART 38

150

125

v 100

c

UJ 75

50

25

f- 4- —4

- — —

1 ( r - —

-

-s

1 i_<

t 4

1

f T

< \ r

\ XV- /

7

^~ -

s X —

-

S -

1.00

.7 5

.50 £

.2 5

350 400 450 500 550 600 650 700 750

Wave Length millimicrons (m/x)

.00

\

(m/i) T E"

* / E 350 0.2O6 H.Z .09H

60 .2.95 Z .9 .0*9 70 .39Z 1 5.1 J 96

80 .t?1 zs.o .Z37

90 .S7? 3 S .6 .359 400 .637 59.9 .917

1 .67 6 73. .660

20 .69 S 93.7 .S9I

30 .67Z 1 I I .9 .993

40 .6 1 1 1 Z3.0 1 .Of 3 450 .5Z3 1 Z6.3 1 .IZ9

60 .937 1 Z5.6 1 .1 11

70 .375 1 Z6.9 1.136

80 .3 ZZ 1 Z.7.3 / .197

90 .2 7/ / 2.3. f 1 JZ5

500 .2 15 1 IZ? 1 .035

1 .169 1 00.9 .137

20 .1315 99.5 SSI

30 .1 Z69 96.0 .916 40 .1 1 96 1 1 .6 .1S3 550 ./ I2Z 1 06. 1 .093

60 JOZC 1 06.5 I .065

70 1 03.1 / .OSO

80 .077? 9 7.7 I .01

3

90 .0675 7Z.5 .971 600 .051? Sf.7 .95~?

1 .05th $7 .6 .9 65

20 .os-os S7.S .951

30 .Of 77 S $ .3 1 .o/s

40 .Of97 $ ? .6 1 .09Z 650 .09Z6 ?1 S i .or/ 60 J>90Z 9 o.o 1 .109

70 .0373 5F.6 1 .1 1 7 80 .03 93 $ 6.C 1 .1 10 90 .0301 S 1 .5 1 .07?

700 .OZ79 77-Z 1 .097

1 .OZ5-/ 7Z.S 1 .01 20 .ozzs 6$.Z .971

T-- Spectral Transmission of Filter at 25°C V — Relative Visibility Function E Relative Energy of 2450° K

E'—Relative Energy of 6500°H

E"(=TxE)* 0000 Relative Energy of 2450°K and Filter Combination

Light Transmission of Filter

for 2450° K = 0.0917

FILTER FORMULA

Copper Sulphate (CuS04 -5H 20) 4.42 8 grams Manmte (C 6 H a (0h%) 4.428 grams

Pyridine (C 5 H 5 N) 30.0 cc Water (distilled) to make 1000. cc

B Cobalt Ammonium Sulphate (Co504 -(NH4) 2 S04 -6H2 0) Copper Sulphate (CuS04-5H2 0) Sulphuric Acid (sp.gr. 1.835) Water (distilled) to make

38.930 grams Z9.000 grams

10.0 cc 1000. cc

These data are for a one centimeter layer each of solutions A and B m a double cell with three plates of borosilicate crown glass

(refractive index. D hne.= l.5l). each 2.5 mm thick.

Adjusted to mane Sum of E - E from 400

to 720 myu. equal practically to zero

Factor to be usee to multiply the candle-power

of the light source to oDtam the candle-power

of the SOurce-and -filter comDinat'On

Bureau of Standards.
REPRODUCIBLE WHITE LIGHT FILTERS 163

XI. BIBLIOGRAPHY 21

1. Ives, H. E., Color Measurement of Illuminants—A Resume, Trans. I. E. S., 5, pp. 189-207; 1910.

2. Priest, I. G., A Precision Method for Producing Artificial Daylight,22 Phvs. Rev. (2), 11, pp. 502-507; 1918.

3. Priest, I. G., Standard Artificial Sunlight for Colorimetric Purposes, 22 J. Opt.

Soc. Am. and Rev. Sci. Inst,, 12, pp. 479-480; 1926. (For correction of printer's errors, see also J. Opt. Soc. Am. and Rev. Sci. Inst,, 14, p. 70; 1927.)

4. Report of Optical Society of America Committee on Colorimetry for 1920-

21, L. T. Troland, chairman, J. Opt. Soc. Am. and Rev. Sci. Inst., 6, pp. 527-596; 1922.

5. Guild, J., A Critical Survey of Modern Developments in the Theory and

Technique of Colorimetrv and Allied Sciences, Proceedings of the Optical

Convention, 1, pp. 61-146; 1926.

6. Report of Optical Society of America Committee on Unit of Photographic

Intensitv, L. A. Jones, chairman, J. Opt. Soc. Am. and Rev. Sci. Inst,, 12, pp. 567-586; 1926.

7. Rapport de la Commission de 1" Optical Society of America" Sur l'Etalon Lumineux pour Sensitometrie, Proceedings, Sixth International Congress of Photographv, Paris, 1925, pp. 60-70; Societe Francaise de Photographie;

1926.

8. Proceedings, Sixth International Congress of Photography, Paris, 1925, pp. 23-24; Societe Francaise de Photographie; 1926.

9. Frehafer, M. Katherine, and Snow, C. L., Tables and Graphs for Facilitating the Computation of Spectral Energv Distribution bv Planck's formula,

B. S. Misc. Pub. No. 56; 1925 (35 cents).

10. Proceedings, International Commission on Illumination, Sixth Meeting,

Geneva, 1924, p. 67; Cambridge University Press; 1926.

11. Gibson, K. S., and Tvndall, E. P. T., The Visibilitv of Radiant Energv,

B. S. Sci. Pap. No. 475; 1923 (15 cents); Trans. I. E. S., 19, pp. 176-196;

1924.

12. Gibson, K. S., The Relative Visibility Function, Proceedings, Sixth Meeting,

International Commission on Illumination, pp. 232-238. (See 10, above.)

13. Ives, H. E., Artificial Daylight, J. Frank. Inst,, 177, pp. 471-499; 1914.

14. Luckiesh, M., and Cadv, F. E., Artificial Davlight—Its Production and Use, Trans. I. E. S., 9, pp. 839-872; 1914.

15. Bradv, E. J., The Development of Davhght Glass, Trans. I. E. S., 9, pp. 937-952; 1914.

16. Gage, H. P., Daylite Glass, Sibley J. Engineering, 30, pp. 247-250; 1916.

17. Macbeth, N., Color Temperature Classification of Natural and Artificial IUuminants, Trans. I. E. S., 23, pp. 302-324; 1928.

18. Mees, C. E. K., and Sheppard, S. E., On the Sensitometrv of Photographic

Plates, Photo. J., 44, pp. 282-303; 1904.

19. Sheppard, S. E., and Mees, C. E. K., Investigations on the Theory of the Photographic Process, Longmans, Green & Co.; 1907.

20. Mees, C. E. K., The Illuminating Engineer, London, 5, pp. 79-80; 1912.

21. Wratten Light Filters, 6th ed. rev., 1924, Eastman Kodak Co.

22. Naumann, H., Ein Spektrales Weisslichtfilter, Zeits. f. Wiss. Photogr., 23, pp. 303-319; 1924.

23. Sinden, R. H., Studies Based on the Spectral Complementaries, J. Opt. Soc. Am. and Rev. Sci. Inst., 7, pp. 1123-1153; 1923.

24. Guild, J., A White Light for Colorimetrv, Trans. Opt, Soc, 27, pp. 122-124;

1925-26.

25. Hilger, Adam (Ltd.), Standard Artificial Daylight, Nature, p. 13, January 14, 1928.

21 National Bureau of Standards official publications may be obtained from the Superintendent of Documents, Government Printing Office, Washington, D. C, postpaid, at the prices given.

22 The values of spectral energy distribution, E, introduced by Priest into the work of the colorimetry

section, National Bureau of Standards, some years ago, now in current use and reproduced in Figure 3 of the present paper, are not exactly the same as proposed in his earliest publication cited above. The

values given here are as shown in Figure 4 of the Standards Yearbook for 1927. They are given by: EccE2,m sin2 a/2 where Ei,m is energy per unit wave length as a function of wave length for the Planckian radiator at 2,848° K. for C2 at 14,350m°, and a is the rotation of the plane of polarization of light, as a function of wave length, per mm in crystalline quartz.
164 BUREAU OF STANDARDS

26. Davis, Raymond, B. S. Sci. Paper No. 409; 1921 (5 cents). Walters, Francis

M., and Davis, Raymond, B. S. Sci. Paper No. 422; 1921 (15 cents). Davis,

Raymond, and Walters, Francis M., B. S. Sci. Paper No. 439; 1922 (35

cents). Davis, Raymond, B. S. Sci. Paper No. 511; 1925 (15 cents). Davis, Raymond, B. S. Sci. Paper No. 528; 1926 (20 cents). 27. Coblentz, W. W., Distribution of Energy in the Spectrum of an Acetylene

Flame, B. S. Sci. Paper No. 362; 1920 (5 cents). 28. Priest, I. G., Preliminary Data on the Color of Daylight at Washington,

J. Opt. Soc. Am. and Rev. Sci. Inst., 7, pp. 78-79; 1923.

29. Kimball, H. H., The Distribution of Energy in the Visible Spectrum of Sunlight, Skylight, and the Total Daylight, Proc, Int. Com. on Illumination,

Seventh Meeting, Saranac Inn, N. Y., 1928, pp. 501-515.

30. Priest, I. G., The Colorimetry and Photometry of Daylight and Incandescent

Illuminants by the Method of Rotatory Dispersion, J. Opt. Soc. Am. and

Rev. Sci. Inst., 7, pp. 1175-1209; 1923.

31. Forsythe, W. E., and Worthing, A. G., The Properties of Tungsten and the

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121; 1924.

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55. Priest, I. G., Apparatus for the Determination of Dominant Wave Length,

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57. Skogland, J. P., Tables of Spectral Energy Distribution and Luminosity for use in Computing Light Transmissions and Relative Brightnesses from

Spectrophotometry Data, B. S. Misc. Pub. No. 86; 1929 (10 cents).

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