wSimaffl

«£§ WEBS®

»2

BOH

mm

1 1 I

I

AM - I *

1

I

US I

,.* .' V <

<••/•
INITED STATES •ARTMENT OF JMMERCE

IBUCATION NBS TECHNICAL NOTE 594-3

Optical Radiation Measurements:

Photometric Calibration

Procedures
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation2—Nuclear Radiation 2—Applied Radiation2—Quantum Electronics3—

Electromagnetics3—Time and Frequency3—Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies;

and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies

upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of a Center for Building Technology and the following

divisions and offices: Engineering Standards Services—Weights and Measures—Invention and Innovation—Product Evaluation Technology—Electronic Technology—Technical

Analysis—Measurement Engineering—Fire Technology—Housing Technology4 —Federal Building Technology4 —Building Standards and Codes Services4—

Building Environment4—Structures, Materials and Life Safety 4—Technical

Evaluation and Application4

. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

1 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Optical Radiation Measurements:

Photometric Calibration

Procedures

Velma I. Burns

and

Donald A. McSparron

Heat Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of

transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

Vau of *

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting D/recfor,

Issued November 1972
National Bureau of Standards Technical Note 594-3

Nat. Bur. Stand. (U.S.), Tech. Note 594-3, 35 pages (Nov. 1972) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C 20402 (Order by SD Catalog No. C13.46:594-3). Price 60cents.
Preface

This is the third issue of a series of Technical Notes entitled

OPTICAL RADIATION MEASUREMENTS. The series will consist primarily of

reports of progress in or details of research conducted in radiometry

and photometry in the Optical Radiation Section of the Heat Division

and will appear ahout every six weeks.

The level of presentation in OPTICAL RADIATION MEASUREMENTS will

be directed at a general technical audience. The equivalent of an undergraduate degree in engineering or physics plus familiarity with

the basic concepts of radiometry and photometry [e.g., G. Bauer,

Measurement of Optical Radiations (Focal Press, London, New York,

1965)] should be sufficient for understanding the vast majority of

material in this series. Occasionally a more specialized background

will be required. Even in such instances, however, a careful reading

of the assumptions, approximations and final conclusions should permit

the non-specialist to understand the gist of the argument if not the

details

.

At times, certain commercial materials and equipment will be

identified in this series in order to adequately specify the experimental procedure. In no case does such identification imply recom- mendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment identified is necessarily

the best available for the purpose.

Any suggestions readers may have to improve the utility of this

series are welcome.

Henry J. Kostkowski, Chief

Optical Radiation Section

National Bureau of Standards

111
TABLE OF CONTENTS

Page

1. Introduction —r~

2. Description of Calibration of Inside Frosted

Lamp Standards of Luminous Intensity 2

2.1 Test Lamps 2

2.2 Photometer 3

2.3 Calibration Procedure \±

2.U Discussion 5 2.5 References 9

3. Color Temperature ]_0

3.1 Description of Calibration of Incandescent

Lamp Standards of Color Temperature 10

3.1.1 The NBS Color Temperature Scale 10

3.1.2 Test Lamps H

3.1.3 The NBS Color Temperature Comparator 12

3.1.U Calibration Procedure 13

3.1.5 Uncertainty l4

3.1.6 Discussion 15

3.1.7 References 18

3.2 Inside-Frosted Lamp Standards of Color Temperature . . 19

3.2.1 Test Lamps 19

3.2.2 The NBS Color Temperature Scale for

Inside-Frosted Lamps 19

3.2.3 References 20

IV
Page

h. Description of the Calibration of Incandescent

Lamps for Luminous Flux 20

k.l Test Lamps 20

k.2 Sphere Photometer 20

4.3 Standards 21

k.k Calibration of the Test Lamps 22

k.5 Uncertainties 23

h.6 Care and Handling 26

h.J References 27

Photometric Calibration Procedures

Velma I. Burns and Donald A. McSparron

The National Bureau of Standards supplies calibrations of luminous intensity, luminous flux and color

temperature on a routine basis. The procedures, equipment and techniques used to perform these calibrations

as of October 1972 are described. Details of the

uncertainty information currently available, including

estimates and procedures for determining uncertainties

of the reported values, are also presented.

Key words: Calibration procedures; color temperature;

luminous flux; luminous intensity; photometry; uncertainty.

1. Introduction

The Optical Radiation Section of the National Bureau of Standards

recently announced a new calibration policy for radiometric and photometric standards (NBS Technical News Bulletin, June 1972). Detailed

descriptions of the procedures and equipment used to perform these calibrations are being prepared as rapidly as possible. For the photometric

calibrations of luminous intensity (candela), geometrically total flux

(lumen) and color temperature, new procedural descriptions are now available and routinely supplied along with the reports of calibration.

This Technical Note has been prepared to bring these descriptions

together in a single document and make them available to people who have

not obtained a recent NBS calibration.

Lamp standards of luminous intensity and lamp standards of color

temperature calibrated in accordance with these calibration procedures

are routinely available as "off the shelf" items from the Optical Radiation Section. Presently such lamps are calibrated twice (three measurements for each calibration) at least six months apart. Prices and

ordering information can be obtained from NBS Special Publication 250.*

Because of the sporadic nature of requests for luminous flux calibrations,

such tests are performed only at infrequent intervals . The calibration

description which appears here applies specifically to a recent luminous

flux test involving some 180 calibrations from 10 laboratories. Requests

for luminous flux calibrations can also be treated on an individual basis

and, for the immediate future, will be performed in a manner very similar

to the present description.

The material presented in this Technical Note has been limited

solely to a description of the methods and procedures presently being

used by NBS. In many cases the presently available uncertainty information is inadequate or incomplete. In the present document the attempt is

*See Measurement Users Bulletin Number h

1
made to present that information which is available and point out its

limitations. Major research efforts in the Optical Radiation Section are now devoted to improving methods and procedures for these measurements

and determining their uncertainties. As improvements are made and as "better estimates of uncertainty become available they will be reported in

this Technical Note series and incorporated into the routine calibrations.

2. Description of Calibration of

Inside Frosted Lamp Standards

of Luminous Intensity

Inside frosted lamp standards of luminous intensity are calibrated

at the National Bureau of Standards by a substitution method on a calibrated horizontal bar photometer. The photometer is calibrated at the

time of the measurements in terms of the unit of luminous intensity

(candela) at approximately the color temperature of CIE standard illuminant A (2856 K - IPTS 68) as maintained at NBS.

2.1 Test Lamps

Description

Gas-filled, inside frosted lamp standards of luminous intensity are calibrated by NBS in 100- , 500- , and 1000-watt sizes. The 100- and 500-

watt lamps have T-20 bulbs; the 1000-watt lamps have T-2U bulbs. They

all have medium-bipost bases and C-13B filaments. The 100-watt and

1000-watt lamps are designed to have approximately 1000 hours life when

operated at 120 volts. The 500-watt lamps are designed to have 500 hours

life at 120 volts.

Preparation

The lamps are seasoned by operating them at 120 volts ac for

approximately 5% of their rated life and an identifying number is etched

on each bulb

.

Orientation

Test lamps are calibrated while burning base down on a horizontal

bar photometer with the identifying number turned away from the detector.

Lamp orientation is accomplished by aligning the lamp socket so that the

lamp posts are held vertically with the plane formed by the axes of the

posts perpendicular to the optical axis of the photometer. The bottom of

the lamp bulb is 10. l6 cm* below, and the lamp pins are equidistant from,

the photometer axis. Source-to-receiver distances are measured from the

*Some of the 500-watt lamps have light center lengths of 7.62 cm

(3 inches). For these a distance of 7.62 cm is used between the photometer axis and the bottom of the lamp bulb. The distance used is stated

in the individual "Report of Calibration" for the lamps.
plane formed "by the axes of the biposts to the sensitive surface of the

detector.

Before calibration the lamps are checked to ascertain that the

luminous intensity varies by less than 0.1$ for (l) a ±1.5° rotation

about a horizontal axis intersecting the photometer axis in the plane

formed by the axes of the posts (pitch) and (2) a ±1.5° rotation about a vertical axis contained in the post plane and intersecting the photometer

axis (yaw)

.

Operation

All measurements are performed with the lamps operated on dc power.

Electrical measurements are made potentiometrically to an accuracy of

0.02$. After positioning and alignment, the lamps are slowly (15-30

seconds) brought up to the designated electrical operating point and

allowed to stabilize for at least 10 minutes before measurements are made. Normally, the lamp current is set and measurements are made of

the luminous intensity and of the potential drop across the pins of the

bipost base.

2.2 Photometer

Detector and Amplifier

The photometer detector is a selenium barrier-layer photocell [l] 1 equipped with a filter which modifies its spectral response to approximately match the CIE luminous efficiency function [2]. Measurements are usually made at a photocell illumination level of approximately 80 lux.

The entire photosensitive surface is directly illuminated by the entire

test or standard lamp, and no auxiliary optics are used. The photocell

is fully illuminated during the 10-minute stabilization period of the

lamps

.

The detector photocurrent is measured with an operational amplifier

(current to voltage converter) arranged in a typical closed loop configuration. Thus the photocell is operating into a near "zero-resistance"

circuit (no voltage across the terminals of the photocell) in accordance

with the formula E output = I input x R feedback. A five place digital

voltmeter is used to measure the output of the operational amplifier.

Optical Bench

All measurements are made on a h. 5-meter optical bench. The

optical bench is housed in an enclosure 6l cm wide and 117 cm high. The

enclosure is covered with black "suedine" cloth on all four sides. The

optical axis is 38 cm above the bench and is located approximately 69 cm

•Figures in brackets indicate the literature references at the end of

each section.
from the bottom, U8 cm from the top, and 30.5 cm from the sides of the

enclosure. Source-to-receiver distances of 1.3 to k.2 meters are typically used. For the 100- and 500-watt lamp sizes, a limiting baffle

with an aperture 7-6 cm wide and 17.8 cm high is placed 25 cm from the

plane of the biposts and centered with respect to the lamp bulb. For

the 1000-watt test lamps, this baffle aperture is 10 cm wide and 23 cm

high. Additional baffles are placed between the source and the detector

to screen the detector from any directly reflected light. Light emitted

by the lamp in the direction away from the receiver is absorbed with a light trap placed kO cm behind the lamp.

2.3 Calibration Procedure

Photometer Calibration

The photometer is calibrated, at the time of the measurements, in

terms of the illuminance produced at the detector by each of a group of

500-watt lamps of the same type and construction as the test lamps . The

500-watt lamps used to calibrate the photometer are periodically compared

with a group of 10 similar lamps (group NBS 56l2) which represents the

unit of luminous intensity at approximately the color temperature of CIE

standard illuminant A (2856 K - IPTS 68) as maintained at NBS.

An illuminance substitution method is used for the measurements.

500-watt lamps are calibrated at a photometric distance of 3 meters for

both standards and test lamps . This produces approximately 80 lux at

the photocell. When calibrating 100-watt lamps the photometer is calibrated with the 500-watt standards at a photometric distance of 3 meters

(approximately 80 lux) and again with a distance of h.l'J meters (approximately hO lux). The test lamps are then measured at two distances, one distance to produce approximately 80 lux and another distance to produce

approximately kO lux. The 1000-watt lamps are measured at h.l"J meters

with the photometer calibrated with the 500-watt standards at a distance

of 2.9 meters (approximately 80 lux).

Measurement Schedule

Test lamps are measured in groups of approximately 18. Normally

8 standards are used to calibrate the photometer. Measurements of the

standards are interspersed before, after and within the group of test

lamps to check for drifts in the detector sensitivity.

Data Reduction

Individual photometer calibration factors (lux per volt output of

the operational amplifier) are computed for each standard lamp. After

checking these factors to ascertain that no significant drift in the

sensitivity of the photometer has taken place during the calibration of

the test lamps, an average photometer calibration factor is computed.

This average factor is used to compute the illuminance produced by the

test lamps. Test lamp intensities are then computed by multiplying the
measured illuminance by the square of the distance "between the lamps and

the detector. The entire measurement procedure is repeated at least

three times, each with a different selenium barrier-layer photocell. The

test lamp intensities reported are the averages of these determinations.

2.k Discussion

Uncertainty

In the measurement of a property of a material object, such as the

luminous intensity of an incandescent lamp, an uncertainty statement is an estimate of the possible discrepancy between the reported value and

the physical parameter this value represents, that is, an estimate of

the possible error in the reported value. Such uncertainties are usually based on a statistical treatment of the random variations of the

measurements and on theoretical deductions together with direct measurements of the possible sizes and types of biases that one recognizes may

be present. Of course, any bias not recognized or not measured is not

considered in determining the uncertainty. Therefore, the true error is never known . Its possible value is merely estimated with varying degrees

of sophistication and assurance ranging from that resulting from a few

measurements and an approximate theory to years of investigation and

continued validation using an "exact" theory. No useful way has been

devised to quantify the reliability of the uncertainty. It can be

judged qualitatively only from the extent and sophistication of its

characterization and the degree to which it has been subjected to con- tinuing validation.

Inherent in an uncertainty statement is the concept that it is

unlikely that subsequent determinations of the same invariant physical

parameter for the same material object will deviate from the original

determination by more than the stated uncertainty. Such subsequent

determinations may be in the form of replications (identical theory,

instrumentation and procedures) or complete redeterminations (totally

different theory, instrumentation and procedures) or any combination of

the two.

The totality of the theory, instrumentation, and procedures which

NBS uses in obtaining a given reported value and its uncertainty is

designated the NBS process. Characteristic of a well-developed process

is a complete and sophisticated investigation that led to its establishment. Characteristic of a well-run process is a continuing validation

of the process parameters (precision, checks for possible biases, etc.)

in the form of replications and periodic redeterminations. The greater

the extent and completeness of the fundamental investigations and the

higher the degree of redundancy in the continuing process, the greater

the confidence that the discrepancy between the sought after value and

the value being reported is not larger than the assigned uncertainty.

The photometric chain, which is used by NBS in realizing and maintaining the illuminant A candela, is subject to a number of biases.
These may be considered in terms of the three major steps used in the

generation of the reported values.

A) Platinum point realization. Specific sources of possible biases

include: the effect of impurities on the freezing point of platinum, the

quality of the blackbody cavity and diffraction effects.

B) Photometric transfer to illuminant A. Specific sources of

possible biases are the nonlinearity and spectral response characteristics of the detectors.

C) Substitution calibration of test lamps. Two distinct types of

bias, constant and time dependent, are possible. If the operation of

the working standard lamps does not duplicate in all essential respects

the conditions under which their assigned values were derived or if

these working standards have changed, a constant bias will be introduced

into all measurements. On the other hand, if the photometric apparatus

is sensitive to variations in an uncontrolled parameter, for instance

environmental temperature or humidity, a time dependent (for example,

day-to-day) bias will result.

The different biases will have varying effect on the uncertainty

with respect to the three bases represented by (l) SI candela, (2) the

vorld mean candela and (3) the NBS candela. For example, biases in the

NBS realization of the platinum blackbody will have a direct effect on the uncertainty with respect to the SI candela, and an indeterminate

effect on the uncertainty with respect to the world mean. The latest

possibility arises because the various national standardizing laboratories have utilized similar equipment and techniques for realizing the

primary standard of light, and thus may all have the same bias. Biases

introduced during the transfer chain will have a direct effect on the

uncertainty with respect to SI and world mean, but no effect on the

uncertainty with respect to the unit as maintained by NBS. Only the

biases of the substitution calibration will affect the uncertainty of a reported value with respect to NBS. The user of a lamp standard of

luminous intensity will usually wish to know his uncertainty with respect

to one or more of these bases. The remainder of this section presents

data and procedures currently used to assign uncertainties to the illuminant A luminous intensity calibrations.

Three biases will affect the uncertainty of a reported value with

respect to the candela as maintained by NBS. These are: (l) constant

biases of the substitution calibration of the test lamps, (2) time

dependent biases of the substitution calibration and, (3) random variation of the measurements. No detailed investigation has yet been made

of the possible constant biases of the substitution calibration. The

uncertainty due to the other two biases was determined in 1970 from a study of 12U measurements on 36, 500-watt lamps and 80 measurements on

20, 100-watt lamps. These data indicate that time dependent biases are present in the process and usually amount to 0.2-0.3% though a few as large as 1% have been observed. These bias percentages include
short-term lamp instabilities. Occasionally, for example, changes of

0.5$ have been observed in merely turning a lamp off and then on again.

Assuming that these time dependent biases are due to the random variation of an uncontrolled parameter, a statistical analysis has been made

of the measurements. A pooled estimate of the standard deviation of a single measurement has been calculated to be 0.3$. In assigning a value

to the uncertainty, an allowance of 1.0$ is made for the time dependent

bias in the data and an additional allowance of 0.5$ is made for the

random variation in the measurements (three times the standard deviation

of the mean of three measurements). The uncertainty assigned to a reported value with respect to the candela as maintained by NBS is the

sum of these two components or 1.5$.

The most recent international intercomparison of the candela at the

color temperature of CIE illuminant A showed a range of 1.7$ among the

eight participating national standardizing laboratories including NBS

[3]. Adding the calibration uncertainty of transferring the NBS candela

to a test lamp (l.5$) to one half this range (0.85$) yields our current

uncertainty of 2.3$ for a reported value with respect to world mean.

The uncertainty of a reported value with respect to the SI candela

will be subject to several sources of bias in addition to those discussed

above. Although extensive investigations of these additional sources of

bias have not been made, some relevant information exists on which to

base an uncertainty. The same intercomparison referred to earlier [3]

showed that the candela at the color temperature of CIE illuminant A, as realized in the national standardizing laboratories of the world, was inconsistent with the candela realized at the color temperature of

freezing platinum by as much as 3.5$ [h] . Also, recent theoretical work

has indicated that previous realizations of the platinum point blackbody

may have been in error by as much as 1.5$ [5]. Thus in our current

process there are six individual biases or sources of bias relative to

the SI candela for which uncertainties have been assigned. These

together with the uncertainties associated with them are listed below:

Biases

Substitution Calibration

A. Time dependent bias

B. Constant bias

C. Random variation

Assigned Uncertainty

1.0$

0.0

0.5

II, Transfer Chain

A. International intercomparison

at 2856 K (1/2 range)

B. International intercomparisons

;

inconsistency in realizing the

candelas at the platinum point

and 2856 K

III. Platinum Point Calibration

0.85

3.5

1.5
It is unlikely that all these individual uncertainties would have the

same sign resulting in a total uncertainty of 1.3%. Therefore a combination in quadrature (square root of the sum of the squares) resulting in

k.1% is considered to give the uncertainty of a reported value relative

to the SI candela.

Recently a program was established at NBS for developing improved

luminous intensity standards and methods of transfer and for more

thoroughly characterizing biases and uncertainties. Results of this

work will be published as they become available.

Precautions

Lamps supplied to other laboratories as standards of luminous

intensity represent the unit of luminous intensity (candela) as maintained at the National Bureau of Standards with the best precision now

available. They are expensive laboratory equipment and deserve the

utmost care in handling and use.

In addition to some obvious precautions, such as keeping the lamps

clean and avoiding mechanical shock to the filament, the following are

a few precautions, sometimes overlooked, which should be used with these

standards.

1. The lamps should be turned on and off slowly (15-30 seconds),

and great care should be taken so that at no time will the current

appreciably exceed the value stated in the report. The lamp should not

be moved while lighted.

2. In order to prolong the useful life of the lamps, it is recom- mended that they be used sparingly and that for general use, working

standards be prepared by calibrating them relative to the lamps supplied

by NBS. When a laboratory procures standard lamps from NBS it is well

to obtain at least three such lamps in order to be able to detect any

changes that may occur.

3. The lamps should be carefully aligned in accordance with the

procedures described above. Photometric measurements should be made

only after the lamp has stabilized (approximately 10 minutes after turn

on) .

h. Stray light must be excluded. One source of stray light that

is sometimes overlooked is the standard lamp itself. The background of

the lamp on the side away from the detector should not reflect light

back along the photometric axis. NBS uses a light trap made of two

pieces of black glass set at approximately 60° to each other and set

behind the lamp. Black cloth positioned h^ to 60 cm behind the lamp is

convenient and usually adequate.

5. Baffle aperture edges should be very thin or beveled so as not

to reflect light to the detector. It is of little use to coat baffle

8
edges vith "black paint or black cloth, since most flat surfaces reflect

light reaching them at large angles of incidence regardless of whether

or not they are blackened.

6. It is preferable not to confine the lamp to a small space,

especially with poor ventilation. Excessive noise in the measurements

often results.

7. The variation of luminous intensity of a gas-filled tungsten

lamp is approximately related to the variation in its current by the

formula:

|^= 6.25 £1

1 i

where I is the luminous intensity and i is the lamp current [6,7]. Thus

a current measurement accuracy of at least 0.032% is required if it is

not to affect the luminous intensity to greater than 0.2%.

2.5 References

[l] V. K. Zworykin and E. G. Ramberg, Photoelectricity , Chapter 11,

John Wiley & Sons , Inc . , New York, N. Y. (19U9).

[2] IES Lighting Handbook, Fourth Edition, p. 1-3, 2-k (1966),

Illuminating Engineering Society, New York, N. Y.

[3] Comite Consult atif de Photometrie, 5 e Session (1962) p. 8U.

[k] Comite Consult at if de Photometrie, 6

e Session (1965),

Annexe 3, p. P28.

[5] Ibid., Annexe 1, p. PIT.

[6] J. W. T. Walsh, Photometry , p. 532, Dover Publications, Inc.,

180 Varick Street, New York, N. Y. (1965).

[T] A. Kings lake, Applied Optics and Optical Engineering , p. 6l,

Academic Press , New York, N. Y. C1965).
3. Color Temperature

3.1 Description of Calibration of

Incandescent Lamp Standards of Color Temperature

Incandescent lamp standards of color temperature are calibrated at

the National Bureau of Standards by a substitution method on a red-blue

ratio, color temperature comparator. The color temperature comparator

is calibrated at the time of the measurements in terms of the color

temperature scale maintained by NBS.

3.1.1 The NBS Color Temperature Scale

NBS has established and maintains a scale of color temperature.

Careful distinctions should be made between the concept of color temperature and the related concepts of correlated color temperature and

distribution temperature. Color temperature is defined as the "temperature of the full radiator which emits radiation of the same chromaticity

as the radiation considered" [l]. That is, the test source and the full

radiator (i.e. blackbody) have the same appearance to a human observer.

Note particularly that the corresponding relative spectral power distributions are not necessarily identical or even similar. If the test source and the corresponding blackbody have identical relative spectral

power distributions in the spectral region of interest, the correct

designation is distribution temperature [2]. If one wishes to assign a "temperature" to describe a source whose appearance, and hence whose

relative spectral power distribution, is markedly different from a blackbody, such as a fluorescent lamp, the correct designation is correlated

color temperature [3]. Mathematical procedures are available for

determining the correlated color temperature of any source [h] . In

applying the values of color temperature reported by NBS, the user should

bear these distinctions in mind. Particular care should be exercised in

utilizing the widespread practice of treating color temperature values

as synonymous with distribution temperatures.

The present NBS scale of color temperature was established in 193*4- [5] by visual comparison of monoplane, coiled tungsten filament, incandescent lamps with three melting point blackbodies: platinum at 20*4-5 K,

rhodium at 2236 K, and iridium at 2720 K. The nine lamps thus calibrated

(three at each temperature) were then used to calibrate three working

standard lamps across the color temperature range. An empirical approach

was adopted for interpolating between the three fixed temperature points

.

Taking advantage of the cavity effect due to filament coiling, pyrometric

determinations of the radiance temperature* were made at a position on the inside of a single turn of the coiled filament. It was observed that

*Radiance temperature, also called luminance temperature and brightness temperature, is defined as the temperature of a blackbody for which

the spectral radiance at the specified wavelength is the same as that of

the radiator considered.

10
the difference between the radiance temperatures so determined and the

color temperature of the lamp as a whole was a smoothly varying function

of the voltage applied to the lamp. Pyrometric determinations of the

radiance temperatures of three different coil turns of the test lamps

yielded consistent color temperature values for the interpolated region.

Further, from this data it was empirically determined that an equation

relating the color temperature (T ) and the applied voltage (V), of the

form:

T = A + B(V) 1 ' 2

(1J

was adequate to express all of the data within the estimated uncertainties. The 193^ color temperature scale was also extrapolated up to

3200 K and down to 1500 K. Sources with known color temperatures in the

extrapolated regions were produced "by modifying incandescent lamp outputs

(assumed to he sources whose distribution temperatures equalled their

color temperatures), with blue and amber filters whose spectral transmittances had been measured. Incandescent lamps and associated equations

of the form of eq. (l) have been used since 193^ to maintain the NBS

color temperature scale. The scale has been adjusted twice, in 19^-9 [6]

and in 1970 [7], to take account of changes in the International Practical Temperature Scale.

The present scale of color temperature resides in nine incandescent

working standard lamps. Lamps BS 9021, BS 9022, and NBS 1005 are used

in the range 2000-2600 K; lamps NBS 1923, NBS 192U, and NBS 1925 are used

in the range 2300-2900 K; and lamps NBS 1926, NBS 1927, and NBS 7875 are used in the range 2700-3200 K. These working standard lamps were originally calibrated by visual comparison with the lamps used to establish

the scale in 193^-. In addition to the calibrations at fixed color temperature points, associated empirical equations of the same form as eq. (l) are used to establish the scale in interpolated and extrapolated

regions. Empirical equations of the form,

i = A + BT

C + CT^ , (2)

relating the color temperature and the current through the lamps , i

,

have also been found to be completely adequate to represent the data

within the associated uncertainties.

3.1.2 Test Lamps

Description

Airway beacon lamps designated 500T20/13, are issued by NBS as lamp

standards of color temperature. They are 500-watt, 120-volt lamps with

clear T-20 bulbs, C-13B filaments and medium bipost bases. They have a rated life of 500 hours at 120 volts.

11
Preparation

Initially, the lamps are seasoned by operating them at 120 volts ac for 20 hours (k% of rated life). An identifying number is then etched

on each bulb

.

Orientation

Test lamps are calibrated while burning base down with the etched

identifying number turned away from the comparator. The lamps are aligned so that their planar filaments are centered on and perpendicular

to the optical axis of the entrance aperture of the collecting sphere

of the color temperature comparator. No auxiliary diaphragms are used,

and thus light from essentially the entire test lamp is viewed by the

comparator.

Operation

All measurements are made with the lamps operating on dc power.

Electrical measurements are made with a digital voltmeter to an accuracy

of 0.1%. After positioning and alignment, the lamp is slowly (15-30

seconds) brought up to approximately the required electrical operating

point and allowed to stabilize for at least 10 minutes before measurements are made. Measurements are then made of the current through the

lamp and the potential drop across the pins of the bipost base necessary

to produce a color match to a comparison source.

3.1.3 The NBS Color Temperature Comparator

At present, all routine color temperature measurements at NBS are made on a device which compares the ratios of the "red" portion of the

visible spectrum to the "blue" portion of the visible spectrum for test

and standard sources. Equality of these two red-to-blue ratios is

interpreted to mean equality in color temperature. Since the measurement

involves the inference of a visual property of the test source (chromaticity match to the standards) from the measurement of a different

physical property, severe requirements are placed on the test and standard sources. Specifically, their relative spectral power distributions

must be identical throughout the entire visible region of the spectrum.

Only test lamps substantially identical to the working color temperature

standard lamps (clear bulbs, monoplane coiled tungsten filaments) are calibrated by this method.

Numerous devices based on the red/blue ratio principle have been

described in the literature [8]. The present NBS instrument has not

been optimized, either in the electronics used for signal sensing or in

the optical components (filters and detector), It has, however, been

shown to possess adequate sensitivity and precision. The standard deviation of a single measurement for repeated measurements of the same color

temperature on the same test lamp is about 2 K. The chief virtues of

the present instrument are precision, speed, and convenience.

12
The figure on page 15 is a block diagram of the major components of the

comparator. Radiation from the test or standard source is collected in

a BaSO^ coated entrance sphere (20 cm diameter, 7-5 cm entrance aperture).

The sphere is mounted so that it can be rotated about the optical axis of

its exit aperture and thus view a comparison source. (The function and

use of the comparison source will be discussed in Section 3.1.U, Calibration Procedure.) Radiation leaves the entrance sphere through a 3.8 cm diameter exit aperture and passes through an infrared absorbing filter

(Corning No. h600) . This infrared absorbing filter is placed in the

optical chain to alleviate the effects of the temperature sensitivity of

the red filter. The radiation next passes through either the red filter

(Corning No. 3^86) or the blue filter (Corning No. 5562) and is detected

by a photomultiplier (S-ll, EMI 951US). The red and blue filter are mounted on a rapidly spinning wheel (lUOO rpm) so that radiation passing

through the two filters is alternately presented to the photomultiplier.

Two synchronously driven electronic gates assure that the signal from

radiation passing the red filter, and only radiation passing the red

filter, is diverted to one amplifier-integrator chain while the signal

from radiation passing through the blue filter is diverted to a second

amplifier-integrator chain. After passing the amplifier-integrator

chains, the two signals are compared on the null meter. Suitable adjustments are made to either the lamp electrical parameters or the signal

attenuator and adjustable voltage divider to obtain equality between the

two signals (see Section 3.1.^, Calibration Procedure). Constant loading

of the photomultiplier over the course of a given test run, is assured

by referencing one of the two signals (either the red or the blue) to a stable dc reference source (0.1% stability) servo controlling the photomultiplier high voltage supply. A front panel reversing switch allows

the operator to select which signal passes which amplifier-integrator

chain and hence which signal references the high voltage supply. The

instrument as described is capable of measurements in the range 2^00 K

to 3000 K. For color temperatures outside this range, there is not

sufficient compensation available in the electronics to achieve a null

between the red and blue signals. For measurements in the range 2000 K

to 2U00 K, a blue "accessory filter" (Corning No. 5900) is introduced

immediately after the infrared absorbing filter. A yellow filter (Kodak

Wratten 2A) is similarly used for measurements above 3000 K.

3.1.^ Calibration Procedure

Color temperature measurements made on the red-to-blue ratio color

temperature comparator are made by a strict substitution procedure. Test

and standard lamps are placed in the same geometrical position with

respect to the comparator, the optical path through the comparator is the

same and finally, at null, the comparator indicates a spectral match

between the sources.

At the beginning of each calibration run the comparator is calibrated against each lamp of the relevant working standard group in turn.

One member of the working standard group is placed in the measuring

position and allowed to operate at the current required for the desired

13
color temperature for 10 minutes. The signal attenuator and adjustable

voltage divider are adjusted to produce an exact equality between the

red and blue signals from the photomultiplier. The entrance aperture of

the collecting sphere is then rotated to view the comparison lamp and

the voltage across the comparison lamp is adjusted to produce the same equality of red and blue signals. This comparison lamp voltage is noted

and the entire procedure is repeated for each member of the standards

group. The average comparison lamp voltage is computed and the comparison lamp set to that voltage. The red and blue signals now obtained when

viewing the comparison lamp are nulled by adjusting the electronic con- trols. During the remainder of the calibration run, the comparison lamp

is periodically viewed to assure that the red-to-blue null condition

still prevails. Thus the calibration of the comparator depends on the

stability of the comparison lamp and not on the stability of the electronics or the photomultiplier. If a drift from null is observed, the electronic controls are again adjusted to restore the null condition. Test

lamps are then placed in turn at the same position previously occupied

by the standard lamps and after warming up for 10 minutes the exact

current required to match the comparison lamp is determined.

Typically nine test lamps are run at one time. The entire calibration procedure, including calibration of the comparator against each

member of the working standard group, is repeated three times on successive half days. In order to check for possible shelf effects (drifts

that occur while the lamp remains unused on the shelf) in the test lamps,

at least one additional set of three runs is made six months later. Thus

the values reported for a test lamp are the average of at least six runs made over a period of at least six months.

3.1.5 Uncertainty

The color temperature scale presently maintained by NBS is con- sidered to be a "gage" standard, that is, a standard which NBS has not

investigated sufficiently to assign defensible uncertainties relative

to SI units. However, control relative to NBS standards does exist and

provides at least a uniformity in measurements relative to these standards. The present scale has not been checked against absolute standards

since it was established in 193^. NBS is presently in the final stages

of a program to realize an absolute scale of spectral irradiance. An

early reevaluation of the present color temperature scale in terms of

this new scale of spectral irradiance is planned. Results of this work

will be published as they become available.

Although nothing definite can be said about the uncertainty of the

NBS color temperature scale with respect to SI, information is available

about the precision of the transfer of the NBS scale to the test lamps.

An analysis of three runs of 7 lamps at 2000 K yielded a standard deviation of a single measurement of 1.1 K. A similar analysis of three runs

on twenty lamps at 2856 K yielded a standard deviation of a single

measurement of l.U K. From these data an allowance of 1.9 to 2.U K is

made for the uncertainty of the transfer of the NBS scale to a test lamp

Ik
UJ

UJ CE O o < H J < 5 < Z Sv

UJ ~

a:

—•

UJ

u. ^

UJ o > 0. CE CC O S 3 1 o O o Q o a."

ccUJO <^ HO Q- uj- 2

LU Ll) a & oe UJ 3

UJ ^ UJ X 1- Q > Q o O

-J UJ moo: 1- _J 1 DC < < UJ < _|0 o I 2i ADJUST

VOLT

DIVID LU 2 I

tO <-> ce iUJ «>

ce 4

toUJ H-ce CEo

UJ h- co <

-J ce

=> e>

a. uj

aoQo

l-oI

£°

3* or °< I-z O

UJ t10 <

_i ce 3 o

a. So a. uj a: o z 11 UJ fo> [ 1 K

LU

z> < £

CD

e> o1 _l ' _i ' UJ _|Q- JOCD

1-<

«*5 < 2 2< 2< o t0_i to _l o 1

=> oz, 1UJ 1 — o

CD

W 0-w£ o-1 <

H°->- <2 1- oi=<" ^ < Uo5

CL COCL r~>

t- = ^ • J^ ZI to o o

a.2

r»-ii / >1 . < (®5O^^^V,

UJ

ceoo<

to

UJ>

_lo>

UJ

ce

toa

15
in a set of three closely spaced runs (three times the standard deviation of the mean of three measurements). The procedure to check for

shelf effects in test lamps has only recently been instituted, and thus

only preliminary data is available. Two sets of three runs each 18

months apart have been made on a group of nine test lamps calibrated at

2856 K. These lamps exhibited a directional drift averaging 2 K, but in one case as large as 7 K. Until a more definitive body of data is

obtained on typical shelf effects, an allowance corresponding to the

maximum observed shift of 7 K is made in assigning an uncertainty to the

transfer of the NBS scale to the test lamps. Thus the estimated uncertainty of the reported value with respect to the NBS scale is 9 K

(precision plus the allowance for shelf effects).

One international intercomparison of national scales of color temperature has been conducted [9]« Two methods of making the color temperature comparisons were used at the Bureau International des Poids et

Mesures: red/blue and spectroradiometric. The table below shows the

range (combined for the two methods of comparison) observed among the

7 participating national standardizing laboratories.

Nominal

Color Observed

Temperature* Range

20U2 K 9 K

2200 9

2353 11

2600 29

285U 31

3000 hi

*Temperature based on IPTS-U8.

The entries in the table are indicative of the range of color temperature measurements that might have been encountered in the world in 1965

.

Since only one such intercomparison has been conducted, no information

is available on the stability of the various scales over long periods of

time. Hence, although NBS was quite close to world mean at that time

(maximum difference between NBS and world mean was 10 K at 3000 K) no inferences are drawn about the relationship of the present NBS scale to

world mean.

3.1.6 Discussion

Lamps supplied to other laboratories as standards of color temperature represent the color temperature scale as maintained by NBS. They

are expensive laboratory equipment and deserve the utmost care in handling and use. In addition to some obvious precautions, such as keeping

the lamps clean and avoiding mechanical shock to the filament, the

16
following are a few precautions, sometimes overlooked, which should be

used with these standards.

1. The lamps should be turned on and off slowly (15-30

seconds), and great care should be taken so that at no time will the current appreciably exceed the value

stated in the report. The lamp should not be moved

while lighted.

2. In order to prolong the useful life of the lamps, it

is recommended that they be used sparingly. For

general use, working standards should be prepared

by calibrating them relative to the lamps supplied

by NBS. When a laboratory procures standard lamps

from NBS it is well to obtain at least three such

lamps in order to be able to detect changes that

may occur.

3. Measurements should be made only after the lamp has

stabilized (approximately 10 minutes after turn on)

.

h. Stray light should be excluded from the measuring

instruments. This includes light from the standard

which may be reflected from the background or from

instruments being used.

5. Differentiation of equation 2 yields:

di = (B + 2CT

C

) dT

c (3)

Typical values of the constants A, B, and C in eq. (2) for the airway

beacon lamps of the type 500T20/13 are, A = -.U5, B = 1.0 x 10 -3 and

C = l.U x 10 -7. Evaluating eq. (3) in the range 2000 K to 3000 K, shows

that the sensitivity of the current setting ranges from 0.0016 to

0.0018 amps/K. Since these lamps typically draw about k amps at 2856 K, a current setting accuracy of at least 0.1$ is required if the color

temperature of the lamp is not to be affected by more than 2 K.

IT
3.1.7 References

[l] International Lighting Vocabulary , 3rd Edition, CIE Publication,

No. IT, Section U5-05-270, p. 29 (1970).

[2] Ibid, Section 1+5-05-265, p. 29.

[3] Ibid, Section ^5-15-260, p. Ik.

[k] K. L. Kelly, Lines of Constant Correlated Color Temperature Based

on MacAdam's (U,V) Uniform Chromaticity Transformation of the

CIE Diagram, J. Op. Soc. Am., 53., No. 8, pp. 999-1002,

August 1963.

[5] H. T. Wensel, D. B. Judd, and W. F. Roeser, Establishment of a Scale of Color Temperature, Bur. of Stand. J. Res., 12 , pp. 527-536, May 193U.

[6] D. B. Judd, The I9U9 Scale of Color Temperature, J. Res. Nat.

Bur. Stand., kh , January 1950.

[7] Color Temperature, Luminous Efficacy and the International

Practical Temperature Scale of 1968, Nat. Bur. Stand. Technical

News Bulletin, pp. 206-207, 5^, No. 9, September 1970.

[8] See for instance:

J. S. Preston, Phys . Soc, Proc. hj_, 1935, p. 1012.

H. G. W. Harding, Phys. Soc, Proc 63, 1950, p. 685.

J. Sci. Inst., 29, 1952, p. 1U5.

W. J. Brown, J. Sci. Inst., 31, 195^, p. ^69.

[9] Comite" Consultatif de Photome"trie , 6

e Session (1965), Annexe 7,

p. hi.

18
3.2 Inside-Frosted Lamp Standards

of Color Temperature

Inside-frosted incandescent lamp standards of color temperature at

2856 K are calibrated at the National Bureau of Standards "by a substitution method in accord with the procedures described in Section 3.1,

"Description of Calibration of Incandescent Lamp Standards of Color

Temperature". For inside-frosted lamps, the exact procedure used differs

from the previously described procedure only in the type of lamp calibrated and the standards used for the comparison.

3.2.1 Test Lamps

The test lamps and their preparation are described in Section 2.1,

"Description of Calibration of Inside-Frosted Lamp Standards of Luminous

Intensity". The test lamps are calibrated while burning base down with

the identifying number turned away from the comparator. Lamp orientation is accomplished by aligning the lamp socket so that the lamp posts

are held vertically with the plane formed by the axes of the posts perpendicular to the optical axis of the comparator. The light center of

the lamp is on, and the lamp pins are equidistant from the optical axis

of the comparator (see calibration report for specific light center

lengths )

.

3.2.2 The NBS Color Temperature Scale

for Inside-Frosted Lamps

Three inside-frosted, 500-watt incandescent working lamp standards

of color temperature have been calibrated at 2856 K and are maintained

by NBS. These lamps (NBS 9171, NBS 9172, and NBS 9173) are of the same type as described above. Initial calibration of these lamps was performed spectroradiometrically against the 500-watt, clear-bulb working

standards of color temperature (NBS 1926, NBS 1927, and NBS 7875). The

spectroradiometer used for this calibration [l] determined the ratios of

the spectral radiant powers of the test and standard lamps at every

10 run from 380 nm to 760 nm with a spectral bandpass of 5 nm. The data

reduction assumed that the relative spectral power distribution of the

clear-bulb standards was the same as the relative power distribution of

a Planckian radiator. The relative spectral power distribution of the

inside-frosted lamps thus determined was used to compute their chromaticity coordinates. The procedure described by Kelly [2] was then used

to compute the correlated color temperature of the inside- frosted lamps.

19
3.2.3 References

[l] D. A. McSparron, K. Mohan, R. C. Raybold, R. D. Saunders, and

E. F. Zalewski, Spectroradiometry and Conventional Photometry

An Interlab oratory Comparison, Nat. Bur. Stand. (U.S.) Tech

Note 559, pp. 5-12, November 1970.

[2] K. L. Kelly, Lines of Constant Correlated Color Temperature

Based on MacAdam's (U,V) Uniform Chromaticity Transformation

of the CIE Diagram, J. Op. Soc. Am., 53., No. 8, pp. 999-1002,

August 1963.

k. Description of the Calibration of

Incandescent Lamps for Luminous Flux

In response to several requests from the users of luminous flux

standards, the National Bureau of Standards has calibrated a group of

general-purpose incandescent lamps. The lamps range in size from

10 watts to 1000 watts and are approximately 120-volt , base-up burning

lamps. They were calibrated in a 2-meter integrating sphere photometer.

k.l Test Lamps

The lamps to be calibrated were submitted by the users and were

stated to have been properly seasoned before submission. Each lamp

bears an identifying number etched on the bulb.

k.2 Sphere Photometer

The lamps were calibrated while burning base up in the center of a 2-meter integrating sphere photometer. The sphere is coated with Burch

sphere paint. A 7.6-centimeter diameter observation port is located in

the sphere wall and is covered with a white diffusing plastic (Plexiglas,

color V/2 U i+ T Rohm and Haas white) flush with the sphere wall. A baffle,

also coated with Burch sphere paint is placed between the lamp and the

window, 0.55 meters from the window. When measuring 10-watt to 500-watt

lamps, the baffle used was a circular disk 17«5 centimeters in diameter.

When measuring the 750-watt and 1000-watt lamps, the baffle was shaped

roughly like a projection of the lamp and was 15 centimeters by 23 centimeters.

Because of the somewhat selective spectral reflectance of the sphere

coating and transmittance of the window, a blue filter is inserted

adjacent to the window. The blue filter was selected from a set of

filters of graded color temperature altering power, so that the net

spectral effect of the sphere reflectance, window transmittance and the

blue filter is that the spectral distribution of the flux arriving at

the detector is approximately the same as that leaving the lamp. In

order to select the proper filter, a lamp was operated at 2856 K color

20
temperature in the sphere. The filter was then selected such that light

from the lamp operating in the sphere and transmitted through the window

and filter had a color temperature of 2856 K. The proper filter was determined just before this test was started (about four months after

the sphere was painted) . The correction at that time was approximately

6h0 K. The correction was determined again after about half of the data

had been taken (three months later) and the correction needed was approximately TOO K ( a change of about 60 K) . At the conclusion of the test

the correction needed was still TOO K.

A selenium barrier layer photocell [l] (Weston model 856) equipped

with a filter to correct its spectral response to approximately match

the CIE luminous efficiency function [2] is placed in a brass tube

fitted to the window. The brass tube is coated on the inside with mat

black paint. The photocell photocurrent is measured with an operational

amplifier (current to voltage converter) arragned in a typical closedloop configuration. Thus the photocell is operating into a near zero resistance circuit (no voltage across the terminals of the photocell)

.

A five-place digital voltmeter is used to measure the output of the

operational amplifier.

Three selenium photocells were used in this test and the deviation

from linearity of the response of the detection system with each photocell was determined. This determination was made on a horizontal bar

photometer by an inverse square method. The photocell was mounted on the bar at a known distance from a stable monoplane straight-wire filament lamp. The filament was approximately 2 centimeters square. The

photocell was moved along the bar and its response noted at various

distances from the lamp. The deviation of the response from that which

one would expect by applying the inverse square law was attributed to

the nonlinearity of the detection system. Distances used were 0.63

meters to h.l'J meters. The lamp was operated at two voltages in order

to cover the desired range within these distances. For illuminance at

the detector of 3 lux to 800 lux, maximum deviations from linearity for

two of the detectors were -k.5% and for the third detector the maximum

deviation from linearity was +6.5%.

U.3 Standards

A group of six 300-watt opal bulb lamps (Group NBS 526) operating at

a color temperature of approximately 2T20 K serves as the NBS standard

of luminous flux. The lamps were calibrated by measuring their luminous

intensity distributions relative to their luminous intensity in a single

direction. The luminous intensity in the single direction was measured

by comparison with the NBS standard of luminous intensity. The total

luminous flux was then determined by integrating this luminous intensity

with respect to angle of view. This work was done about 30 years ago

and some check measurements were made about lU years ago and about

5 years ago. The lamps are used very infrequently to calibrate NBS

working standards.

21
The NBS working standards were recalibrated at the start of this

test by comparing them in six runs, using four photocells (the three

photocells mentioned above and one silicon photodiode), with the 300-

watt opal-bulb lamps. The NBS working standards are listed in the

following table. Each group of standards consists of six lamps.

Lamp

Group

Designation

Nominal

Wattage

NBS 162 500

NBS 6807 200

NBS 3158 100

BS 5^70 60

Bulb

PS 35 clear gas-filled

PS 30 frosted gas-filled

A-21 frosted gas-filled

S 21 clear vacuum

Approximate

Color

TemperFilament ature Lumens

C-7 2800 K 6hh0

C-9 2900 K 3260

C-9 2900 K 16U1

Squirrel 21+50 K hk6

Cage

Corrections for nonlinearity of the detection system were applied.

These corrections varied between -0.58% and +1.25%. In addition, corrections for the difference in the lamp absorption between the opal-bulb

lamps and the working standards were determined and applied. In order

to determine the correction due to lamp absorption, a 200-watt lamp was placed at the bottom of the sphere and held at constant voltage. Light

from the 200-watt lamp was baffled from the sphere window and from

unlighted lamps placed in turn at the center of the sphere. Readings

were made of the detector output when each of the opal-bulb lamps

(unlighted) was in the socket at the center of the sphere and again

when each of the working standards (unlighted) was in the socket at the

center of the sphere. The correction is the ratio of the average reading

taken with each group of working standards to the average reading taken

with the opal-bulb lamps in the sphere. For these lamps the corrections

varied between -0.l6% and -0.32%.

k.k Calibration of the Test Lamps

The test lamps were calibrated in groups of approximately twenty.

Three measurements were made on each lamp in the group, one on each of

three days. Each lamp was placed in the sphere and allowed to operate

at its designated current or voltage for ten minutes before measurements

of voltage, current, and luminous flux were made. The lamps were

measured in the following order: 2 standards, one half of the test

lamps, 2 standards, the other half of the test lamps, and the remaining

two standards. The lamps submitted by any single laboratory were distributed throughout the group and not measured successively. The three

photocells described above were used, one on each of the three days.

Test lamps were compared with standards which were closest to them in

wattage. Corrections for the non-linearity of the photocell were applied.

These corrections were greatest for the 10-watt lamps. For one photocell

the correction was as high as 2.U%. Corrections for the difference in

22
absorption were determined by the same method as described for the calibration of the working standards. Absorption corrections ranged from

+0.66$ to -0.07$.

U.5 Uncertainties

The geometrically total luminous flux output of the prime reference

group (the six 300-watt opal-bulb lamps) was obtained by measuring the

luminous intensity of the lamps in a large number of directions relative

to the intensity in a reference direction (distribution photometry).

Determination of the luminous intensity of the lamp in this reference

direction, together with integration of these relative intensity measurements then allowed computation of the total luminous flux output of the

lamps. Distribution measurements were made three times, twice in 1938

and once in 1958, on three different instruments. No detailed error estimates were made for these measurements. In the absence of more

fundamental error estimates, the maximum observed range among the three

sets of measurements, for any member of the prime reference group, will

be taken as the estimate of the uncertainty of the distribution photometry measurements. This range is 0.7$.*

The uncertainties associated with the direct substitution comparison of the luminous intensities of two identical inside-frosted lamps

have been discussed in section 2 . U L3]. For this type of direct substitution transfer the uncertainty assigned is k.1% relative to SI (individual

components combined in quadrature). For the luminous intensity determination on the prime reference flux group, an additional uncertainty

must be allowed because of the difference between the luminous intensity

standard lamps and the flux standard lamps. Specific differences that

will affect the measurements are: (l) the different physical sizes of

the lamps (23 cm long, PS-UO bulbs for the flux lamps versus 17.8 cm long, T-20 bulbs for the luminous intensity standards); (2) different

spectral distributions (2720 K for the flux lamps versus 2856 K for the

luminous intensity standards); and (3) inability to repeat the alignment

of the flux lamps due to inadequate reference positions on the lamps.

Inverse square law considerations lead to an estimate of 0.5$ for the

uncertainty introduced by the differing physical sizes of the lamps.

Spectral differences of the magnitude encountered here have been

observed to produce differences of up to 0.25$ in other similar luminous

intensity measurements. Finally, misalignment, particularly a systematic

difference in alignment between the distribution photometers and the

photometers used for the luminous intensity determinations might produce

an error of 0.25$ (based on a study of the distribution curves and an estimated ability to align the lamps to ±2°). It is unlikely that these

uncertainties would have the same sign and therefore, they are combined

in quadrature. This results in an uncertainty relative to SI of k .2% .

*The uncertainties being discussed are listed in the table on page 25.

23
The -uncertainty of transfer from the prime reference flux group to

the NBS working standards may be divided into three parts, those

uncertainties due to: (l) geometric effects; i.e., the presence of

objects, such as the lamp, its support, and the baffle, in the sphere

and their geometrical arrangement; (2) spectral effects, i.e., differences in the spectral distribution of the lamps being compared or a difference between the spectral response of the detector being used and

the CIE spectral luminous efficiency function; and (3) random variations

of the measurements. No detailed analysis of the geometric and spectral

effects have been made. However, regarding the geometric effects, differences as large as 1% have been observed when using a sphere geometry

described above compared to a very different geometry requiring more

objects in the sphere during the measurements. An allowance of 1% has

been made for this source of error. Uncertainties due to the spectral

effects have not been extensively evaluated. An allowance of 1% is the

best guess estimate for these. For random variations, the standard

deviation of a single measurement obtained from a population of lUU

measurements (6 measurements on each of the 6 lamps on each of the h

groups of working standards) was 0.lU%. The uncertainty assigned for

the random variations was three times the standard deviation of the

mean of the 6 measurements on each lamp or 0.2%. These uncertainties

combined in quadrature give an uncertainty of l.k% relative to the

prime reference group and when combined in quadrature with the SI

uncertainty of the prime reference group yield k.k%.

The uncertainty of the transfer from the NBS working standards to

test lamps supplied to other laboratories depends on the size and

operating color temperature of the test lamps relative to the standard

group used to calibrate them. For the test lamps which are the same size and color temperature as the standards used, the uncertainty is

estimated from the random variations. This was computed as three times

the standard deviation of the mean of the three measurements made of

each of the test lamps and is 0.k%. For the uncertainty of the transfer

from NBS working standards to test lamps of other sizes and color temperatures , additional uncertainties as large as 1.0% for geometric

differences and 1.0% for spectral differences must be included. Combining these in quadrature results in an uncertainty of 1.5%.

The total uncertainties of the test lamps relative to SI or to the

NBS prime reference group are obtained by combining in quadrature either

the above 0.4% or 1.5% with the k.k% or the 1.4% referred to above and

listed in the table. The resulting SI total uncertainty is 4.4% to 4.7%

and the total uncertainty relative to the prime reference group is 1.5%

to 2.0%. The specific value depends on the relative size and color

temperature of the test lamps and the NBS working standard lamps used to

calibrate them.

24
TABLE OF UNCERTAINTIES

Uncertainty of the Prime Reference Group

Uncertainty of the distribution measurements 0.7%

Uncertainty of the NBS luminous intensity scale

relative to SI (individual uncertainties

combined in quadrature) k.1%

Additional uncertainty of the luminous intensity

transfer because of:

(1) different physical sizes of the lamps 0.5%

(2) spectral differences 0.5%

(3) alignment of flux lamps 0.25%

Total uncertainty relative to SI U.2%

Uncertainty of the NBS Working Standards

Uncertainty of the transfer from the prime

reference group because of:

(1) geometric differences 1.0$

(2) spectral differences 1.0%

(3) random variations 0.2%

Total uncertainty relative to the NBS prime

reference group l.k%

Total uncertainty relative to SI k.k%

Uncertainty of the Test Lamps

Uncertainty of the transfer from the NBS

working standards

(1) for lamps of the same size and color

temperature as the working standards 0.k%

(2) for lamps of other sizes and color

temperatures 1.5%

Total uncertainty relative to SI k.k% to U.7%*

Total uncertainty relative to the

NBS prime reference group 1.5% to 2.0%

Total uncertainty relative to the NBS working standards 0.k% to 1.5%

*depending on the relative size and color temperature of the test lamps

and the NBS working standard lamps used to calibrate them.

25
NBS participated in the fifth international intercomparison of

national standards of luminous flux in 19 69. The results of this intercomparison [k] showed the NBS luminous flux standards to be 0.7% above

world mean. However, later investigations have shown that the technique

used for calibrating the lamps for this intercomparison resulted in the

assignment of luminous flux values which were low by 0.6% relative to

the technique used in the present test. Thus the values assigned to the

lamps calibrated at this time are estimated to be higher than world

mean by 1.3$.

The NBS 200-watt group of working standards (group NBS 6807 ) and

200-watt lamps of the same type which have been submitted for calibration several times appear to be drifting in luminous flux output relative to lamps of other sizes and relative to the NBS prime reference

group (approximately 1% since 1962). Therefore, these lamps should be

checked frequently.

h.6 Care and Handling

Lamps supplied to other laboratories as standards of luminous flux

represent the unit of luminous flux (lumen) as maintained at the

National Bureau of Standards with the best precision now available. They

are expensive laboratory equipment and deserve the utmost care in handling

and use.

In addition to some obvious precautions, such as keeping the lamps

clean and avoiding mechanical shock to the filament, the following are

a few precautions which should be used.

1. The lamps should be turned on and off slowly (15-30 seconds),

and great care should be taken so that at no time will the current

through a lamp appreciably exceed the value stated in the report.

2. In order to prolong the useful life of the lamps as standards,

it is recommended that they be used sparingly and that for general use,

working standards be prepared by calibrating them relative to the lamp

standards supplied by NBS. When a laboratory procures standard lamps

from NBS, it is well to obtain at least three such lamps in order to be

able to detect any changes that may occur.

3. A baffle should be used between the lamp and the observation

window. The baffle should be as small as possible but large enough to

prevent any light from the lamp reaching the window without undergoing

at least one reflection. The baffle surface should be coated with the

same material as the sphere wall.

h. The variation of luminous flux of a tungsten filament lamp is

approximately related to the variation in its current by the formula:

[5,6]

d£_ _ K di

* i

26
where (j> is the luminous flux, i is the lamp current and K is 6.25 for

gas-filled lamps and 6.05 for vacuum lamps. Thus a current measurement

accuracy of at least 0.032% is required if it is not to affect the

luminous flux to greater than 0.2%.

5. Most detectors are not linear in their response. Detectors of

a single type made by a single manufacturer sometimes differ significantly

in this respect. If lamps of different flux outputs are to be compared,

the response of the detector should be studied to determine its departure

from linearity.

6. Most sphere coatings are somewhat spectrally selective in

reflectance. Diffusing windows commonly used in spheres also have

spectrally selective transmittances . Corrections should be made when

comparing lamps operating at different color temperatures. In addition

to this, a detector whose spectral response agrees with the CIE luminous

efficiency function should be used.

7. Lamps of different physical size and blackening absorb different

amounts of the flux reflected to them by the sphere wall. The amount of

flux absorbed also varies with the size of sphere being used and with the

reflectance of its coating. Therefore, when lamps of different physical

size, bulb material and blackening are being compared, corrections for

the lamp absorption should be made. The absorption correction should be

determined in the sphere in which the comparison is made.

8. When comparing lamps which are nearly identical in physical

size, absorption, luminous flux, and color temperature, the corrections

listed above in 5 S 6, and 7 may not be significant.

k.7 References

[l] V. K. Zworkin and E. G. Ramberg, Photoelectricity , Chapter 11,

John Wiley & Sons, Inc., New York, N.Y. (19^9).

[2] IES Lighting Handbook, Fourth Edition, pp. 1-3, 2-U (1966),

Illuminating Engineering Society, New York, N.Y.

[3] National Bureau of Standards, Technical News Bulletin, p. UU,

(February 1972)

.

[h] Bureau International des Poids et Mesures, Rapport Sur La

Cinquieme Comparaison des Etalons Nationaux D'Intensite et

de Flux Lumineux (1969).

[5] J. W. T. Walsh, Photometry , p. 532, Dover Publications, Inc.,

180 Varick Street, New York, N. Y. (1965).

[6] A. Kingslake, Applied Optics and Optical Engineering , p. 6l,

Academic Press, New York, N. Y. (1965)

.

27

form NBS-114A u-71)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEETS

1. PUBLICATION OR REPORT NO. NBS TN-594-3

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

OPTICAL RADIATION MEASUREMENTS:

Photometric Calibration Procedures

5. Publication Date November 1972

6. Performing Organization Code

7. AUTHOR(S)

Velma I. Burns and Donald A. McSparron

8. Performing Organization

10. Project/Task/ Work Unit No. 9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No . 9 .

13. Type of Report & Period Covered Final

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

The National Bureau of Standards supplies calibrations of luminous intensity,

luminous flux and color temperature on a routine basis. The procedures, equipment

and techniques used to perform these calibrations as. of October 1972 are described.

Details of the uncertainty information currently available, including estimates

and procedures for determining uncertainties of the reported values, are also

presented.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Calibration procedures; color temperature; luminous flux; luminous intensity;

photometry; uncertainty

»

18. AVAILABILITY STATEMENT

IXI UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

35

22. Price

.60<j>

USCOMM-DC 66244-P7I

NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National

Bureau of Standards research and development in

physics, mathematics, and chemistry. Comprehensive

scientific papers give complete details of the work,

including laboratory data, experimental procedures,

and theoretical and mathematical analyses. Illustrated with photographs, drawings, and charts. Includes

listings of other NBS papers as issued.

Published in two sections, available separately:

• Physics and Chemistry

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $9.50; $2.25

additional for foreign mailing.

• Mathematical Sciences

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design,

numerical analysis, theoretical physics and chemistry, logical design and programming of computers

and computer systems. Short numerical tables. Issued

quarterly. Annual subscription: Domestic, $5.00;

$1.25 additional for foreign mailing.

TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's measurement, research, developmental, co- operative, and publication activities, this monthly

publication is designed for the industry-oriented

individual whose daily work involves intimate contact with science and technology

for engineers, chemists,

physicists, research managers, product-development

managers, and company executives. Includes listing of

all NBS papers as issued. Annual subscription: Domestic, $3.00; $1.00 additional for foreign mailing.

Bibliographic Subscription Services

The following current-awareness and literature- survey bibliographies are issued periodically by

the Bureau: Cryogenic Data Center Current

Awareness Service (weekly), Liquefied Natural

Gas (quarterly), Superconducting Devices and

Materials (quarterly), and Electromagnetic

Metrology Current Awareness Service (monthly) . Available only from NBS Boulder Laboratories. Ordering and cost information may be

obtained from the Program Information Office,

National Bureau of Standards, Boulder, Colorado 80302.

NONPERIODICALS Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries,

professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality, and methods for testing various industrial products. These standards are developed co- operatively with interested Government and industry

groups and provide the basis for common understanding of product characteristics for both buyers and

sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other-agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards

Publications. This series is the official publication

within the Federal Government for information on

standards adopted and promulgated under the Public

Law 89—306, and Bureau of the Budget Circular A—86

entitled, Standardization of Data Elements and Codes

in Data Systems.

Consumer Information Series. Practical information, based on NBS research and experience, cover- ing areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's tech- nological marketplace.

CATALOGS OF NBS PUBLICATIONS NBS Special Publication 305, Publications of

the NBS. 1966-1967. When ordering, include

Catalog No. CI 3.10: 305. Price $2.00; 50 cents additional for foreign mailing.

NBS Special Publication 305, Supplement 1, Publications of the NBS, 1968-1969. When ordering, include Catalog No. C13.10:305/Suppl. 1. Price

$4.50; $1.25 additional for foreign mailing.

NBS Special Publication 305, Supplement 2,

Publications of the NBS, 1970. When ordering, include Catalog No. C13.10:305/Suppl. 2. Price $3.25; 85 cents additional for foreign mailing.

Order NBS publications (except Bibliographic Subscription Services)

from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

215