NBS SPECIAL PUBLICATION 260-51

y*£AU of

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

GLASS FILTERS AS A STANDARD REFERENCE

MATERIAL FOR SPECTROPHOTOMETRY--

SELECTION, PREPARATION, CERTIFICATION,

USE SRM 930

C.3L
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OP STANBARDB

LIJJRASY Standard Reference Materials: NOV 2 6 1975

Glass Filters as a Standard Reference Material for

Spectrophotometry— Selection, Preparation, Certification,

Use SRM 930

R. Mavrodineanu

J. R. Baldwin

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued November 1975
Library of Congress Cataloging in Publication Data

Mavrodineanu, Radu, 1910- Glass Filters as a Standard Reference Material for Spectro- photometry.

(Standard Reference Materials) (NBS Special Publication260-51) "CODEN : XNBSAV."

Bibliography: p. Supt. of Docs. No.: C 13.10:260-51.

1. Spectrophotometer—Standards. 2. Light niters—Standards.

3. Glass, Optical—Standards. I. Baldwin, John Raymond, 1918-

Joint Author. II. Series. United States. National Bureau of Standards. Standard Reference Materials. III. Series: United

States. National Bureau of Standards. Special Publication; 260-51 QC100.U57 No. 260-51 [QC465] 389'.08s [535'.84'028] 75-619308

National Bureau of Standards Special Publication 260-51

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-51, 118 pages (Nov. 1975) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:260-51). Price $1.90 (Add 25 percent additional for other than U.S. mailing).
PREFACE

Standard Reference Materials (SRM's) as defined by the

National Bureau of Standards are "well-characterized materials, produced in quantity, that calibrate a measurement

system to assure compatibility of measurement in the nation."

SRM's are widely used as primary standards in many diverse

fields in science, industry, and technology, both within the

United States and throughout the world. In many industries

traceability of their quality control process to the national

measurement system is carried out through the mechanism and

use of SRM's. For many of the nation's scientists and technologists it is therefore of more than passing interest to

know the details of the measurements made at NBS in arriving

at the certified values of the SRM's produced. An NBS series

of papers, of which this publication is a member, called the

NBS Special Publication - 260 Series is reserved for this

purpose

.

This 260 S er i e s i s d e d i c a t e d to th e d i s s em ina t i on of

inf o rma t i on on all phases of th e prep ar a t ion , measurement

,

and certificat ion o f NBS- SRM ' s . In g en e r al, much more d e - tai 1 will be f ound in the s e p ap e r s t h an i s g enerally allowed

or d e s i r ab le , in s c ient if ic j o u r na 1 a r t i c 1 e s This enables

the user to as s e s s the va lidi ty and a c c ur acy of the measuremen t processes emp 1 oyed , to j ud ge the s ta t is tical analysis,

and to learn d e tail s o f t e c hn iq ue s an d me tho ds utilized for

wor k entailing the g r ea t e st c ar e and a c c u r a c y. It is also

hop e d that the s e pa p e r s w ill pr ov ide s u f f ic i ent additional

information not found on the certificate so that new applications in diverse fields not foreseen at the time the SRM

was originally issued will be sought and found.

Inquiries concerning the technical content of this

paper should be directed to the author(s). Other questions

concerned with the availability, delivery, price, and so

forth will receive prompt attention from:

Office of Standard

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.

Reference Materials

S tandar d s 20234

J. Paul Cali, Chief

Office of Standard Reference Materials

i i i
OTHER NBS PUBLICATIONS IN THIS SERIES

NBS Spec. Publ. 260, Standard Reference Mate- NBS Misc. Publ. 260-12, Standard Reference

rials: 1973 Catalog, April 1973, $1.25.* Materials: Homogeneity Characterization of

NBS Misc. Publ. 260-1, Standard Reference NBS Spec trometric Standards III: White Cast

Materials: Preparation of NBS White Cast Iron and Stainless Steel Powder Compact,

Iron Spectrochemical Standards, June 1964. September 1966. NBS Misc. Publ. 260-12.**

COM 74-11061.** NBS Misc. Publ. 260-13, Standard Reference

NBS Misc. Publ. 260-2, Standard Reference Materials: Mossbauer Spectroscopy Standard

Materials: Preparation ''of NBS Copper-Base for the Chemical Shift of Iron Compounds,

Spectrochemical Standards, October 1964. July 1967. NBS Misc. Publ. 260-1 3.**

COM 74-11063.** NBS Misc. Publ. 260-14, Standard Reference

NBS Misc. Publ. 260-3, Standard Reference Materials: Determination of Oxygen in FerMaterials: Metallographic Characterization of rous Materials - SRM 1090, 1091, and 1092, an NBS Spectrometric Low-Alloy Steel September 1966. NBS Misc. Publ. 260-14.**

Standard, October 1964. COM 74-11060.** NBS Misc. Publ. • 260-15, Standard Reference

NBS Misc. Publ. 260-4, Standard Reference Materials: Recommended Method of Use of

Materials: Sources of Information on Stand- Standard Light-Sensitive Paper for Calibratard Reference Materials, February 1965. ing Carbon Arcs Used in Testing Textiles for

COM 74-1 1059.** Colorfastness to Light, June 1967. (Replaced

NBS Misc. Publ. 260-5, Standard Reference by Spec. Publ. 26041.)

Materials: Accuracy of Solution X-Ray Spec NBS Spec. Publ. 260-16, Standard Reference

trometric Analysis of Copper-Base Alloys. Materials: Homogeneity Characterization of

March 1965. 25 cents. (Out of print). NBS Spectrometric Standards IV: PreparaNBS Misc. Publ. 260-6, Standard Reference tj on an d Microprobe Characterization of

Materials: Methods for the Chemical Analysis W-20% Mo Alloy Fabricated by Powder Metof White Cast Iron Standards, July 1965. allurgical Methods, January 1969. COM

COM 74-1 1068 ** 74-11062.**

NBS Misc. Publ. 260-7, Standard Reference NBs Spec. Publ. 260-17, Standard Reference

Materials: Methods for the Chemical Analysis Materials: Boric Acid; Isotopic and Assay

of NBS Copper-Base Spectrochemical Stand- Standard Reference Materials, February

ards, October 1965. COM 74-1 1067.** 1970 . 65 cents.*

NBS Misc. Publ. 260-8, Standard Reference NBS Spec Publ 260-18, Standard Reference

Materials: Analysis of Uranium Concentrates Materials: Calibration of NBS Secondary

at the National Bureau of Standards, Decern- Standard Magnetic Tape (Computer Ampliber 1965. COM 74-11066.** tu de Reference) Using the Reference Tape

NBS Misc. Publ. 260-9, Standard Reference Amplitude Measurement "Process A," NoMaterials: Half Lives of Materials Used in the vember 1969. (See Spec. Publ. 260-29.)

Preparation of Standard Reference Materials NBS Spec. Publ. 260-19, Standard Reference

of Nineteen Radioactive Nuclides Issued by Materials: Analysis of Interlaboratory Meathe National Bureau of Standards, November surements on the Vapor Pressure of Gold

1965. COM 74-1 1065.** (Certification of Standard Reference Material

NBS Misc. Publ. 260-10, Standard Reference 745), January 1970. PB 190071.**

Materials: Homogeneity Characterization on NBS Spec. Publ. 260-21, Standard Reference

NBS Spectrometric Standards II: Cartridge Materials: Analysis of Interlaboratory MeaBrass and Low-Alloy Steel, December 1965. surements on the Vapor Pressures of CadCOM 74-11064.** mium and silver, January 1971. COM

NBS Misc. Publ. 260-11, Standard Reference 74-11359.**

Materials: Viscosity of a Standard Lead-Silica NBS Spec. Publ. 260-22, Standard Reference

Glass, November 1966. NBS Misc. Publ. Materials: Homogeneity Characterization of

260-10** Fe-3Si Alloy, February 1971. COM

74-11357**

IV
NBS Spec. Publ. 260-23, Standard Reference NBS Spec. Publ. 260-35, Standard Reference

Materials: Viscosity of a Standard Boro- Materials: Thermal Conductivity of Austensilicate Glass, December 1970. 25 cents.* itic Stainless Steel, SRM 735 from 5 to NBS Spec. Publ. 260-24, Standard Reference 280 K, April 1972. 35 cents.*

Materials: Comparison of Redox Standards, NBS Spec. Publ. 260-36, Standard Reference

January 1972. $1.* Materials: A Referee Method for the DeterNBS Spec. Publ. 260-25, Standard Reference mination of Calcium in Serum. SRM 915,

Materials: A standard Reference Material May 1972.COM 72-50527.**

Containing Nominally Four Percent Austen- NBS Spec. Publ. 260-37, Standard Reference

ite, February 1971. COM 74-11356.** Materials: Methods of Analysis of NBS Clay

NBS Spec. Publ. 260-26, Standard Reference Standards, June 1972. COM 72-50692.**

Materials: National Bureau of Standards-U.S. NBS Spec. Publ. 260-38, Standard Reference

Steel Corporation Joint Program for Deter- Materials: Preparation and Calibration of

mining Oxygen arid Nitrogen in Steel, Feb- Standards of Spectral Specular Reflectance,

ruary 1971. 50 cents.* May 1972. 60 cents.*

NBS Spec. Publ. 260-27, Standard Reference NBS Spec. Publ. 260-39, Standard Reference

Materials: Uranium Isotopic Standard Refer- Materials: The Eddy Current Decay Method

ence Materials, April 1971. COM for Resistivity Characterization of High74-1 1358.** Purity Metals, May 1972. 55 cents.*

NBS Spec. Publ. 260-28, Standard Reference NBS Spec. Publ. 260-40, Standard Reference

Materials: Preparation and Evaluation of Materials: Selection of Thermal Analysis

SRM's 481 and 482 Gold-Silver and Gold- Temperature Standards Through a CooperaCopper Alloys for Microanalysis, August tive Study (SRM 758, 759, 760), August

1971. COM 71-50365.** 1972. 65 cents.*

NBS Spec. Publ. 260-29, Standard Reference NBS Spec. Publ. 260-41, Standard Reference

Materials: Calibration of NBS Secondary Materials: Use of Standard Light-Sensitive

Standard Magnetic Tape (Computer Ampli- Paper for Calibrating Carbon Arcs used in tude Reference) Using the Reference Tape- Testing Textiles for Colorfastness to Light,

Amplitude Measurement "Process A-Model August 1972. COM 72-50775.**

2," June 1971. COM 71-50282.** NBS Spec. Publ. 260-42, Standard Reference

NBS Spec. Publ. 260-30, Standard Reference Materials: The Characterization of Linear

Materials: Standard Samples Issued in the Polyethylene, SRM 1475, September 1972.

USSR (A Translation from the Russian), 45 cents.*

June 1971. COM 71-50283.** NBS Spec. Publ. 260-43, Standard Reference

NBS Spec. Publ. 260-31, Standard Reference Materials: Preparation and Homogeneity

Materials: Thermal Conductivity of ElectroCharacterization of an Austenitic Iron- lyticIronSRM 734from4to 300 K,Novem- Chromium-Nickel Alloy, November 1972.

ber 1971 COM 71-50563 ** cents. NBS Spec. Publ. 260-32, Standard Reference NBS

^

S Pec

: P"bL 26044' Standard Reference

Materials: Standard Quartz Cuvettes For

Materials: Preparation,and Use;of SuperconHigh Accuracy Spectrophotometry. Decernduct

,lve l°mt DeviCes' SRM 767' De" ber 1973 55 cents * cember 1972" 75 cents-* NBS Spec. Publ. 260-33, Standard Reference NB^

S P

ec

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Materials: Comparison of Original and SupMateriaIs: Powdered Lead-Based Paint, SRM

plemental SRM 705, Narrow Molecular NTJ I

' p w o^'n £ c+ r**

Weight Distribution Polystyrene, H. L. Spec. Publ. 260-46 Standard Reference

Wagner, May 1972. COM 72-50526.** Materials: Thermal Conductivity and ElectnNBS Spec. Publ. 260-34, Standard Reference f

Standard Reference Materials:

Materials- Thermoelectric Voltage Anril

Austenitlc Stainless Steel, SRM's 735 and

1972 COM 72 50371 .** 798' from 4 to 1200 K' March 1975- $L0°-
NBS Spec. Publ. 260-47, Standard Reference

Materials: Electrical Resistivity of Electrolytic Iron, SRM 797, and Austenitic Stainless

Steel, SRM 798, from 5 to 280K, February

1974. 55 cents.*

NBS Spec. Publ. 260-48, Standard Reference

Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory,

SRM 933 and SRM 934, May 1974. 60

cents.*

NBS Spec. Publ. 260-49, Standard Reference

Materials: Calibrated Glass Standards for Fission Track Use, November 1974. 75 cents.*

S Spec. Publ. 260-50, Standard Reference

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials:

Electrolytic Iron, SRM's 734 and 797 from 4

to 1000 K, June 1975. $1.00

NBS Spec. Publ. 260-51, Standard Reference

Materials: Glass Filters As a Standard Refer- ence Material for Spectrophotometry; Selection, Preparation, Certification, Use - SRM

930. (In press)

NBS Spec. Publ. 260-52, Standard Reference

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials

730 and 799, from 4 to 3000 K. (In press)

*Send order with remittance to: Superintendent of Documents, U.S. Government Printing Offic

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22151

vi
CONTENTS

Page

1. GENERAL 2

2. INTRODUCTION. . 4

Factors Affecting the Proper Functioning of

a Spectrophotometer . . .

3. MEANS AND PROCEDURES TO ASSESS AND CONTROL SOME OF

THE FACTORS WHICH AFFECT THE PROPER FUNCTIONING

AND ACCURACY OF A SPECTROPHOTOMETER

5 3.1 The Short and Long Term Stability 5

3.2 Wavelength Accuracy 11

3.3 Spectral Bandpass 13

3.4 Stray Radiations 17

3.5 Accuracy of the Transmittance or Absorbance

Scales .......... 17

4. STANDARD REFERENCE MATERIALS FOR TRANSMITTANCE IN

SPECTROPHOTOMETRY 21

4.1 Selection and Preparation of Materials Which

Can Be Used as Standard Reference Material

in Spectrophotometry ..... 21

4.2 Preparation of the Glass Filters ...... 24

4.3 Packaging of the Glass Filters 31

5. RESULTS 31

5.1 Stability of SRM 930 31

5.2 Interlaboratory Transmittance Measurements

on SRM 930 37

6. SUMMARIZING REMARKS 4 4

7. REFERENCES 4 7

8. APPENDICES

.

48

9. ACKNOWLEDGEMENT 4 8

vii
LIST OF TABLES

Table Page

1. Influence of temperature on the transmittance

(T) of seven Schott NG-4 glass filters at five

wavelengths (nm) 32

2. Percent transmittance of three Schott NG-4

neutral glass filters before and after exposures

to accelerated radiation tests 33

3. Stability of transmittance (T) as a function of

time, measured on SRM 930 35

4. Comparison of percent transmittance (%T)

measurements performed on a set of three Schott

NG-4 glass filters over a period of two years. . . 36

5. Comparison between the percent transmittances

(%T) measured on three Schott NG-4 glass filters

at NPL and NBS in 1971 , 1973 , and 1975 38

6. Comparative transmittance measurements performed

by four laboratories on three Schott NG-4 colored

glass filters 39,40

7. Transmittance of SRM 930, set 106, after two

years of use in a laboratory 42

8. Comparison of %T measurements performed on a Schott NG-4 filter using the NBS high-accuracy

instrument and a conventional spectrophotometer. . 43

LIST OF FIGURES

Figure Page

la, Control chart for a spectrophotometer showing

lb. the variation of transmittance as a function

of time for a neutral glass filter at 635.0 nm

(a) and 590.0 nm (b) and 24.0 °C 6

2. Transmittance of a didymium glass filter 9

3. Transmittance of a holmium oxide glass filter. . . 10

4. Spectral characteristics for five transparent

materials from 200 nm to 700 nm 12

viii
LIST OF FIGURES (continued)

Figure Page

5. Influence of spectral bandpass on the trans

mittance of a Chance ON-10 glass filter measured

at 400 nm; 44 5 nm; 49 0 nm; and 590 nm 14

6. Influence of spectral bandpass on the transmittance of a Schott NG-4 glass filter measured

at 440 nm; 465 nm; 590 nm; and 635 nm 15

7. Illustration of the relation between radiation

flux intensities F and corresponding photocurrent

I in an ideal case 19

8. Instrumentation to examine, clean, and mount the

glass filters used to produce SRM 930, a, b. . . . 23

9. Stainless steel holder for six 50 mm by 50 mm,

and twelve 30 mm by 10 mm glass filters 25

10. Soxhlet extractors used for the final cleaning

of the 50 mm by 50 mm, and 30 mm by 10 mm glass

filters, using isopropyl alcohol (electronic

grade) 26

11. Schematic description of the aluminum alloy

holder with its bronze retaining spring used

to hold the glass filters 28

12. View of the assembled and unassembled glass

filter holder 29

13. Plastic box for SRM 930b containing the three

glass filters in their holders 30

ix

Glass Filters as a Standard Reference Material

for Spectrophotometry

Selection, Preparation, Certification, Use

SRM 930

R. Mavrodineanu and J. R. Baldwin

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

This publication describes various factors which can

affect the proper functioning of a spectrophotometer and

suggests means and procedures to assess and control these

factors. Particular consideration is given to the long and

short term stability of a spectrophotometer, to the wavelength accuracy, the spectral bandpass, the stray radiation,

and the accuracy of the transmit tance or absorbance scale.

A description is given of the means and Standard Reference

Materials (SRM's) which can be used to control these factors,

together with the methods for the preparation, certification,

and use of such materials (SRM 930) . The results obtained

in actual use of SRM 930 are examined in some detail. An

Appendix contains the reproduction of several publications

relevant to the subject discussed in this work.

Key Words: Accuracy in spectrophotometry; glass filters;

spectral bandpass; spectrophotometry; standard reference

materials; stray radiation; testing of spectrophotometers;

transmittance (absorbance) accuracy; wavelength accuracy.

l
1. GENERAL

Among the measuring techniques available to the analyst

for the detection and determination of a large number of

chemical species in a large variety of matrices, spectrophotometry constitutes a valuable tool of major significance.

This is due largely to the inherent characteristics of high

sensitivity and good specificity of the method, associated

with the need for only relatively simple instrumentation and

straightforward sample preparation methodology. For these

reasons spectrophotometric procedures are used in practically

all fields of science, where chemical analytical measurements

are required. As mentioned by Rand (1) of 147 analytical procedures used in clinical chemistry laboratories, 84 require

the use of spectrophotometry, and of 4S0,000 analyses per

year, performed only in one hospital, 300,000 required measurements by spectrophotometric procedures. In such a laboratory, 800 to 1,000 determinations are made daily by

spectrophotometry

.

From the information given by Rand in 1972 (1) , a conservative estimate of analyses made by spectrophotometry

indicates that more than 1,000,000 such tests are performed

daily in the clinical laboratories in this country, and this

figure increases by approximately 15 percent per year.

These considerations refer to clinical laboratories

alone, and to these one should add the numerous other areas

such as the general field of scientific research, industry,

agriculture, environmental sciences, etc., where spectrophotometry represents a major analytical technique.

In spite of this wide use of spectrophotometry, no suitable standards were available to verify the stability of the

spectrophotometer, and no adequate reference material was

available to verify the accuracy of the transmittance or

absorbance scale of the instrument. This situation is even

more surprising when one considers the wide discrepancies

obtained in interlaboratory comparisons of spectrophotometric

data (2,3,4,5,6,7). Under these circumstances a program was

initiated at the National Bureau of Standards (NBS) in the

Analytical Chemistry Division of the Institute for Materials

Research in July 1969, with the objective to provide the

analytical chemist with means to verify that his spectrophotometer functions properly and that it produces reliable and

meaningful data. This decision was taken in response to the

requests received from scientists using spectrophotometry in

all areas of science, and more specifically from the analytical chemists who use spectrophotometry in clinical and

pathological laboratories (8)

.

2
The planned program was implemented in 1970 through the

design and construction, in the Analytical Chemistry Division,

of a high-accuracy spectrophotometer (8,10) which was used to

produce two Standard Reference Materials (SRM's), SRM 930 (a

and b) , Glass Filters for Spectrophotometry, and SRM 931,

Liquid Filters for Spectrophotometry. These SRM's were issued

in 1971. A third SRM 932, Quartz Cuvettes for Spectrophotometry, was added in 1973 and was produced in association with

the Optical Physics Division of the Institute for Basic Standards at NBS. SRM 932 was intended to be used in association

with SRM 931 and for all measurements requiring accuracy in

the determination of transmittance in solutions. A detailed

description of this SRM and of the techniques developed at

NBS for its production and certification is given in reference

9 which is reproduced in the NBS Special Publication 260-32,

issued in December 1973. This publication is available from

the U. S. Government Printing Office, Washington, D.C. 20402,

under SD Catalog No. C13 . 10 : 260 - 32 , price 55 cents.*

SRM 930 (a,b) consists of a set of three glass filters

having nominal transmittances of 10, 20, and 30 percent from

440 nm to 635 nm. These glasses are calibrated and certified

for optical transmittance at 440.0 nm; 465.0 nm; 590.0 nm;

and 635.0 nm. These wavelengths were selected in accordance

with the transmission characteristics of the glass. An

additional certification at 546.1 nm, corresponding to the

mercury green radiation, was provided with the more recent

SRM 930b. SRM 930 is a transfer standard reference material

which can be used to determine and monitor the short and long

term stability of a spectrophotometer and to verify the

accuracy- of its transmittance and absorbance scales . This publication is intended as a practical guide for

the proper calibration of spectrophotometers and the use of

SRM 930 and contains data presented in a simple and straightforward manner. The reader interested in the more fundamental

problems associated with high-accuracy spectrophotometry and

luminescence measurements is advised to consult the publications listed in reference 10 together with the papers

mentioned in those publications. Reprints from several papers

closely related to the subject discussed in this publication

are reproduced here in the Appendix Section as a convenience

to the reader.

See Appendix 1 for NBS -SRM ordering information.

3
2. INTRODUCTION

Factors Affecting the Proper Functioning of a Spectrophotometer

There are a number of factors which can affect the proper

functioning of a spectrophotometer and prevent the acquisition

of meaningful data. Some of these factors are:

a. The short and long term stability.

It is related to the instruments mechanical, optical, and

radiation measuring design and to the quality of the materials

and workmanship used to implement the design. The conditions

under which the instrument is operated, such as vibration,

humidity and temperature, dust and corrosion, will also affect

its stability. Other critical factors are the short and long

term stability of the radiation source, the reproducibility

of the positioning of the sample holder and carriage, and the

stability of the read-out system.

b. The wavelength accuracy.

This is a function of the stability of the optical system

and of the accuracy of the wavelength scale, or wavelength

cam, and its reproducibility.

c. The spectral bandpass of a spectrophotometer.

This is the spectral interval AX in wavelength which dX

emerges through the exit slit w of the instrument: AX = w t—, where is the linear dispersion of the spectrophotometer.

It is a parameter which depends on the optical design and of

the associated elements: grating, prism, or filters used in

the instrument. The magnitude of the spectral bandpass for

a specific spectrophotometer over the whole spectral range of

the instrument is important information that is provided by

the manufacturers of the particular instrument.

d. The accuracy of the transmittance or absorbance

scales

.

This is the capability of a spectrophotometer to measure

ratios of radiant fluxes with a known accuracy and to produce

true transmittance (T) or' absorbance (A) measurements . This

fundamental parameter is determined ultimately by the linearity

of the detection system, when the other parameters such as wavelength accuracy, adequate spectral bandpass, and photometric precision are under control.

A
e. Stray radiations.

They can originate in the spectrometer and in the sample

compartment. The stray radiation produced in the spectrometer

is the radiant energy at wavelengths different from those of

the nominal spectral bandpass transmitted through the instrument at a particular wavelength. The stray radiation in the

sample compartment is the radiant energy which falls on the

photosensitive detector without passing through the absorbing

s amp 1 e

.

f. Conditions associated with the stability of the

analytical sample (color reaction) and its homogeneity.

Dissociation and association reactions, radiation scatter

inside the sample, polarization, fluorescence, temperature,

particulate matter, and surface conditions can affect, in a significant manner, the accuracy of the measurements.

These conditions are characteristics of the analytical

sample and of the chemical reactions involved in the spectrophotometry process and are to a large extent independent of

the spectrophotometer used for the measurements. However,

since these factors can affect the precision and accuracy of

the measurement, they are mentioned here to remind the analytical chemist that he must be aware of their existence, and

that he must evaluate their magnitude in the particular analytical process considered before proceeding with the actual

measurement. As a result of his knowledge, he must be able

to select the most adequate analytical procedure for the

matrix and chemical species under consideration, and to

establish ways to eliminate, minimize, or compensate for

these causes of errors.

From the factors mentioned above, those which are relevant to the subject discussed in this publication will be

examined in more detail.

3. MEANS AND PROCEDURES TO ASSESS AND CONTROL SOME OF THE

FACTORS WHICH AFFECT THE PROPER FUNCTIONING AND ACCURACY

OF A SPECTROPHOTOMETER .

3 . 1 Short and Long Term Stability

The short and long term stability of a spectrophotometer can be verified and monitored by using glass filters

specially selected for this purpose. Stable radiation sources,

such as tritium-activated luminescent materials, could be

considered for the same purpose (11) . The use of such a source, however, is not always possible due mainly to problems

associated with the design and operation of the instrument.

5
6
From the' several kinds of glass filters which can be

used, those constituting SRM 930 (a or b) are most suitable.

The optical characteristics of these glass filters are discussed in Section 4; however, since the stability of a spectrophotometer is the first test which should be carried

out, we will describe their use now.

As a general rule, it is desirable to place the spectrophotometer in a clean laboratory and in an area where the

temperature is as constant and reproducible as possible,

protected from direct sunlight, sudden changes of temperature,

corrosive atmosphere, dust, and vibration.

The tests should be carried out after the instrument was

verified by the manufacturer, and was found to perform well

and to meet all the required specifications. After an initial

warming-up period, the "0" and "100" reading on the scale is

verified, following the procedure described by the manufacturer

of the particular instrument, the wavelength dial is set at a selected value, usually the one at which the transmittance of

the analytical sample will be determined, e.g. 635.0 nm, and

one of the glass filters in its metal holder having the

nominal transmittance of 30 percent is inserted in the sample

compartment. The reference compartment or "blank" is left

empty. The transmittance of the glass filter is measured

following the method specified by the manufacturer, and the

results recorded. This procedure is repeated at all wavelengths at which the analytical samples will be measured.

Similar transmittance measurements are performed using the

glass filters having a nominal transmittance of 20 percent

and 10 percent. The temperature at which these measurements

are made should also be recorded. In this manner, the performance of the instrument is determined over the working

spectral range for a transmittance interval and at a temperature which covers most of the conditions of analytical

determinations under consideration.

The values obtained are then displayed in a graphical

form, by plotting the transmittance found for every wavelength

on the ordinate, and on the abscissa the date at which these

measurements were made. An example of such a graph is given

in figures la and b for only two wavelengths: 635.0 nm and

590.0 nm. Similar graphs should be made for the wavelengths

of interest and for the 10 percent, 20 percent, and the 30

percent nominal transmittance filters. Since the laboratory

in which the measurements shown in figure 1 were made is

provided with controls which maintain the surrounding temperature at 24.0 °C ± 1 °C, this was recorded on the graphs as

a common value for all measurements. When solutions are

used, the temperature of the liquid in the cuvette should be

determined. Where the temperature changes from measurement

7
to measurement, its value should be recorded and written on

the graph under the corresponding transmittance value. The

procedure described above should be performed every time the

spectrophotometer is used, before, and preferably after, the

analytical work is carried out.

The data plotted in figures la and lb were obtained with

a commercial spectrophotometer provided with a quartz prism

double spectrometer, and a transmittance scale divided in

1000 units between 0 and 100 transmittance readings. The

size of each division and the stability and reproducibility

of the instrument is such that it permits visual interpolation

between each division, to fractions of a division; hence, the

ordinate scale on the graphs (figs, la and b) was selected

to reflect this resolution.

An examination of the data shows that the stability of

the instrument as expressed by the transmittance value,

operated at 24.0 °C, and from October 22 to February 5, varied

from 32.17 percent T to 32.28 percent T with an average of

32.23 T percent at 635.0 nm. At 590.0 nm the equivalent values

were 30.79 percent T and 30.90 percent T with an average of

30.85 percent T. The spread is in both cases 0.11 percent

of T. This spread results from the inherent instability of

the instrument and of the glass filters used. This situation

changed markedly on February 8 when a transmittance of 31.94

percent and 30.59 was found at 635.0 nm and 590 . 0 nun respectively. Since these values largely exceeded the specified

spread established for this particular instrument by the manufacturer, a field engineer from the manufacturer was called

in. , An examination of the spectrophotometer revealed that a potentiometer was malfunctioning. After replacement of this

faulty part the instrument was tested again on February 18,

and the transmittance values, measured on the same glass

filters, were within the established spread of the instrument,

as seen in figures la and lb, indicating that the spectrophotometer was again performing according to its specifications.

This example illustrates the usefulness of the short and long

term stability tests described in this paragraph since this

permits detection and diagnosis of possible malfunctions in

an instrument before such malfunctions would affect the

validity of measurements. It is obvious that similar control

charts can be prepared using absorbance values.

The periodic verification of the stability of spectrophotometers as described above and illustrated in figures la

and lb, is used in these laboratories routinely. This procedure

is highly recommended to all users of such instruments, and is

performed, as has already been mentioned, with the help of

specially selected glass filters. It should be added here

that the precision of such tests, however, cannot exceed the

8
1.00

90

.80

.70 LUo

1 .60 E

S 50 COz

<£ .40

.30

.20

.10

CORNING 5120 DIDYMIUM GLASS, 3.01 mm THICK

350 360 370 380 390 400 420 440 460 480 500 550

WAVELENGTH. NANOMETERS

600 650

1.00

90|-

.80

.70

|

.60 % .50 z

<

SE .40

8.8 nm

8.4 nm

8.7 nm

3.8 nm

9.7 nm

8.7 nm

j i i i i

' ' ' i_ 0.56 0.60 0 70 0 80 1.00 1.20 1.40 1 60 1 80 2.00 WAVELENGTH, MICROMETERS

220 240 260

Figure 2. Transmittance of a didymium glass filter. The

spectral bandpass used is marked at the corresponding transmittance minimum. Courtesy

Corning Glass Works.

9
Tr an smi t t an ce of a holmium oxide glass filter.

The spectral bandpass used is marked at the corresponding transmi t tance minimum. Courtesy Carning

Glass Works

10
stability of the glass filters; this parameter will be discussed in Section 5.1 for the material used in this work.

3 . 2 Wavelength Accuracy

Although the test discussed in paragraph 3.1, may show

that a spectrophotometer exhibits a good stability, large

errors in the accuracy of its wavelength scale may produce

unreliable measurements.

The wavelength accuracy of a spectrophotometer can be

verified by the use of (a) an emission source capable of

producing discrete radiations of suitable intensity and

adequately spaced throughout the spectral range of interest,

e.g., 200 to 800 nm, or (b) by using glass filters with sharp

absorption bands.

(a) Adequate sources of discrete radiation, of wellknown wavelengths, most suitable for use in the calibration

of the wavelength scale of a spectrophotometer, are the low

pressure quartz discharge tubes containing mercury vapor,

helium, or neon. Such tubes are available commercially from

the manufacturers of laboratory instrumentation at reasonable

prices. The discharge tube has a cylindrical shape with a total length of 120 mm from which a 50 mm portion constitutes

the quartz discharge tube; this tube should be placed immediately in front of the entrance slit of the spectrometer.

(b) Wavelength calibrations can be made by using a glass

filter having a number of strong and narrow absorption or

transmission bands suitably spaced over the spectral range of

interest. Two materials have been used or suggested for this

purpose: glasses containing rare-earth oxides, such as

didymium glasses and holmium oxide glasses. They have been

used for many years at the National Bureau of Standards. The

transmission characteristics of these two glasses are illustrated in figures 2 and 3.

A detailed description of the properties and use of low

pressure discharge tubes and of glass filters with sharp

absorption bands is given in an NBS Letter Circular LC-1017:

"Standards for Checking the Calibration of Spectrophotometers"

by K. S. Gibson, H. J. Keegan, and J. C. Schleter, reissued

in January 1967. Part of this paper is reproduced in the

Appendix Section of this publication (reprint 3) . Further

recommended practice can be found in the Manual on Recommended

Practices in Spectrophotometry published by the American

Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa., 19103.

11
(J'<

CD

CTO

CO

CD<

CORNING

8364

CHANCE

ON-IO

SCHOTT

NG-4

EVAPORATED METAL ON QUARTZ

[INCONEL]

RONCHI RULING ON GLASS

100 LINES/in.

_L 200 300 400 500 600

X, NANOMETERS

700

Figure 4. Spectral characteristics for five transparent

materials from 200 nm to 700 nm (8).

12
The Bureau is in the process of certifying as Standard

Reference Materials didymium and holmium oxide glasses. These

SRM's will have increased wavelength accuracy in comparison

to and will replace the didymium and holmium oxide glasses

offered by NBS through the Office of Measurement Services.

For those who wish further information on this subject, the

reader should consult W. H. Venable of the Institute for

Basic Standards at NBS.

3 . 3 Spectral Bandpass

As mentioned in Section 2, the spectral bandpass over

the entire spectral range for various slit widths is an

instrumental parameter which should be provided by the manufacturer. For spectrophotometers with prisms or gratings,

the spectral bandpass (AX) is related to reciprocal linear

dispersion (dX/dx) and slit width (w) by AX = w dX/dx.

For example, for an instrument with a reciprocal dispersion

of 15 nm/mm and a slit width of 0.2 mm, the bandpass is

3.0 nm. For prism instruments, the reciprocal dispersion

and therefore the bandpass vary with wavelength. The following discussion will be limited to the influence of this often

neglected and important parameter on the measurements of

transmittance for the glass filters discussed in this publication . Two types of filters were used having a transmittance

which varies only moderately with wavelength: the Chance*

ON-10 and Schott NG-4 optically neutral glasses. A neutral

glass #8364 produced by Corning was also included in this

study. However since this filter produced results similar

to those found for the Chance ON-10 glass it will be discussed

together with the later glass filter.

The transmission characteristics of these glasses as a function of wavelength in the spectral interval of 350 nm to

700 nm were obtained using a Cary 14 spectrophotometer and an

adequate instrumental bandpass. The results are illustrated

in figure 4, where it can be seen that the Corning 8364 and

Chance ON-10 glasses exhibit similar characteristics. Figure

4 also shows that these glasses have an absorbance, or transmittance, more dependent on wavelength than the Schott NG-4

glass. The transmittance of these last two glasses was then

* The identification of commercial instruments and products

is given only to permit reproduction of the work described in

this publication. In no instances does such identification

imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the particular equipment or

product is necessarily the best available for the purpose.

13
\*£1A I I I I I I L_

(==)0.0 10.0 20.0 30.0 40.0

SPECTRAL BANDPASS, NANOMETERS

Figure 5. Influence of spectral bandpass on the transmittance of a Chance ON-10 glass filter measured at 400 nm; 445 nm;

490 nm; and 590 nm. The graph at left, upper corner, describes

in general the relation between spectral transmittance and

spectral bandpass. Measurements made with a smaller bandpass

|

| are susceptible to provide accurate T values when compared

with those made with a larger spectral bandpass j J (8)

.

14
SCHOTT GLASS FILTER NG-4

X, NANOMETERS

400 500 600 700

OPTICAL TRANSMITTANCE VERSUS SPECTRAL

BANDPASS FOR A SCHOTT GLASS FILTER NG~4

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 nm

%T

| 20h

1 1 1 ——,—^^_X465

/

//

^^^X590 J

/

/

/

i

1

i

\\\\

i i i i ,>t

i

i

\\

\\

1 4 1 1

1 0.0 200 40.0 600 80.0 100.0

SPECTRAL BANDPASS, NANOMETERS

Figure 6. Influence of spectral bandpass on the t r ansm i t t an c

e

of a Schott NG-4 glass filter measured at 440 nm;

465 nm; 590 nm; and 635 nm (8).

15
measured at four wavelengths as indicated in figure 5 and 6,

using this time variable slit widths, and consequently various

spectral bandpasses, from 0.5 nm to 100.0 nm, using a Cary 16

spectrophotometer

.

Figure 5 illustrates the dependence of the transmittance

for the Chance ON-10 (and Corning 8364) on the spectral

bandpass at 400.0; 445.0; 490.0; and 590.0 nm. It can be

seen that the transmittance varies with the bandpass, and

that this dependence is stronger at the wavelengths where

the glass has a stronger and sharper absorption band. For

instance, at 400.0 nm a rather narrow spectral bandpass of

0 . 5 nm will be required for this type of glass to obtain an

accurate transmittance value, everything else being equal

and in particular the wavelength and photometric scale

accuracy, while at 490.0 nm a spectral bandpass of 15 nm

would be adequate. The same dependence, but to a smaller

extent, is illustrated in figure 6 for a Schott NG-4 glass,

and here it can be seen that larger spectral bandpasses

could be used to produce accurate transmittance values.

This transmittance - spectral bandpass dependence is

summarized in the graph from figure 5 at the upper left

corner, where it can be seen that the use of wider spectral

bandpasses will produce the inaccurate Ti and T 2 values

when compared with T{ and T£, which are theoretically the

correct values. As a general rule the value of the spectral

bandpass required to produce accurate transmittance measurements with an uncertainty not greater than 0.1 percent, should

be yTy of the symmetrical natural spectral bandwidth, at half

intensity, of the material to be analyzed and at the wavelength at which the measurement is performed. When this

information is not available, it can be determined by measuring the transmittance (or absorbance) of the material at the

wavelength of interest, using various spectral bandpasses

(or slit widths) as illustrated in figures 5 and 6. The

largest bandpass or slit width which can be used is given by

the value found at the end of the horizontal portion of the

transmittance -spectral bandpass curve.

The selection and use of adequate spectral bandpass is

one of the indispensable conditions which must be fulfilled

to obtain true transmittance values.

16
3 . 4 Stray Radiations

The measurement of stray radiation in the monochromator

,

that is, the radiation energy at wavelengths different from

those of the nominal spectral bandpass transmitted through

the instrument, is not easy or infallible. A detailed

discussion of this instrumental parameter was given in an

ASTM Standard Method (13), which is reprinted in the Appendix,

reprint 7. The procedure recommended in this work should be

used to determine this parameter in the spectral range of

interest

.

The stray radiation generated inside the photometric

system is defined as the radiant energy which falls on the

photosensitive detector without passing through the absorbing

sample. This stray radiation is usually produced by reflections and scattering of radiations on the optical and

mechanical parts located between the exit slit of the monochromator and the photodetector . The measurements can be

performed using a maximum slit opening and by placing a front surface mirror at the sample position, which reflects

to the instrument all radiations received from the exit

slit imaged at the mirror surface. In this way, a maximum

stray radiation condition will be generated in the spectrophotometer. The measurements should be performed at the

wavelength of interest by determining the dark current of

the photomultiplier with the shutter in the closed position

at the photodetector. The mirror is then placed at the sample

position, the shutter is opened and measurements should be

made again. The differences between the two measurements are

an indication for the presence and amount of stray radiation

generated under these circumstances in the sample compartment

of the "spectrophotometer

.

3 . 5 Accuracy of the Transmittance or Absorbance Scales

This is an essential condition which must be fulfilled

by a spectrophotometer for production of accurate transmittance or absorbance values

.

Optical transmittance is due to an intrinsic property

of matter and characterizes a particular transparent material. Since this parameter is not known a priori, it must

be determined by experimental procedures

.

True transmittance values can be obtained only by using

accurate measuring techniques and by taking into consideration all factors which can affect and distort the data. In

this discussion the optical transmittance T of a solid material includes the reflection losses which occur at the airsolid interface. The internal transmittance Ti is defined

17
as the transmittance of the material corrected for reflection

losses. This internal transmittance can be calculated in

principle from the transmittance by using the well-known

Fresnel equations as shown in reprint 4 in the Appendix. The

internal transmittance is obtained experimentally when the

measurements are made using a blank sample in the reference

beam of the spectrophotometer. When a blank is not available,

as in the case of SRM 930, the measurements are made against

air in the reference beam. Under these circumstances the

resulting transmittance value includes the reflection losses

(which are about 8 percent for a glass filter having an index

refraction of 1.5). A discussion of this subject by Mielenz

is given in reference 10. The absorbance A of a material is

related to the internal transmittance Ti by the expression

A = -logioTi while the optical density D is related to the

transmittance T by the expression D = -logioT.

Transmittance is the ratio of two radiant fluxes. It is

therefore necessary that the transmittance scale of the

spectrophotometer be accurate. The transmittance of a particular material is also a function of wavelength; hence the

wavelength scale of the monochromator should also be accurate,

and appropriate spectral bandpasses should be used. These

conditions were examined in the previous paragraphs. The

measurements should be made using collimated radiation. Such

radiations define unambiguously the actual path length through

the transmitting medium, the reflection losses, and eliminate

the effects of polarized radiations that are produced at the

surface of the sample (See Appendix, reprints 4 and 5) . Other

important factors, already mentioned, which must be considered

are: homogeneity and stability of the sample, radiation

scatter inside the sample, interference phenomena, stray

radiation, polarization, fluorescence, temperature, particulate matter, and surface conditions. Since transmittance

measurements depend on a diversity of factors, meaningful

values can be obtained only by defining the experimental

conditions for obtaining transmittance data.

Spectrophotometers are used to perform two types of

measurements

(1) Quantitative determination of chemical species

using the relation between optical transmission of the

material, and the concentration as a measuring parameter.

Under these circumstances, the photometric scale of the

spectrophotometer is calibrated in meaningful units, using

a series of reference solutions having known concentrations

of the species to be determined, rather than values of

optical transmittance.

The accuracy of the measurements is related solely to

the accuracy with which the concentration of the reference

18
19
solutions is known and to the precision (stability, sensitivity) of the spectrophotometric method and instrument used.

The accuracy of the transmittance or absorbance scale per se , is not a critical factor in such measurements.

The precision and stability of the instrument are, however, important parameters which should be verified before

and after each series of measurements. This can be done, as

already discussed, by the careful use of selected solid or

liquid reference filters having well established and stable

transmittance values.

(2) Determination of the optical transmission characteristics of solid or liquid materials, and the determination of

certain phys icochemical constants such as molar absorptivity

and equilibrium constants. In all of these cases the accuracy

of the transmittance or absorbance scales of the measuring

instrument, among other things, is essential to provide true

values. Ways to establish and check this important parameter

are critically needed. For example, current interest in molar

absorptivity values, as an index of the purity of biological

clinical materials, requires greater accuracy of measurement.

Also, the accuracy of the determination of equilibrium constants of chemical reactions in solutions and the determination of enzyme activity in international units is dependent

on true values of their molar absorpt ivities

Since transmittance is the ratio of two radiant fluxes

intensities, true transmittance values will be produced only

when this ratio is measured accurately. These measurements

are usually performed by using photodetectors capable of

generating signals proportional to the two fluxes. Trans

mittances are measured accurately when there is a linear

relation between the two radiant fluxes and the resulting

photoelectric signals. Hence, linear photoelectric measurements result in accurate transmittance values. This is

illustrated in the simple graph (fig. 7) on which the

abscissa represents arbitrary radiation flux intensities F

with the relative intensities I on the ordinate. In an

ideal case, when F=0, 1=0; for a certain value of F, called

here arbitrarily 100 percent, there corresponds a maximum

value of the photocurrent , I. These determine two fixed

points on the graph, and any measurement of radiant flux

intensity will be accurate if it falls on the straight line

which connects the origin 0 with the 100 percent point . Photodetector linearity can be measured by various

methods (11) , but of these only the use of the inverse square

law and the radiation addition principle using a plurality

of sources or multiple apertures will be mentioned here.

This last method was selected and used in conjunction with

a spectrophotometer designed and built in the Analytical

20
Chemistry Division at NBS (14) . This instrument is described

in detail in the Appendix (reprint 4) . Its adaptation for

work in the ultraviolet is described in reprint 6 of the

Appendix

.

Using this high-accuracy spectrophotometer, a series of

glass filters was developed as Standard Reference Materials

and were issued as SRM 930. SRM 930 consists of three neutral

glass filters having transmittances of approximately 10, 20,

and 30 percent. Each filter is individually calibrated and

certified for transmittance over a spectral wavelength range

from 440 to 635 nm. These filters are intended to verify the

accuracy of the transmittance or absorbance scales of spectro-.

photometers as well as their short and long term stability

and to provide a means for interlaboratory comparisons of

data. A further purpose of these filters is to assure that

systematic errors due to a particular characteristic or condition of an instrument can be recognized.

SRM 930 (a and b) should be valuable in checking the

calibration of instruments used to obtain accurate physicochemical constants such as molar absorptivity and equilibrium

constants. For example, current interest in molar absorptivity

values, as an index of the purity of biological clinical materials, requires greater accuracy of measurement. Also, as discussed earlier in this section, the accuracy in the determination of equilibrium constants of chemical reactions in

solutions is dependent on true values of their molar

absorpt ivit ies

.

4. STANDARD REFERENCE MATERIALS FOR TRANSMITTANCE IN

SPECTROPHOTOMETRY ,

4 . 1 Selection and Preparation of Materials Which Can Be Used

as Standard Reference Material in Spectrophotometry . Such materials should fulfill the following conditions:

(a) be transparent in the spectral range of interest, usually

between 200 nm and 800 nm; (b) have a transmittance independent of wavelength (optically neutral) ; (c) have a spectral

transmittance independent of temperature; (d) have low reflectance and be free of interferences; (e) be non- fluorescent

;

(f) be stable, homogeneous, and free of strain; (g) have

mechanical stability for the size used (thickness, length,

width) and be easy to fabricate by conventional techniques

used in optical shops; (h) be simple to use in conjunction

with the conventional spectrophotometers available today in

analytical laboratories; (i) be readily available and

relatively inexpensive.

21
These conditions are fulfilled to a smaller or larger extent

by the following materials:

<colored glass filters

evaporated metals -on-quartz

Transparent liquids

inorganic

organic

Grids <

non- supported : metal or plastic

grids

supported Ronchi ruling on

glass or quartz

Rotating

Sectors <

concentric with the radiation

beam: Pool sector

excentric to the radiation beam

Polari zers

From these materials, the colored glass filters, mentioned in

the first group, most nearly satisfied the required conditions

and constitute the most acceptable compromise when compared

with the other materials mentioned above.

Several colored glasses were examined, and from these

the Corning 8364, the Chance ON-10, and the Schott NG-4

were initially selected. The spectral transmittance of

these glasses is illustrated in figure 4 in comparison

with an evaporated metal -on-quartz filter (non- fluorescent

fused silica of optical quality) and a Ronchi ruling on a glass substrate. The transmittance of these two last materials exhibits the least dependence of wavelength from 200 nm

to over 700 nm (the Ronchi ruling transmittance below 350 nm

is limited only by the transmittance of the glass substrate

and not by the nature of the ruling itself). However, since

the attenuation of radiations is produced by the evaporated

metal-on-quartz , through reflection rather than absorption,

this material was not selected for the purpose considered

here, since there is a possibility that reflected radiation

is generated in conventional spectrophotometers. The Ronchi

ruling was rejected since it is subjected, by its nature, to

22

diffraction phenomena. From the remaining glasses, the Schott

NG-4 type was finally selected since this material exhibits

the best optical neutrality, and, excluding its limited spectral transmittance range to the visible and near - infrared

spectral domain, it is the best material which satisfies the

other conditions enumerated above.

A number of procedures were developed and used for the

preparation of SRM 930 (a and b) and were tested in a variety

of conditions. From these, only those which were finally

selected and are used currently for the preparation of this

SRM will be described here.

The optically neutral NG-4 and NG-5 glasses for the

filters were produced by Schott of Mainz, Germany, and are

designated as "Jena Colored and Filter Glass." The NG-5

glass is similar to the NG-4 glass in its spectral transmittance but is less absorbing. This permits the use of a thicker glass (1.5 mm) for the nominal 30 percent filter.

These glasses are provided by the Fish-Schurman Corp., 70

Portman Road, New Rochelle, New York, 10802, in the form

of rectangular pieces 30.5 mm long, 10.4 mm wide and

2.0 mm and 1 . 5 mm thick for the NG-4 glass and 1 . 5 mm thick

for the NG-5 glass. Corresponding to these thicknesses are

nominal transmittances of 10, 20, and 30 percent, respectively. These transmittances were selected to provide a means to verify the short and long term stability of the

instrument, and the accuracy of its transmittance or absorbance scale at the three different levels of transmittance

at which most spectrochemical analyses are performed. A

further specification for these filters is freedom of visible

defects such as inhomogeneity , stains, inclusions and scratches.

The parallelism should be within 0.02 mm and flatness should

not exceed three fringes (mercury 546.1 nm radiation) over

the entire glass surface.

4 . 2 Preparation of the Glass Filters

The glass filters are received from the supplier wrapped

individually in lens paper and contained in small individual

envelopes to avoid damage during transportation. Each glass

filter is submitted to a preliminary manual cleaning, with

distilled water using a piece of lens paper. This is followed

by a preliminary examination for defects using a stereomicroscope with a magnification X12. This examination as

well as the following cleaning procedures are performed in

an all -polypropylene hood provided with a vertical laminar

flow of filtered air, illustrated in figure 8.

24
Stainless steel holders for six 50 mm by 50 mm,

and twelve 30 mm by 10 mm glass filters.

25
o

COc

CO om

>i2 —\ O CU m nd

CO

cu J-i 00 u o

M-l •H O O

oo

U c u

•H CJ0)

a)

QJ 0> iH

CJ rH

rH

O

CO J=IO

•H o

iH

CO

01

.-1

U

ao o j_i O.O

T3 CO

01 •H

GO3 00

co •H

S-i CO o o uo #* co CO M M

4J cu

4-1 0) rH

•H

u 14-1 01

r-\ CO X. CO X cO o rH

CO 00 d

26

aiM3

00

•H
Each glass filter which has passed this initial examination is engraved at its upper end with a serial number

using a diamond scriber and is placed in a stainless steel

holder. Two types of such holders were developed and are

illustrated in figure 8, at left and lower right for twelve

30 mm by 10 mm units and at rear right for six 50 mm by 50 mm

glass filters. These filter holders are shown in more detail

in figure 9, at left for the 50 mm by 50 mm and at right

for the 30 mm by 10 mm glass filters. The holder is placed,

in a borosilicate beaker which is then placed in an ultrasonic

bath (80 w) containing distilled water (shown at right in the

figure (fig. 8)). Isopropyl alcohol of electronic grade is

added to the beaker to cover the filters and the unit is

operated for 5 min. This operation eliminates all the glass

particles produced by the engraving which could damage the

glass surface of the filter during the subsequent cleaning

operations. Each glass filter is then washed manually with

a diluted aqueous solution of a nonionic detergent such as the alkyl phenoxy polyethoxy ethanol (Triton X-100, Rohm and

Haas), rinsed with distilled water, and dried in air.

Each glass filter is then re-examined carefully under the

stereomicroscope , and those which pass this final inspection

are transferred in the 12 unit stainless steel holder and

placed in a Soxhlet extractor illustrated in figure 10. An

aluminum slug is inserted at the bottom of each extraction

flask to reduce the volume of solvent required for the functioning of the extractor and to shorten the syphoning time.

To avoid bumpy boiling, followed sometimes by an undesirable

overflow of the isopropyl alcohol from the boiling flask into

the extraction vessel, a side tube provided at the bottom with

a coarse glass frit is inserted through the side of each flask.

A steady stream of clean air from a compressed gas cylinder

equipped with a gas regulator and needle valve, is passed

through the tube to produce a continuous and gentle bubbling.

The glass filters are then washed for 3 hours, at a rate of

three syphoning per hour, using isopropyl alcohol of electronic

grade. After this period the glass filters are dried in air

in the laminar flow hood, placed individually in the special

aluminum alloy holder designed in collaboration with L.

Monostori and made at NBS in the Instrument Shops. This

operation is performed by using the tools illustrated in

figure 8, which includes an antistatic brush and plastictipped tweezers. The mounted filters, placed in a covered

plexiglass box, are aged for at least two weeks in the laminar

flow hood where they are exposed continuously to the radiation

of the three fluorescent lamps provided at the top (total 120 w,

arbitrarily chosen) . Since the individual glass filter metal

holder contributes significantly to the usefulness of the

filters, it will be described below.

27
28
Figure 12. View of the assembled and unassembled glass

filter holder.

29
Figure 13. Plastic box for SRM 930b containing the three

glass filters in their holders. A fourth empty

holder is provided for use in the reference beam.

The grooved insert is made of polypropylene.

30
The shape and dimensions of these metal holders are

given in figures 11 and 12. They were established in conformance to the dimensions of the rectangular sample

compartments of most conventional spectrophotometers, and

were designed to hold the glass filters without producing

mechanical strains in the material, as indicated by a polariscope examination. The metal used for the holder

is an aluminum alloy 2024 (4.51 Cu; 1.5% Mg ; 0.61 Mn) ; the

retaining two-pronged spring is made of phosphor bronze,

and both parts are anodized black (flat) . The spring is

secured by a screw and washer made of nylon (black) . The

dimensions and shape of the window were selected to avoid

vignetting even for spectrophotometers having a low pass

radiation beam.

4 . 3 Packaging of the Glass Filters

Each set, consisting of three glass filters in their

metal holder and having a nominal transmittance of 10, 20,

and 30 percent, is placed in a plastic box similar to that

shown in figure 13. The insert is made of polypropylene and

the glass filters are placed in the metal holder described

in figures 11 and 12; this constitutes SRM 930b.

This SRM is produced currently, and replaces SRM's 930

and 930a. It differs from SRM 930 and 930a by an additional

metal holder without glass filters. This empty filter holder

should be placed in the reference beam when performing

measurements. SRM's 930 and 930a were certified at 440.0 nm;

465.0 nm; 590.0 nm; and 635.0 nm. An additional measurement

at 546.1 nm was included for SRM 930b.

5 . RESULTS

The transmittance measurements used in the certification

of the glass filters discussed previously as well as those

measurements which were made to assess the various characteristics of this material, together with inter laboratory

comparative transmittance measurements, were made with the

high-accuracy spectrophotometer described in Appendix 4,

except the data from table 6. Some of the measurements

relevant to the topic covered in this publication will be

given here.

5 . 1 Stability of SRM 930

As a rule, before taking measurements with the spectrophotometer, a warmup period of one hour is required. The

room temperature is kept at 24 ± 1 °C, and the relative

humidity is 35 percent.' The particulate matter is controlled

through special filters which rates the room in the 100,000

class

.

31
Tab 1 e 1. Influence of temperature on the transmit tahce (T) of seven Schott N'G-4 glass filters at five wavelengths fnn") .

at

2 5.5 °C 2 0.5 °C

Hate

Aver a ge

Wavelength 1 - 1 4 1

- 1

5

1

- 1

7

1

- 2 3 1-2 2 1- 2 4 Pi f f erence

9 . 30 9 . 29 9 . 30 Q t 32 Q 32 0 .32

' + 0 . 0 2

1

8

. 70 18 . 69 18 . 70 1 8 . 73 1 8 . 72 1 8 .73 1 + 0 . 0 3

2 9 . 08 29 . 0 7 2 9 . 09 2 9 1

1

29 . 1 0 29 .11 + 0 . 0 3 440.0 32 . 95 32 . 96 32 . 95 3 2 . 96 32 . 9 5 3 2 .96 + 0 . 01

9 . 89 Q . 89 9 . 89 9 . 91 9 . 91 9 .91 + 0 .02

18 . 53 18 . 5 3 18 . 5 3 18. 54 18. 54 1 8 54 | + 0 . 01

29 . 1

5

2 9 . 1 5 29 . 1

7

2 9 . 20 2 9 . 2 0 2 9 • 20 + 0 . 04

11 . 0 3 11 . 04 1

1

. 04 11

.

0 5 1 1 . 0 5 1

1

[ .06 ; + 0 . 0

1

21 . 06 21 . 0 8 21 . 08 21 . 0 9 2 1

.

10 21 .09

| + 0 . 0 2

4 6 5.0 32 . 20 3 2 . 21 3 2 . 22 32 . 24 3 2. 2 5 32 . 24 + 0 . 0 5 3 5 . 62 3 5 . 61 3 5 . 61 3 5 . 61 3 5

.

61 3 5 .61 j 0 . 0 11 .69 11 .69 11 . 69 11

.

70 11

.

70 11 . 70 + 0 .01

20 .92 20 . 91 20 .92 20 . 9 3 20 . 92 20 .92 0 . 0

3 2 . 2 7 32 . 2 7 3 2 . 3 0 3-2 . 32 32 . 33 32 . 3 3 + 0 . 0 5

Q . 94 0 . 95 Q . 9 5 9

.

9 3 9

.

9 3 9 . 93 - 0 . 0 2 546.1 19 . 5 9 19 . 60 1 9 . 61 1 9

.

5 7 1 9 . 58 1 9 . 57 - 0 . 0 3 30 . 69 30 . 6 9 3 0 . 7 0 30

.

67 30 . 68 3 0 . 6 .' - 0 . 0 2 3 3 . 76 3 3 . 76 33 . 76 3 3

.

70 33

.

71 33 . 71 - 0 . 0 5 10 . 55 10 . 57 10 . 57 10. 54 10 . 53 10 . 54 -0 . 02

19 . 43 19 .47 19 . 48 19. 43 19. 43 19 . 44 -0 .03

30 . 74 30 . 76 30 . 78 30

.

7 5 30 . 7 3 30 . 7 5 - 0 . 0 2

8 . 82 8 . 8 2 8 . 8 3 8 . 79 8 . 8 0 8 . 80 -0 . 0 2

1

8

. 0 2 1

8

. 04 1 8 . 04 1 8 . 0 0 1 8 . 00 1 8 . 0 0 - (J . U 3

2 7 . 62 2 7 . 65 2 7 . 65 2 7 . 6 2 2 7

.

6 2 2 7 . 6 2 - 0 . 0 L 590.0 31 . 2 0 31 . 2 2 31 . 22 31

.

14 31 . 1

4

31 1 A. 1 4 -0 . U / 9 . 37 9 . 38 9 . 38 9

.

34 9 > 35 9 . 35 -0 .03

17 .89 17 .90 17 . 90 17 . 86 1 7 . 86 17 . 86 -0 .04

? 7 . 69 27 . 70 27 . 71 27. 68 27 . 67 27 . 68 -0 .02

9 .85 q . 86 9 . 86 9 . 83 9. 83 9 .83

:? .03

19 .47 19 . 48 19 .49 19. 45 19. 44 1 9 .45 .03

27 . 74 27 . 74 27 . 76 27. 72 27. 71 27 . 73 -0 .03

635 .0 32 .65 32 . 67 32 . 66 32 . 59 32. 60 32 . 60 -0 . 06

10 . 44 10 . 44 10 . 44 10. 40 10. 41 10 . 42 -0 .03

19 . 34 19 . 35 19 . 35 1 9. 30 19. 31 19 . 31 -0 . 04

. 78 27 . 80 27 .80 2 7 . 77 27 . 77 27 .78 -0 . 02

32
33
Seven glass filters were placed in the sample holder

of the high-accuracy spectrophotometer and measured in six

replications. The results are given in table 5 (see Appendix

4) and show that these measurements can be reproduced with a pooled relative standard deviation of 0.010 percent.

The temperature dependence of the transmittance was

measured for the same type of glass by determining this

value at a room temperature of 20.5 °C and 25.5 °C and at

440.0 nm; 465.0 nm; 546.1 nm; 590.0 nm; and 635.0 nm; and

table 1 gives the results.

From these data it can be concluded that the transmittance dependence on temperature over the tested temperature

range, in the spectral interval from 440.0 nm to 635.0 nm,

for the type of glass used is insignificant at the .95 confidence level. For a variation of ± 1 °C to 2 °C the dependance

is minimal and averaged less than 0.2 percent of the measured

transmittance values

.

The stability of the Schott NG type of glass to visible

radiations was tested by exposing three filters having a nominal transmittance of 10, 20, and 30 percent to the high

intensity radiation of a source with a spectral distribution

similar to the conventional fluorescent lamps used for

illumination of laboratories. Each high intensity exposure

lasted 21 hours and was equivalent to 21,000 hours of normal

exposure. The transmittance measurements assembled in table

2 were made before and after each of the two exposures.

Column 2 gives the transmittance of the glass filters at

440.0 nm before the exposure; column 4 gives the same

measurements after the 21 hours of accelerated exposure.

The difference between these two measurements is given in

column 3. The same filters were exposed again for another 21

hours, and column 5 contains the transmittance values before

this exposure, while column 7 gives these values after this

second 21 hours exposure. The difference between these data

(columns 5 and 7) is given in column 6. Measurements were

also made at 465.0 nm, 590.0 nm, and 635.0 nm. From these

data it can be concluded that the type of glass tested

exhibits an acceptable stability when exposed to the visible

radiation experiment described. Similar stability tests

performed on other neutral glasses have indicated that these

glasses were less stable by a factor of four when compared

with the Schott NG-4 glass, and consequently this latter

material was selected for SRM 930 (a and b)

.

34
Table 3. Stability of transmittance (T) as a function of time,

measured on SRM 9 30.

1

r

t.t i . j Wavelength

nm j 3 PATF.

32 . 87 19.80 ] 1 . 59 5-18-71

32 . 88 19.83 11.62 7-12-72

32 . 91 19.81 11.59 8-30-73

440.0 32 . 96 19.84 11.61 11-16-7 3 32 . 98 19.84 11.62 ] - 9-74

32 . 94 19.84 11.61 8 - 1-74

32 . 95 19.84. 11.62 1-13-75

35 . 53 22 . 59 13.56 5-18-71

35 . 5 4 22.62 13.59 7-12-72

3 5 . 54 2 2.62 13.59 8-30-73

465.0 35

.

62 2 2.62 13.58 11-16-73

3 5 . 6 3 2 2.63 13.58 1 - 9-74

3 5 . 60 2 2.62 13.57 8- 1-74

35 . 60 22.62 13.58 1-13-75

31 . 13 19.16 10. 37 5-18-71

31 . 14 19. 20 10.41 7-12-72

3 1 . 1

4

19.20 10.41 8-30-73

5 9 0.0 31 . 2

1

19.19 10.40 11-16-7 3 31 . 2

1

19.20 10.41 1 - 9-74

31 . 1

8

19.20 10.40 8 - 1-74

31 . 1

9

19.20 10.41 1-13-75

32 . 55 20 . 60 11 . 37 5-18-71

32 . 59 20 . 65 11 . 39 7-12-72

32 . 59 20 . 65 11 . 39 8-30-73

635.0 32. 66 2 0.65 11.40 11-16-73

32 . 68 20.67 11.41 1- 9-74

32 . 63 2 0.66 11.41 8- 1-74

32 . 64 20. 66 11.40 1-13-75

35
Table 4. Comparison of percent transir.it ranee (~°?T) measurements

performed on a set of three Sc. holt N f 1 - 4 glass f ilters

over a period of tv.o yea r s

.

Filter

Identification

Wave 1 enr.th

fnm)

9

T~

1 9 7

1

May

1972

' Pecemfc I'- 1973

Ap r i 1

J (J <.

440.0 11.59 ll .62 11 .60

465.0 13.56 13.59 13.57

590.0 1 0 . 37 10.40 10.39

1 6 35 ,0' 11. 36 11 .40 11 . 39

? n

°-

440 . 0 19. 80 19.83 19.81

46 5.0 22 . 59 22.62 HHB|

590 . 0 19.16 19.20 19 .'l

9

6 3 5.0

. ._ .... ._ . « 20.60 20.65 2 0.65

i— '

1

!

i

30%

440 . 0 32.87 32 .88 3 2.92

465 . 0 3 5.53 3 5.54 35 .59

590.0 31 .13 , 31 .14 31 . 20

6 35 . 0 32 . 55 32.58 3 2.64

36
The variation of transmi ttance as a function of time

for the Schott NG type glasses (NG-4 and NG-5) was determined by using three filters which constitute our own N reference set, and measuring them over a period of 4 years.

Table 3 presents the results obtained at 440.0 nm, 465.0 nm,

590.0 nm, and 635.0 nm. The largest difference observed

after 4 years of use did not exceed about 3 parts in 1000.

The measurements also seem to indicate a trend toward an

increase of the transmittance of these glass filters which

are in accordance with the known property of glass surfaces

to produce a layer of SiO at its surface when exposed to

atmospheric conditions. This phenomena is called "blooming"

of the glass, and the thin SiO deposit which is formed acts as

an antiref lection layer which results in an apparent increase

of the transmittance of the glass. Similar measurements

were made on another set of three glass filters over a period of two years; table 4 contains the resulting transmittances, which confirm the previous data.

5 . 2 Inter laboratory Transmittance Measurements on SRM 930

A set of SRM 930 was measured at NBS and at the National

Physical Laboratory (NPL) in England by F . J. J. Clarke and

his associates using their high-accuracy instrument. These

measurements were made with noncollimated convergent beam

geometry. A rectangular surface of the filter about 3 mm by

8 mm was used at NPL and the beam was only slightly convergent. At NBS an area of about 8 mm by 0 . 5 mm was used for

the transmittance measurements. The results given in table

5 indicate that an average difference of about 0.20 percent

of the measured values was obtained between the measurements

carried out at NPL and at NBS. Agreement to within the

instrumental accuracy of about 1 part in 10

1* was also

obtained in comparative measurements, using another highaccuracy spectrophotometer built at NBS (See Mielenz in

reference 10). The average reproducibility of these measurements performed in 1971 and repeated in 1973 and 1975 was

within this value.

37
Table 5. Comparison between the percent transmit tances (IT)

measured on three Schott NG-4 glass filters at NPL

and NBS in 1971, 1973, and 1975.

Wavelength NBS, %T NPL, IT NBS, IT

nm 1971 -\ All 1971 1973 1975

440.0 12 . 91 12 . 93 12.91 12 . 90

465.0 14.97 15. 01 14.98 14 . 97

590 .0 11.67 11.67 11.65 11.62

635 .0 12.70 12.72 12.71 12.69

440 . 0 19.60 19. 62 19.60 19 . 56

465.0 22 .37 22.43 22.39 22.34

590.0 19.00 19. 01 19. 00 18 .94

635.0 20.41 20.47 20.46 20 .40

440.0 32 .87 32.98 32 . 86 32.87

465. 0 35. 53 35. 66 35. 52 35.51

590 .0 31.13 31. 21 31.11 31 . 07

635 .0 32 . 54 32.62 32.57 32 . 53

Another comparative study was performed to include three

clinical laboratories to determine the reproducibility of

transmittance measurements on the Schott NG-4 neutral glass

filters. Again three filters having nominal transmittances

of 10, 20, and 30 percent were measured at four wavelengths

on a conventional spectrophotometer at the NBS. This instrument included a double monochromator with quartz prisms

used in a double beam mode, and a transmittance scale with

1000 divisions between 0 and 100 percent transmittance. The

transmittance accuracy of this scale was established with

SRM 930. The same filters were then sent to laboratories A,

B and C describing the technique to be used in measuring

their transmittances. The results obtained are summarized

in table 6 and include the percent transmittance (IT) values,

the standard deviation (S . D.), the percent standard deviation

(I S. D.), and the percent difference (I difference). This

difference was calculated using the NBS data as a reference.

38
Table 6. Comparative transmittance measurements performed by

four laboratories on tbree Schott NG-4 colored glass

filters

.

Laboratory A

Measurement

No . 4 4 0 nm 4 6 5 nm 590 nm 6 35 nm Ave . 12 . 91 14 . 99 11 . 68 12.71

i c n 1 £> . l)

.

U . UUoo v . 0 U 4

n n n o 0 . 0 0 8

A a a r 0.005

% s.n. . 049 . 027 .070 .041

niff. % - . 4 - .07 - .6 - . 5 Ave

.

19.61 2 2.37 19. 06 20.50

9 q n U . U U 4 U . U 1 u U . U (J 4

% s.n. .021 . 046 .022

niff. % - . 5 - . 3 - . 3 -0.1

X 9 91 7 r A A

£> b . 4 4

7 1 1 O .U . 1 Z 11 CO

3 s.n. 0.0080 0 . 0080 0 . 0050 0 .0050

<1 b . J ) . n 9 "z

. U Z .5 n 9 ?.. 0 Z J) . (J 1 / . 0 1 o niff. % + .3 + . 3 + . 1 + . 2

Laboratory B

Measurement

No . 44 0 nm 4 6 5 nm 5 9 0 nm 6 35 nm Ave . 12.91 14.87 11 . 56 12.59

i1 q n o . JJ . U . U 4

1

U . U 4

1

% s.n. _ .276 . 36 _ niff. % -0.4 - . 9 -1.6 -1.4

Ave . 19 . 59 22 . 39 18 . 90 20.51

2 s.n. 0.053

I s.n. . 281

niff. % -0.6 -0.2 -1 .

1

-0.05

Ave . 32.58 • 35 . 23 30 .90 32 . 36

3 s.n. 0 .093

% s.n. . 264

niff. ?, -0.4 - . 3 -0.6 -0.4

39
Table 6 (Cont'd)

Laborato ry r Measurement 0. T

No .

. 4 4 0 nn 4 65 nn 590 nm 3 5 nm

Ave . 12. 88 1 4 . 96 ] 1 . 66 1 2 . 6 5

n0 . 0 J. 4 0.011 0 , 015

0 9 . 09 • i 5 Dill i

~ U • r0 } - . 8 • O

Ave . 19. 54 22 . 34 19.12 2 0 . 3 7

Z o . 1 )

.

0.024

o tS

. 1 3 Dirt. ?

— 0 Q - 0

.

+ .05 - 0 . Ave . 32 . 88 35. 4 0 30.97 3 2 . 4 3

3 S.D.

?

0 S.D.

Diff. % + 0 . 5 + 0 . 1 -0.4 -0 . 7

Laboratory NBS

Measurement % T

No . Ave . 1 S.D.

% S.D.

Diff. %

440 nm 12.96

0 . 0053

.041

4 65 nm

15.00

0.0037

.025

5 90 nm

11.75

0 .0048

.041.

6 3 5 nm 12.77

0.0057

.04 5

Ave . 19. 71 22 . 44 19.,11 2 0.52

2 S.D. 0 . 0049 0 .0052 0 ,, 0038 0 .0049

% S.D. .025 .023 .020 .024

Diff. % Ave

.

32 . 72 3 5 . 35 31 . , 09 32 . 50

3 S.D. 0.0045 0 . 0056 0 ..0040 0 . 00 4 3

% S.D. .014 . 016 , 013 .013

Di ff

.

40
In this study, the cooperation of George N. Bowers, Jr.,

M. D., of Hartford Hospital, Hartford, Connecticut, Royden

N. Rand, Ph.D., of the Hospital of the University of

Pennsylvania, Philadelphia, Pennsylvania* and Donald S. Young,

M. D., Ph.D., of the National Institutes of Health, Bethesda,

Maryland, is gratefully acknowledged.

This study demonstrates that transmittance data can be

reproduced in actual laboratory conditions with a general

uncertainty of less than one percent. From the total of

36 transmittance measurements only 3 exceed this value, and

this was due probably to limitations in the spectral bandpass of the particular spectrophotometer used.

The transmittances of several sets of SRM 930 were

determined, after several years of use in routine laboratory

conditions, to assess the effect of time and handling on

this parameter. Table 7 presents the results obtained on

set no. 106 certified in 1972. The column marked 1 contains

the transmittance values as certified in November 29, 1972.

This set was measured again in February 27, 1974, in an "as

received" condition, and the transmittance values obtained

are given in the column marked 2. The difference between

these measurements is given in the column marked 3. As it

can be seen, the transmittance of these glass filters, after

about two years of use in a laboratory, is lower than the

certified values, suggesting a possible contamination of the

glass surface. This supposition was verified by submitting

the filters to the cleaning procedure described previously.

The filters were remeasured, and the data obtained are given

in the column marked 4, which present the average of four

series of measurements carried out over a period of three

days. These results, which indicate this time a positive

difference, given in the column marked 5, confirm the

hypothesis of a surface contamination of the glass during

their use, and verify the tendency of the glass to cover

itself with a layer of SiO as was already discussed. The

results of this phenomenon is an apparent increase of the

transmission of the material.

* Present address: Eastman Kodak Company, Research Laboratories, Rochester, New York, 14650.

41
1—1 1—

1

LO CM ° O to

c_, LO V

^r o o o o o O O o O o o '

1 + + + + + + + + + +

-

c

O

cn oo -3- oo cm cm to CM o

</) c- I cn oo LO CO cn to iH -=i- cn en c CSi cS ID Cn o oo cn vO OO LO r - O to cn o

<D e r-1 I—

1

1—

1

rH i—

i

to to CM to

<u

<_; r-t

•—

I

to to to CM LO CO oo o cm cm o o o o o o o o o o i—

i

i—

1

<i fO rH o o o o o o o o o o o o 1—1 1 1 1 1 1 1 1 1 1 1 1

c! ^J-

,

•r-l r-- LO G". n) = tO CM o t-H to LO CO cn oo

a) > CM cn oo LO oo CM cn CM oc r- •H CM

<u c. o oo cr. VD oo LO r cr. to cn Q

ri '/l o i—

i

i—

1

i—

1

rH CM to CM to

cs o X) o S-i 0

CM

<b rH

U-l H to LO r~- to to oo o to CM o

i—

i

+-> c Cn CM cn OO [ LO r— cn to O to C cn

c,a> cn o oo cn VO oo LO r-- o to cn o

u • rH 1—

)

rH rH rH to to CM to

in c

<:

.c

•MM o o o o o O O o CD o o o

C

<D o LO o LO o LO o LO O LO o LO rH E Cn to cn to cr. to

c; LO LO vO ^r LO >

(/i c

o

•H

CO

U; o

2: vO o O o

+-> rH rH

<u CO CM to

42
In another series of tests, a set of SRM 930 was used to

verify and correct the transmittance scale of a conventional

spectrophotometer, by measuring the transmittance of the

certified glass filters on the tested instrument, and by comparing the results with the certified values, and Table 8

presents these results.

Table 8. Comparison of % T measurements performed on a Schott NG-4 filter using the NBS high-accuracy

instrument and a conventional spectrophotometer.

Average

Wavelength NBS Conventional Relative Relative

nm Instrument Instrument Diffj % Diff. % Diff.

440.0 32.82 32.75 +0.07 +0.21

465.0 35.51 35.40 + .11 + .31 +0.22

590.0 30.98 30.93 + .05 + .16

635.0 32.38 32.32 + .06 + .19

From these data it can be concluded that the transmittance scale of the conventional spectrophotometer provides

transmi ttances which are lower than the accurate values by an

average difference of 0.22 percent. This average difference

can be added to the measurements made with the conventional

instrument, to obtain the accurate transmittance value.

Another method to perform the correction is to use an interpolation procedure using a graph established by plotting on

the abscissa the accurate transmittance (or absorbance)

values of SRM 930 and on the ordinate the values found on

the instrument for this material. In this discussion it was

implicitly assumed that all other parameters which can affect

the transmittance accuracy have been considered and minimized,

and especially the stability of the instrument, the wavelength

accuracy, the spectral bandpass, and temperature. Correction

of the transmittance or absorbance scale of a spectrophotometer

should be applied only when these conditions are fulfilled.

The glass which is used to produce SRM 930 has a spectral

transmittance limited to the visible range from about 400 nm

to 800 nm, as illustrated in figures 4 and 6, and this obviously limits the usefulness of this standard reference material.

Appreciable efforts have been made at NBS in the past and are

continuing now, to find a transparent material which could

extend this spectral range to the ultraviolet. At the present

time, however, only one material has been found which satisfies

these required characteristics; this material is the evaporated metal -on-quartz filter which was already discussed

(figure 4). The evaporated metal -on-quartz filter meets the

43
requirements for spectral neutrality and good stability and

is not susceptible to the production of an SiO layer. When

chromium is used to produce the evaporated metal layer, the

filter exhibits a good resistance to abrasion. The only

limitation of the evaporated metal-on-quartz results from

its intrinsic property of attenuating the incident radiation

by reflecting part of it rather than by absorbing it. Although interlaboratory tests between NBS and NPL, in which

evaporated metal-on-quartz filters have been used, have produced results similar to those obtained with the glass filters

used in SRM 930, it should be noted that these results were

obtained with specially designed high -accuracy spectrophotometers. The use of such filters in conjunction with the large

variety of conventional instruments is now being studied to

determine to what extent and under what circumstances the

metal-on-quartz filters could be used as standard reference

material in spectrophotometry.

6 . SUMMARIZING REMARKS

The acquisition of meaningful measurements by spectrophotometry requires that every phase of the chemical processes

involved in the analytical procedure, and every parameter of

the instrumental spectrophotometr ic technique be well-known

and understood, and under the complete control of the analytical chemist. He must be able to assess the importance of

these two factors in a quantitative manner, and to evaluate

their individual contribution to the uncertainty of the final

measurement. Some of the parameters involved in the instrumental spectrophotometric technique have been examined in this

publication, and particular attention was given to those instrumental parameters which play a more significant role: the

short and long term instrumental stability, the wavelength

accuracy, the spectral bandpass problem, and the transmittance

accuracy. Means and methods have been discussed, and, standard

reference materials have been described and produced, which

permit to determine to a large extent the magnitude of these

important parameters and to assess and correct their individual

contributions toward the deterioration of spectrophotometric

precision and accuracy.

While the use of SRM 930a and b does not solve all problems in spectrophotometry, its availability should contribute

appreciably to the production of more accurate and therefore

more meaningful measurements in all fields where spectrophotometry is applied.

Instrumental stability, wavelength accuracy, adequate

spectral bandpass, and transmittance accuracy are basic parameters which play a determinant role in the production of

accurate transmittance measurements of a transparent material

44
in the spectral range of interest. Without the fulfillment

of these conditions no accurate transmittance measurements

could be performed. SRM 930 seems to be a useful means to

assume that the measurements are accurate.

The production of accurate chemical analytical data,

that is the capability to determine accurately the amount of

a particular chemical element in a given matrix , requires,

on the other hand, only spectrophotometric precision, as determined by the instrumental stability and sensitivity.

The accuracy of the analytical measurements is determined in

this case solely by the accuracy achieved in the preparation

of the analytical samples and in the establishment of the

reference curve. These in turn depend on the stability of

the chemical reactions involved in the spectrophotometric

process, and on the ability to prepare adequate reference

solutions having accurately known concentrations of the

chemical element of interest in a matrix identical to that

of the analytical sample. Under these circumstances, analytical accuracy can be achieved using spectrophotometers

which exhibit good stability and sensitivity. The wavelength accuracy, the spectral bandpass, and the accuracy of

the transmittance or absorbance scale play a secondary role

in this analytical application. The lack of accuracy of

these parameters could affect the sensitivity and linearity,

and in some cases, the specificity of spectrophotochemical

analyses, rather than their chemical accuracy.

Standard Reference Material, "SRM 930, a and b, Glass

Filters for Spectrophotometry," is a transfer standard which

was calibrated for transmittance in the spectral range of

440.0 nm to 635.0 nm, with a high-accuracy spectrophotometer

which represents the primary transmittance standard. The

transmittance values were certified with an uncertainty of

0.5 percent. The validity of these transmittance values with

time will depend on the stability of the glass and the conditions under which it is used. The measurements from table

3 prove that the stability of the certified transmittance

values are reproducible, within the uncertainty indicated,

over a length of time exceeding 4 years. This assumes that

the filters have been used with proper care, following the

instructions given in the certificate. Deviations from

these instructions could result in degradation or loss of

accuracy. In particular, the glass surface should never

be touched with the fingers, the filters should not be

exposed to dust or corrosive reagents, and when not in use

they should be kept always in the box provided for this

purpose. Should the glass surface of the filters become

accidentally contaminated, no attempts should be made to

wash it, since the user does not have means to determine if

this cleaning treatment has not altered the properties of

45
the glass surface and degraded the accuracy of the certified

transmittance values. Since SRM 930 is a transfer standard,

the only means available to determine its integrity is by

remeasuring its transmittance with a primary standard instrument, and this can be done only with the high-accuracy

spectrophotometer described in this work. Should there be a need to verify the accuracy of the certified transmittance

data, the glass filters should be returned to NBS for verification of the transmittance data assigned to that particular

set, and for cleaning and recalibration if necessary. It is

recommended that before sending the glass filters, proper

arrangements be made by writing the Office of Standard

Reference Materials at NBS to establish the best conditions

in which such a verification can be made.

SRM 930 is a delicate and costly unit which has required

an appreciable amount of careful work for its preparation and

certification. This material should be handled with great

attention to insure production of accurate measurements and

long life. This is particularly true when the working conditions in certain laboratories require frequent use of SRM

930. To protect this reference material it might prove

desirable in such cases to produce "in house" secondary glass

standards derived from SRM 930, and use these secondary

standards for daily control of the spectrophotometers used

in the laboratory. When these standards indicate a significant change in the measurements, only then should SRM 930 be

used as a final verification of the data.

Secondary glass standards could be produced by purchasing the optically neutral glass used for the production of

SRM 930, cut to size as described in this publication and

placed in holders similar to those illustrated in detail in

figures 11 and 12. The transmittance of these filters should

be established at the desired wavelengths with the best spectrophotometer available in the laboratory after its transmittance scale was verified with SRM 930 (a or b)

.

46
7. REFERENCES

1. Rand, R. N., J. Res. Nat. Bur. Stand. (U.S.) 76A,

No. 5, 499 (1972).

2. Belk, W. P., and Sunderman, F. W. , Amer. J. Clin.

Pathol. 1_7, 853 (1947) .

3. Wooton, I. D. P., and King, E. J., Lancet I, 470 (1953).

4. Hendry, P. I. A., Report Adelaide Meeting, p. 8. College

of Pathologists of Australia (1961) .

5. Tonks, D. B., Clin. Chem. 9, 217 (1963).

6. Desmond, F. B. , New Zealand Med. J. 63, 716 (1964).

7. Rand, R. N. , Clin. Chem. 1_5, 839 (1969).

8. Mavrodineanu, R. , in NBS Technical Note 544 , 6-17 (July

1969 - June 1970); and 584 , 2- 21 (July l$7u* - June 1971).

9. Mavrodineanu, R. , and Lazar, J. W. , Clin. Chem. 19 , No. 9, 1053-1057 (1973).

10. NBS Special Publication 378, "Accuracy in Spectrophotometry and Luminescence Measurements," 255 pp. (1973).

R. Mavrodineanu, J. I. Shultz, and 0. Menis, editors.

This volume contains the 18 papers presented at the

Conference on "Accuracy in Spectrophotometry and Luminescence Measurements" held at NBS from March 22 to 24

(1972) . The same papers were published previously in:

J." Res. Nat. Bur. Stand. (U.S.) 76A , Nos . 5 and 6,

375-510; 547-654 (1972).

11. Mavrodineanu, R. , in ref. 10.

12. Gibson, K. S., "Spectrophotometry," NBS Circular 484 , 48 pages (1949)

.

13. ASTM Standard Method of Estimating Stray Radiant Energy,

ASTM: E387-72, in "1972 Annual Book of ASTM Standards."

14. Mavrodineanu, R. , in ref. 10.

47
8. APPENDICES

These appendices contain a reproduction of the notice

introducing the Standard Reference Material 930, and the

calibration certificate issued for SRM 930b. It differs

from SRM 930 and 930a by the addition of an empty filter

holder and an additional tr ansmi ttance certification at

546.1 nm. Also included are reprints from five publications which should provide further and more detailed

information on the subject discussed in this publication.

9. ACKNOWLEDGEMENT

The authors gratefully acknowledge the assistance of

the following persons

:

- J. R. DeVoe , Chief, Special Analytical Instrumentation

Section; R. W. Burke, Research Chemist; K. D. Mielenz, Analytical Chemistry Division, NBS; T. W. Mears , Standards

Coordinator, Office of Standard Reference Materials, NBS;

and R. N. Rand of Eastman Kodak, Research Laboratories, and

President Elect, American Association of Clinical Chemists,

for reviewing the manuscript and for making many valuable

comments and constructive suggestions.

- H. L. Hoover, Corning Glass Works, for providing the

transmi ttance information of the didymium and holmium oxide

glass filters.

- American Society for Testing and Materials for permission to reproduce "Standard Method of Estimating Stray

Radiant Energy" reprint for Appendix 7.

48
Appendix I

NBS STANDARD

REFERENCE MATERIALS

March 26, 1971

Standard Reference Material 930

Glass Filters for Spectrophotometry

The Office of Standard Reference Materials announces

the availability of Standard Reference Material 930, Glass

Filters for Spectrophotometry. This SRM consists of three

glass filters having transmittances of approximately 10, 20,

and 30 percent. Each filter is individually calibrated and

certified for absorbance and transmittance over a spectral

wavelength range from 440 to 635 nanometers.

These filters are intended to check the accuracy of

the photometric scale of spectrophotometers, and to provide

a means of interlaboratory comparisons of spectrophotometry

data. It is probable that in the field of clinical chemistry

a large amount of data are being obtained on precise instruments whose accuracy is unknown. To make these data more meaningful and universally applicable, the biases between

instruments must be eliminated or at least determined. I. major purpose of these filters will be to assure that

systematic errors due to a particular characteristic or condition of an instrument can be recognized.

This SRM should be valuable in the calibration of

instruments used to obtain accurate physico-chemical constants

such as molar absorptivity and equilibrium constants. For

example, current interest in molar absorptivity values, as

an index of the purity of biological clinical materials, re- quires greater accuracy of measurement. Also, the accuracy

in the determination of equilibrium constants of chemical

reactions in solutions, is dependent on true values of their

molar absorptivities

.

Standard Reference Material 930 is available from the

Office of Standard Reference Materials, National Bureau of

Standards, Washington, D. C. 20234. The price is $395. for

a unit of three filters.

49
50
Appendix II

. S. Department of Commerce

Frederick B. Dent

Secretary

National R,h.d wBureau R^rt.

of Standards d.«- A* I 'M t SZtl iff ^attonal bureau of standards

(Certificate

Standard Reference Material 930b

Glass Filters for Spectrophotometry

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for the calibration of the photometric scale of spectrophotometers. It consists of three individual filters and one empty

filter holder. Each filter bears an identification number, and the upper left comer has been removed to indicate correct placing in I he metal holder.

Filter & Set

Ident. Number

ABSORBANCE (A) TRANSMITTANCE (%T)

Wavelength and (Bandpass) nm

Wavelength and (Bandpass) nm

440.0

(2.2) 465.0

(2.7) 546.

1

(6.5) 590.0

(5.4) 635.0

(6.0) 440.0

(2.2) 465.0(2.7) 546.

1

(6.5) 590.0

(5.4) 635.0

(6.0)

The transmittance values given arc certified with a relative uncertainty of ±0.5 percent (example:

a nominal value of absorbance of 0.500 ± 0.0022). This uncertainty is the sum of the random error of ± 0.1 percent (2SI) limit) and of estimated biases which are ± 0.4 percent. These biases are due to possible systematic errors. Measurements were made at 25 °C. Room temperature variations within

several degrees Celsius of this temperature will not significantly affect the calibration of these

filters.

It is recommended that the filters be handled onlv bv the edges and with soft plastic (poly- ethylene) gloves and optical lens tissue. W hen not in use thev should be stored in their holders and

in the box provided for this purpose. Extended exposure to lahoralon atmosphere and dusty sur- roundings should be avoided.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of (). Menis and |. I. Shultz.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials by T. W. Mears.

Washington. I). C. 20234 J. Paul Oali, Chief Fehruan 24, 1975 Office of Standard Reference Materials

(o\er)

51
The transmittance measurements were made with the high-accuracy spectrophotometer designed and constructed at the National Bureau of Standards [1]. The accuracy of this instrument was

established by light-addition measurements.

The neutral NG-4 and NG-5 glasses for the filters were provided by Sehott of Mainz, Germany,

and are designated as "Jena Golored and Filter Glass." Nominal transmittancc for a filter 1.5 mm

thick is 20 percent at 400.0 nm wavelength and 32 percent at 700.0 nm wavelength. Between these

limits the transmittancc varies in a monotonic manner [2)

.

The filter holder is black anodized aluminum alloy and has the following nominal dimensions:

height: 57 mm, width 13 mm, depth: 13 mm. This holder and the size and shape of the filters were

selected to conform to the dimensions of the sample compartment of most conventional spectro- photometers. The filters are approximately 30.5 mm long. 11 mm wide, and 2.0 mm and 1.5 mm

thick for the NG-4 glass and 1.5 mm thick for the NG-5 glass. Corresponding to these thicknesses

are nominal transmittances of 10, 20, and 30 percent, respectively. These thicknesses were selected

to provide a means for calibrating the photometric scale at three different levels. The exposed

surface of the glass filter is approximately 29 mm by 8 mm, starting from a distance of 1.5 mm

from the filter holder base (see figure).

The transmittancc of filters depends on the intrinsic properties of the material. Spectral bandpass, wavelength [2,3,5) , geometry of the optical beam, surface conditions, and positioning of the

filter also affect the transmittancc values, and can lead to further biases. The certified data will be reproduced when transmittance measurements are made under similar conditions. The effective

spectral bandpasses used to determine the certified values are given on the face of the certificate and

the transmittance measurements are made bv producing the vertical image of the slit (about 8 mm

by 0.5 mm), using a convergent beam geometry with an opening of f:10, in the middle of the entrance face of the glass filter. (The filter should be positioned in the spectrophotometer as shown

in the figure, to obtain correct values.)

Prior to the certification measurements, each filter was examined for surface defects and then

thoroughly cleaned. If. through handling, the surface of the filter becomes contaminated with dust,

it may be cleaned with a small soft brush attached to a rubber tube connected to a vacuum source

[2,4] . If the surface becomes contaminated with fingerprints, they must be eliminated before making measurements. This may be accomplished by removing the filter from its holder, breathing

lightly on it. and rubbing the surface gently with optical lens tissue. The clean filter is then replaced

in its proper position in its holder. To remove and replace the filter in the metal holder, the

spring-loaded plate should be removed with care to prevent damage to the filter. As little handling

as possible is recommended.

NOTE: The check of the calibration of photometric scales defines only one of the parameters

required for obtaining accurate transmittance values and molar absorptivities. Other factors that

also must be established are wavelength accuracy, stray light, cell parameters, fluorescence, polarization, reflection, and temperature coefficient. Some of these variables are discussed in NBS publications [1,2,5]. It is planned to summarize various aspects of accurate spectrophometric measure- ments in an NBS-260 Special Publication that would provide additional data on specific Standard

Reference Materials. In the interim, SRM 930b, should be used as described in the certificate. Consult the manufacturer of the instrument if differences are obtained that exceed those specified

by the manufacturer. We wish to acknowledge the cooperation of George N. Bowers, Jr., M.D., of Hartford Hospital. Hartford, Connecticut: Rovden N. Rand, Ph.D., of the Hospital of the University of Pennsylvania.

Philadelphia, Pennsylvania;' and Donald S. Young, M.B., Ph.D., of the National Institutes of Health. Bcthesda, Mary land.

52
L. R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid ami Liquid Materials, NBS Journal of Research 76A, No. 5, 405-425 ( 1972).

2. R. Mavrodineanu. Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS

Tech. Note 544, (). Menis and J. [. Shultz, ed., pp 6-17, U.S. Government Printing Office, Washington, D.C. 20402 (Sept. 1970), ibid NBS Tech. Note 584, pp. 2-21 (December 1971).

3. K. S. Gibson, Spectrophotometry. NBS Circ. 484 (Sept. 1949).

4. J. R. Edisbury, Practical Mints on Absorption Spectrophotometry, Plenum Press, New York

5. Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence

Measurements, NBS Journal of Research 76A, No. 5, 375-510 ( 1972).

(1967).

METAL HOLDER FOR THE COLORED GLASS FILTERS

FRONT TOP METAL HOLDER

57 X 13 x 13

EXPOSED FILTER SURFACE

29 X 8

T COLORED GLASS FILTER 30 5 X II X 2 0 (Of I 5)

DIMENSIONS IN mm

53
Appendix III

January

1955

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C. 20234

Letter

Circular

LC-1017

Re-issued

January 1967

STANDARDS FOR CHECKING THE

CALIBRATION OF SPECTROPHOTOMETERS

(200 to 1000 nm)

Contents

1. Introduction.

2. Checking the wavelength scale.

2.1 Non-recording spectrophotometers.

2.2 Recording spectrophotometers.

2.2.1 General Electric recording spectrophotometers.

2.2.2 Cary Model 14 recording spectrophotometers.

References

.

1. Introduction.

In continuation of a type of activity carried on for many years at

the National Bureau of Standards, there is described in this letter circular the various types of standards that are available for issuance by

the Bureau for the purpose of checking or maintaining the over-all reliability of spectrophotometers in the ultraviolet, visible and near-infrared regions of the spectrum. Certain other information of similar purpose

is also included.

Experience has shown that spectrophotometers can easily get out of

adjustment. Although repeated trials may give the sa'me values over and over again, indicating high sensitivity and precision, and the instrument

may otherwise appear to be functioning perfectly, gross errors in wavelength may nevertheless be rendering the values obtained highly unreliable.

Other causes of error may likewise be present and unsuspected, particularly with the photoelectric spectrophotometers now comprising so large a percentage of the total in use. The use of the various standards described

herein has been found of considerable assistance in detecting and eliminating errors that would otherwise be present, or in confirming that the

instrument is in fact giving reliable results.

Reference is made in this letter circular to the NBS test fee

schedules currently in effect. Those relating to spectrophotometry are designated as 202.105, Spectrophotometry Standards, and 202.106,

Spectrophotometry Measurements. These schedules are obtainable from the

National Bureau of Standards.

54
2. Checking the Wavelength Scale.

Most of the present-day spectrophotometers have a direct-reading

wavelength scale; that is, the scale, instead of being divided in uniform

linear or circular measure, is divided and engraved directly in nanometers (formerly called millimicrons). This greatly facilitates setting

the instrument at any desired wavelength. The accuracy of many of these

direct-reading wavelength scales is remarkably good, when put in the best

average adjustment, considering the difficulties of quantity production

of such scales. When so adjusted, it is not uncommon to find them in

error by not more than 1 nm throughout the ultraviolet and visible spectrum.

However, if one wishes the uncertainties in his wavelength settings to be

of the order of 0.1 nm, a careful check of these direct-reading scales is

necessary.

Certain sources and wavelengths that have proved especially suitable

for the calibration of various types of spectrophotometers are listed in

tables la and lb which are similar in scope and purpose to table 1 of

Circular 484 (1)*.

*Numbers in parentheses refer to the References

All values of tables la and lb are consistent with those published in the

M.I.T. wavelength tables (2). Which of these sources to use, and which

wavelengths of the several sources are the most suitable, will depend on

the type of instrument. Furthermore, the procedure will vary importantly

depending on whether the instrument is a non-recording or a recording

spectrophotometer.

55
Table la. Wavelengths of Sources Suitable for Calibration of

Spectrophotometers.

Wavelength

(nm)

Note Wavelength

(nm)

Note Wavelength

(nm)

Note Wavelength Note

205.29

222.47

223.41

225.88

230.21

232.32

!35.25

237.83

239

239

244.69

246.41

248.

2

$

248.27

248 o 38,

. 0-> '.947

I.97J

1*

2*

.32

. 20*|

.37V

.5lJ

253.48

253.65

257.63

260.32

265

265

265

269.95

275.28

275.97

280.35

280.45

284.78

289.36

292.54

I

3*

4*

5*

296.73

302.15^

302.35

302.56

302.75

312.57

313.15

313

334

349.28

365. Of

365.48

366.29

366. 33^ 390.64

;.18\

.15/

6*

*

7-

8*

(nm)

Mercury Arc in Quartz (Same wavelengths in glass above 300 nm)

398.40

400.63

404.66

407.78

435.83

491.60

546.07

576,

579.

623.44

671.62

690.72

1014.0

1128.7

. u/

.96") I.07J 9*

Helium Discharge Tube in Glass

318.77

361.36

363.42

370.50

381.96

388.86

396.47

402.62

412.08

414.38

438.79

443.75

447 . 15

471.31

492.19

501.57

504.77

587.56

667.81

706.52

728.13

1083.0

* These lines have been found most useful on the Beckman DU

spectrophotometer.

1. A value of 239.95 is recommended for the unresolved pair.

2. A value of 248.3 is recommended when the 3 lines are unresolved.

3. The intensity of 253.48 is negligible compared to that of 253.65.

The latter value should be used when the lines are unresolved.

4. The 265.20 line is somewhat stronger than the others and a value

of 265.3 is recommended when the three lines are unresolved.

5. ' These two lines are of approximately the same intensity and a value of 280.40 is recommended for the unresolved pair.

6. The two shorter lines are considerably stronger than the other

two. It is probable that a value of 302.25 should be used for the unresolved

lines

.

7. A value of 313.16 is recommended for the unresolved pair.

8. With the arc used on the Beckman DU spectrophotometer the ratio of

intensities for 365.01 : 365.48 : 366.33 is 100 : 48 : 36, approximately.

The intensity of the 366.29 line appears negligible relative to that of

366.33.

9. These two lines are of approximately the same intensity and a value of 578.0 is recommended for the unresolved pair.

56
Table lb. Wavelengths of Sources Suitable for Calibration of Spectrophotometers

Neon Discharge Tube Aluminum Hydrogen Arc

Spark in Air

Wave- Relative

length Intensity Wavelength Wavelength

(nm) (ran) (ran)

585.25 5 216.88 434,05

588.19 4 217.40 486.13

594.48 8 220.46 656.28

597.55 2 221.00

603.00 2 226.35

607.43 8 226.91 Sodium Arc

609.62 13 236.71 Wavelength

614.31 25 237.21 ( nm^

616.36 6 237 .31 589. 00

621.73 4 237.34 589.59

626.65 11 237 .84

630.48 4 256.80

633.44 20 257.51 Cesium Arc

638.30 23 257.54 W^vpI (1 GL V V_' X pn^tH \— L L f—, Ull 640.22 100 263. 16 \ lull J 650.65 39 265.25 852.11

653.29 8 266 . 04 894.35

659.90 12 281.62

667. ,83 23 308.22

671.70 14 309.27

692.95 23 358.69

702.41 2 394.40

703.24 45 396.15

705.91

717.39 5 724.52 17

743.89 4

748.89

753.89

754.40
2.1 Non-Recording Spectrophotometers .

The best procedure for checking the wavelength scale of a non- recording spectrophotometer is by direct use of a source of radiant

energy having spectral lines of suitable intensity and adequately

spaced throughout the spectral range of interest. Various sources

are available and can be recommended for such purpose. How many sources,

or how many wavelengths, to use in such a calibration depends, of course,

on the desires of the individual investigator.

In this connection it should be noted that the number of significant figures of importance in spectrophotometry (including "absorption

spectroscopy") is of a different order of magnitude than that used in

emission spectroscopy or in standard wavelength tables. In the visible

spectrum with the usual type of spectrophotometer it seems impossible

to maintain the wavelength calibration with uncertainties less than about

0.1 nm. While the uncertainty may be less in the ultraviolet with a prism instrument, there seems no purpose served in giving standard wavelengths to better than 0.01 nm for spectrophotometry calibration.

Two suitable sources for wavelength calibration are the mercury

lamp and the helium lamp. A mercury lamp in a quartz envelope is by far

the best single source for wavelength calibration from 205 to 1014 nm. A mercury lamp in a glass envelope provides the same spectral lines except

that below about 300 nm they are not transmitted by the glass envelope.

The helium lines are especially well placed for wavelength calibration

in the visible spectrum, and the strong lines at 388 and 1083 nm are also

often very useful. Many other sources, flame or arc, are available for

visual wavelength calibration (2, 3) but most of these are too unstable for

accurate calibration with a photoelectric detector.

These same sources and many others are also useful for the wavelength

calibration of spectrographs used in photographic spectrophotometry. Between 200 and 400 nm the series of doublets obtained from the aluminum

spark in air is very useful because they are so readily recognized.

Not all of the lines for any of the sources are given in tables la

and lb but only those that are considered especially suitable for the purpose. Furthermore, not even all of those listed for any one source may

be suitable for any one particular instrument. The mercury arc in quartz

is an example. All of the lines listed (and still others) can be used for

wavelength calibration of a photographic spectrophotometer over the range

of sensitivity of the plate used. The lines from 404.7 to 690.7 nm can

be used for visual calibration of a spectrophotometer. But not all of the

lines are suitable for calibration of a photoelectric instrument, and

those that prove adequate will depend on the sensitivity and slit widths

characteristic of any particular instrument. One must be very careful

that other lines are not included, in addition to the one on which the

settings are supposedly being made, of sufficient intensity to affect

the wavelength setting.

58
Special attention should perhaps be called to the use of a cesium

arc at 852.1 and 894.3 nm (4). From tables la and lb it is apparent that

there are few suitable lines between 706.5 and 1014.0 nm, particularly

from steady sources necessary or desirable in the calibration of photoelectric spectrophotometers. The neon discharge tube gives many lines

between 750 and 1000 nm (2) but these have not been found satisfactory

in the calibration of photoelectric spectrophotometers. In the orange

and red the neon lines are useful for visual calibration and many of

these can be used to calibrate photoelectric spectrophotometers (5) if

the sensitivity is such that very narrow slits can be used. The relative

intensities (6) given in table lb will help in case of overlapping.

The best technique to use in wavelength calibration of non-recording

spectrophotometers, given a suitable source, will vary from instrument to

instrument and method to method. A few general principles can be given

here, however.

In photographic spectrophotometry it usually is sufficient to

photograph a known spectrum at the top and bottom of the plate, unless

the source used for the absorption spectra itself carries such known

reference lines. A few of these reference lines will then serve to

correlate that particular plate with whatever complete calibration curve

has previously been established by more extensive measurements, with the

various sources

.

On visual and photoelectric non-recording spectrophotometers, it

usually is necessary, for highest precision, to have a basic reference

line to which all of the other wavelengths are compared by direct check.

At the Bureau the Hg yellow lines have proved most suitable for the

Konig-Martens visual spectrophotometer (7). At the slit widths used the

overlapping of the two lines gives a central brighter "line" taken as 578.0 nm with a luminous background against which the slit jaws are readily seen. A luminous background, or slight illumination of the

ocular slit, always facilitates calibration when an eyepiece is used.

Visual calibration without an eyepiece is usually less precise unless

very narrow slits are used.

Two techniques have been used at the Bureau in the calibration of

non-recording photoelectric spectrophotometers. On the Gibson spectrophotometer (8) the slits are always 0.1 mm wide or greater and the most

reliable calibration is obtained by plotting galvanometer deflections

at closely adjacent wavelengths. The most probably value for the wavelength reading is given by the intersection of the two straight lines

resulting from a plot of the data for any given line, the correction

being given by the difference between this value and the true wavelength.

This is illustrated in reference (1).

59
On the Beckman DU spectrophotometer the same method has been used

(9), but at the Bureau it has seemed preferable and is much more rapid,

to calibrate with a narrow slit and record the wavelength dial reading

for the maximum left deflection of the galvanometer needle as the wavelength dial is slowly turned. The most suitable reference line on two

of the Bureau's instruments has proved to be the Hg green line at

546.07 nm (5).

2.2 Recording Spectrophotometers .

The initial wavelength calibration of a recording spectrophotometer,

such as the manufacturer must f carry out in connection with cutting his cams or preparing his reading scale, is not here considered, but only

the check of such a calibration by the user of the instrument.

Such a user can, of course, follow the procedure prescribed above

for checking the wavelength calibration of non-recording spectrophotometers. However, there are two important reasons for following a different procedure for recording spectrophotometers. For such instruments it is desirable to have a calibration that is made with the instrument operating. It is further desirable in most kinds of work to have

this calibration appear on the graph sheet so that difficulties connected

with positioning of the sheet, expansion or contraction of the paper with

humidity or temperature, or instrumental variations can be eliminated.

Wavelength calibrations of this kind can be made if a material is

available having a number of strong and narrow absorption or transmission

bands suitably spaced over the spectral range of interest. Two materials

have been used or suggested for this purpose: (a) Glasses containing

rare-earth oxides, such as didymium glasses and holmium oxide glasses,

have been used for many years at the National Bureau of Standards (10, 11),

(b) quartz -Polaroid combinations have been proposed (12) and may prove

useful for such work.

The use of a didymium glass or a holmium oxide glass in this manner

would not in general be accurate unless it is calibrated at nearly the same slit widths as are to be used» Most of the absorption bands that are usable for the purpose are multiple bands and the wavelengths of maximum

absorption often depend on the slit widths. This has been illustrated in

previous publications (1, 10).

While the use of a didymium glass or a holmium oxide glass for checking the wavelength calibrations of a recording spectrophotometer is highly

recommended, there are two other uses of these glasses which are not

recommended. First, these glasses are not well suited for checking the

photometric scale of any spectrophotometer, recording or non-recording.

Transmittances at the peaks of the absorption bands are too dependent on slit widths, and transmittances on the steep parts of the curve are too

dependent on slight wavelength errors, both as illustrated in Fig. 1 of

reference (10) and in Fig. 8 of reference (1). Second, the use of these

glasses to check the wavelength calibration of a non-recording spectro60
photometer is considered much inferior from the standpoints of time,

convenience and reliability to the direct use of line sources as described in Sec. 2.1. The National Bureau of Standards has consistently refused to accept didymium glasses or holmium oxide glasses for

calibration for either of these two purposes

„

2.2.1. General Electric Recording Spectrophotometer .

The NBS didymium glass standards were carefully calibrated by

point-by-point measurements on the Konig-Martens visual and Gibson

photoelectric spectrophotometers with slit widths approximating the

10- and 20- nanometer slits used on the NBS General Electric spectrophotometer. Some of these values have been published (13).

The most suitable didymium glass for the purpose, considering

type of curve and availability, is a Corning 5120 glass of 3.0 mm

thickness. While it is not known how much the wavelengths of maximum

absorption of this 5120 glass might vary from melt to melt, glasses

from at least three melts have been measured, and there has never been

any certain variation among the samples tested. For much work it is

probably safe to ,use the values given in table 2.

Table 2. Wavelengths of maximum absorption for Corning 5120 glasses

of 3.0 mm thickness as obtained at the National Bureau of

Standards for the slit widths indicated.

Wavelength

of

Maximum Absorption

(nm)

Approximate Spectrum

Interval

Transmitted by Slits

(nm)

441..

475.,

528.:

585.,

684.

743.

745.-

808.

883.

1067.

8

10

10

10

10

10

10

20

20

20

20

For those who wish greater certainty, however, the Bureau has

obtained a supply of Corning 5120 glass in 2-inch polished squares

and of 3.0 mm thickness. These are measured and the values reported

in accordance with NBS test fee schedule 202.105, items d to f. The

measurements consist of recording a curve of the test glass on the same sheet as the curve of the NBS standard glass and deriving values

of the wavelengths of minimum transmittance of the former relative to

61
those of the latter. The over-all uncertainties of the values so reported are considered to be not greater than +1 nm from 441.0 nm

to 743.5 nm, and not greater than + 2 nm from 745 to 1067 nm.

Methods of use of a calibrated didymium glass on a G. E. recording

spectrophotometer are described in references (1), (10), and (13).

2.2.2. Cary Model 14 recording spectrophotometer .

The NBS holmium oxide glass standards were carefully calibrated

by using a Cary Model 14 recording spectrophotometer, the wavelength

indicating dial of which had previously been calibrated by means of

a number of sources having wavelengths throughout the ultraviolet and

visible spectral regions. Measurements of the wavelengths of minimum

transmittance were made as functions of slit width over the range 0.06

to 5.0 nanometers of spectral width„ Eleven sharp absorption bands were found to be sufficiently symmetrical that the wavelengths of

minimum transmittance indicated by the recorder remained constant for

slit widths up to about 2 nanometers.

The most suitable holmium oxide glass for the purpose is a Corning

3130 glass of approximately 2.5 mm thickness. It is not known how much

the wavelengths of minimum transmittance of this 3130 glass might very

from melt to melt. It is known that, for some 3130 glasses, the absorption of the base glass prevents the use of the glass for wavelength

calibration in the ultraviolet near 241 nanometers. For much work it is

probably safe to use the values given in the following table.

Wavelengths of minimum transmittance for Corning 3130

glasses of 2.5 mm thickness as obtained at the National

Bureau of Standards for slit widths less than 2 nm.

Useful wavelengths Useful wavelengths

between 240 and 370 nm between 360 and 650 nm

241.5 360.8

279.3 385.8

287.6 418.5

333.8 453.4

360.8 459.9

536.4

637.5

For those who require that the base glass of the standard transmit

sufficiently for the standard to be useful at 241 nanometers, the

Bureau has obtained a supply of Corning 3130 glass in 2-inch polished

squares and of 2.5 mm thickness. These are measured and the values

reported in accordance with NBS test fee schedule 202.105, items g to i.

62
The measurements consist of recording a curve of the test glass on

the same sheet as the curve of the NBS standard glass and deriving

values of the waveleng/ s of minimum transmittance of the former

relative to those of the latter 0 The over-all uncertainties of the

values so reported are considered to be not greater than + 0.5 nm. The present supply of holmium oxide glass contains striae, and in some cases strains, -which have not appreciably altered the wavelengths of minimum transmittance.

The methods of use of a calibrated holmium oxide glass on a Cary Model 14 recording spectrophotometer are similar to those

described in references (1), (10), and (13) relating to the method

of use of a didymium glass on a G. E. recording spectrophotometer.

63
References.

1. Kasson S. Gibson, Spectrophotometry (200 to 1000 millimicrons) NBS Circular 484 (1949).

2. Massachusetts Institute of Technology Wavelength Tables,

measured and compiled under the direction of George R» Harrison (John Wiley & Sons, Inc., New York, 1939) .

3. Spectrophotometry, report of 0.S o Ao Progress Committee

for 1922-23, K. S. Gibson, Chairman, Jo Opto Soc. Am0 &

Rev. Sci. Inst. I0j 169 (1925).

4o No Co Beese, Cesium vapor lamps, J, Opto Soc, Am0 36, 555

(1946)

o

5. Kasson S. Gibson and Margaret M. Balcom, Transmission

measurements with the Beckman quartz spectrophotometer,

J. Research NBS 38_, 601 (1947) RP1798; also J. Opto Soc»

Am. 37, 593 (1947).

6. M. Garbuny, Relative Intensitaten und Ubergangswehrscheimlichkeiten der rotgelben Neonlinien, Zeit. Physik 107, 362

(1937).

7. Ho Jo McNicholas, Equipment for routine spectral transmission and reflection measurements, BS Jo Research

V, 793 (1928) RP30.

8. K. S. Gibson, Direct-reading photoelectric measurement of

spectral transmission, J. Opto Soc. Am. and Rev. Scio Inst.

Tj 693 (1923); also described in J. Opt. Soc. Am0 21_, 564

(1931) o

9. H." H. Cary and Arnold 0. Beckman, A quartz photoelectric

spectrophotometer, J. Opt. Soc. Am. 31, 682 (1941).

10. Kasson S. Gibson and Harry J. Keegan, Calibration and operation

of the General Electric recording spectrophotometer of the

National Bureau of Standards, J. Opt. Soc. Am0 28, 372 (1938).

11. H. J. Keegan, J. C. Schleter, and V. R. Weidner, Ultraviolet

wavelength standard for spectrophotometry, J. Opt. Soc. Am.

51, 1470 (1961).

12. George L. Buc and E. I. Stearns, Uses of retardation plates in

spectrophotometry, II, Calibration of wavelength, J. Opt. Soc.

Am. 35, 465 (1945).

13. Harry J. Keegan and Kasson S. Gibson, On the use of working

standards of didymium and Vitrolite glasses for spectrophotometry measurements, J. Opt. Soc. Am. 34, 770 (1944).

64
Appendix IV

JOURNAL OF RESEARCH of the Notional Bureau of Standards - A. Physics and Chemistry

Vol. 76A, No. 5, September-October 1972 An Accurate Spectrophotometer for Measuring the

Transmittance of Solid and Liquid Materials

R. Mavrodineanu

Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234

(May 31, 1972)

The optical transmittance of solids and liquids as well as the molar absorptivity of various chemical

species are parameters of fundamental significance in characterizing these materials. Meaningful trans- mittance data can be obtained only when the measurements are performed with well-known accuracy and precision. To perform such measurements, a high accuracy spectrophotometer was designed and

assembled at NBS, Analytical Chemistry Division, and will be described in this paper. This single- beam instrument is composed of a constant radiation source, a monochromator, a sample carriage, an

integrating sphere-photomuJtiplier assembly followed by appropriate electronics, and a read out system

consisting of a digital voltmeter and a computer data acquisition and handling provision. The accuracy

of transmittance measurements is determined by the light-addition principle used in conjunction with a two-aperture arrangement. The spectrophotometer can be used in manual or automatic modes of operation. A detailed discussion of the data obtained with this instrument, used in both modes, will be pre- sented together with its application to the certification of solid and liquid Standard Reference Materials

for checking the photometric scales of conventional spectrophotometers.

Key words: Absorbance; automation of accurate spectrophotometer; instrumentation, spectrophotometric; spectrophotometry, high accuracy; standard reference material in spectrophotometry;

transmittance.

Transmittance is the ratio of two radiation flux in- tensities. It is therefore necessary that the photometric

scale of the spectrophotometer used to perform the measurements be accurate. The transmittance of a particular material is also a function of wavelength; hence

the wavelength scale of the monochromator should also be accurate, and appropriate spectral bandpasses

should be used. The measurements should be made

using eollimated radiations. Such radiations define un- ambiguously the actual path length through the trans- mitting medium, the reflection losses, and eliminate

the effects of polarized radiations that are produced at the surface of the sample. Other important factors which must be considered are: homogeneity and stability of the sample, radiation scatter inside the sample,

interference phenomena, stray radiation, polarization,

fluorescence, temperature, particulate matter, and sur- face conditions. Since transmittance measurements

depend on a diversity of factors, meaningful values can

be obtained only by defining the experimental conditions for obtaining transmittance data [1, 2].

2 Spectrophotometers are used to perform two types of measurements:

(1) Quantitative determination of chemical species

using the relation between optical transmission of the

material, and the concentration as a measuring parameter. Under these circumstances, the photometric scale

' Figures in brackets indicate the literature references at the end , if tins paper.

I. Introduction

Optical transmittance is due to an intrinsic property

of matter and characterizes a particular transparent

material. Since this parameter is not known a priori, it must be determined by experimental procedures. True transmittance values can be obtained only by

using accurate measuring techniques and by taking

into consideration all factors which can affect and distort the data. 1

1 The optical transmittance of a solid material includes the reflection losses which occur

at the air-solid interface. The internal transmittance isflenned as the transmittance of the materia) corrected for reflection losses (2). This internal transmittance can be calculated in principle from the transmittance by using the well known Fresnel equations (1, pp. 98 to 100). For eollimated radiation the reflectance R, for a material with an index of refraction, n, and an absorptivity, a, at wavelength. X. is given through:

„ (B>-l)' + n; Bj (B. + D' + nioJ

absorbing material and eollimated radiation:

(n. + l) For glass, n is approximately 1.5 in the visible region of the spectrum, and R will be about 4 percent at every air-glass interface. When noneollimated radiation is used: s,nMa-flK

» sinMa + /3).

for perpendicular polarized radiation, and

., tan2 (a-B)x

» lanMa + 0),

for parallel polarized radiation, where a and 0 are the angles of incidence and refraction, respectively.

In eollimated radiation and in air, a = fl = 0 and /?' = R" = R.

65
of the spectrophotometer i9 calibrated in meaningful

units, using a series of refpr*>n<^ solution" having

known concentrations of the species to be determined,

rather than values of optical transmittance. The accuracy of the measurements is related solely

to the accuracy with which the concentration of the reference solutions is known and to the precision (stability, sensitivity, reproducibility) of the spectrophotometry method and instrument used. The accuracy of the photometric scale per se, is not a critical factor in such measurements. The precision, stability, and reproducibility of the instrument can be checked before each series of meas- urements by careful use of solid or liquid reference

filters having well established transmittance* values.

(2) Determination of the optical transmission characteristics of solid or liquid materials, and the determination of molar absorptivities of chemical compounds.

In both cases the accuracy of the photometric scale of the measuring instrument, among other things, is essential to provide true values. Ways to establish and check this important parameter are critically needed.

Since conventional spectrophotometers do not pro- vide means to check photometric accuracy or to evaluate the possible sources of systematic errors, it was decided in 1969 to design and construct a re- search spectrophotometer on which transmittance measurements could.be performed with well defined accuracy. Such an instrument would be used to deter- mine optical transmittance of selected solids and

liquids at various wavelengths. These materials can be used as standard reference materials (SRM's) to check

the accuracy of the photometric scale of conventional spectrophotometers. The same certified SRM's could

likewise be used to monitor the precision, stability, and reproducibility of those instruments [3, 4], After a comprehensive examination of the literature

in this field [5 to 34] arranged in chronological order, an instrument was developed which is similar in principle to the instrument at the National Physical

Laboratory (NPL), Teddington, England, where a long

tradition of high accuracy spectrophotometry exists. The instrument described in this work performs' measurements of radiant energy in the visible and

ultraviolet region of the spectrum, with well established and high photometric accuracy. Transmittance meas- urements on solids and liquids can be made with this instrument using collimated as well as noncollimated

beam geometry. The wavelength accuracy and spectral

bandpass achievable are adequate to avoid degradation of photometric accuracy, and the other inter- ferences mentioned have been given careful consideration, and, in most cases, have been assessed quantitatively. The transmittance measurements on the optically neutral glass filters discussed in this work have been

made with a noncollimated beam geometry corresponding to an aperture of about f : 10. The image of the exit

slit of the monochromator (8 mm x 0.5 mm) was

produced at the center of the entrance face of the

filter. All measurements have been made against air for the nonattenuated radiation flux, and no correction for reflection losses was made. Transmittance measurements made with noncollimated radiation by

projecting the image of the exit slit of the monochromator on the entrance face of the sample using an opening of f:10 (total angle of about 7° or 8°), may

differ by several parts in 104 of the value when compared with similar measurements made with colli- mated radiations, as indicated in this Journal by

K. Mielenz. Noncollimated beam geometry was applied in this work to approach the measuring conditions used in most of the conventional spectrophotometers which

are available today. A brief description of this instru- ment was given earlier in reference [3].

II. Description of the Instrument3

The high accuracy spectrophotometer, completed

and tested in 1970, is a single beam instrument which

contains the following components: (a) a constant

radiation source, (b) a monochromator, (c) a sample

holder, (d) a system to check the accuracy of the photometric measurements, (e) an integrating sphere

attached to a photomultiplier-digital voltmeter unit, and (f) the data presentation system. Figure 1 illus- trates schematically the arrangement of these various components. A circular neutral wedge is placed after the light source to select various levels of radiation

intensities required for measurements. A description

of the components is presented in the following

sections.

a. The Radiation Source. Since the instrument is a single-beam type, it is essential that the radiation source be constant and homogeneoirs. Additional

desirable conditions are: capability of monitoring the current supplied to the source and radiation similar

to that from a Planckian radiator. The source is similar in design to that developed and used at NBS

by H. J. Kostkowski and R. D. Lee of the Institute for Basic Standards. This source was duplicated in our instrument with the kind assistance of its developers.

The source is used in the spectral range 360 nm to 800 nm and consists of a tungsten incandescent fila- ment lamp with a tungsten ribbon 8 mm long by 2 mm

wide. The connections to the lamp terminals are soldered to minimize contact problems (see fig. 6). The direct current required to operate this lamp at approximately 3000 K is 18 A across a 6 v drop; our source is operated at 5 V and 15 A. The d.c. power supply is capable of delivering 15 V and 50 A,

and can be operated in constant current or constant

voltage modes. To achieve the constant current mode

an external sensing resistor of 0.1O and 50 A and

a current control circuit are placed in series with the power supply. A feedback voltage across this resistor

is connected to the sensing system. The character

3 The commercial instruments and parts used in the construction of the spectrophotometer are identified in the addendum.

66
Precision Optical Bench

us sum mi

III! 1CC0S1CT sricTiiriiriiiru

Clrcular|Neutral Wedge

^ Source

NPL Aperture tor Linearity Control

(After F.J.J. Clarke}

BLZZI

(A) +(Bl=(A + Bl

Constant Power

Supply

Sample

1m Grating Monochromator (Czerny -Turner

Precision Optical Bench in Lightproot Enclosun

I

Blank

Integrating Sphere

FIGURE 1. Principle of the single beam high accuracy spectrophotometer. The aperture unit is placed on the optica] bench only when linearity measurements are performed.

istic of this function is the ability to automatically

change its output voltage to maintain a constant

current to the load resistor, which, in our case, is the lamp source. The rjominal current regulation obtained is better than 0.01 percent, and the stability over an 8 hour period, at constant load temperature,

is better than O.02 percent. The stability of the current

delivered to the lamp is monitored with a high accuracy

potentiometer used in conjunction with a null meter. This meter is sensitive to variations in the current supplied to the lamp from 1 part in 1000 to 1 part in 1,000,000 per division (fig. 1 and fig. 14). The po- tentiometer is connected to the current source across

a resistor (0.01O and 100 A) placed in series with the lamp.

The demagnified (2 to 1) image of the ribbon fila- ment is projected on the entrance slit of the pre- disperser by a fused quartz (nonfluorescent SiCM

lens whose focal distance is 254 mm and diameter is 44 mm. This and the other lenses used in the optical

system, were calculated by K. Mielenz of the Institute

for Basic Standards at NBS. The lenses are mounted

in carriers which permit orientation in any position.

A circular neutral wedge is placed between the light source and the predisperser. This wedge, evaporated

inconel on a fused quartz disc (150 mm diam), is linear

in density and provides a light attenuation of 100 to 1. The wedge is motor driven (1 rev. per s) to select proper radiation intensity levels as required by the measurements (figs. 2, 3, and 4). The radiation source used for measurements in the ultraviolet region to 275 nm is a single coil tungsten-bromine incandescent

lamp (fig. 5) supplied by an adequate power source; below 275 nm, a deuterium discharge lamp is con- templated.

b. The Monochromator. The monochromator is a 1-m Czerny-Turner type grating instrument with a dispersion of 0.8 nm/mm. The flat grating has 1200

grooves per mm covering a surface of 100x100 mm.

The monochromator is provided with a predispersing attachment to reduce the stray light (fig. 3). This preFlGURE 2. (>eneral views of the spectrophotometer. Rear: optical bench carrying the tungsten-halogen radiation source used for checking the alignment of optical components, followed by a quartz

lens, the circular quartz neutral wedge, and a flat mirror. Left: the 1-m Czerny-Turner grating monochromator (the predisperser

is not illustrated here). Front: optical bench carrying a quartz

lens, the single sample and blank carriage, a second quartz lens, and the integrating sphere with the photomultiplier housing.

67
Figure 3. Close view of the tung&ten ribbon filament lamp on its adjustable holder, followed by The quartz lens — circular neutral wedge assembly, and by the flat mirror in its adjustable holder. The 30° quartz prism l.ittrow type predisperser is located at the entrance slit of the 1-m grating monochromator. Extreme left: neon gas laser used to check the optical alignment, and mercury

discharge lamp for wavelength calibration. When in use, the tungsten ribbon lamp is surrounded by an enclosure with black

walls (50 cm x 50 cm x 70 cm high). Rear: enclosure containing the

optical units illustrated in figure 2.

disperser is a small quartz prism monochromator

connected to the scanning system of the 1-m instru- ment. A wavelength counter permits readings to

0.1 nm and the scanning speed can be varied from 0.05 nm to 200 nm/min by a 12 speed synchronous electric motor. The optical components are placed on precision lathe bed type optical benches which are 160 and 120 cm

long, and are equipped with appropriate carriers provided with x-y-z adjustments.

c. Sample Carrying Systems. The spectro- photometer is provided with two sample carrying sys- tems. One system measures one sample and its blank, while the other system permits sequential measure- ments for seven samples and eight reference reading

positions against air, and can be operated manually

or automatically through a computer interfaced with the instrument. The single sample carrying unit consists of a plat- form provided with two vertical holders which can accept J-in (14 mm) rods and a variety of sample

supports (fig. 2). These holders can be moved laterally through a rack and pinion arrangement. The platform

is mounted on 4 ball bushings which ride on two

horizontal rods and can be moved pneumatically

across the optical axis. The pneumatic operation was recommended by G. E. Moore and J. T. Sterling of the Institute for Materials Research at NBS and

by L. Owen, a guest worker at NBS. The travel dis- tance is 8 in (20 cm) and the linear movement is smooth; the position of the platform and the sample

in and out of the optical beam, can be reproduced

within 0.025 mm. This unit is illustrated in figure 2

FIGURE 4. Close view of the circular, neutral wedge. The front plate which carries the fused silica lens was removed to show the fused

silica disc with the evaporated metal layer.

and is located between the two quartz lenses. The

sample holder is designed to accept conventional

solid or liquid filter holders which fit most spectro- photometers. These holders are provided with a thermostating jacket, and can be rotated in the

horizontal plane through a 10 cm diameter rotating

table. A filter holder which permits the rotation and

scanning of the sample in the x—y direction is also available (fig. 7). It is provided with micrometer

screws having a total linear motion of 25 mm with

0.01 mm per division. The seven-sample carrying

unit is illustrated in figures 8 and 9 and consists

of a semicircular aluminum-alloy plate placed hori- zontally on an appropriate carrier on the optical bench

along the optical axis. This plate, which is 32 cm in diameter and 2.5 cm thick, can be rotated clockwise through a pneumatically operated precision ratchet system in increments of 12°. The stepwise rotation

utilizes a solenoid valve which is operated electrically

by a switch located outside the enclosure. This

switch can be operated manually or automatically by

computer (fig. 14).

68
Figure 5. Single coil tungsten halogen lamp in the adjustable holder

The semicircular plate carries seven sample holders

similar to those used for the single sample system

described earlier. The holders are placed at 24°

intervals and are separated by blank spacings. About

1 atm of air pressure is used to operate the plate and

the rotation is set at 2 s per 12° step when the auto- matic computer operating mode is used.

d. System to Check the Accuracy of the

Photometric Reading. Since the high accuracy

spectrophotometer is single beam, accurate photo- metric data are obtained when there is a linear relation between the measured radiation flux and the corre- sponding response of the photodetector.

Linearity of photodetectors can be measured by

several means: the inverse square law [7, 15); the use of

optical elements having a known transmittance which

can be determined by other means [17| and the light addition principle of Elster and Ceitel using a plurality of light sources [5, 6, 8, 9, 10, 13. 18, 19, 20, 28, 31, 33,

34] or multiple apertures [11, 12, 14, 16, 21, 23, 25, 26,

27, 30 1. A novel approach to the problem of accurate

FIGURE 6. The tungsten ribbon filament lamp in the newly designed

adjustable holder, the platform which carries the lamp is similar

to that described in figure 5 and can be oriented in the horizontal plane through the six screws spaced around the edges of the platform at 6(f intervals. Three screws push the platform while

the other three pull. The current-supplying wires are soldered directly to the lamp

terminals to eliminate contact problems.

photometric measurements was described by O. C. Jones and F. J. J. Clarke [24. 29| and by F. Desvignes

and J. Ohnet [32]. A critical discussion of some

aspects of accurate spectrophotometry will be found

in an NBS manuscript by Cibson and associates [22]. The radiation addition principle, using two apertures

with one source of radiation, was chosen for our work. The aperture method for checking the linearity of photometric data was in use at the National Physical

Laboratory from about 1930 onwards, and one form of it was described by Preston and Cuckow [11 1 in conjunction with a single beam spectrophotometer, using a

five aperture screen. One year later, Buchmiiller and

Konig [12] described and used a two aperture unit. At NBS, Barbrow [14| used a 10 aperture arrangement,

while Harding [16[ and Cordle and Habell [25] a. NPL

described a two aperture system. Multiapertures were

used by Hoppmann [21 1, BischofT [23], Sanders [26|

and Nonaka and Kashima [27]. Finally. Clarke 130]

69
Figure 7. Same as figure 2 except lor the sample holder which

in this case is capable of rotating the sample 360° and to displace

it in the x-y direction through the micrometer screws.

Figure 8. Circular platform carrying seven filter holders. The \i> position switches (7 sample positions and 8 blank positions) are

visible along with the two quartz lenses. The exit slit of the monoehromator is at left.

discussed in detail the use of a two aperture system to check the accuracy of photometric data obtained on the spectrophotometer at NFL. It is this two aperture

system which is used at NBS.

The two aperture unit consists of a metal plate (130 mm by 100 mm) containing two rectangular

FIGURE 9. Same as figure 8. The pneumatic cylinder which rotates the circular platform through a ratchet mechanism is visible at the rear of the platform. The integrating sphere with its pneumatic

shutter is seen at right.

Figure 10. Same as figure 2. In this case the dual-aperture unit

for linearity control is located on the optical bench after the exit

slit of the monoehromator.

windows, A and B, (20 mm by 8 mm) located one above

the other (figs. 10, 11, 12). Each aperture can be closed by a light-tight shutter which is operated pneumatically

by remote control (fig. 14). The aperture plate is placed

in the optical path after the exit slit of the monoehromator and within the optical solid angle of the instrument. The image of the apertures is then pro- jected on the target of the integrating sphere. A fused

quartz lens with a focal distance of 190 mm and a diameter of 60 mm is used for this purpose. The arrange- ment is illustrated in figure 10. No optical element

7 0
FIGURE 11. Detail of the dual-aperture unit showing its construction and the pneumatic system which operates the two shutters. One aperture is open, the other is ciosed.

should be placed between the aperture plate and the monoehromator. The linearity check consists of measuring the photocurrent produced when aperture

A is open then closed, and then aperture B is open and

then closed. The value of (A) + {B) is compared with the values obtained with both apertures (A + B) open. If the system is linear these two values should be identical:

(A) + (B) = (A + B).

If this is not the case, the system shows nonlinearity

which is proportional to the amount by which the sum of (A) + (B) differs from (A + B). This difference is then used to correct the transmittance values measured

on the solid or liquid filters.

e. Integrating Sphere and Photomultiplier

Arrangement. The radiations emitted from the exit

slit of the monoehromator and passing through the aperture or the filter are received on the target of the integrating sphere. This sphere is illustrated in figures 2, 7, 9, and 10. A block of aluminum made

from identical halves was cut to produce a half sphere

in each block. The halves were joined together to form

a hollow sphere. Its diameter is 125 mm and a target, made from a circular plate, 35 mm in diameter, is located at the center of the sphere. The front surface

of the sphere has a 20 mm diameter opening. This

FIGURE 12. Front view of the dual-aperture unit, with both

apertures open.

opening can be closed by a shutter which is operated

remotely by a pneumatic system. A 50 mm diameter

opening is at the opposite end to which the housing of the photomultiplier is attached by an "O" ring to pro- vide a light-tight joint. The inside of the sphere is coated using a suspension of BaSC^; the outside is painted black. Under these circumstances the sensitive surface of the photodetector receives the radiations originating

from the exit slit of the monoehromator only after these radiations have undergone at least two diffuse

reflections. The photomultiplier is a 50 mm flat-faced, silica end window tube with a 44 mm cathode and 11 Venetian blind dynodes having CsSb secondary emitting

surfaces. The cathode is an S-20 or tri-alkali type. The spectral range of this tube is from below 200.0 nm

to 850.0 nm. The operating voltage used is 850 V. The

photomultiplier output is supplied to a current-to- voltage converter consisting of an operational amplifier with high precision feedback resistors with

values of 106

, 3 x 106

, 10 7

, 3 X 107, and 10*n. Dark

current compensation is also available. This electronic system, described in figure 13 was designed and

assembled by K. W. Yee of the NBS Electronic In- strumentation Section. The output from the currentto-voltage unit is connected to a digital voltmeter,

illustrated in figure 14, with one microvolt resolution on the 1 V full scale range.

71
I

. _J

OUIPUT

IV m IIDICUEQ

FULL SCALE e°'..i)

' SLLECIED IW ? iil I HtlO 10 WITHIN 0 1%

UiLOt DEVICES

Figure 13. Schematic of the current-to-voltage circuitry. Courtesy of K. W. Yee.

The optical components located after the exit slit of the monochronr*ator, including the photomultiplier

tube, are enclosed in a light-tight box 200 cm long, 70 cm wide and 76 cm deep (fig. 3). The removable

front panel is provided with a sliding door to permit

rapid access to the filter-holder system. The box

contains outlets for the compressed air which operates

the apertures, sample carriage and integrating sphere

shutter, and for the electrical connection from the photomultiplier. The inside walls are lined with thermal

insulation painted black. When in use, all nonblack

metal parts are covered with a black cloth to reduce

stray light. The entire equipment is placed on a vi- bration isolation table 3.66 m by 1.52 m. The optical

benches and the monochromator are secured by stops which are attached to the table surface. The align- ment of the optical parts is made and checked periodically with a low-power laser shown in figure 3 (CW gas

laser, output power 2 mW, A. 6328 A) and with a high

intensity tungsten-halogen lamp shown in figure 5.

f. Data Collection and Presentation Systems.

The data output from the digital voltmeter (DVM).

corresponding to the current generated at the photomultiplier tube by the radiations passing through the aperture system {A, B, A + B) or the samples (/) and

blanks (Io), can be obtained by visual means or computer operation. Both methods have been used in this work with good results. In the visual mode, the operator examines the digital voltmeter display and

takes a mental average of the data. The display rate

is adjusted to about one reading per second. When measurements are taken by computer, the display of the digital voltmeter is adjusted to a faster

rate; for instance. 10 to 20 data per second, depending

on the capabilities of the instrument and measurement

requirements. In our work, we use 10 data per second

and collect 50 individual data for each measurement.

This information is fed to the computer which calculates and prints the results as averages with the cor- responding standard deviation, relative standard

72
CAl IRhATinN OF SKM930 AT 440 NM SAMPLE NUMBFR AND POSITION:

1-70 IN 2; 1-79 IN 4; 2-79 IN 6; 1-91 IN 12; 3-91 IN 14 REFERENCE IN 8; 3-79 Ih

Figure 14. Console containing the power supply for the photomultiplier tube, the dc null detector, the current-to-voltage converter, the digital voltmeter, the command panel for computer

operation, and the command panel for the pneumatic operation

of the shutter, aperture system, and single sample carriage. The

electric switches for operating the circular neutral wedge are also located on this panel. Middle right: potentiometer for monitoring the dc current supplied to the tungsten ribbon filament lamp.

At bottom left: teletype for data presentation.

Right: light panel which indicates the position of the automatic seven sample holder.

deviation, and sample position number to identify the measurement. When transmittance measurements

are made on individual samples or when linearity

cheeks are performed, the readings are initiated manually for every position. When the seven sample

holder is used for sequential measurements, the operation is performed automatically by the computer. It is programmed to take a predetermined

number of individual DVM readings (50), print the arithmetic average, followed by the standard deviation, relative standard deviation, percent transmit- tance and sample position (fig. 15). At the conclusion

of each measurement, the computer initiates a signal

which rotates the holder to the next position. This is followed by the data taking and sample changing sequence until the measurements are stopped manually or automatically by a provision made in the computer program.

au .2001700E 01 .64 1344 IE 00 .2001230E 01 .6S28346E 00 .2001040E 01 .4221U0F 00 .P000995F 01 •6560142F 00 •2001402F 01 .23SB762F 00 .2001539E 01 -6739426F 00 .2001205F 01 .2386761E 00 .2001 101E 01 -2001622F 01 .6412983E 00 .2002043E 01 .6S264R2E 00 .2001551E 01 .42222R2E 00 .2001507F 01 .6S62004E 00 .P00I5P9F 01 .2359933E 00 -2001634E 01 .6741099F 00 .20015R9F 01

• 2 3B6R2R F 00 .20018P9F 01 .2002214F 01 .6413472F 00 .2001RHPF 01 .6526177F 00 •2001741F 01

• 42P.341BF 00 .2002075F 01 •65620R3F 00 .2001871F 01 .23SR786F 00 .2002130F 01 .6740S63E 00 .P001997F 01 .23883S4F 00 •200247RF 01

.43R8F-03

.3256E-03

.4320E-03

.4804F-03

.42S4E-03

.2293E-03

.3041E-03 -3189F-03

• 4476F.-03

. 142SF-03

.4 1 I4F-03 .2848F-03

.4678F-03

. 1 064E-03

.4150E-03

.3187E-03

. 1 32 1 E-03 .6139E-03

.2474F-03

•4756E-03

.2 1 74F -03 .4355F-03

.3344F-03

.5337E-03

. 1 686F-03 .4S32E-03

.3418F-03

.636RF-03

. 1604E-03 .33RRF-03

.PR26E-03 .3S18E-03

.3BRPF-03

-2513F-03

. 384 1F-03 .29S7E-03

.4892F-03

. 1 365F-03

.3708F-03

. 1 S96F-03

. 5036E-03

. 3022F-03

.4545F-03

. 1 704F-03

.2975F-03

S/AV

.21 92F-03

.5077F-03

.2 1 59F-03 73S9F-03

.2 126F-03

. 5432E-03

. 1520F-03

.4861 F-03 .PP36F-03

.6039F-03

.20S5F-03

.42P.6F-03 .2338F-03

•4456E-03

.2074F-03

. 1 592F-03 .2060F-03

. 3067E-03

. 3790F-03

-2376F-C3

. 5149F.-03

.21 76F-03

. 509SF-03

-2667F-03

. 7992F-03

•2264F-03

. 5O70F-03

. 3 182F-03 .6720F-03

. 1 693F-03

. 1 4 12E-03

. 54R6F-03

. 1939F-03

. 385 I F-03

. 1919F-03

. 7000F-03

.2443F-03 .2081E-03

. 1852F-03 .6767E-03

.25 1 SE-03 .4464F-03

.2270F-03

. 71 33E-03

. I48SF-03

FFRCFNT T

32 .044

32 .623

2 1 .095 32.781

1 1.785 33.674

1 1 .927

32 .036

32 .603

2 1 .095 32.785

1 1 . 790

33 .678

1 1 .924

32 .035

32 .601

2 1 .097 32.778

11 .782 33.668

1 1 .928

FIGURE 15. Computer data presentation. The programming of the entire computer operation was developed by J. Aronson, R. Freemire, and J. Wing. The computer-instrument interfacing was

performed by F. Ruegg and R. Shideler of the NBS

Analytical Chemistry Division, Technical Service Group, under the supervision of J. DeVoe.

III. Stability of the Electronic System

As a rule, before taking measurements with the spectrophotometer, a warmup period of one hour is required. The room temperature is kept at 24 ± 1 °C,

and the relative humidity is 35 percent. The particulate matter is controlled through special filters which

rates the room in the 100,000 class. The dark current of the photomultiplier tube was

measured by taking 15 replications each consisting of the average of 50 individual digital voltmeter readings.

These measurements were made using 850 V at the anode. The average dark current under these cir- cumstances produced 0.000682 V with a relative standard deviation of 0.71 percent.

In all of our work, a dark current buck-out arrange- ment was used. A series of measurements were per- formed to determine the stability of this dark current compensation. To this effect, 15 consecutive measurements, each representing the average of 50 indi73
vidual digital voltmeter readings, were made and the average dark current value was 0.000024 V with a

relative standard deviation of 23.1 percent. Four tests were made to determine the stability of

the electronic system and the radiation source using

the computer data acquisition mode.

a. Stability of the Current-to-Voltage Converter. A constant voltage was supplied to the converter using the dark current compensation provided on the unit. Fifty individual measurements were taken

every 5 seconds and the average value was printed

along with its percent standard deviation. The meas- urements were then repeated 15 times and an average

of the 15 values was calculated along with the cor- responding percent standard deviation. These meas- urements were then repeated three times. The results are summarized in table 1. This table also presents

the values for the first group and the average values and corresponding percent standard deviation for the two consecutive groups. It can be seen from the

stability of the current-to-voltage unit that measure- ments can be performed with a reproducibility of about 0.0012 -(at the 67% confidence level) expressed

as percent standard deviation for a single determination. The time interval between the first and last group of measurements was 15 min.

Table 1 Stability of the current-to-volfage converter alone measured in three groups of 15 replications each

Average of 50 individual Percent standard

plication measurements; volts deviation

1 1.003494 0.00136 2 1.0034% 0.0012,

3 1.003482 0.00146 4 1.003507 0.0011,

5 1.003515 0.0013^

6 1.003508 0.0013B 7 1.003497 0.001 ]„ 8 1.003498 0.0013^ 9 1.003505 0.001

1

0 10 1.003510 0.00142 11 1.003518 0.0013.,

12 1.003521 0.0012o

13 1.003522 0.0013,

14 1.003507 0.00126 15 1.003527 0.0013 7

Average of replications Percent standard

deviation

1.003507

0.0012

First group

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

1.003535

0.0015

1.003545

0.0010

Second group

Third group

b. Stability of the Current-to-Voltage Converter Plus the Photomultiplier Tube Supplied

with 850 V and in Total Darkness. The measure- ments were made as previously described and the

results are presented in table 2.

c. Stability of the Current-to-Voltage Converter and the Photomultiplier Tube Supplied

with 850 V and Exposed to the Radiation of a Tritium Activated Fluorescence Source. A con-,

stant radiation source consisting of a tritium activated phosphor was placed before the integrating sphere

and a series of measurements were taken following

the technique described above. Table 3 shows the

results.

d. Stability of the Current-to-Voltage Converter, the Photomultiplier Tube Supplied

with 850 V, and the Tungsten Ribbon Filament

Lamp. The same measuring procedure as mentioned

in a, b, and c was used here. In this case, however,

the incandescent tungsten lamp was used as the source of radiation. Table 4 summarizes the results of four groups of measurements over a period of 20

min. This last series of measurements indicate that

'the single-beam spectrophotometer is capable of producing measurements of radiation fluxes with a percent standard deviation of about 0.022s for single measurements with 2.00 V at the photomultiplier tube anode.

74
Table 2. Stability of the current-to-voltage converter and the photomultiplier tube at 850 V in total darkness

Average of 50 individual Percent standard

Replication measurements; volts deviation

1 1 . UlZoZV ft ft97

2 1 .Ul Zo'+Z ft ft 1ft

io1 ft 1 9 Q 9 9 1 .UlZoZZ ft ft97

A 1 ftl 939ft ft t\%'\ cD i . u izoyf Uft iW 'X

. U405 O 1 ft 194.91 1 .UI Z4-Z 1 ft ftl ^

7 1 ftl 94 ft/1

1 . UI Z'+lr+ ft ftl ft U.UI o4 QO 1 ftl 94.ftA

I . UI Z'+UO ftU.UZVg ft9Q Q i

I . ni9^#^Q U 1 ZoOo ftU.U1V6 ftl 0

1 u

1 ft.194.ft9

1 . U 1 Z4-UZ ftu. ftl ui Qv 1

1 ftl 94 fx ^ ft ft9R U.UZ02 12 1.012412 0.061s

13 1.012451 0.0235 14 1 .01241

7

0.0298 15 1 . \J 1 1 0.024»

Average of replications 1.012395 "I Percent standard Ir Firs, r irst group

devi at ion 0 0050

Average of replications 1.012467 I Percent standard I s

.

v Second group

deviat ion 0 0033

Average of replications 1.012510 1 Percent standard Third group

deviation 0.0035

In these measurements the stability of the direct current (nominal 5 V; 14 A) supplied to the tungsten

ribbon lamp was monitored with the potentiometer,

and the variation of this current was less than one

part in 10 5 during a series of 15 consecutive meas- urements (5 min).

Following the four stability tests discussed earlier, a consecutive s«ries of six measurements were made

to determine the reproducibility of transmittance measurements. To this effect seven Schott NG-4

neutral glass filters were placed in the automatic

sample carrying system and the data acquisition and

sample changing operations were performed auto- matically through the computer unit. As mentioned

previously, the sample carrying system can accept seven samples in positions 2; 4; 6; 8; 10; 12; 14, and

eight intermediate positions 1; 3; 5; 7; 9; 11; 13; 15. The odd numbers correspond to measurements of the nonattenuated radiation beam passing through

air and are marked /o, while the even numbers cor- respond to measurements of the attenuated radiations

after passing through the absorbing material and are marked /. The uncorrected transmittance, T, is then

'o The radiation flux from the tungsten ribbon filament lamp was attenuated with the circular neutral wedge

until a photocurrent corresponding to about 2.0020 V

was obtained for the nonattenuated beam /() . The photomultiplier tube was supplied with 850 V and the 30

Mfl resistor was used at the current-to-voltage converter. For every position, 50 digital voltmeter readings were taken by the computer at a rate of 10 to 15 per second. The average value was printed along with

the sample position, the standard deviation, the rela- tive standard deviation, and the transmittance values

for the glass filters 2; 4; 6: 8; 10; 12; and 14: h

/!' + /'.'

; 7V

U

/.? + /"

; T«

I, p + /»

5 7

; etc.

2 2

until the seven glass filters were measured. This sequence was repeated six times and the results are given in table 5. As can be seen from these data, the reproducibility

of sequential transmittance measurements can be performed with an average standard deviation of 0.010 percent for a single determination.

IV. Wavelength Calibration

The wavelength scale of the monochromator is provided with a counter which indicates wavelength

directly in angstroms. This counter is checked for

7 5
Table 3. Stability of the current-to-voltage converter and the photomultiplier tube at 850 V and exposed to the

radiation of a tritium activated fluorescent source

Average of 50 individual Percent standard

Replication measurements; volts deviation

1 1.536345 0.0275 2 1.536326 0.0302 3 1.536196 0.022„

4 1.536289 0.0235 5 1.536106 0.0309 6 1.536117 0.031 4 7 1.535916 0.025,

8 1.536065 0.0238 9 1.536179 0.0293 10 1.536003 0.0233

11 1.536083 0.021 5 12 1.535961 0.026,

13 1.536052 0.031 3 14 1.536095 0.0266 15 1.536092 0.0262

Average of replications 1.536122 Percent standard I First group

deviation 0.0082 J

Average of replications 1.535768 Percent standard > Second group

deviation 0.0095 J

Average of replications 1.535522 Percent standard f Third group

deviation 0.0054 J

accuracy with a low pressure mercury discharge

lamp placed before the entrance slit of the monochromator. The' following wavelengths were used

for calibration: 3650.2 A: 4046.6 A: 4077.8 A: 4339 2 A: 4347.5 A: 4358.4 A; 4916.0 A; 5460.7 A: 5769.6 A;

and 5790.7 A. If additional reference wavelengths

are needed, a Cd-Hg or a He-discharge lamp could be

used for calibration. The wavelength counter was

then checked using the procedure recommended by

Gibson [2]. and a slit of 0.1 mm which is equivalent to an effective spectral bandpass of 0.08 nm. The deviation of the wavelength counter from the true value was found to be less than ±0.1 nm; hence no wavelength correction was applied to the measurements

discussed here.

V. Stray Radiation

Tests were made to determine the stray radiant energy (SREl in the monochromator proper, as

well as in the photometric arrangement. The measure- ment of stray radiation in the monochromator. that is. the radiation energy at wavelengths different from

those of the nominal spectral bandpass transmitted through the instrument, is not easy or infallible. A

detailed discussion of this instrumental parameter was

given in an ASTM Tentative Method [35] and the procedure recommended in this work was used to deter- mine SRE in the blue and yellow spectral range. In this procedure, a solution of methylene blue, which has a strong absorption in the range from X 600 to 660 nm is used. The SRE using a slit of 1 mm (0.8 nm) was equal

to or less than five parts in 10 s. The SRE generated inside the photometric system is defined as the radiant energy which falls on the photosensitive detector without passing through the absorbing sample. This SRE is usually produced by

reflections and scattering of radiations on the optical

and mechanical parts located between the exit slit of the monochromator and the integrating sphere. The

measurements were performed using a slit of 1 mm by

placing a front surface mirror at the sample position,

which reflects to the instrument all radiations received

from- the exit slit imaged at the mirror surface. The size of this image was about 8 mm high and 1 mm wide. In

this way, a maximum SRE was generated in the spectrophotometer. The measurements were then performed at A 577.3 nm. using a radiation flux intensity

five times greater than that used in routine trans- mittance measurements, by determining the dark cur- rent of the photomultiplier with the shutter in the closed position at the integration sphere. An average dark current of 0.040 mV was observed. The mirror was then placed at the sample position, the shutter

7 6
Table 4. Stability of the current-to-voltage converter, the photomultiplier tube at 850 V, and the tungsten ribbon

filament lamp

Replication Average of 50 individual Percent standard measurements; volts deviation

1 2.002395 0.0380 2 2.001356 0.0226 3 2.002145 0.024, 4 2.000975 0.0262 5 2.001944 0.0207 6 2.000925 0.028,

7 2.001832 0.026, 8 2.000825 0.023s 9 2.001551 0.0263 10 2.000960 0.021 2 11 2.001739 0.023,

12 2.000851 0.024„

13 2.001729 0.0282 14 2.000825 0.0230 15 2.001557 0.024.,

Average of replications 2.001441 Percent standard 1 Vr irst, group

deviation 0.026

Average of replications 2.001517

Percent standard Second group

deviation 0.012

Average of replications 2.000826

Percent standard I nird group

deviation 0.025 I

Average of replications 2.001268 1 Percent standard >

1 Fourth group

deviation 0.027 J

Table 5. Reproducibility of transmittance measurements on seven Schott NG-4 glass filters No. 2; 4; 6; 8; 10;

12; and 14

Percent transmittance

Replication No.

Average

Percent a

2 4 6 8 10 12 14

1 33.327 21.711 12.236 50.990 33.377 20.906 13.473

2 33.325 21.710 12.237 50.983 33.377 20.903 13.471

3 33.321 21.711 12.241 50.992 33.383 20.900 13.474

4 33.320 21.708 12.240 50.988 33.375 20.901 13.470

5 33.323 21.710 12.239 50.983 33.379 20.901 13.474

6 33.325 21.710 12.238 50.986 33.377 20.904 13.470

33.32 21.710 12.238 50.987 33.378 20.902 13.472

0.0080 0.0051 0.0150 0.0072 0.0083 0.0108 0.0141

Average percent cr was opened and measurements were made again.

The average value found was 0.037 mV. This indicated

that no SRE could be detected under the experimental

circumstances.

0.010

VI. Linearity Control

The single-beam static optical system described in this work permits the unequivocal use of the radiation

77
addition principle by means of the double-aperture

method for determining departure from linearity of the

entire optical, photometric, and electronic system, and

thus of the photometric accuracy of transmittance measurements. The double-aperture and its positioning on the

optical bench was described earner. Its use will now

be illustrated, and follows the procedure developed

and used at the National Physical Laboratory.

Since the linearity of photometric data for a given

photomultiplier tube depends on the anode voltage, the values at the current-to-voltage converter, and

the ambient temperature, all measurements were made

using identical experimental conditions. These same

conditions were maintained when transmittance meas- urements were performed. Since the linearity is, within 1 part in 104

, not usually a function of wavelength [36], all measurements were performed at A

565.0 nm. A recent study of this parameter at NBS by

Mielenz and Eckerle indicates that there may be a

relation between wavelength and linearity at the level of 1 part in 105 [38]. The intensity of the radiation flux produced by the tungsten ribbon lamp was attenuated with the circular neutral wedge until a photocurrent equivalent to 2.0020 V was obtained when both apertures, A and

B, were open. A setting of 850 V was used at the photomultiplier tube with a 30 MCI resistor at the currentto-voltage converter. Fifty individual DVM readings were taken and the average value for (A + B) was

printed. Aperture B was then closed, and 50 DVM

readings were taken. The average value for aperture

A was printed. The average value for aperture B was

then obtained in a similar manner by closing aperture

A and opening aperture B. This sequence was repeated

three times, ending with an {A + B) value. Identical measurements were made over a range of attenuation corresponding to 4 cascaded steps of 2

to 1 as illustrated in the actual example which follows:

Step 1

Av.

Diff. % Corr.

Step 2

Av.

Diff. % Corr.

Step 3

(A + B)

2.00147 2.00156 2.00206 2.0021,

B

0.9864! 0.9864o

0.9862.

1.01596 1.01599 1.01609 1.01598 + 0.98635 = 2.00233

2.00181 2.00233-2.00181 = -0.026

0.00052

(A+B)

1.00049 1.00078 1.00066 1.00090 1.00071 1.00112- = -0.041

(A + B)

0.50062

B

0.4910s

0.4909, 0.4911s

0.5102 0 0.50996 0.51004 0.51007 + 0.49105=1.00112

1.00071=4-0.00041

A

0.2565,

B

0.24430

0.50056 0.25673 0.2443s 0.5O07o 0.2566s 0.24432 0.50062 0.25665 + 0.24433

Av. 0.50063

UlII. U.oUUvo — — V/. V >\J\_f, > .

)

% Corr. = -0.069

Step 4 (A + B) A B

0.2502o 0.12872 0.1217* 0.25028 0.12856 0.1216g

0.250U 0.12854 0.12166 0.25023 0.12861 + 0.12170

Av. 0.25023

Diff. • 0.25031 - 0.25023 = + 0.00008 % Corr. = -0.031

= 0.50098

= 0.25031

The correction curve is established from these data by plotting voltages on the abscissa and the corre- sponding additive, correction value on the ordinate. These are tabulated below and illustrated in figure 16.

Figure 16. Linearity correction curve.

Voltage % T % Correction

2.00 100 0.0 1.00 50 0.026

0.50 25 0.067

0.25 12.5 0.14 0.125 6.25 0.167 When transmittance measurements are performed,

the /o reading is initially set with the circular neutral wedge to a value near 2.0020 V. The / value is then measured. If the initial 70 = 2.00214 V and final /o=2.00228 V and 7=0.54220 V, then percent T is:

0.54220 2.00214+2.00228

x 100=27.081

which is the noncorrected value. To correct this value, one takes from the ordinate of figure 16 the value corresponding to 0.54220 on the abscissa which, in this case, is 0.072. The corrected percent T value is then: 27.081-

F

Q81 nX0072

) =27.061.

\ 100 )

7 8
Mielenz and Eckerle have studied recently the double aperture method for testing photometric

linearity, and have used a curve -fitting procedure

for finding the nonlinearity correction rather than

the method described earner (38).

VII. Sample Position

A series of measurements were performed to deter- mine the magnitude of error which could occur when

the sample is oriented with its entrance face at an

angle to. the incoming radiation beam. The single

sample holder provided with the rotating table, as described in section II, paragraph c, was used. Trans- mittance measurements were performed by producing

the image of the exit slit of the monochromator at the entrance face of the sample (aperture f:10). The data

are shown in table 6. The consequence of this condition on transmittance measurements is discussed by

Mielenz in this Journal.

Table 6. Percent transmittance (%T) , measured on

three neutral glass filters 1.0; 1.5; and 2.0 mm

thick at k = 440 nm, at three angles of incidence

Angle of Filter Filter Filter incidence 1 .0 mm 1 .o mm 2.0 mm

Normal incidence 32. 91 5 19.83b 1 1 606 1° 32.899 19.83a 11. 60^

2° 32.89, 19.81 9 11.599 3° 32.88, 19.81 2 11.589

Similar measurements were made to determine the

identity of positions on the seven-sample automatic

changer described in section II, paragraph c. For

this experiment, seven neutral glass filters A; B; C;

D; E; F; and G were used and were positioned in holders 2; 4; 6; 8;' 10; 12; and 14 in three different arrangements as described by Garfinkel, Mann and

Youden [39].

Table 7. Evaluation of the identity of the seven stations of the automatic sample changer

Percent Transmittance {%T) at station number (St. No.) 2; 4; 6; 8; 10; 12; and

14 for filters A, B, C, D, E, F, and G at \ = 465.0 nm

Run No.

St. No.

27.08 16.44 46.39 26.12 15.22

10

37.34 23.23

14

St. No.

27.07

14

16.40 46.39 26.11 15.21 37.34

10

23.23

12 %T

III St. No.

27.08

10

16.43

12

46.39

14

26.11 15.21 37.33 23.23

Transmittance measurements were then performed

on all filters for the three different arrangements and

the results are given in table 7. From these data it can

be concluded that the seven stations are interchangeable and will produce measurements which will not

differ by more than one part in one thousand.

VIII. Influence of Polarized Radiations on

Transmittance Measurements

This effect was determined by measuring the trans- mittance of a Schott NG-4 neutral glass filter at four wavelengths using radiations emerging from the pre- disperser-monochromator unit, and by projecting the image of the exit slit (8 by 0.5 mm) at the entrance

face of the filter with a convergent beam geometry

corresponding to an f : 10 opening. The glass filter was

checked prior to measurements with a polariscope for freedom of internal tensions. Column one of table 8 shows the results obtained when transmittance meas- urements were made using the radiations produced by

the spectrophotometer. Column two shows the results

obtained when a polarizing sheet, with the vibration plane horizontal, was placed in front of the glass filter. The measurements obtained with the vibration plane

in vertical position, are given in column three. These measurements show that polarized radiations :an affect transmittance measurements of solid glass filters when noncollimated beam geometry is used. This effect is predicted by the Fresnel equations

mentioned in the introduction and should disappear

when collimated radiations are used (1, pg. 100).

XI. Comparison of Transmittance

Measurements

Two sets of solid filters were used in a comparative

test to determine the reproducibility of transmittance measurements between two laboratories. One set was made from three neutral glass Schott NG-4 filters having nominal percent transmittances of 10; 20; and 30. The second set was made as described elsewhere [4]. Three evaporated metal (Inconel) on fused quartz

(nonfluorescent) plates having nominal percent

79
Table 8. Effect of polarization on percent transmittance {%T) measured at four wavelengths on a Schott NG-4

glass filter

Wavelength nm

Produced by

spectro- photometer

Polarizer, plane of vibration horizontalPolarizer, plane of vibration

vertical

% Difference

%r, - %t3

1

440.0

465.0

590.0 635.0

19.81 H 22.59, 19.178 20.61,19.80o 22.60,, 19.170 20.602

19.78« 22.56s 19.09a 20.54 7

- 0.15 - 0.12 - 0.41 - 0.31

transmittances of 25; 50; and 75 were used. The trans- mittance measurements were performed on two sets of

filters at the National Physical Laboratory (NPL) in England using their high accuracy spectrophotometer, and at NBS on the instrument described in this paper. The measurements at NBS were carried out before and after the measurements at NPL. All measurements were made with noncollimated conver- gent beam geometry. A rectangular surface of the

filter about 3 mm by 8 mm was used at NPL and the beam was only slightly convergent. At NBS an area about 8 mm by 0.5 mm was used for the transmittance measurements. The results given in table 9 indicate that an average

difference of —0.19 percent of the values was obtained between the measurements carried out at NPL and at NBS. An average difference of —0.30 percent of the value was found when similar measurements were

performed on the inconel-on-quartz filters, as shown in table 10.

X. Standard Reference Materials for Spectrophotometry

The need for providing means and materials t< check the proper functioning of a spectrophotometer was discussed in some detail in previous publications [3, 4]. At that time it was established that the accuracy of the photometric scale is a critical and most demanding parameter in spectrophotometry. Hence,

particular attention was given to a number of ways for checking this parameter. Investigations showed that

solid colored glass filters, exhibiting optical neutrality over the spectral range from 400.0 nm to 700.0 nm,

would constitute an acceptable Standard Reference

Material (SRM). From the various colored glass

Table 9. Comparison between the percent transmittances (%T) measured on three Schott NG-4 glass filters at NPL and NBS

/ avelength NBS, %T NBS. %T NBS, %T NPL, %T % Diff. nm March 12, May 18. average February NBS to Nl

1971 1971 1971

440.0 12.92 12.91 12:91s 12.93 - 0.11 465.0 14%, 14.98 14.97.T 15.01 - 0.25 590.0 11.70 11.64 11.67 11.67 0.0 635.0 12.72 12.68 12.70 12.72 - 0.16

440.0 19.62 5 19.58 19.60, 19.62 - 0.09 465.0 22.385 22.35 22.367 22.43 - 0.28 590.0 19.06 18.95 19.005 19.01 - 0.03 635.0 20.455 20.37 20.41

3

20.47 - 0.23

440.0 32.89 32.86 32.875 32.98 - 0.32 465.0 35.52 35.54 35.53 35.66 - 0.36 590.0 31.16s 31.10 31.133 31.21 - 0.25 635.0 32.565 32.52 32.543 32.62 - 0.24

between NBS and NPL percent T values = — 0.19 percent.

filters available, Schott NG-4 "neutral glass" was

selected, prepared and characterized. It is now offered by NBS as a means to check the photometric scale of spectrophotometers.

80
Table 10. Comparison between the percent transmittances (%T) measured on three inconel-on-silica filters at NPL and NBS

Wavelength NBS, %T

nm 1 2

450.0 24.87 24.88

550.0 23.78 23.82 650.0 23.38 23.39

450.0 49.35 49.33 550.0 47.60 47.60

650.0 46.85 46.85

NBS, %T % Diff. average NPL, %T NBS to NPL

24.875 24.93 - 0.18 23.80 23.86 - 0.25

23.38s 23.46 - 0.32

49.34 49.56 - 0.44* 47.60 47.81 - 0.44 46.85 47.14 - 0.64

450.0 72.17 550.0 72.05 650.0 72.20

72.20 72.18s

72.11 72.08 72.34 72.27

72.30 - 0.16 72.20 - 0.17 72.33 - 0.08

Average difference between NBS and NPL percent T values = — 0.30 percent.

"This filter had a flaw in the form of a crack which was sometimes visible and other times invisible. The larger differences found in the measurements of this filter may be due to this flaw.

SRM 930, developed in the Analytical Chemistry

Division and available since March 1971 consists of three glass filters. Each filter bears an identification number, and the upper left corner has been removed

to indicate correct orientation in the metal holder

(fig. 17). The transmittance measurements were made with

the high accuracy spectrophotometer described in this paper, and are certified with an uncertainty of ±0.5 percent of the value. This uncertainty is the sum of the random errors of ±0.1 percent (2SD

limit) and of estimated biases which are ±0.4 per- cent. These biases are due to possible systematic

errors originating principally from the inherent inhomogeneity and instability of the glass as well

as from positioning of the filter. Measurements were made at 24 °C, and variations within several de- grees Celsius of this temperature will not signifi- cantly affect the calibration of the filters. The neutral NG—4 glass for the filters was provided by Schott

of Mainz, Germany and is designated as "Jena Colored and Filter Glass." Nominal transmittance for a

filter 1.5 mm thick is 20 percent at 400.0 nm wavelength and 32 percent at 700.0 nm wavelength. Be- tween these limits the transmittance varies in a monotonic manner. The filter is held in a frame and the size and shape

of the filters and frames were selected, for practical

considerations, to conform to the dimensions of the standardized cuvettes for which holders are supplied

in most conventional spectrophotometers. The filters are approximately 1.0, 1.5, and 2.0 mm thick. Cor- responding to these thicknesses are nominal trans- mittances of 30, 20, and 10 percent, respectively.

These thicknesses were selected to provide a means for calibrating the photometric scale at three different

levels.

The effective spectral bandpasses used to determine

the certified values were equal to or smaller than 2.2 nm

at 440.0 nm; 2.7 nm at 465.0 nm; 5.4 nm at 590.0 nm;

and 6.0 nm at 635.0 nm. The transmittance measure- ments are made by producing the image of the slit (about 8 mm by 0.5 mm) using a convergent beam

geometry with an opening of f:10 corresponding

to an angle of 7° to 8° in the middle of the entrance

face of the filter. This beam geometry was used to reproduce the average experimental conditions found in most of the conventional spectrophotometers available today. Prior to the certification, each

filter is examined for surface defects and thoroughly

cleaned. If, through handling, the surface of the

filter becomes contaminated, it may be cleaned

with a small soft brush attached to a rubber tube connected to a vacuum source [40]. If contamination

results from fingerprints, they must be removed

before making measurements. This may be accomplished by removing the filter from its holder, breathing lightly on it, and rubbing the surface gently with optical lens tissue. The clean filter is then properly

positioned in its holder. To remove and replace the

filter in the holder, the spring-loaded plate should

be lifted with care to prevent damage to the filter. As little handling as possible is recommended. SRM

930 should be used according to the directions on

the certificate; consult the manufacturer of the instrument if differences are obtained that exceed

those specified by the manufacturer. Under no circumstances should other cleaning

procedures which make use of detergent solutions,

organic solvents, etc. be applied.

When a filter has become contaminated beyond

cleaning by the procedure described in the certifi- cate, it should be forwarded to NBS. After proper

cleaning, the filters will be checked and, if needed.

81
Figure 17. NBS Standard Reference Material 930 and calibration certificate.

recalibrated using the high accuracy spectrophotometer described in this work.

It was already stated that the accuracy of photo- metric scales defines only one of the parameters re- quired for obtaining accurate transmittance values and molar absorptivities. Other factors must also be

established. These are wavelength accuracy, adequate

spectral bandpass, stray light, cell parameters (when

solutions are measured), fluorescence, polarization,

reflection, and temperature coefficient. Some of these

variables were discussed in NBS Technical Notes 544

and 584 and are also examined in this paper. The transmittance data given in the certificate which accompanies each SRM 930 depend not only on the intrinsic properties of the glass and the ex- perimental measurement conditions, but on the surface

state of the glass. This parameter varies with time and exposure conditions. When glass is exposed to normal room atmosphere and temperature, its surface

is corroded to an extent depending on the composition, time of exposure, concentration, temperature

and nature of the glass surface acting agents. This

action produces a change in the reflecting and trans- mitting properties of the material [41 1. For instance,

a well-known phenomenon called "blooming" of the

glass is due to the formation of an Si() layer at the surface of the glass. This layer, which increases the transmittance. acts as an antireflection coating. The

speed with which such a layer is formed varies with

the composition of the glass, the atmosphere and time,

(.enerally speaking, several years are needed for a fresh surface to reach equilibrium. This, and similar

82
phenomena are presently being studied, along with means to stabilize the surface state of glass filters. Until more information is acquired in this field, we

recommend that the colored glass filters issued as SRM 930 be rechecked annually to determine whether

any physicochemical changes, which might affect the transmittance values, have occurred. Another important factor is the need for defining

and producing a clean glass surface. Until now the

final cleaning of the NG-4 filters was made with re- distilled ethyl alcohol and pure water (thermally

distilled and deionized). Other cleaning procedures

are under consideration. The use of isopropyl alcohol

in vapor or liquid form associated with mild ultra- sonic action is being investigated [41]. The transmittance characteristics of the SRM 930

limit the use of this material to the visible region of the spectrum from about 400 nm to the near infrared. Since the ultraviolet region, from about 200 nm is also important to the analyst who uses spectrophotometric

methods, exploratory work is underway to select and

certify solid material for checking the photometric

scale in this spectral region. Optical filters exhibiting small transmittance-wavelength dependence in the

spectral range 200 nm to near infrared can be obtained

by evaporating thin semitransparent layer of a metal on a suitable transparent substrate [42, 4], and such

filters have been considered in this work. The metals

selected were inconel and chromium which exhibit adequate transmission characteristics and good ad- hesion to the substrate. The substrate was nonfluo- rescent fused quartz. A series of filters were prepared

by the optical shop at NBS according to the following

specifications: a number of nonfluorescent optical

quality fused quartz plates, 10 mm X 25 mm and 1 mm

thick, were cut and polished, Inconel or chromium

metal was evaporated on the surface to produce

nominal transmittances of 25, 50, and 75 percent.

The surface bearing the evaporated metal was coated

with a layer of optical cement which was transparent

to the visible jjnd ultraviolet radiations down to 230

nm. A clear plate of the same material was used to cover and protect the evaporated metal layer. The filter assembly was then marked at one corner

to insure its proper positioning and the finished filter was placed in a metal holder of conventional size (approximate o.d. 13 x 13 x 57 mm) fitting the cuvette holder found in most spectrophotometers. The metal

holder was also marked at one side to permit positioning of the filter in a reproducible manner.

In addition to the evaporated metal filters, a number

of units were prepared using only the clear uncoated

fused quartz plates and assembled with the same

optical cement. When desired, these clear filter assemblies could be used as reference samples in the blank compartment.

Before submitting the evaporated metal filters to transmittance measurements, a study was made of the

effect of radiations on their transmittances. A filter was exposed to an accelerated test in which radiations had the same spectral distribution as the fluorescent

lighting of the laboratory, except that they were 1000

times more intense. The filter was exposed for an

equivalent of 36,000 hours of continuous irradiation. This test was made on a radiation accelerator made

available by the Building Research Division of NBS.

The percent transmittance was measured before and

after the exposure and gave the following results:

Transmittance, perc- it

Wavelength, nm 250 380 500 650

Before exposure 44.48 51.35 48.90 47.41

After exposure 44.11 51.34 48.92 47.47

The relative standard deviation for a single deter- mination of these measurements was 0.01 percent. As can be seen, the only significant relative change in transmittance of about 0.84 percent of the value occurred at 250 nm.

Several sets of these filters were calibrated at five selected wavelengths, 250 nm; 350 nm; 450 nm; 550 nm; and 650 nm, using the cleaning and measuring

procedures outlined for Schott NG-4 colored glass neutral filters. The results indicated that the reproducibility of transmittance measurements is good

(percent standard deviation 0.009 to 0.024) and is comparable to those obtained for the colored glass

filters at all wavelengths except 250 nm. From the experimental data, it is evident that the transmittance

of the evaporated metal filter at 250 nm is critical and, at present, no satisfactory explanation for this phenomenon can be given. A limitation of the evaporated metal filters is that they attenuate the intensity

of radiation by reflecting a part of it, rather than ab- sorbing. This can produce, in certain circumstances,

undesirable stray light in the instrument and make the transmission measurements dependent on the geometry of the optical beam. However, since these filters are closer to optical neutrality than the colored glass

filters, and since they can be used in the ultraviolet region as well, they were included in this work.

This limitation was apparent from the data ob- tained in a cooperative study conducted at C. Zeiss by A. Reule using conventional spectrophotometers.

On the other hand, a similar comparative test, made

on the same filters by F. J. J. Clarke at NPL has produced the results presented in table 10. One can

observe that, in spite of the limitations mentioned

above, an agreement within —0.30 percent of the value was obtained between NBS and NPL measure- ments at the indicated wavelength. Further studies will be needed to assess unambiguously the transmittance characteristics of evaporated

metal-on-quartz filters, with or without a protective

quartz plate, and to assess their suitability as Standard

Reference Materials to check the photometric scale

of spectrophotometers in the ultraviolet and visible part of the spectrum.

83
XI. Addendum

The identification of commercial instruments and

products, is given in the Addendum only to permit

reproduction of the work described in this paper.

In no instances does such identification imply recom- mendation or endorsement by the National Bureau of Standards, nor does it imply that the particular equip- ment or product is necessarily the best available for the purpose. Radiation source for visible — Microscope lamps,

type 18A/T10/1P-6V: General Electric Co., Lamp

Division, Nela Park, Cleveland, Ohio 44112. For ultra- violet: Atlas single coil halogen (Bromine) lamp,

type Pl/8, 30V, 250W: GTE Sylvania, Inc., 6610

Electronic Drive, Springfield, Virginia 22151. Power supply for microscope lamp, Kepco, Model

JQE 15-50-M-VP: Kepco, Inc., 131-38 Sanford Avenue, Flushing, New York 11352. For tungstenhalogen single filament lamp: same manufacturer,

Model JQE-36-30 Mt-VP.

Potentiometer: Leeds and Northrup Model K3 with

null meter and power supply. Resistors: Leeds and

Northrup 0.1, 50 A and 0.01, 100 A: Leeds and

Northrup, Sumneytown Pike, North Wales. Pa.

19454. Nonfluorescent fused silica: Dynasil Corporation of America, Berlin, New Jersey 08009. Neutral Density Attenuator and BaS04 white paint: Eastman Kodak Co., Special Products Sales, Kodak

Apparatus Division, Elmgrove Plant, Rochester, New

York 14650. Monochromator with predisperser: McPherson In- strument Corp.. 530 Main Street, Acton, Massachusetts 01720. Optical benches with carriers and x — y sample

holder with micrometer control: Gaertner Scientific Corp., 1201 Wrightwood Ave., Chicago, Illinois 60614. Lens holders: Ardel Instrument Co., Inc., P. O. Box 992, Jamaica. New York 11431.

Ball bushing and rails: Thompson Industries, Inc., Manhasset, New York 11030. Pneumatic cylinders and accessories: Clippard Instrument Laboratory, Inc., Cincinnati. Ohio 45239. Rotating table: Ealing Optics Division, 2225 Massachusetts Avenue, Cambridge, Massachusetts 02140. Thermostating holders for glass cells and glass

filters: Cary Instruments, 2724 South Peck Road,

Monrovia. California 91016. Pneumatic ratchet system: Allenair Corp., P. O. Box 350, 255 East 2nd Street, Mineola, New York

11501. Black paint — Nextel 101-c 10 Black: Reflective Products Division 3M, 2501 Hudson Road, St. Paul,

Minnesota 55101. Photomultiplier tube EMI-9558QA: Gencom Divi- sion, 80 Express Street, Plainview, New York 11803. Power supply for photomultiplier tube: Model 415B

and digital voltmeter 8400A: John Fluke Manufacturing

Co., P.O. 7428, Seattle, Washington 98133.

Thermal insulation: Photoshroud. Shumway

Optical Instruments Corp., 2118 Beechgrove Place,

Utica, New York 13501. Vibration isolation table: Lansing Research Co., 705 Willow Avenue, Ithaca, New York 14850. Low power laser; Model 195 cw gas laser, output power 2 mw: Optics Technology, Inc., 901 California Avenue, Palo Alto, California 94304. Tritium activated fluorescent source. Beta light Marker HM-110: Canrad Precision Industries, Inc.. 630 Fifth Avenue, New York, New York 10020. Colored glass neutral filters, Schott NG-4: Fish- Schurmann Corp., 70 Portman Road. New Rochelle,

New York 10802. Spectral lamp: Oriel Optics Corp., 1 Market Street, Stamford, Connecticut 06902.

Polarization filters: Polaroid Corp., 119 Windsor

Street. Cambridge, Massachusetts 02139. Computer: 24K memory and 16 bit words. EMR

computer. Division of Weston Instruments, Inc., Schlumberger Co., 8001 Bloomington Freeway,

Minneapolis, Minnesota 55420.

The author gratefully acknowledges the assistance

of the following persons, in addition to those already

mentioned in the text: F. J. J. Clarke, for the considerable assistance given

through personal discussions on the philosophy and

principles of high accuracy spectrophotometry at NPL.

Anne Compton, for advice and help during the ex- perimental work carried-out at the high accuracy

spectrophotometer at NPL.

G. E. V. Lambert, for information concerning glass

filters (NPL).

K. D. Mielenz (NBS), for constructive discussions on

the principles of spectrophotometry and for critically reviewing the manuscript.

W. H. Venable (NBS), for critically reviewing the manuscript.

K. L. Eckerle (NBS), for advice in measuring stray radiations and polarization produced in the monochromator.

C. R. Yokley (NBS), for advice on the radiation

stability of incandescent tungsten filament lamps.

W. K. Haller (NBS), for valuable information on the

properties of glass surfaces and optical glass filters. H. H. Ku (NBS), for advice on interpretation of the data discussed in this paper. NBS Instrument Shop, for constructing the mechanical parts described in this work. NBS Optical Shop, for grinding the quartz lens used

in this work.

D. S. Bright (NBS guest worker), for the programs

written at various times during the initial phase of this work.

V. E. Gray (NBS), for assistance in performing the accelerated radiation tests. H. J. Hofert and A. Reule (C. Zeiss, Oberkochen,

Germany), for valuable information on linearity of

84
photodetectors and for demonstration of the radiation addition method.

R. J. Carpenter (NBS), for constructive discussions on electronic circuitry and photodetectors.

L. Marzetta (NBS), for advice and assistance in assessing various functioning parameters of photomultiplier tubes. O. Menis, Section Chief, Analytical Coordination

Chemistry (NBS), for approving and encouraging the work reported in this paper.

J. I. Shultz, Assistant Section Chief, Analytical

Coordination Chemistry (NBS), for critically reviewing

the manuscript.

XII. References

[1] Kortiim, G., Kolorimetrie-Photometrie und Spektrometrie 464 p., 4th ed. (Springer-Verlag. 1962).

[2] Gibson, K. S., Spectrophotometry, Nat. Bur. Stand. (U.S.). Circ. 484,48 pages (1949).

[3] Mavrodineanu, R., Spectrophotometry, instrumental develop

ment, O. Menis and J. I. Shultz, Eds., Nat. Bur. Stand.

(U.S.), Tech. Note 584, pp. 9-21 (1971).

[4] Mavrodineanu, R., Solid materials to check the photometric

scale of spectrophotometers, O. Menis and J. I. Shultz, Eds., Nat. Bur. Stand. (U.S.), Tech. Note 544, pp. 6-17 (1970).

[5] Elster, J. and Geitel, M., Un tne comparison of light intensities by photoelectric methods, Ann. Physik, Chemie 48,625 - 635

(1893).

[6] Elster, J. and Geitel, H., 1 he photoelectric effect on potassium

at low light levels, Phys. Ztsch. 13 , 468-976 (1912).

[7] Roller, L. R., and Breeding, H. A., Characteristics of photoelectric tubes. General Electric Rev. 31, 476-479 (1928).

[8] Campbell, N. R., The experimental proof of a fundamental

photoelectric law. Trans. Opt. Soc. (London) 32, 61—65

(1930-1931).

[9] Fleury, P., An addition method for the precise study of the

variation of the current output of a photoelectric cell as a function of the incident luminous flux, Compt. Rend. Acad.

Sci. (France) 199, 195-197 (1934).

[10] Preston, J. S., and McDermott, L. H., The illumination re- Eonse characteristics of vacuum photoelectric cells of the

ster-Geitel Type, Proc. Phys. Soc. (London) 46, 256-272

(1934).

[11] Preston, J. S.. and Cuckow, F. W., A photoelectric spectro- photometer of high accuracy, Proc. Phys. Soc. (London) 48,869-880 (1936).

[12] Buchmtiller, F., and Kiinig, H., Precision measurements on

incandescent lamps using selenium photocells, Assoc. Suisse Electriciens, Bulletin 28, 89-99 (1937).

[13] Atkinson, J. R., Campbell, N. R., Palmer, E. H., and Winch,

G. T., The accuracy of rectifier-photoelectric cells, Proc. Phys. Soc. (London) 50, 934-946 (1938).

[14] Barbrow, L., A photometric procedure using barrier-layer photocells, J. Res. Nat. Bur. Stand. (U.S.), 25, 703-710

(1940) RP1348.

[15] Kaiser, H., Photographic-photometric calibration of step

filters, Spectrochimica Acta 3, 518-537 (1947-1949).

[16] Harding, H. G. W., Precautions necessary for accurate measure- ments of optical density standards. Photoelectric Spectrometry Group Bulletin 4, 79-86 ( 1951 )

[17] Kortiim, G., and Maier, H., Dependence of photocurrent and

illuminating intensity in photocells and photomulti pliers, Z. Naturforschung8A, 235-245 ( 1953).

[18| Hansen, G., Check of photometric function of spectrophotometers, Mikrochimica Acta 1955, 410-415.

[19] Hermann, W., Noise and linearity check on photomultipliers, Z. Naturforschung 12A, 1006-1013 (1957). [20 1 Reule, A., Checking the photometer scale of absorption instru- ments, Zeiss-Mitt. 1, 283-299 (1959).

[21 1 Hoppmann, H., An instrument for checking the proportionality

of physical radiation receivers, Technisch-Wissenschaftliche Abhandlungen Der Osram-Gesellschaft 7, 306- 312 (1958).

[22] Gibson, G. L., Hammond, H. K., Ill, Holford, W. L., and

Nimeroff, I., Calibration of photometers. NBS Manuscript,

Nov. 1, 1960.

[23| Bischoff, K., Measurement of proportionality of radiation re- ceivers over a large radiation intensity range, Z. Instr. 69,

143-147 (1961).

[24] Jones, O. C, and Clarke, F. J. J., A new photometric technique

using a variable shutter device. Nature 191, 1290 (1961).

[25] Cordle, L. C, and Habell. K. J., Photometry of telescopes and

binoculars. National Physical Laboratory. Notes on Applied Science No. 14. 18 pp. (1962).

[26] Sanders, C. I.., A photocell linearity tester, Appl. Optics 1, 207-271 (1962).

[27] Nonaka, M., and Kashima, T., Linearity characteristics of multiplier phototubes, Japanese Journal Applied Physics 2, 785-

791 (1963)

[28] Hiifert, H. J., and Loof, H., Calibration of photometric scales of

a reflection photometer, Z. Instrumentenkunde 72, 139-143

(1964).

[29] Clarke, F. J. J., Time ratio photometry, Lecture to the Optical Group of the Institute of Physics and the Physical Society (June 9, 1966).

[30] Clarke. F. J. J., High accuracy spectrophotometry of three filters, NPL, Report No. 3042, 14 pp. (Nov. 4, 1968).

[31] Reule, A., Testing spectrophotometer linearity, Appl. Optics 7, 1023-1028(1968).

[32] Desvignes, F., and Ohnet, J., Characteristics and measurement

of properties of radiation receivers, Techniques Philips N.6,

1-18 (1968).

[33] Kunz. H., Representation of the temperature scale above 1337.58 K with photoelectric direct current pyrometers. Metrologia 5, 88-102 (1969).

[34] Kostkowski, H. J., Persona] communication (NBS 1970).

[35] ASTM Tentative Method of Estimating Stray Radiant Energy, ASTM:E 387-69T.

[36] Clarke, F. J. J., Personal communication (NPL, August 1970).

[37] Mielenz, K. D.. and Eckerle, K.. Accuracy of Dolarization attenuators (to be published), Appl. Optics (1972).

[38] Mielenz, K. D., and Eckerle, K., Spectrophotometer linearity testing using the double-aperture method, Appl. Opt. 11,

2294 (1972).

[39] Garfinkel, S. B., Mann, W. B., and Youden, W. J., Design and

statistical procedures for the evaluation of an automatic gamma-ray point-source calibrator, J. Res. Nat. Bur. Stand. (U.S.),70C(Eng. and Instr.). No 2.53-63 (1966).

[40] Edisbury, J. R., Practical Hints on Absorption Spectrophotometry (Plenum Press, New York, 1967).

[41] Holland, L., The Properties of Glass Surfaces (Chapman and

Hall, London, 1964).

[42] Hass, G., and Thun, R. E., Editors, Physics of Thin Films (Academic Press, 1967). (Paper 76A5-729)

85
Appendix V

JOURNAL OF RESEARCH of the Notional Bureau of Standards — A. Physics and Chemistry

Vol. 77A, No. 6, November-December 1973 Reflection Correction for High-Accuracy Transmittance

Measurements on Filter Glasses

K. D. Mielenz and R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(July 27, 1973)

Multiple reflections in the sample compartment of a spectrophotometer constitute a source of systematic bias in transmittance measurements on filter glasses. This bias may be removed by applying

a numerical correction obtained from measurements on tilted samples in polarized light. For a high- accuracy spectrophotometer, this correction was found to be of the order of several 10" 4 transmittance

units, independent of polarization, but slightly wavelength dependent.

Key words: Correction for reflections; reflections, multiple; spectrophotometry, high accuracy;

systematic bias in spectrophotometry; transmittance, correction.

1. Introduction

The sample compartment of most spectrophotometers is designed such that the monochromator exit

slit is focused into the sample by a lens, and is re- focused into the detector by another lens (fig. 1). This beam geometry is preferred by instrument designers

for intensity reasons and other convenience gains, but may constitute a source of significant systematic

errors of the measured data. One of these errors is caused by multiple reflections between the two lenses and the sample surfaces [1.2. 3].' For high-accuracy applications this error can be

eliminated by means of a numerical correction, AT,

which converts the measured transmittance, T.m, into the correct valuet=Tm + AT. (1)

1 Figures in brackets indicate the literature references at the end of this paper.

A simple but accurate method to determine this cor- rection is described in this paper. The specific spectrophotometric application dis- cussed is the measurement of the transmittance of

glass filters to an accuracy of ± 10"4 transmittance

units. For the particular high-accuracy spectropho- tometer used [4], the correction AT was found to be

of the order of several 10 4 transmittance units, independent of polarization throughout) the visible spectrum, and applicable for filters with widely dif- ferent indices of refraction. The correction was found

to be slightly wavelength dependent.

2. Theoretical Background

A detailed discussion of the reflection errors

affecting the measurement of filter transmittances may be found in reference [3]. A summary is given here in order to provide the theoretical foundation

for the experimental work described in section 3.

SAMPLE ENTRANCE

APERTURE

FIGURE 1. Sample-compartment optics of a focused-beam spectrophotometer.

86
The purpose of the measurement is assumed to be

the determination of the normal-incidence trans- mittance,

Tf (l-r)«

l-r2^

2 (2a)

of a glass filter surrounded by air. The filter is characterized by its thickness t and complex refractive index ti(1 + ik), so that for each wavelength X the normal-incidence internal transmittance t, and

Fresnel reflectance r appearing in eq (2a) are given by

(2b)

(2c)

(a) / \—

)

(b) A

T,T 2

T, TT, — T,TR 2 TR,TT 2

T| TR 2 RT 2

T, RR.TT,

FIGURE 2. Reflected beams in a spectrophotometer with lenses. AT=- (R X + R 2 )RT, (4b)

This quantity r can be measured accurately with

specially designed instrumentation in which the sample

is placed normal to a collimated beam of light, the

collimation being effected by off-axis mirror optics in a manner which reduces reflection errors to a negligible

level [3, 5]. A conventional spectrophotometer does not permit such a direct measurement of t because of the above-mentioned reflection error, even if its lenses were arranged to produce a collimated rather than the usual focused beam.

The cause of this error is illustrated in figures 2a and

2b. According to the first of these, the radiant flux received by the detector during the clear-space measurement is

<t>„ = T1 T2 (\+R l R2 + . . .), (3a ) where the initial flux is taken as unity, and where 7\

Ri, Ti, and R2 are the fractional fluxes transmitted or

reflected into the detector by all components (lenses,

slit jaws, detector housing, etc.) in front of and behind

the sample area, respectively. When a sample with transmittance T and reflectance R is inserted, the

flux into the detector is

4> = Tl TT2 (l + R l R2 T2 + RR2 + R l R + . . .), (3b)

as indicated in figure 2b. Within the approximation

that all contributions due to four or more reflections are considered negligibly small, the measured trans- mittance is Tn — </>/ <f>0 = T[i + (R i +R2 )R-R,R2 (\-T^)]. (3c)

The correction which must be applied to Tm to eliminate the reflection bias is T-TM =AT+AT', (4 a ) where

AT' = R 1 R2 T(1-Ti ). (4c)

The first of these two error terms, AT, is due to the

fact that the insertion of the sample has created the two additional beams shown at the bottom of figure 2b. This error is greatest when the sample is normal to the

optic axis, and may be eliminated by sufficiently tilting the sample so that these two beams no longer reach the

detector. The other error component, AT", arises from

the fact that the second beam from the top in figure 2b

passes the sample three times, whereas the direct beam passes it only once. Since this residual error cannot be avoided by tilting the sample, it is clear that

tilting is an effective means to eliminate reflection errors only if A7" is substantially smaller than AT.

Equations (4b, c) suggest that this should be the case

for any well-designed spectrophotometer for which

these reflection errors are not unduly large. To show

this, we assume that Ri and R2 are both of the order

of a few percent, and that glass filters with an average

transmittance T ~ 0.5 and reflectance R ~ 0.08 are used as samples. This leads to ATIAT' ~ 10, showing

A7" to be a significantly smaller error in such circumstances. Whether or not this estimate holds must be

ascertained individually for any given spectrophotometer.

Tilting the sample introduces obliquity errors, and

therefore does not yield an accurate measurement of the normal-incidence transmittance t unless further

corrections are applied. These errors arise from a de- crease of internal transmittance due to increased

pathlengths in the sample for oblique light incidence

and, secondly, from a variation of reflection losses at the sample boundaries with angle of incidence. The

latter also introduces a dependence of measured

transmittance on polarization. For glass filters measured with a conventional spec- trophotometer in which the monochromator exit slit is imaged onto the sample, these errors are given by

T=r[\+ (l/2/i2

) (hmX^+f 0§) ± (4r/n) 6* + . . .], ,5)

87
where T is the measured transmittanee (corrected for reflection errors); t, x,-, and r are the normal-incidence

values of transmittanee, internal transmittanee, and

Fresnel reflectance as given by eqs (2a, b, c); 6t and

#o are the tilt angle and the cone angle of the incident

light; and where the upper and lower signs of the last term pertain to the two cases in which the light is S or P polarized in the tilt plane. Equation (5) was

derived in reference [3] under the assumption that the monochromator exit slit is sufficiently short to approxi- mate a point source, and that the focusing lens is underfilled so that the grating or prism constitutes the limit- ing aperture. This aperture is assumed to be square, subtending the same angle 26o in the horizontal and

vertical planes. The sample is assumed to be tilted only

in one of these planes. The small-angle approximation

given in eq (5) is estimated to be accurate to 10~4 transmittanee units for angles 6t and do up to about 10°. For average transmittances and tilt angles of a few

degrees, these obliquity effects are of the order of 10~3 transmittanee units. Since this is the same magnitude as estimated above for the reflection error, it is clear that for a spectrophotometer with lenses an

accurate measurement of the normal-incidence trans- mittanee t cannot be achieved without numerical cor- rection for either source of error. If the sample is tilted in order to reduce the reflection error, an obli- quity correction must be applied. Vice versa, a reflec- tion correction is necessary if the sample is normal to the optic axis so that obliquity effects are minimized. Although these two alternatives are equivalent, the

latter was chosen in this work since it constitutes the simpler approach for routine measurements of trans- mittanee.

.556

T

- 1

•

1 1

• j

—*r

i

i

T

•X • • •

(b)

i i I I 0

FIGURE 3. (a) Measured transmittanee T M versus for 55 percent filter in S andP polarized light at K = 5

(6) The same data after application of tilt

tilt angle 0 r 74 nm.

correction.

3. Experimental Procedure

The preceding theoretical discussion does not permit

a precise calculation of the required reflection correction. Representing fractional fluxes which actually reach the detector, the quantities/?! and R* appearing

in equations (4b, c) are complicated and generally

unknown functions of the relative positions of sample

compartment elements, lens curvatures, location of aperture stops, and other beam-geometry factors. This makes it necessary to measure the correction. Such a measurement may be performed as follows. Each of a series of filters with different transmit- tances is measured for different known tilt angles

&t, and for S and P polarization of the incident light. This yields a set of raw data 7V, such as plotted in figure 3a. These are transformed into corrected data T by adding the tilt-correction given by eq (5),

(Ar) tilt =-rA/ (1/2 »«)ln( r^)±(4^» (6)

where the approximate values T\i and 7\;/(l— 2r) were substituted for x and x, on the right-hand side, and

where estimates of n and r may be used. As indicated by figure 3b, these corrected transmittances T are constant for sufficiently large positive or negative tilt

angles, but near normal incidence exhibit the 're- flection hump' whose height represents the desired

reflection correction A T to be applied for this particular value of transmittanee. From the measurements

performed on all of the filters one may then determine

the dependence of AT1 on T, as illustrated by figure 4. According to eq (4b), this dependence should be linear. In the work described here, these measurements were made as follows. A series of four Schott NG~4 glass filters with nominal transmittances 0.1, 0.2, 0.3, and 0.5, an evaporated

metal-on-glass filter with a nominal transmittanee

0.4, and a clear glass plate were used. All of these samples were 50 mm X 50 mm X 2 mm in size, and flat and plane-parallel to within a few fringes of mercurygreen light. The spectrophotometer used to perform

the measurements was fully described in a previous

issue of this journal [4], and will not be discussed here. The filters were placed in the sample compartment of

this spectrophotometer on a vertical rectangular holder adapted to a rotary horizontal table with 10 cm

diameter. This table had a scale divided in 360° with

a vernier reading to 5 min, and was held on the single- sample carriage of the spectrophotometer (see ref.

[4], fig. 2) with a vertical mounting rod. This sample

carriage unit consists of a platform provided with

vertical holders which can be moved laterally by a

88
!

Figure 4. Reflection correction AT versus T for K = 574 nm.

rack and pinion arrangement. The platform, mounted

on four ball bushings which ride on two horizontal

rods, can be moved pneumatically across the optical

axis. This remote-controlled movement is smooth, and

the sample position in and out of the beam is repro- ducible within 0.025 mm. A sheet polarizer of 9.5 cm diam was used to produce the S and P polarized radiation. This polarizer was held in a ball bearing rotating mount with a circular scale divided from 0 to 360°. It was placed on the

optical bench in front of the entrance slit of the pre- disperser-monochromator aggregate of the spectro- photometer between the circular neutral wedge and

the flat front-surface mirror (see ref. [4], fig. 3). This placement of the polarizer on the source side of the spectrophotometer avoids the additional reflection errors which would occur if the polarizer were put in the sample compartment of the spectrophotometer,

but requires that the monochromator does not signifi- cantly depolarize the incident radiation or rotate its plane of polarization. This was ascertained by testing the monochromator between crossed polarizers at the wavelengths at which the measurements were per- formed. Each transmittanee measurement was performed

in S and P polarized radiation and for tilt angles varying

by ± 10° about the normal position. Each measurement

resulted from the ratio ///<> of the attenuated to the unattenuated radiation value, each / and I„ being

the average of 50 individual measurements. The

measurements were repeated four times.

4. Results

Using the procedures outlined in section 3, the numerical values of the reflection correction AT

were determined for the four neutral-density glass

filters and the clear glass plate, at the wavelength

\ = 574 nm. This yielded the result represented by the

straight line in figure 4; namely,

Ar/r=-8.9X 10-", for\ = 574 nm. (7a)

Each of the individual values of A71 could be measured

to a precision better than 10~4 transmittanee units. There was no evidence suggesting a dependence of AT on polarization.

Secondly, the measurements on the clear glass

plate were repeated for A. = 400 nm and 650 nm.

Together with the previously obtained result for 574 nm, this showed the wavelength dependence indi- cated in figure 5. This dependence is also linear, and may be expressed as A77r=-1.26xl0-,i (X-574 nm), for 7=0.92.

(7b)

These results were then combined into the final empirical expression.

A77r=-8.9X 10" 4 [l + 1.4x 10- 3 (A-574 nm)].

(8)

for the correction AT to be applied for different trans- mittances T and wavelengths K. This formula agrees

with all individual measurements of AT1 to within ± 5 X 10~5 transmittanee units. The correction AT so obtained represents only the

first of the two error terms appearing in eq (4a), above. The exact value of the second term AT" cannot be determined within the context of this paper, but a quantitative estimate may now be obtained as follows. From eqs (4b) and (7a) it may be seen that, for this spectrophotometer and for the appropriate value R ~ 0.08 for the filters used,

2 (R1 + R2) ~.6X 10-3

. (9a)

Provided that this arithmetic average of R 1 and R 2 is not substantially different from their geometric average (RiR 2 ) 112

, it follows from eq (4b) that AT ~ 3.6 x 10"5 7X1 -T2 ), (9b)

1.2- -<AT/T)XIO'

400 500 600 700

FIGURE 5. Reflection correction AT versus A for T = 0.92

89
or A7"~1.4X10-5 for 7= 0.577, for which this error is a maximum. Although this argument is not mathematically exact, it suggests that the residual

reflection error AT" is entirely negligible for the spectrophotometer used here. According to eq (4b), the correction AT depends on

the reflectance R and, therefore, on the refractive index

n of the sample. Using eqs (4b) and (2c) it is easily seen

that

(AT) f =- (T/3n*

d(AT)

8r(ft, + R 2 )T

(n + iy dn. (10)

For n = 1.5, r=0.04, 7"= 0.5, and the above value

(9a) for (Rj + Ri). this is reduced to d(AT) ~ 3 X 10-" dn, showing that AT will not be affected by more than 10~4 transmittance units as long as dn < 0.3, or 1.2 < n < 1.8. Hence it may be concluded that the above

values of AT are applicable to virtually all filter glasses.

Other types of samples, such as coated glasses, will of course require a different correction. For example, measurements on the evaporated metal-on-glass filter with transmittance 0.4 at 474 nm yielded a value, AT = — 1.4 X 10~3

, which is about four times larger than indicated by figure 3 for uncoated samples.

5. Conclusion

Based upon this work, all transmittance measure- ments on glass filters with this high-accuracy spectro- photometer are performed with the sample accurately

aligned normal to the optic axis. In addition to the re- flection correction given by eq (8) and the detector

nonlinearity correction discussed in reference [4], an /-number correction is applied to remove the resid- ual bias due to the nonparallel radiation in the sample

compartment. This latter correction is given by eq

(5) as

ft 2 -

its magnitude is 1.3 X 10 4 transmittance units, or less, for the//10 cone of light (0O =O.O5 rad) used. With

these corrections applied, all measurements made with

this high-accuracy spectrophotometer yield the normalincidence transmittance r defined by eq (2a) within a 10~4 limit of instrumental accuracy. This spectrophotometer was designed and con- structed to permit calibration of solid and liquid filters for transmittance with a well-defined accuracy (4). These filters are used as Standard Reference Materials (SRM's) to check the photometric scale of conventional spectrophotometers. SRM 930a consists of a set of three glass filters (Schott NG-4) with nominal trans- mittances 0.1, 0.2, and 0.3. Their transmittance values

are certified to ±0.5 percent of the value, this uncertainty being the sum of random and systematic errors. The latter are principally due to the inherent in- homogeneity and instability of the glass, surface

effects, and positioning the filters. They exceed by a large margin the instrumental corrections discussed

in this paper. Hence, the certified transmittances of previously issued standard filters remain unaffected by these corrections.

6. References

[1] Gibson, K. S., and Balcom, M. M.. J. Res. Nat. Bur. Stand.

(U.S.). 38, 601 (1947).

[2] Goldring, L. S., Hawes, R. C, Hare, G. H.. Beckman, A. O., and Stickney, M. E., Anal. Chem. 25 , 869 (1953).

[3] Mielenz, K. D., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and

Chem.), No. 5, 455-467 (Sept.-Oct. 1972).

[4] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No. 5, 405-425 (Sept.-Oct. 1972).

[5] Mielenz, K. D., Eckerle, K. L., Madden, R. P.. and Reader, J.. Appl. Optics 12, 1630 (1973).

(Paper 77A6-792)

90
Appendix VI

JOURNAL OF RESEARCH of the Notional Bureau of Standards - A. Physics and Chemistry

Vol. 78A, No. 5. September-October 1974 Adaptation of a High-Accuracy

Spectrophotometer for Ultraviolet Work

K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik*

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 31, 1974)

A high-accuracy spectrophotometer, originally designed for work at visible wavelengths, was

modified to permit measurements in the ultraviolet without degradation of its original performance.

This was accomplished by equipping the spectrophotometer with a stable deuterium arc source, a highly efficient averaging sphere with fluorescent wavelength converter, a new grating, and achromatic sample-compartment optics. The modified spectrophotometer will be used for the development of new

Standard Reference Materials, as well as for materials research, in the region between 200 and 300 nm.

Key words: Averaging sphere; deuterium arc lamp: flunrescent wavelength converter: grating: spectro- photometry: standard reference materials; ultraviolet; UV achromats: visible.

1. Introduction

A primary goal of the current NBS program in spectrophotometry has been the design of high- accuracy instrumentation for the certification of cali- bration standards and the development of improved measurement techniques. For transmittance meas- urements at visible wavelengths, this goal was accomplished by specially constructing two highly accurate single-beam spectrophotometers [1, 2],' which are presently used in the Institutes for Materials Research

(IMR) and Basic Standards (IBS) for standards work

at visible wavelengths (400 to 800 nm, approximately).

The instrumental accuracy achieved with these spectrophotometers is 10~4 transmittance units (0.01 percent). Although similar in some design aspects, these two spectrophotometers differ significantly in others, and therefore can be employed for in-house comparisons to ensure the accuracy and consistency

of all spectrophotometric measurements at NBS. The

purpose of this work was to extend these capabilities

into the ultraviolet spectral region by developing the means for modifying either spectrophotometer for applications down to about 200 nm. The particular instrument that was modified was

the IMR spectrophotometer described in reference

[1]. As originally designed, this spectrophotometer

employed a current-stabilized, 100-W tungsten ribbon lamp as the radiation source; a Tmeter, f/8.7 Czerny-

•Tliis paper describes a cooperative project mi the pan i »f the three aulliurs. The liasir design concepts ami tin- accomplishment ..I' a functioning instrument an- itur t.i K I) Mielen2 anil K. Mavrodineanu. Tin- optimization of the fluorescent wavelength converter for the averaging sphere was achieved b> K.f). Cehelnik.

1 figures in brackets indicate the literature references at the eml of this paper.

Turner grating monochromator (1200 lines/mm, 500-nm

blaze) with a fused-silica prism predisperser as the dispersing element; and an 11 -stage, S-20 photomultiplier tube attached to a 125-mm. BaS04 -coated averaging sphere as the signal detector. The tungsten

lamp is imaged on the predisperser entrance slit by

a fused-silica lens. Two additional fused-silica lenses

are used in the sample compartment to focus the monochromator exit slit at the sample, and to refocus

it into the averaging sphere [3]. The photomultiplier

signal is measured by means of a current-to-voltage converter and a digital voltmeter, interfaced with a computer.

It may be estimated that, for a spectrophotometer of

this type, a clear-space signal of at least 10"' A is required if measurements are to be made with a stand- ard deviation of at least 10 4 transmittance units |4|. The actual clear-space signal current obtained with

this spectrophotometer at different wavelengths and

for typical values of tungsten-lamp power, monochromator slit width, photomultiplier anode voltage,

etc. is shown as curve (a) in figure 1, and thus indi- cates an inadequate performance of the instrument in the spectral region below approximately 370 nm. In order to effect the desired improvement of ultraviolet performance, the spectrophotometer was equipped

with a deuterium arc source, a re-designed averaging

sphere, and a differently blazed grating. These components were chosen to achieve a wide spectral range

(200 to 800 nm) of the modified spectrophotometer,

requiring only a change back to the tungsten lamp

for work above 400 nm. In order to facilitate the usage

of the instrument throughout this extended range, it was necessary to replace the two sample-compart- ment lenses by fused-silica, lithium-fluoride achromats.

91
2. Modification of Spectrophotometer

2.1. Deuterium Arc Lamp

To a significant extent, the sharp drop of ultraviolet

is attni

300WAVELENGTH.

Ki*;ure 1. Successive improvement of clear-space signal level of

original spectrophotometer (curve a) obtained by:

ib) deuterium arc lamp, (c) improved averaging sphere, and id) new grating.

utable to the tungsten lamp of the original spectro- photometer. A rapid decrease of radiant intensity with decreasing wavelength is typical for incandescent

filament lamps, and therefore makes any source of

this kind a poor choice for work at short wavelengths

[5]. In contrast, a deuterium arc lamp exhibits an in- crease of radiant intensity with decreasing wavelength, and thus provides much better power levels

in the ultraviolet. A 60-W deuterium lamp with high-purity fusedsilica window was chosen for the modified spectro- photometer. This lamp has a line-free continuous

emission in the ultraviolet extending to below 180 nm,

and is rated to have the same output at 200 nm as a 150-W xenon arc lamp. In comparison to a 250-W

tungsten-bromide filament lamp, its output is about 1000 times greater at 250 nm, and 4 times greater at 300 nm. For work above 400 nm, tungsten lamps are preferable because of the decreasing intensity of the deuterium-arc spectrum and its line structure in the

visible. The clea^r-spaee signal obtained after the deu- terium lamp was installed in the spectrophotometer

is shown as curve (b) in figure 1. The stability of the radiant-intensity output of the deuterium lamp was tested by performing regularly

repeated measurements of the spectrophotometer

signal over extended periods of time. The long-term

stability of lamp power so observed may be seen from

figure 2. showing a gradually decreasing signal drift at the rates of 1.5, 0.6, and 0.08 percent per hour after the lamp had been operated for 1, 3, and 5 h, respectively. The average short-term instability (random

noise plus drift) of the deuterium lamp was found to be

0.02 percent for a total of 15 sets of 20 individual readings, taken at 5-s intervals and at arbitrary times dur- ing an 8-h period of lamp operation. Since all data ob- tained with this spectrophotometer are derived by

averaging a large number of individual measurements

of transmittance, performed in a time-symmetrical sequence which eliminates the effects of drifting, this stability of the deuterium lamp is adequate for routine

OPERATING HOURS

FIGURE 2. Long-term signal drift observed

with deuterium lamp.

92
measurements with a precision better than \0~ A transmittance units. The above results for the deute- rium lamp compare favorably with the stability data found for a current-stabilized tungsten lamp [2].

2.2. Averaging Sphere

The original averaging sphere of the spectrophotometer was designed as shown in figure 3a. It has an

internal diameter of 125 mm. and circular entrance and

exit ports 20 and 50 mm in diameter, respectively.

The circular target, 35 mm in diameter, is located at the center of the sphere. The inside of the sphere,

both sides of the target, and the thin target support

rod are coated with several layers of specially pre- pared barium sulfate and polyvinyl alcohol coatings

as developed by Grum and Luckey [6]. The photomultiplier tube is attached to the sphere, so that its 50-mm cathode is located 16 mm behind the exit port. The efficiency of this sphere was determined by suc- cessively measuring photomultiplier signals obtained

at different wavelengths with and without sphere,

using the deuterium arc lamp. These measurements showed a steep decrease of sphere efficiency from about

15 percent for visible wavelengths near 500 nm to

SH

Kna'RE 3. Geometry of (a) original averaging sphere and ib) improved

sphere.

A: rnlram-r port. A': «'xil port. I): ilpteilor. K: fluor.-sci-nt dye fell, S: sphere wall. SH:

shinier. T: larjiel

less than 0.1 percent at 200 nm, and thus indicated the necessity to equip the spectrophotometer with an improved, more efficient sphere [7]. According to theory [8], the efficiency of an averaging sphere of the type considered is, approximately,

4>nl4> = [r-l ( 1 - r ) ] ( 1 - e/d ) dV 16s 2

( 1

where

</> = radiant flux into sphere,

d)D = flux reaching detector. r= wall reflectance (assumed close to, but not equal

to unity),

s = sphere radius,

d = radius of sensitive area of detector (assumed

equal to radius of exit-port)

e = distance between exit port and detector.

It may be seen from this equation that, to a large extent, the poor efficiency of the original spectropho- tometer sphere is due to inadequate coupling of exit port and detector. The collection efficiency of the detector (i.e.; the fraction of the diffuse flux from the

exit port which actually reaches the photocathode) appears in Eq (1) as the factor (l-e/d). which in this case has the numerical value 0.64 and thus shows

that more than one third of the available flux is lost through the gap between exit port and detector. Therefore, a large increase in efficiency could be

gained by modifying the sphere as shown in figure 3b. The sphere wall was tapered in a pear-like fashion and was extended into the detector housing, so that the exit port is butted directly against the photo- cathode. This simple modification of the sphere re- sulted in a large increase of efficiency (from 15 to 70 percent at 500 nm). However, the efficiency in the

ultraviolet (now 2.5 percent at 200 nm) was still inadequate.

The averaging effectiveness of this sphere was

tested by Mr. K. L. Eckerle of the Optical Radiation

Section of NBS. The sphere was found to be insensitive to beam displacements from the target center of ±1.5 mm within a ± 10" 4 limit of signal variation. This is to be compared with a ±5 percent variation

for a ± 1 mm beam displacement on the same photomultiplier used without the sphere.

The high efficiency of the modified sphere in the

visible suggested that a near-optimal sphere geometry

had been achieved, and that the limiting factor in the

ultraviolet was now the reduced reflectance of the bari- um-sulfate sphere coating. In order to overcome this re- maining difficulty, a fluorescent dye was used to shift the incident short-wave radiation into the longer

wavelength region for which the reflectance of barium

sulfate is high. This dye, which is contained in a cell placed in front of the sphere target as shown in figure

3b, had to be chosen such that virtually all of the inci- dent flux at short wavelengths is absorbed and is converted into fluorescence with a quantum efficiency as nearly equal to unity as possible. In order to avoid impairing the high sphere efficiency at longer wavelengths, it was also required that visible light be not absorbed, so that it passes through the dye unaffected and is reflected from the target as before. The fluores93
cent material selected was a 0.923 g/1 solution of 2,5- diphenyloxazole (PPO) in n-hexane in a 10 X 10 X 2 mm high-purity fused-silica cell. It is effectively trans- parent for wavelengths above 360 nm. and below 340 nm absorbs more than 99 percent of the incident radiation. Its estimated quantum efficiency is 0.9. with the peak emission occurring near 370 nm. The use of this dye proved highly successful, and resulted in a sphere

efficiency of 20 percent at 200 nm. The marked enhancement of signal level obtained

with this improved sphere is shown by comparing

curve (c) in figure 1 with curve (b). The measured

efficiency of the sphere is roughly constant and equal

to 20 percent between 200 and 300 nm, followed by a gradual increase to a constant level between 60 and

65 percent for wavelengths from 400 to beyond 650 nm. A further increase of sphere efficiency was obtained in a final design by reducing the sphere diameter and

further optimization of the dye [8]. It was also ascertained that the fluorescence wavelength converter did not affect the linearity of the detection system. Theoretically, the radiant intensity

of fluorescence from the dye can be expressed as

I, =01,,. (2a)

where Q is the quantum efficiency of the dye and /„ is the intensity absorbed. The latter is given by Beer's law as

/„ = /„(l-10-«-ft

) (2b)

where /„ is the initial intensity, and where e, c, and b denote molar absorptivity, concentration, and path

length, respectively. Thus, Ie is proportional to /„ under the applicable assumptions that Beer's law is valid and that the quantum efficiency of the dye is independent of intensity. This conclusion was verified by measuring the linearity of the detection system at one wavelength (250 nm) for which the dye is effective, and at another (400 nm) for which it is not. These two measurements, which were made using the double- aperture method [9], yielded indistinguishable results within the limits of experimental uncertainty, and also agreed with previously performed linearity measure- ments of the same photomultiplier tube at 575 nm [1].

2.3. Grating

The previously obtained clear-space signal level, curve (c) in figure 1. was still judged inadequate for accurate work at short wavelengths. Since the original

grating of the spectrophotometer (blazed for 500 nm in the first order) had been chosen for work in the visible,

it was obvious that a further improvement could be

effected by substitution of another grating. This new

grating has 600 lines/mm, is blazed for 200 nm in the

first order, and resulted in the final clear-space signal current plotted as curve (d) in figure 1. Since the signal

level near 200 nm can easily be raised by adjusting the photomultiplier gain, the original goal of achieving a

clear-space signal of at least 10 " A throughout the

ultraviolet had thus been accomplished.

The loss of signal level in the visible due to the change of gratings [curves (d) and (c) in figure 1 1 is relatively small, and is more than offset by the higher

efficiency of the new sphere [curves (c) and (b)|. Thus

the modified spectrophotometer can be used above 400 nm by merely interchanging deuterium and tungsten

lamps, without need to substitute gratings.

2.4. Achromatic Sample-Compartment Optics

In view of the wide spectral range of the modified spectrophotometer, the chromatic aberration of the

original sample-compartment lenses proved to be a

limiting factor of instrumental performance. Significant losses of signal level were caused by the fact that these lenses, when initially focused by eye. were out

of focus in the ultraviolet and thus caused less-than- optimal illumination of sample and detector.

If the monochromator exit slit is focused at the sample with unit magnification at a wavelength An. the diameter of the blur circle in the sample plane for a wavelength k < k (i can easily be shown to be equal to

8 = D[2/(Ao)//(\)-l], (3) where /is the focal length and D is the effective diameter of the focusing lens. For the particular lenses of the

original spectrophotometer [/(200 mm)=168 mm.

/(500 nm) = 200 nm, 0 = 38 mm|, and assuming that the focusing was done at 500 nm, the computed blur- circle diameter at 200 mm is 14 mm. This numerical

example illustrates the large defocusing caused by

chromatic lenses in the sample compartment, although

it should be pointed out that due to the opposing effect of the second lens a smaller blur circle is incurred at the detector. Nevertheless, it was found empirically

that the ultraviolet signal level of the spectrophotometer could be improved as much as 30 percent by re- positioning the sample-compartment lenses. The most effective way to overcome this deficiency

of the spectrophotometer would have been to use

mirror optics, but since this would have required a redesign of the sample compartment and its enclosure

it was more expedient to employ achromatic lenses. The lenses selected are f/3 lithium-fluoride, fusedsilica (non fluorescent) achromats with 200 mm focal length, best correction for longitudinal chromatic aberration between 200 and 400 nm. and with anti-reflection coating for 400 nm. On account of the limited spectral regions for which

the deuterium and tungsten lamps of the modified spectrophotometer are employed, it was not found

necessary to use an achromat in the source compart- ment as well.

3. Conclusion

As a result of the work reported here, the modified high-accuracy spectrophotometer is now used, throughout the spectral region between 200 and 800 nm, for

94
the certification of solid and liquid Standard Reference

Materials for routine applications in spectrophotometry, as well as for materials research aimed at establishing inorganic and organic molar absorptivity standards. The modified spectrophotometer may also serve as a model for improved commercial instrumentation. In

particular, it is anticipated that the development of a highly efficient averaging sphere for visible and ultra- violet wavelengths will lead to a more widespread

use of spheres in routine spectrophotometry.

4. References and Notes

[1| Mavrodineanu. R.. J. Res. Nat. Bur. Stand. (U.S.]. 76A (Phvs. and Chem.). No. 5. 405-425 (Sept.-Oct. 1972).

[2| Mieienz. K. D.. Eckerle. K. L.. Madden. R. P.. and Reader. J.. Appl. Optics 12, 1630-1641 (1973); Mieienz. K. D.. and

Eckerle. K. L.. Nat. Bur. Stand. (U.S.). Tech. Note 729. 60

pages (June 1972).

|3| Tlie systematic error due to interreflection between the sample- compartment lenses is accounted for in all measurements made with this spectrophotometer. See Mieienz. K. D.. and

Mavrodineanu. R.. J. Res. Nat. Bur. Stand. (U.S.), 77A (Phys. and Chem.). No. 6. 699-703 (Nov.-Dec. 1973).

[4] K. D. Mieienz. J. Res. Nat. Bur. Stand. (U.S.). 76A (Phys. and

Chem.). No. 5. 455-467 (Sept.-Oct. 1972).

(5| Although tungsten-halogen lamps are frequently used in ultraviolet spectrophotometry, such a source was unsuitable for

this application. A 250-W. single-coil tungsten-bromide lamp

was tried, but merely resulted in an over-all increase of signal

level which could also have been achieved by choosing a more

intense tungsten lamp or increasing the photomultiplier gain. The tungsten-bromide lamp exhibited essentially the same

steep drop of signal toward shorter wavelengths as the tungsten lamp, and was found to be inadequate for accurate work

below 320 lira. A further difficulty is that tungsten-halogen lamps are generally too unstable for high-accuracy spectro- photometry in a single-beam geometry. See Clarke. F. J. J..

J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No.

5, 375-403 (Sept.-Oct. 1972).

[6| Crum. F.. and Luckey. C. W.. Appl. Optics 7, 2289-2294 (1968).

[7| A significant improvement of the visible and ultraviolet signal

levels was found with a flux averager consisting of two double- ground fused-silica plates. 1 mm in thickness and spaced by a diffusely reflecting, cylindrical Al_.0:i tube with 50 mm length and internal diameter. Although this type of averager has been

used successfully for other applications (See Budde. W., Proc. Fourth Imeko Symposium. Prague 1969. p. 167). the averaging

effectiveness of the particular unit used in this work was in- adequate for this application since a beam displacement of ±1 mm at the entrance window of the averager still caused a 0.4 percent change in the photomultiplier signal. These meas- urements were performed by Mr. K. L. Eckerle of the Optical Radiation Section of NBS.

|8| Mieienz. K. D.. Mavrodineanu. R.. and Cehelnik. E. D.. Appl. Optics (to be published).

[9] Mieienz. K. D. and Eckerle. K. L.. Appl. Optics 11, 2294-2303

(1972). (Paper 78A5-838)

95
Appendix VII

Designation: E 387 - 72

AMERICAN SOCIETY FOR TESTING AND MATERIALS

1916 Race St., Philadelphia, Pa., 19103

Reprinted from the Annual Book of ASTM Standards. Copyright ASTM Standard Method of

ESTIMATING STRAY RADIANT ENERGY1

This Standard is issued under the fixed designation E 387; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of

last reapproval

1. Scope

1.1 Stray energy (SRE) can be a significant source of error in spectrophotometry measurements. The clanger of significant error is enhanced since its presence is often unsuspected. Also, SRE frequently increases with passage of time. Therefore, thorough periodic

testing is highly important.

1.2 This method affords an estimate of ra- diant energy at wavelengths remote from

those of the nominal pass band transmitted through the monochromator of an absorption

spectrophotometer. Test-filter materials are described that discriminate between the desired wavelengths and those which contribute most to SRE for conventional commercial

spectrophotometers used in the ultraviolet, the visible, the near infrared, and the alkali halide infrared ranges. These procedures apply to instruments of conventional design, with usual sources, detectors, and optical designs. The vacuum ultraviolet and the far in- frared present special problems that are not discussed herein.

1.3 These procedures are neither all-in- clusive nor infallible. Because of the nature of

readily available filter materials, with a few

exceptions the precedures are insensitive to SRE of shorter wavelengths in the ultraviolet or visible, or of lower frequencies in the in- frared, and they are not necessarily valid for "spike" SRE nor for "nearby SRE." (See Appendixes for general discussion and definitions of these terms.) However, they are adequate in most cases and for typical applications. They do cover instruments using

prisms or gratings, in either single or double monochromators.

1.4 The proportion of SRE encountered with

a well-designed monochromator, used in a favorable spectral region, is a small fraction of 1 percent. With a double monochromator

it may easily be less- than 1 ppm even with a broad-band continuum source. Under these conditions, it may be difficult to do more than determine that it falls below a certain level. Actual measurement often requires special techniques and instrument operating conditions that are not typical of those occurring

during use. When absorption measurements

with continuum sources are being made, it

is frequently true that, owing to the effect of

slit width on SRE in a double monochromator, these test procedures tend to give "conservative" results; that is, because larger

slit widths than normal must be used to ad- mit enough energy to the monochromator to permit evaluation of the SRE, the stray pro- portion indicated is greater than would nor- mally be encountered in use.

1.5 The principal reason for a test pro- cedure that is not exactly representative of normal operation is that the effects of SRE

are "magnified" in sample measurements at high absorbance. It is usually necessary to increase sensitivity in some way during the

test in order to evaluate the SRE adequately. This is usually accomplished by increasing

slit width and so obtaining sufficient energy

to allow meaningful measurement of the SRE

after the monochromatic energy has been re- moved by the SRE filter. A further reason for increasing energy or sensitivity can be that

1 This method is under Ihe jurisdiction of ASTM Committee E-13 on Molecular Spectroscopy. Current edition approved June 29, 1972. Published Sep- tember 1972. Originally published as E 387 69 T. Last pre- vious edition E 387 - 69 T.

By publication of this standard no position is taken with respect to the validity of any patent rights in connection there- with, and the American Society for Testing and Materials does not undertake to insure anyone utilizing the standard

against liability for infringement of any Letters Patent nor assume any such liability. "Reprinted by permission of the American Society

for Testing and Materials from the 1972 ASTM

Book of Standards, Copyright 1970."

96
# E387

many instruments have only absorbance

scales, which obviously do not extend to zero transmittance. Even a SRE proportion as large as I percent may fall outside the meas- urement range.

1.6 Many instruments that use optical at- tenuators to balance sample absorption make

relatively inaccurate measurements below 10 percent transmittance, because of poor atten- uator linearity. All instruments should be carefully and frequently checked if used below

about I percent transmittance because of the

possibility of zero shift due to electrical pickup

or other causes. For these reasons, the test procedure specifies that the measurements be made within 100 percent and 10 percent on

the scale. Obviously this is not required if re- liable photometric linearity and zero checks

are made under the conditions prevailing

during the test, and use of the 10 and 1 percent range may give lower and more ac- curately representative SRE values. In marginal cases this more elaborate test procedure,

incorporating accepted ASTM tests for photometric linearity, may be justified.

1.7 High accuracy in SRE measurement is not required. A measurement reliable within

10 or 20 percent is always sufficient, but often many readings must be cascaded to obtain the

final result, so that painstaking measurements

are desirable.

2. Summary of Method

2.1 The following test procedure is written

for transmittance-recording double-beam in- struments. Modification for absorbance re- cording will be obvious. It is readily adapted

for point-by-point operation by measuring at enough points to allow reasonably accurate

interpolation between the points so as to provide essentially continuous information over

the wavelength region covered, or by direct measurement at the desired specific wavelength.

2.2 After establishing that the instrument

is adequately free of zero transmittance error, and measuring the baseline (100 percent) over

the appropriate spectral range, a filter is in- serted into the sample beam and its transmittance curve is recorded, starting in a region where it is nearly transparent and scanning

slowly toward increasing absorption. The filter materials are selected for sharp cutoff, free- dom from fluorescence, and sufficiently high

absorption that their transmittance in the stop band can be neglected. They should be vis- ually clear and free of bubbles and striae. SRE will then set the limit to the minimum

transmittance observed, unless an adverse

signal-to-noise ratio, a scale limitation of the photometer, or a false electrical zero inter- venes.

2.3 These limitations may usually be over- come by increasing the slit width to admit more incident radiant energy on the filter. In a double-beam instrument, this may be ac- complished by attenuating the reference beam

with a screen, mask, or neutral filter. The fol- lowing procedure assumes the use of etched

screens, and measures their transmittance by

their effect on the instrument reading.

2.4 The apparent transmittance of the filter

is sensitive to spectral bandwidth. Since bandwidth increases with slit width, it is necessary

that the transmittance of the screen be measured at a fixed slit width each time. The slits may then be allowed to open for the following

steps of the test. The effect of such changes in

slit width on the apparent transmittance of

the filter becomes insignificant at the end of

the test when the SRE becomes the dominant

source of signal. 2.5 The scanning is continued, and the ref- erence beam is attenuated further in as many

steps as required, until the steeply falling transmittance curve levels off because of SRE. The product of all the reference atten- uation fractions times the value of the trans- mittance reading at the plateau, corrected by

the baseline value and the filter reflection

loss, is taken as the proportion of SRE.

2.6 The proportion of SRE can in some

cases increase with increasing slit width (see Appendix A4). Therefore, the smallest practical value of slit width should be used at the

last step of the reference beam attenuator,

as this corresponds most nearly with the nor- mal operating conditions of the spectrophotometer.2.7 Some instruments scan automatically

only in one direction, which may not be the one specified in the test procedure. Manual

97
control or scanning by hand may be necessary. Particularly note any precautions called for by the manufacturer's instructions in this case. If, for mechanical reasons, manual scan- ning is not feasible, the test procedure can be

reversed, after making the checks of 100 per- cent line and zero, by going to the desired wavelength within the filter stop band, insert- ing neutral attenuators in the reference beam

until the pen lies between 10 and 100 percent transmittance (preferably near 10 percent, widening the slits while doing so to restore normal pen response, scanning in reverse until 100 percent transmittance is reached, and removing the reference beam attenuators one at a time while measuring their attenuation, until all are out and a region where the

filter is transparent is reached.

2.7.1 The only difficulty with this procedure

lies in finding the narrower slit width that will give normal pen response after each reference beam attenuator is removed, and finding the transmittance change at that slit width. Several trials may be required.

2.7.2 When done carefully, this procedure

gives the same result as the normal one, and

in fact affords a way of checking the standard

test results whenever both scan directions are permitted.

3. Apparatus and Material

3.1 Screens1 of Etched Nickel, useful for reference beam attenuators are listed in Table

1 and footnote 2. They may also be convenient for "quick check" transmittance "standards." The screens should be placed as far as possible from each other, and preferably at a point along the beam where it is as wide as

possible. Usually a 1-cm spacing between

screens suffices to allow acceptable reproducibility of results. Their exact transmittance

varies with location in the beam, hence they should be mounted so they can be positioned

reproducibly, and mechanical stability of all parts of the spectrometer should be established. The transmittance of two or more

screens used in cascade or tandem is rarely equal to the product of their separate trans- mittances, and may be quite sensitive to lat- eral displacements with respect to each other, due to the moire effect.

3.2 Test Filter Materials, shown in Table

2, provide an array capable of satisfactorily covering nearly all normal ultraviolet and in- frared spectral ranges. The first column shows

the spectral range over which critical remote SRE measurements can be made. The second column shows the approximate 80 percent transmittance wavelength or wavenumber.

Scanning for the following procedure should always begin at this poinl, or at one more re- mote from the test spectral range.

3.2.1 Filter diameters are not specified. They should be large enough to cover the entire beam with substantial safety margin. Be- cause of the danger that radiant energy outside the nominal beam may bypass the filter and be scattered back into the beam it is advisable to mount them against a cell compart- ment window or in a much larger metal holder

so as to block any such paths. If in doubt, test with an opaque object of the same size as the

filter. 3.2.2 If there is any possibility that fluores- cence of windows, cells, or sample solvents may be contributing to SRE in the ultraviolet range, locate the test filter immediately fol- lowing the sample position in the beam, and

test in the presence of such cell or solvent. Note that fluorescence of optical elements be- tween the sample and the detector merely

modifies the detector sensitivity. It does not

constitute an effective source of SRE, since

this fluorescent emission is not differentially absorbed or transmitted by the sample.

3.2.3 The useful spectral range of the sharp

cutoff filters can be extended toward consider- ably more remote wavelengths, but if this is

Available in sheet form from Perforated Products.

Inc.. 68 Harvard St., Brookline, Mass 02146. Also as cut disks in holders from Cary Instruments. Monrovia. Calif. These holders fit the Cary cell blocks for cylindrical cells They do not fit holders for rectangular cells. The holders

are also useful for glass filters for Bouguer's law absorbance

linearity tests.

Screen and

Holder Part Number

14041 1

1

14041 12 14041 13 14041 14

Mesh Number Absorbance

30 25 K 0.5

60 30 R 1.1 120 40 T 1 .5 270 50 W 2.0

98
E 387

done, it must be recognized that the meas- urement becomes progressively less signifi- cant. The apparent proportion of SRE may be reduced in such regions because the filter ab- sorbs energy at wavelengths contributing sigr nificantly to SRE.

3.2.4 Plates of Alkali Halide, about 6 mm

thick for absorption cell windows are commonly on hand in analytical laboratories, and

the 80 percent transmittance points are specified for this thickness. However, other thicknesses, over a range from about 4 to 15 mm,

can be substituted without invalidating the

test.3.2.5 Fused Silica,'

1 in the form of cell win- dows, is commonly available and useful over

a thickness of 1 to 5 or 6 mm. Crystal quartz

is less desirable because of its birefringence, which may cause apparent cyclical trans- mittance variations with wavelength.

4. Procedure

4.1 Verify that pen response is "normal,"

that is, that normal damping (preferably

slight underdamping) and designated re- sponse time and low (less than V 2 percent) dead zone exist. Follow the manufacturer's in- structions, or test by moving a mask a small

distance into and out of first the sample, then

the reference beam.

4.2 Check the zero transmittance error with an opaque shutter or sample. Adjust the zero control or note the indicated zero and

subtract it from subsequent readings.

4.2.1 In some infrared instruments the ra- diant energy from a rotating beam modulator

can be compensated by an adjustable electri- cal signal. The amount of compensation re- quired varies with wavelength and slit width. Make frequent zero checks during the SRE

tests. It is desirable to interpose the opaque

shutter ahead of the filter, and only briefly, so that radiation from the filter is self-com- pensated. A shutter made of thick insulating

board, such as Celotex, 4 or several thicknesses of cardboard separated by air spaces, is recom- mended, preferably brought to instrument temperature by being stored in the sample

compartment.

4.3 Record a 100 percent line over the desired wavelength range for a single filter. If the slit system is controlled by a servo system

to maintain a constant reference signal, leave

this control on; otherwise put the slits under manual control or leave off (immobile). In the

latter cases, check pen response frequently

during the scan, and change the slit width if and when pen response becomes sluggish or markedly underdamped. Check by quickly in- serting and removing an opaque object in the sample beam. At each change point, record

the wavelength and new slit width, so that they can be reproduced in step 4.5. 4.3.1 If the spectrophotometer is provided

with a single beam mode in which the sample

signal is compared with a fixed electrical ref- erence signal, use this provision to check and

readjust slit width instead of the less reliable pen response test. 4.4 Insert the filter in the sample beam.

4.5 Scan slowly from high transmittance to low. When about 10 percent transmittance is reached, stop the scan but leave the pen on and the chart running, recording the pen position for several full pen periods (multiples of five times the time constant for an exponential response function). 4.6 Insert in the reference beam a "neutral" beam attenuator, such as a screen se- lected from the list shown in Table 1, and

having about 12 to 20 percent transmittance.

4.7 Record the resulting increased apparent transmittance for at least 20 times the nominal response period of the instrument. (The lengthened time is required because the reduced reference beam energy results in sluggish pen response.) If possible, displace

the optical attenuator, or the pen, or both, by

hand first in one direction, then the other, and

allow to recover to make sure that the photo- metric system still functions.

4.8 Note on the record the ratio of trans- mittance values, or "setback," that corre- sponds to the displacement at fixed slit width.

'Corning 7940 glass. General Electric Type 151 glass. Hereaus Suprasil, Ultrasil. or Thermal American Spectrosil have been found satisfactory.

4 Celotex® is a trademark of the Celotex Corp.

99
E 387

4.9 Turn on the slits or otherwise adjust them to give normal reference signal levels. Note 1 —As indicated in Appendix A4. the change in slit width may sometimes change the value of SRE. The displacement resulting from slit width change may be due in small part to this, or to possible shift of zero resulting from the change in il- lumination conditions resulting from the slit width

change, but is mainly due to the accompanying

change of spectral bandwidth. 4.10 Continue scanning in the direction to- ward decreasing transmittance of the filter until the indicated transmittance again falls to about 10 percent.

4.11 Repeat steps 4.5 through 4.9, replacing the reference beam attenuator of step 4.6 by another with about 12 percent of the trans- mittance of the previously used filter, or about

0.9 absorbance more; or add a second atten- uator, spaced from the first and having about

the same transmittance as the first. 4.12 Again measure and note the new

transmittance change resulting from the ref- erence beam attenuation at fixed slit width.

4. 1 3 Repeat steps 4.11 and 4. 1 2 with a suc- cession of additional reference beam attenuators until the indicated transmittance reaches

a minimum, or until the slits reach their max- imum value (Note 2). If the indicated trans- mittance does reach a minimum, select the reference beam attenuator for the last set- back such that the scale reading is about 10 percent (or even less, if the zero point is verified). If necessary, after selecting the attenuator for the last setback, return to the preceding attenuator and recording conditions before measuring the final setback. Note 2—To qualify the instrument for a particular application, it is usually only required that the SRE fall below a given proportion. It is then of course not necessary to continue adding reference attenuators beyond the point required to demonstrate compliance.

5. Calculation

5.1 Calculate the SRE proportion as the product of all the "setbacks" times the final transmittance fraction at the desired wavelength, divided by the transmittance frac- tion of the 100 percent line at this wavelength, measured in step 4.3, and divided by the fil- ter transmittance in the pass band, due to re- flection loss, of about 0.90. Note 3—Since the instrument SRE often in- creases with increasing slit width, the value obtained will ordinarily indicate a greater proportion

of SRE than is normally encountered in use. If the

final slit width corresponds to a normal use value,

the SRE proportion will be correct for that value. Such a condition may sometimes by obtained by

starting the series of measurements at unusually

high values of amplification or at high photomultiplier dynode voltage, accepting more than normal

noise on the records of the SRE test. If this is done,

it is doubly important to be sure that the photometer scale still reads correctly near zero transmittance under the higher gain condition.

6. Illustrative Example

6.1 A stray radiant energy test was done

on a Cary Model 15 Spectrophotometer fol- lowing the procedure described in Section 4. The resulting recording is shown as Fig. 1. The chart reads from right to left. The steps of the procedure are marked by the corre- sponding numbers of the paragraphs in Section 4. Step 4.1 was performed but was omitted on the chart. However, it is evident from

the pen test shown in step 4.7 that the pen dead zone is small and its response sufficiently

fast even with a reduction of total radiant energy to less than Vg of normal.

6.2 Calculate in accordance with Section

5 as follows: SRE = 0.108 x -0.118 X 0.111 X 0.151 X 0.183 X

U

° 128/o 996 X Vo.885 = 5.67 x 10- 7 or 0.000057 percent

APPENDIXES

Al. General Concepts

A 1.1 Stray radiant energy in a spectrophotom- ant energy of wavelengths outside the nominal pass

eter is difficult both to define and to measure. It is band of the monochromator to the total energy

often defined as the proportion of transmitted radi- transmitted. However, since only signal-producing

100
# E387

radiant energy is significant, it is obvious that the relevant measure is the integral of the product of energv and detector sensitivity over all wavelengths

except the pass band, divided by the corresponding

total. In practice it is impossible to make the test conform exactly to this definition, so a more empirical definition, based upon an accepted test pro- cedure, such as that described herein, must be used. A 1.2 Only for a Raman spectrophotometer is SRE measurement relatively straightforward. Here, it is possible to employ a very narrow, very

intense band of frequencies. The SRE is deter- mined bv the *'slit function" normalized to unity peak height: that is, by the intensity of this monochromatic radiant energy which is transmitted by

the monoehromator. over the spectral interval of

interest. The intensity of frequency-shifted radiant energv is so feeble that it does not itself constitute

a signiticant source of SRE. A 1 .2.1 Yet even in this case problems arc en- countered. It is not legitimate, for example, to beam a laser directly into the monoehromator for the measurement, because in so doing only a small

fraction of the aperture is illuminated, and the slit

is coherently illuminated, so that neither typical scattering nor typical aberrations are produced. The

test illumination and the monoehromator control

settings must fairly reproduce the conditions during

use. But if the exc .ing energy is scattered into the entrance optical train, the scattering medium must

itself be free of appreciable fluorescence and Raman scattering. Very few substances are suit- able, if a well designed double monoehromator is being evaluated. A 1.2. 2 Eor a Raman spectrophotometer, in com- mon with most emission instruments, values of SRE

observed are usually small compared with absorption instruments, because they arise only from the

single line, or a few isolated lines, rather than from

the integrated effect of all the wavelengths of a continuum source. A 1.3 For absorption spectrophotometers, the definition and measurement problems are much

more difficult. SRE is not uniquely a function of the monoehromator itself, but varies with the spectral distribution of the source and of the detector sensitivity, and with other factors discussed below. Also,

suitable test materials are scarce. The ideal filter for measuring SRE should have intense absorption

over a spectral region of adjustable nominal wavelength and bandwidth, and negligible absorption at other wavelengths. Such absorbers do not exist. One takes advantage of absorption edges such as those seen with alkali halide crystals, certain

liquids, and sharp cutoff glass filters, and supple- ments these by finding a variety of substances having narrow, intense absorption bands. There are

spectral regions for which no fully satisfactory filter material has been proposed. A 1.3.1 The tests herein are of limited scope be- cause sharp cutoff filters transparent for the ultraviolet but absorbing in the visible, or transparent in the lower frequency infrared but absorbing higher

frequencies, are not in general available. Fortunately, the available filters, from which a recom- mended set was selected, usually suffice to disclose

significant SRE of remote wavelengths. This is be- cause SRE caused by gross scattering arises prin- cipally from spectral regions where detector sensitivity and source intensity is high, and such sharp

cutoff filters transmit efficiently these regions. If. however, there is any reason to suspect the pres- ence of SRE of wavelengths within the stop band

of the filter, for example when a grating is used in second order in the higher frequency infrared and

the stray might be of first order frequency, or if Beer's law departures are observed when preparing

calibration curves (1)," the following supplemental

test should be used: A 1.3. 1.1 Obtain a filter that transmits efficiently

all wavelengths within the desired monoehromator

pass band, but rejects the frequencies outside this band that might be causing trouble. Interference

filters having suitable characteristics are commercially available for the infrared, visible, and near

ultraviolet ranges. Measure the absorbance of samples with and without the filter, setting the zero absorbance level also with and without the filter, respectively. Appreciable differences of measured

absorbance, especially at high sample absorbance,

indicate trouble from SRE. Samples should then be measured with the narrow band filter in the beam, or filters effective for rejecting the SRE can

be employed (see A 1.3. 7). This test is recommended

only as a supplement because of high cost for the array of test filters if it is to be applied for general instrument evaluation, as well as the large amount

of testing time required. It is nevertheless strongly recommended for critical applications. A 1.3. 2 Depending on the particular measure- ment to which the instrument is to be applied, one may be concerned only with SRE of relatively re- mote wavelengths, or may find so-called "nearby

scattering" (sometimes defined as radiant energy

outside the pass band but within several bandwidths

of the nominal wavelength), of significance. For ex- ample, measurements of aromatic compounds in the gas phase may impose very strict requirements on

scattering of wavelengths adjacent to those ab- sorbed (2). A monoehromator. entirely suited for liquid-phase measurements on the same compounds

might give highly erratic and inaccurate measure- ments in the gas phase application. A 1.3. 3 Because of these considerations, it is not

practical to specify SRE in an absorption spectro- photometer in absolute terms. Nevertheless, test procedures have been developed that give definite values: ones that are valid for certifying the suit- ability of a particular monoehromator or spectro- photometer for most of the important applications, and that additionally allow instruments to be compared when similar detectors and sources are used. SRE is then defined as the fraction of the total en- ergy that is contributed by wavelengths different

11 The boldface numbers in parentheses refer' to the

list of references at the end of this method.

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E 387

from those of the spectral band passed by the monochromalor. a.\ indicated by the lest. A 1.3.4 SRE if present in significant amounts is dangerous because it is often unsuspected. With the passage of time, increasing SRE frequently accompanies gradual deterioration of the optics in a spectrophotometer. Thorough testing annually is recommended, with more frequent tests for certain

critical applications. A 1. 3. 5 It might be assumed that it is only necessar\ to know the magnitude of SRE in order to make corrections on indicated absorbance values. A little reflection, however, will demonstrate the hazard of attempting such corrections except in special circumstances. Samples may absorb even more strongly in spectral regions where SRH is intense than they do in the region being measured.

Linder such circumstances, the effect of SRE will be to increase rather than to decrease indicated absorbance values. An accurate coireclion for SRE

rtould require a detailed knowledge of the sample

absorption spectrum and of the spectral distribu- tion of SRE and detector spectral response, and is rarely feasible. A 1.3. 6 For situations in which the sample does

not absorb in the spectral region over which the SRE is distributed. Slavin (3) shows a plot of ab- sorbance error versus absorbance. While it may

seem too obvious to mention, it must be realized that the relative error in absorbance is in this case

larger than the proportion of SRE. Small relative errors in absorbance (A) are given by the following

equation:

A2. Test

A2.I Sharp culolf filters are the most general!) available for SRE evaluation. As noted in A1.3. SRE caused by random scattering within the monochromator is principally of longer wavelengths when instruments are used in the ultraviolet, and

of shorter wavelengths when instruments are used

in the infrared. Fortunately, sharp cutoff filters having high transmission efficiencies in the wavelength

regions principally responsible for SRE are avail- able in both instances. A2.2 Glass filters for ultraviolet use can be valuable, but must be regarded with suspicion, because most glasses exhibit significant fluorescence (6). Unfortunately phototubes are frequently more

sensitive to the fluorescent wavelengths than to the short ultraviolet wavelengths that excite fluo- rescence efficiently. If the apparent SRE is found

to be increased by locating the filler close to the phototube, or is decreased b\ preceding the glass

filter with a solution filter of slightly shorter wavelength cutoff, glass fluorescence may be limiting

the SRE readings. For less exacting applications, and for tests on single monochromators. glass fil- ters are very useful and convenient. Since differX4/A = [-0.434(10 4 ).v]/.4 (1)

where .v is the fraction of SRE to energy within

the nominal pass band. For example, if the frac- tion of SRE is 0.001 for a measurement made at an absorbance of 1.5. the relative erroi in absorb- ance is —0.0091. or about nine times the proportion of stray to monochromatic radiant energy. A 1.3. 7 While the data obtained with optical

filters do not allow one to correct for SRE. their use to reduce SRE can be very valuable in improving performance, particularly of single monochromator instruments. An example is the use of the Corning No. 9863 (7-54) red-purple Core.x filter for work between 260 and 380 nm (4). In general, the same tests for SRE that serve to measure the per- formance of the monochromator aione give correct values for a monochromator-filter combination. A 1.3.X It is also, possible to use sharp cu'off

filters to provide a kind of automatic SRE correction by substituting the sharp cutoff filler for an opaque shutter when establishing the zero trans- mitlance point of a spectrophotometer (5). If this

is done with the sample in place, it also automatically corrects for errors due to sample absorption

at other wavelengths, provided they fall outside

the rejection band of the filter. This technique is especially useful in infrared measurements, using

alkali halide crystal plates as the absorption filters. Allowance is made for the reflection losses of the

filter, for instance. b\ using a transparent plate of matching reflection loss to transmit the desired wavelengths.

Materials

ent batches of glass mas exhibit different degrees

of fluorescence, caution with the use of glass fil- ters is indicated. A2.3 Sharp cutoff solution filters have been in- vestigated by several workers (7,8,9). as have

crystal filters for the infrared (10). A2.4 Fillers for reducing infrared SRE have been mainlv of two types: scatter fillers (11,9). in- cluding grating fillers (12). and ihin film inlerler- ence filters (13,14,15). A2.5 In certain cases, narrow pass band absorbers have proved useful for evaluating scattering of both nearbv and remote wavelengths. Examples are

the most intense bands in the benzene vapor spec- trum near 260 nm (16). hot mercurv vapor at 254 nm (2). and polystyrene films at 13 3 and 14.4

(17). Other substances found useful are 1 cm of

0.05 percent aqueous solution of methylene blue, near 650 nm, and 5 cm of methylene bromide

liquid at 1.43 ym (18). Of course the bands used must be well resolved in order to give reproducible

results. Because of large variations in resolution be- tween commercial instrument models, no general methods based on such bands can be recommended

102
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A3. Stray Radiant Energy as Affected by Optical System Design

A3.1 No part of the optical system of a spectro- photometer is completely free of influence on SRE.

For example, masks on the source side of the monochromator will reduce SRE if located optically conjugate to the aperture stop of the monochromator

and made slightly smaller than the aperture stop image traced by reversed rays. In this way illumi- nation of the mask that defines the aperture stop

inside the monochromator is limited so that dif- fraction or scattering from mask edges and mask

surface imperfections is lessened. Similarly, in the beam between the monochromator and the detector, a mask conjugate with the monochromator

aperture stop (if such a position exists) will trap

radiant energy that may be scattered within the monochromator and that passes through the exit

slit from some region other than the area of the aperture stop whence the monochromatic radiant energy is intentionally passed to the external optics. A3. 2 Within the monochromator itself, a critical consideration is the freedom from scattering imperfections of the optical elements, and the surface

perfection of mirrors, lenses, and dispersing ele- ments. Even with the best techniques for polishing

optical surfaces, departures from perfect smoothness occur (19), and in prism monochromators are

principally responsible for the small-angle deviations of the emergent beams which are responsible

for "nearby scattering," or tailing off of the slit function of the monochromator.

A3. 3 Even if the first optical elements within

the monochromator and the entering aperture stop

are not overfilled, the process of dispersion causes

rays to be deviated in such a way as to illuminate much of the interior of the monochromator with

energetic radiation. In a monochromator in which

the off-axis angle of a collimator is too small, energy can be returned from the collimator to the dispersing element and after again being dispersed can fall by specular paths directly on the exit slit. This is often called "double dispersion", or "sec- ondary dispersion." The best remedy is to increase

the off-axis angle of the collimator, if the result- ing aberrations can be tolerated. If not, such specular ray paths may be interrupted by judiciously masking off a part of the aperture stop, often with- out excessive loss of monochromator transmission

efficiency. If this problem is ignored, it can easily turn out that it produces significant SRE only over

a narrow spectral region, so that a monochromator

that gives excellent tests for SRE over much of its working region, and possible in all regions where

such tests are easily made, may be seriously deficient over some particular narrow spectral range. A3.4 An advantage of the Czerny-Turner or Wadsworth-type monochromators over the Littrow monochromator is that only a part of the dispersed energy falls on the exit collimator. The flux density on this mirror is therefore less than one half that of the Littrow arrangement, and its scattering imperfections produce a correspondingly reduced amount of trouble. A3. 5 Grating monochromators in general trans- mit efficiently other orders than the intended order. This source of SRE is troublesome in the infrared, since it leads to relatively efficient transmission of a number of wavelengths at which usual sources emit

strongly and detectors are sensitive (20). It is es- pecially troublesome when the grating is operated

near the blaze angle, where quite narrow, easily overlooked bands of stray energy may arise. A3. 6 As Keahl et al (20) have shown, the inter- ference filters that are commonly used for SRE re- duction in infrared grating monochromators may

have "spike" leaks which can cause very serious narrow band SRE problems. The same authors

further note that higher orders of SRE can also be

efficiently transmitted by an "order sorting" prism monochromator in tandem with the grating monochromator, if the prism monochromator is operated under such low resolution conditions that more than one order falls within its spectral pass band. A3. 7 Another often overlooked source of SRE

in a spectrophotometer is fluorescence from the absorption cell or sample itself It is entirely possible to set up conditions under which a solution can show apparent negative absorption because of

the higher sensitivity of the detector for the fluo- rescence radiant energy generated on absorption of

the ultraviolet energy, than for the monochromatic

radiant energy itself. Glass or solution sharp cutoff

filters provide a quick lest for such difficulties when

located alternately first ahead of the sample then

following it in the optical train. Also, as noted for glass filters themselves, a shift of apparent sample

transmittance with a change of its proximity to the phototube strongly suggests significant fluorescence, although it can also be caused by scattering, beam

deflection, or pathlength change. A3. 8 Another source of SRE may be lack of masking in the sample compartment to confine the beam within the sample cuvette. Even if the beam

falls well inside the cell windows when examined

by the rules of geometrical optics, diffraction at nar- row slit widths, or sample turbidity, can cause it to spread so that appreciable energy is transmitted through the cell walls or otherwise to the detector. A check for the error caused by diffraction can be made with India ink or a similar "total absorber"

in the cell. A3. 9 Similarly, a leak past the shutter used for determining instrument zero, or a leak admitting room light, can give erroneous results. A3. 10 Electrical pickup can cause reading errors very similar to those due to SRE. A check can be made by turning off the slit and source and looking

for drifts of the pen in a recording spectrophotometer, or following procedures given by the manufacturer.

103
# E387

A4. Variation with Slit Width and Height

A4.1 If the assumption is made that the scatter- ing process follows the Lambert distribution law.

it is possible to state a simple theory for the ideal monochromator which allows prediction of the way

in which the proportion of SRE varies with monochromator slit width and slit height, and which is at least approximately followed in practice. Several

cases occur: A4.I.1 Single Monochromator with "While" Continuum Source—The proportion of SRE does

not vary with slit width (when the entrance and exit

slits are opened and closed simultaneously). The

energy within the pass band is proportional to the square of the slit width because it is linearly pro- portional to the width of the entrance slit, which

admits energy to the monochromator, and also to the spectral bandwidth, is proportional to the slit width. The SRE also is proportional to the square

of the slit width, because it is linearly proportional

to the width of the entrance slit, which controls aperture illumination, and also to the width of the

exit slit, which affect the solid angle for the trans- mission of scattered energy. A4. 1.1.1 Transmission of monochromatic energy

varies only linearly with slit height, because slit height has negligible effect on bandwidth; whereas SRE varies with the square of the slit height, just as with slit width. The proportion of SRE therefore

increases linearly with slit height. (Actually, experi- ments to confirm this relationship show that, with

graftings, the scattering is predominantly in the direction perpendicular to the grating rulings, and

the proportion of scattering varies as a fractional power of the slit height.) A4.1 .2 Single Monochromator with Pure Narrow

Line Source—Ideally, all of the energy in a monochromatic line that is admitted by the entrance slit

is transmitted by the exit slit when the monochromator is set at the wavelength of the line; thus the monochromatic energy is proportional to the en- trance slit width and height. When the monochromator is displaced from the nominal wavelength,

the intensity of the scattered energy is varied by

both entrance and exit slit width and height, and

thus is proportional to the square of these parameters. The proportion of SRE at remote wavelengths to monochromatic energy at the nominal

wavelength is proportional to slit width and height. A4.1.3 Double Monochromator with Continuum Source—In the first monochromator section, as indicated in A4.1.1, the proportion of energy of' the nominal wavelength is independent of the slit width and varies with the first power of slit height.

(It is assumed that all slits, including tfie intermediate slit, are varied in width by the slit control.) The

scattered energy from the first monochromator that

is transmitted into the second monochromator is re- scattered by its optical surfaces. The amount emerging varies with the solid angle subtended bv the

exit slit as viewed from the scattering surface. Hence, it is proportional to both width and height of the exit slit. Thus, the fraction of stray-to-mono- chromatic energy is very 'much reduced by the second monochromator, and it varies with the slit width and with the square of the slit height. The

sensitivity to slit height makes clear the utility of a

slit-height control. A4.I.4 Double Monochromator with Line Source —The same argument as for the double monochromator with continuum source applies, except that

the monochromatic energy varies only with en- trance slit height and width. Therefore, the pro- portion of stray-to-monochromatic radiant energy

varies with the square of both the slit width and

the slit height. A4.2 Because of these relationships, it is im- portant that tests for SRE be made in most cases

at slit widths and heights representative of those

actually used in the application of the instrument. Unfortunately many published results on SRE

measurements are made with nontypically narrow

slits or with shortened slits, or both. It is often easy

to arrange test conditions that give SRE proportions substantially different from those which would

be found under conditions appropriate to applications. Hence, a SRE proportion given without speci- fying test procedure is essentially meaningless.

References

(1) Hogness, T. R., Zscheile. F. P., Jr., and Sidwell, A. E., Jr., "Photoelectric Spectrophotometry. An Apparatus for the Ultra-Violet and

Visible Spectral Regions: Its Construction,

Calibration, and Application to Chemical

Problems," Journal of Physical Chemistry,

JPCUA, Vol 41, 1937, p. 379.

(2) Tunnicliff, D. D. "Measurement of Nearby

Stray Radiation in Ultraviolet Spectrophotometry." Journal of the Optical Society of

America, JOSAA, Vol 45, 1955, p. 1963.

(3) Slavin, W., "Stray Light in Ultraviolet, Visible and Near-Infrared Spectrophotometrv," Analytical Chemistry, ANCHA. Vol 35,' 1963, p. 561.

(4) Cary, H. H., and Beckman, A. O., "A Quartz

Photoelectric Spectrophotometer," Journal of

the Optical Society of America, JOSAA, Vol

31, 1941, p. 682.

(5) Wright, N„ and Randall. H. M., "Far Infra- red Absorption Spectra of NH, and Phosphine Gases," Physical Review, PHRVA,

104
E 387

Vol 44, 1933, p. 391.

(6) French. C. S., "Fluorescence Spectra of Sharp-Cut Filters," Applied Optics, APOPA,

Vol 4, 1965, p. 514.

(7) McBride, W R., "Optical Materials for Transmission Filters in the Middle Ultraviolet," Journal of the Optical Society of America,

JOSAA, Vol 53, 1963, p. 519.

(8) Paulson, R. E., "Test Methods in Spectro- photometry: Stray Light Determination," Applied Optics, APOPA, Vol 3, 1964, p. 99.

(9) Pfund, A. H., "Infrared Filters of Controllable Transmission," Physical Review, PHRVA,

Vol 36, 1930, p. 71. (10) Smith, R. A., Jones, F. E., and Chasmar, R.

P., The Detection and Measurement of Infra- Red Radiation," Oxford University Press, London, 1957.

(11) Gorton, A. F., "Reflection From and Transmission Through Rough Surfaces," Physical Review, PHRVA, Vol 7, 1916, p. 66. (12) White, J. U., "Gratings as Broad Band Filters for the Infra-Red, " Journal of the Optical Society of America, JOSAA, Vol 37, 1947, p. 713.

(13) Bates, B., and Bradley. D. J., "Interference

Filters for the Far Ultraviolet," Applied OpT ABLE 1 Perforated Screens

Identification Number'

Nominal

Transmittance NominalAbsorbance

20G 0.50 0.30 15H 0.41 0.39

15L 0.29 0.54 ION 0.20 0.70

I2'/2 P 0. 14 0.84

10S 0.07 1 .20 30T 0.03 1 .50 20W 0.01 2.00

° The identification number is that of Perforated Products, Inc., 68 Harvard St., Brookline. Mass. 02146. Screens from this manufacturer are the only ones so far tested in committee work. Screens may be available from other manufacturers.

105

tics, APOPA, Vol 5, 1966, p. 971.

(14) Hadley, L. N., ana Dennison, D. M., "Reflection and Transmission Interference Fil- ters," Journal of the Optical Society of America, JOSAA, Vol 53, 1963, p. 519.

(15) Wolfe, W. L., Ed., "Handbook of Military In- frared Technology," Superintendent of Documents, U. S. Government Printing Office, Washington, D. C. 20402.

(16) Cary, H. H., Cary Instruments, Monrovia,

Calif., unpublished work.

(17) "Tables of Wavenumbers for the Calibration of Infrared Spectrometers," International Union of Pure and Applied Chemistry, Butterworths, 1961

(18) Hawes, R. C, Cary Instruments, Monrovia,

Calif., unpublished work.

(19) Koehler, W. F., "Multiple Beam Fringes of Equal Chromatic Order. Part VII. Mechanism

of Polishing Glass," Journal of the Optical Society of America, JOSAA, Vol 45, 1955, p. 1015.

(20) Keahl, J., Sloane, H. J., and Ulrich, W. F., "The Origin and Effect of Stray Light," The

Analyzer, Beckman Instruments house organ, ANAYA, November, 1965, p. 9.
• E387

TABLE 2 Filters for Tests of Stray Radiant Energy

Transmittance.'

Spectral Range" 80 Percent Filter' Source1* Detector' Wavelength or Wavenumber

A. Sharp Cutoff Types

165 to 173.5 nm 183 nm 0.01 cm H 2 0' UV UV

170 to 183.5 nm 195 nm 1 .00 cm H2 0' UV UV

175 to 200 nm 214 nm 1.00 cm 12 g/liter KG aqueous' UV UV

195 to 223 nm 232 nm 1.00 cm 10 g/liter NaBr aqueous' UV UV

210 to 259 nm 271 nm 1.00 cm 10 g/liter Nal aqueous UV UV

250 to 320 nm 339 nm 1 .00 cm acetone UV UV

300 to 385 nm 420 nm 1.00 cm 50 g/liter NaN02 aqueous VIS UV

2050 to 1200 cm ' 2800 cm

_ ' 2.0-mm fused silica" (2) IR IR

1140 to 800 cm

' 1760 cm"' 6 mm LiF 1R IR

760 to 600 cm

' 1240 cm

"

'

6 mm CaF2 IR IR

630 to 400 cm

"

'

1030 cm

~ 1 6 mm NaF IR IR 410 to 250 cm

" 1 650 cm

"

'

6 mm NaCl IR IR 240 to 200 cm

1 420 cm

"

1 6 mm KBr IR IR

B. Pass- Band Filters

600 to 660 nm 1.00 cm 0.005 percent methylene VIS VIS or NIR

blue aqueous"

1 .66 to 1 .75/im 5.0 cm CH2 Br2

' NIR NIR

0 Longer wavelength (or higher wavenumber, for infrared range) gives 10

~ A transmittance point.

' Transmittance value not corrected for reflection loss.

' Solution fillers should be placed in sample cuvettes appropriate to the range covered. Solid filters are best retained

in metal holders.

d Under "source" is tabulated the usual and appropriate source for each spectral range: UV— Source is either a deuterium or hydrogen discharge lamp or a xenon short arc. VIS— Source is a tungsten filament lamp at 2850 to 3300 K. If a xenon arc is substituted, precautions to avoid UV

stray should be taken, such as interposition of a Corning No. 7380 or No. 3391 filter, for both test and use of the instru- ment at wavelengths longer than 410 nm. NIR -Source is a tungsten filament lamp at 2500 to 3200 K. IR— Source is a Nernsl glower at 1500 to 1800 K, or Globar, or coated Nichrome ribbon, at 1300 to 1500 K Because

the proportion of SRE varies with source temperature, the source current should be set to the value used for analysis. 'Considerable flexibility in detectors selected is common. The following lists only some of those most frequently employed: UV— Photornultiplier or diode phototubes with S-5. S-19. or similar spectral response function, such as types 1 P28, 7200, or R-106. VIS— UV types, plus photornultiplier or diode phototubes with S-4, S-5, or S-19 response; extended red threshold

types such as 7265 (S-20), or R-136. Photoconductor (PbS) detectors. NIR Photobubes with S-l response to 1.2 (im, photoconductor (PbS, PbSe or PbTe) detectors. IR—Thermocouples, bolometers, Golay detectors.

' Apparent absorbance is strongly affected by dissolved oxygen Bubble pure nitrogen through liquid for several minutes immediately before use. Use only recently distilled (not demineralized) water

* Filters such as these, which absorb over a wide range in the infrared, may be warmed sufficiently by the source beam

to reradiate. and so produce significant zero shifts which vary with wavelength and with time of exposure to the beam.

This effect is greatly reduced by using two filters, separated by at least I cm along the beam axis. The reradiation from

the first is then mostly absorbed by the second A slightly less effective alternative is to use a LiF disc for the first filter. If zero shift is troublesome with the LiF filter, a CaF2 disk can be used ahead of the LiF filter

* Passes blue to yellow light efficiently The 0.005 percent methylene blue solution must be made up freshly from a 0.5 percent stock solution in 2 percent KFi 2 PO«, preserved with I part of phenylmercuric acetate (Eastman No. P4267 in 50,000 parts of solution). 'Distillation Products Industries, Eastman Organic Chemicals, No. 1903. Passes from ultraviolet to 1.5 «m radiant energy efficiently, except for a narrow, intense band at 1.4 jim. which is suitable for "nearby stray" evaluation in NIR

grating monochromalors.

106
107
NBS-1 14A (REV. 7-73)

U .S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-260-51

2. Gov't Accession

No.

3. Recipient's Accession No.

4. title and subtitll standard Reference Materials:

Glass Filters as a Standard Reference Material

for Spectrophotometry

Selection, Preparation, Certification, Use SRM 930

5. Publication Date November 1975

6. Performing Organization Code

7. AUTHOR(S)

R. Mavrodineanu and J. R. Baldwin

8. Performing Organ. Report Nc

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Library of Congress Catalog Card Number: 75- 619308

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.) This publication describes various factors which can affect the proper

functioning of a spectrophotometer and suggests means and procedures

to assess and control these factors. Particular consideration is given

to the long and short term stability of a spectrophotometer, to the

wavelength accuracy, the spectral bandpass, the stray radiation, and

the accuracy of the transmittance or absorbance scale. A description

is given of the means and Standard Reference Materials (SRM's) which

can be used to control these factors, together with the methods for

the preparation, certification, and use of such materials (SRM 930)

.

The results obtained in actual use of SRM 930 are examined in some

detail. An Appendix contains the reproduction of several publications

relevant to the subject discussed in this work.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Accuracy in spectrophotometry; glass filters;

spectral bandpass; spectrophotometry; standard reference materials; stray

radiation; testing of spectrophotometers; transmittance (absorbance)

accuracy; wavelength accuracy.

18. AVAILABILITY fyf Unlimited

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