A11103 065126

\

CO /

NBS SPECIAL PUBLICATION 260-68

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

Standard Reference Materials:

Metal-On-Quartz Filters

as a Standard Reference Material

for Spectrophotometry—SRM 2031
NATIONAL BUREAU OF STANDARDS

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

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
NATIONAL BTJREAti OF STANDARDS

LIBRARY

> APR 2 1 19B0

Standard Reference Materials: not aceMetal-On-Quartz Filters

as a Standard Reference Material

for Spectrophotometry—SRM 2031

R. Mavrodineanu* and J. R. Baldwin

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Supported in part by: National Institute for General Medical Sciences National Institutes of Health 9000 Rockville Pike

Bethesda, MD 20205

*Retired, current address: 227 Almeria Road

West Palm Beach, FL 33405

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

^S. NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

| ,0£&C€ cA 6^)&a^T?eWence Materia/*.

Issued April 1980

!
Library of Congress Catalog Card Number: 79-600192

National Bureau of Standards Special Publication 260-68

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-68, 116 pages (Apr. 1980) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1980

For siile by tile Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $4.25
Preface

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

Bureau of Standards are "wel 1 -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 Pub! i cation - 260 Series is

reserved for this purpose.

This 260 Series is dedicated to the dissemination of information

on all phases of the preparation, measurement, and certification of

NBS-SRM's. In general, much more detail will be found -in these papers

than is generally allowed, or desirable, in scientific journal

articles. This enables the user to assess the validity and accuracy of

the measurement processes employed, to judge the statistical analysis,

and to learn details of techniques and methods utilized for work

entailing the greatest care and accuracy. It is also hoped that these

papers will provide sufficient 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 Reference Materials

National Bureau of Standards

Washington, D. C. 20234

George A. Uriano, Chief

Office of Standard Reference Materials

iii
Foreword

A variety of solid and liquid materials which have characteristic optical transmission

properties have been used in the past in the analytical laboratories to verify the "proper"

functioning of conventional spectrophotometers. During the past few years the optical

transmittance of such materials could be measured with an increased accuracy. This is due

in large part to the initiation of an extensive program of research and development in the

field of standard reference materials for spectrophotometry at the National Measurement

Laboratory of the National Bureau of Standards (NBS). As a direct consequence of this

program, two high-accuracy spectrophotometers were designed and implemented, one provided

with quartz optics, the other with off-axis mirrors. These instruments, which are capable

of producing optical transmittance measurements with an accuracy of better than 1 x 10~4 transmittance units, were used to study quantitatively the physical parameters involved in

the measurements of optical transmittance. The results from the studies were published

in two NBS Special Publications, No. 378 (May 1973) and No. 466 (May 1977).

The NBS high-accuracy spectrophotometers, which constitute the primary optical trans- mittance standards available in this country was the basis for the selection and certification of several solid and liquid substances having an accurately determined transmittance,

which could be used as standard reference materials (SRM's) in spectrophotometry. Those

materials which have been issued thus far include SRM 930 and 2030, Glass Filters for

Spectrophotometry, SRM 931, Liquid Filters for Spectrophotometry, SRM 932, Quartz Cuvette

for Spectrophotometry, and SRM 935, Potassium Dichromate for Spectrophotometry. These

materials now make it possible to transfer to the users the accurate transmittance data

established at NBS, thus providing reliable means for verifying the accuracy of the trans- mittance scales of conventional spectrophotometers, as well as monitoring their stability

and reproducibility. Such standards also provide an ideal way to establish baselines for

i ntercompari sons of spectrophotometry data between interested laboratories.

The present publication describes in detail the results of a work initiated in 1969 for

the production of a new SRM which could be used in the ultraviolet and visible (and possibly

the infrared) spectral regions, and would, at the same time, exhibit good optical neu- trality. This standard is evaporated metal on fused silica, using chromium metal as the

semi-transparent film.

The optical characteristics of this newly issued SRM 2031 are such that it should prove

useful in all chemical laboratories where spectrophotometry is used as a measuring tool.

SRM 2031 is but one more of the materials developed by NBS to assist in the standardization of spectrophotometers. The work in this field is continuing and should result in the

production of further useful SRM's. In particular, SRM's to measure heterochromatic and

isochromatic stray radiant energy are now in the final stage of development, and should be

issued in late 1979 as SRM's 2032 and 2033.

By the production of these, and the many other SRM's as well as through the preparation

of detailed scientific information on the preparation and utilization of these SRM's in the

form of NBS 260 Series Special Publications, the National Bureau of Standards fulfills part

of its essential roles designated in the Organic Act which established the Bureau in 1901.

It is hoped that this activity will provide a solid foundation for future progress in the

field of accurate measurements.

The work reported in this publication was supported in part by the National Institute

of General Medical Sciences.

Curt W. Reimann, Acting Director

Center for Analytical Chemistry

iv
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference Materials

(1979-80 edition), R. W. Seward, ed., NBS

Spec. Publ. 260 (April 1979) $3.00* SN003-003-

02048-6

Michaelis, R. E., and Wyman, L. L., Standard

Reference Materials: Preparation of White Cast

Iron Spectrochemical Standards, NBS Misc.

Publ. 260-1 (June 1964). COM74-1 1061**

Michaelis, R. E., Wyman, L. L., and Flitsch, R.,

Standard Reference Materials: Preparation of

NBS Copper-Base Spectrochemical Standards,

NBS Misc. Publ. 260-2 (October 1964). COM74-

11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A.,

Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric LowAlloy Steel Standard, NBS Misc. Publ. 260-3

(October 1964). COM74-1 1060**

Hague, J. L., Mears, T. W., and Michaelis, R. E., Standard Reference Materials: Sources of

Information, NBS Misc. Publ. 260-4 (February

1965). COM74-11059

Alvarez, R., and Flitsch, R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys, NBS

Misc. Publ. 260-5 (March 1965). PB168068**

Shultz, J. L, Standard Reference Materials:

Methods for the Chemical Analysis of White

Cast Iron Standards, NBS Misc. Publ. 260-6

(July 1975). COM74-11068**

Bell, R. K., Standard Reference Materials:

Methods for the Chemical Analysis of NBS

Copper-Base Spectrochemical Standards, NBS

Misc. Publ. 260-7 (October 1965). COM74-

11067**

Richmond, M. S., Standard Reference Materials:

Analysis of Uranium Concentrates at the

National Bureau of Standards, NBS Misc. Publ.

260-8 (December 1965). COM74-1 1066**

Anspach, S. C, Cavallo, L. M., Garfinkel, S. B., Hutchinson, J. M. R., and Smith, C. N., Standard Reference Materials: Half Lives of Materials

Used in the Preparation of Standard Reference

Materials of Nineteen Radioactive Nuclides

Issued by the National Bureau of Standards,

NBS Misc. Publ. 260-9 (November 1965).

COM74-11065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F. J., and

Michaelis, R. E., Standard Reference Materials:

Homogeneity Characterization on NBS Spectrometric Standards II: Cartridge Brass and LowAlloy Steel, NBS Misc. Publ. 260-10 (December

1965). COM74-11064**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-1 1 (November 1966). NBS Misc. Publ. 260-11**

Yakowitz, H., Vieth, D. L., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards III: White Cast Iron and Stainless Steel Powder Compact, NBS Misc. Publ. 260-12

(September 1966). NBS Misc. Publ. 260-12**

Spijkerman, J. L., Snediker, D. K., Ruegg, F. C,

and DeVoe, J. R., Standard Reference Materials: Mossbauer Spectroscopy Standard for the

Chemical Shift of Iron Compounds, NBS Misc.

Publ. 260-13 (July 1967). NBS Misc. Publ.

260-13**

Menis, O., and Sterling, J. T., Standard Reference

Materials: Determination of Oxygen in Ferrous

Materials - SRM 1090, 1091, and 1092, NBS

Misc. Publ. 260-14 (September 1966). NBS Misc.

Publ. 260-14**

Passaglia, E., and Shouse, P. J. Standard Refer- ence Materials: Recommended Method of Use

of Standard Light-Sensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Misc. Publ. 260-15

(June 1967). (Replaced by NBS Spec. Publ.

260-41.)

Yakowitz, H., Michaelis, R. E., and Vieth, D. L., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards IV: Preparation and Microprobe Characterization of W-20% MO Alloy Fabricated by

Powder Metallurgical Methods, NBS Spec.

Publ. 260-16 (January 1969). COM74-1 1062**

Catanzaro, E. J., Champion, C. E., Garner, E. L., Marinenko, G., Sappenfield, K. M., and Shields,

W. R. Standard Reference Materials: Boric

Acid; Isotopic and Assay Standard Reference

Materials, NBS Spec. Publ. 260-17 (February

1970). Out of Print

v
Geller, S. B., Mantek, P.A., and Cleveland, N.

G., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic Tape

(Computer Amplitude Reference) Using the

Reference Tape Amplitude Measurement

"Process A, "NBS Spec. Publ. 260-18 (November

1969). (See NBS Spec. Publ. 260-29.)

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold (Certifica- tion of Standard Reference Material 745). NBS

Spec. Publ. 260-19 (January 1970). PB190071**

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and

Silver, NBS Spec. Publ. 260-21 (January 1971).

COM74-11359**

Yakowitz, H., Fiori, C. E., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of Fe-3 Si Alloy, NBS Spec.

Publ. 260-22 (February 1971). COM74-1 1357**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23

(December 1970). COM71-00157**

Sappenfield, K. M., Marineko, G., and Hague, J. L., Standard Reference Materials: Comparison

of Redox Standards, NBS Spec. Publ. 260-24

(January 1972). COM72-50058**

Hicho, G. E., Yakowitz, H., Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials:

A Standard Reference Material Containing

Nominally Four Percent Austenite, NBS Spec.

Publ. 260-25 (February 1971). COM74-1 1356**

Martin, J. F., Standard Reference Materials:

National Bureau of Standards-US Steel Corportion Joint Program for Determining Oxygen and

Nitrogen in Steel, NBS Spec. Publ. 260-26

(February 1971). 85 cents* SN003-003-00786-2

Garner, E. L., Machlan, L. A., and Shields, W. R., Standard Reference Materials: Uranium

Isotopic Standard Reference materials, NBS

Spec. Publ. 260-27 (April 1971). COM74-

11358**

Heinrich, K. F. J., Myklebust, R. L., Rasberry, S. D., and Michaelis, R. E., Standard Reference

Materials: Preparation and Evaluation of

SRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM71-50365**

Geller, S. B., Standard Reference Materials: Cali- bration of NBS Secondary Standard Magnetic

Tape (Computer Amplitude Reference) Using

the Reference Tape Amplitude Measurement 1 "Process A-Model 2," NBS Spec. Publ. 260-29

(June 1971). COM7 1-50282

Gorozhanina, R. S., Freedman, A. Y., and

Shaievitch, A. B. (translated by M. C. Selby),

Standard Reference Materials: Standard

Samples Issued in the USSR (A Translation \ from the Russian). NBS Spec. Publ. 260-30 (June

1971). COM71-50283**

Hust, J. G., and Sparks, L. L., Standard Reference

Materials: Thermal Conductivity of Electrolytic Iron SRM 734 from 4 to 300 K, NBS Spec. Pubi. , 260-31 (November 1971). COM7 1-50563**

Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes, for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents*

1 SN003-003-O1213-1

Wagner, H. L., Standard Reference Materials:

Comparison of Original and Supplemental

SRM 705, Narrow Molecular Weight Distri- bution Polystyrene, NBS Spec. Publ. 260-33

(May 1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference

Materials: Thermoelectric Voltage, NBS Spec.

Publ. 260-34, (April 1972). COM72-50371**

Sparks, L. L., and Hust, J. G., Standard Refer- ence Materials: Thermal Conductivity of \ Austenitic Stainless Steel, SRM 735 from 5 to 280 K, NBS Spec. Publ. 260-35 (April 1972.)

35 cents* COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D.

S., Standard Reference Materials: A Referee \ Method for the Determination of Calcium in Serum, NBS SRM 915, NBS Spec. Publ. 260-36 ! (May 1972). COM72-50527**

Shultz, J. I. Bell., R. K. Rains, T. C, and Menis,

O., Standard Reference Materials: Methods of Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence Materials: Preparation and Calibration of j Standards of Spectral Specular Reflectance,

NBS Spec. Publ. 260-38 (May 1972). COM72-

50528**

Clark, A. F., Denson, V.A., Hust, J. G., and

Powell, R. L., Standard Reference Materials

The Eddy Current Decay Method for Resistivity : Characterization of High-Purity Metals, NBS

Spec. Publ. 260-39 (May 1972). COM72-50529**
McAdie, H. G., Garn, P. D., and Menis, O., Standard Reference Materials: Selection of Thermal

Analysis Temperature Standards Through a Cooperative Study (SRM 758, 759, 760), NBS

Spec. Publ. 26(M0 (August 1972.) COM-72

50776**

Wood, L. A., and Shouse, P. J., Standard

Reference Materials: Use of Standard LightSinsitive Paper for Calibrating Carbon Arcs

Used in Testing Textiles for Colorfastness to Light, NBS Spec. Publ. 260^1 (August 1972)

COM72-50775**

Wagner, H. L., and Verdier, P. H., eds., Standard

Reference Materials: The Characterization of

Linear Polyethylene, SRM 1475, NBS Spec.

Publ. 260-42 (September 1972). COM72-

50944**

Yakowitz, H., Ruff, A. W., and Michaelis, R. E., Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260^43 (November 1972). COM73-50760**

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A.,

Jr., Standard Reference Materials: Preparation

and Use of Superconductive Fixed Point

Devices, SRM 767, NBS Spec. Publ. 260-44

(December 1972). COM73-50037**

Greifer, B., Maienthal, E. J., Rains, T. C, and

Rasberry, S. D., Standard Reference Materials:

Powdered Lead-Based Paint, SRM 1579, NBS

Spec. Publ. 260-45 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 K, NBS Spec. Publ. 260-46

(March 1975). SN003-003-0 1278-5

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797,

and Austenitic Stainless Steel, SRM 798, from 5

to 280 K, NBS Spec. Publ. 260-47 (February

1974). COM74-50176**

Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory,

SRM 933 and SRM 934, NBS Spec. Publ. 260-48

(May 1974). 60 cents* SN003-003-0 1278-5

Carpenter, B. S., and Reimer, G. M., Standard

Reference Materials: Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ.

260-49 (November 1974). SN003-003-0 1344-7

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials:

Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K, NBS Spec. Publ. 260-50 (June 1975). 1.00* SN003-003-0 1425-7

Mavrodineanu, R., and Baldwin, J. R., Standard

Reference Materials: Glass Filters As a Standard

Reference Material for Spectrophotometry;

Selection; Preparation; Certification; Use -SRM

930, NBS Spec. Publ. 260-51 (November 1975).

$1.90* SN003-003-01481-8

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials

730 and 799, from 4 to 3000 K, NBS Spec. Publ.

260-52 (September 1975). $1.05* SN003-O03-

01464-8

Durst, R. A., Standard Reference Materials:

Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1975, Revised).

$1.05* SN003-O03-O1551-2

Burke, R. W., and Mavrodineanu, R., Standard

Reference Materials: Certification and Use of

Acidic Potassium Dichromate Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (August 1977). $3.00* SNO03-O03-

01828-7

Ditmars, D. A., Cezairliyan, A., Ishihara, S., and

Douglas, T. B., Standard Reference Materials:

Enthalpy and Heat Capacity; Molybdenum

SRM 781 , From 273 to 2800 K, NBS Spec. Publ.

260-55 (September 1977). $2.20* SN003003-

01836-8

Powell, R. L., Sparks, L. L., and Hust, J. G.,

Standard Reference Materials: Standard

Thermocouple Material, Pt.-67:SRM1967, NBS

Spec. Publ. 260-56 (February 1978). $2.20*

SN003-003-0 18864

Cali, J. P. and Plebanski, T., Guide to United

States Reference Materials, NBS Spec. Publ.

260-57 (February 1978). $2.20* SN003-003-

01883-0

Barnes, J. D. and Martin, G. M., Standard Refer- ence Materials: Polyester Film for Oxygen Gas

Transmission Measurements. SRM 1470, NBS

Spec. Publ. 260-58 (in press).

Chang, T. and Kahn, A. H., Standard Reference

Materials: Electron Paramagnetic Resonance

Intensity Standard: SRM 2601, NBS Spec.

Publ. 260-59 (August 1978). $2.30* SN-003-

003-01975-5

i i
Velapoldi, R. A., Paule, R. C, Schaffer, R.,

Mandel, J., and Moody, J. R., Standard Refer- ence Materials: A Reference Method for the Determination of Sodium in Serum, NBS Spec.

Publ. 260-60 (August 1978). $3.00* SN003-003

01978-0

Verdier, P. H., and Wagner, H. L., Standard Refer- * ence Materials: The Characterization of Linear

Polyethylene (SRM 1482, 1483, 1484), NBS

Spec. Publ. 260-61 (December 1978). $1.70*

SN003-003-02006-1

Soulen, R. J., and Dove, R. B., Standard Reference

Materials: Temperature Reference Standard

for Use Below 0.5 K (SRM 768), NBS Spec.

Publ. 260-62 (April 1979). $2.30* SNO03-O03-

02047-8

Velapoldi, R. A., Paule, R. C, Schaffer, R.,

Mandel, J., Machlan, J. L., and Gramlich, J. W.,

Standard Reference Materials: A Reference

Method for the Determination of Potassium in Serum, NBS Spec. Publ. 260-63 (May 1979).

$3.75* SN003-003-02068

Velapoldi, R. A., and Mielenz, K. D., Standard

Reference Materials: A Fluorescence Standard

Reference Material Quinine Sulfate Dihydrate

(SRM 936), NBS Spec. Publ. 260-64 (January

1980). $4.25* SN003-003-02 148-2 Marinenko, R. B., Heinrich, K. F. J., and Ruegg,

F. C, Standard Reference Materials: MicroHomogeneity Studies of NBS Standard Reference

Materials, NBS Research Materials, and Other

Related Samples, NBS Spec. Publ. 260-65

(September 1979). $3.50* SN003-003-021 14-1 Venable, W. H., Jr. and Eckerle, K. L., Standard

Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of Spectrophotometers (SRM 2009, 2010, 2013), NBS Spec.

Publ. 260-66 (October 1979). $3.50*SN003-003-

02127-0

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel,

J., Murphy, T. J., and Gramlich, J. W., Standard

Reference Materials: A Reference Method for the Determination of Chloride in Serum, NBS Spec.

Publ. 260-67 (November 1979). $3.75* SN003-

003-02136-9

Mavrodineanu, R. and Baldwin,J. R., Standard

Reference Materials: Metal-On-Quartz Filters as a Standard Reference material for Spectrophotometry^ RM 2031, NBS Spec. Publ. 260-68 (in

press).

* Send order with remittance to Superintendent

of Documents, US Government Printing

Office, Washington, DC 20402. Remittance

from foreign countries should include an

additional one-fourth of the purchase price for postage.

* May be ordered from: National Technical

Information Services (NTIS), Springfield,

Virginia 22151.

viii
'I

TABLE OF CONTENTS

PAGE

Preface iii

Foreword iv

Abstract xiii

Disclaimer xiii

1. General 1

1.1 Standard Reference Materials for Spectrophotometry Issued by the

National Bureau of Standards 1

1.2 Conditions to be Satisfied by SRM's for Spectrophotometry 8

2. Introduction 9

2.1 Initial Studies 9

2.2 Characteristics of the Metal -on-Fused Silica Filters 9

2.3 Comparison of Transmittance Measurements at the National Bureau of

Standards and the National Physical Laboratory 11

3. Further Studies 12

3.1 The High-Accuracy Spectrophotometer at the National Bureau of

Standards, Center for Analytical Chemistry 12

3.2 Description of the Metal-on-Fused Silica Filters Made at the

National Bureau of Standards 17

3.3 Measurement of Isochromatic Stray Light 17

3.4 Multiple Reflections 17

3.5 Influence of Positioning of the Filters on Transmittances . . 23

3.6 Intercomparison of Transmittance Measurements at the National Bureau

of Standards 23

3.7 Intercomparison of Transmittance Measurements Performed at the National

Bureau of Standards and the Michel son Laboratory of the Naval Weapons

Center 25

3.8 Scatter, Homogeneity, and Polarization of Inconel -on-Fused Silica

Filters and Schott NG4 Glass Filters 26

3.9 Further Evaluation of the Metal-on-Fused Silica Filters 29

3.10 Metal-on-Fused Silica Filters at the National Physical Laboratory 31

3.11 Limitation in the Use of the Metal-on-Fused Silica Filters as SRM's in

Spectrophotometry and Removal of this Limitation 31

4. Semi-Transparent Metal-on-Fused Silica Filters Protected by a Clear Fused

Silica Plate Assembled by Optical Contact 31

4.1 Preparation of the Chromium-on-Fused Silica Filters 32

4.2 Optical Transmittance of the Filters and Spectral Bandpass Dependence ... 32

ix
Table of Contents, continued

PAGE

4.3 Inspection of the Chromium-on-Fused Silica Filters 34

4.4 Tests to Determine the Transmittance Stability, in Time, for the

Chromium-on-Fused Silica Filters 34

4.5 Conclusions from the Time Stability Tests 40

4.6 Issuance of SRM 2031, Description of the Standard Reference Material,

and Certificate 41

Reproduction of the NBS Certificate, Standard Reference Material 2031,

Metal-on-Quartz Filters for Spectrophotometry 43

5. Summarizing Remarks 47

Acknowledgements 48

6. References 49

7. Appendices 49

Appendix I, Comments on Spectrometry Nomenclature 51

Appendix II, An Accurate Spectrophotometer for Measuring the Transmittance of

Solid and Liquid Materials 53

Appendix III, Adaptation of a High-Accuracy Spectrophotometer for Ultraviolet

Work 75

Appendix IV, Considerations for the Use of Semi-Transparent Metallic Thin

Films as Potential Transmittance Standards in Spectrophotometry 81

Appendix V, Reflection Correction for High-Accuracy Transmittance Measurements

on Filter Glasses 87

Appendix VI, NBS Certificate, Standard Reference Material 930D, Glass Filters

for Spectrophotometry 93

Appendix VII, NBS Certificate, Standard Reference Material 2030, Glass Filter

for Transmittance Measurement 97

Appendix VIII, NBS Certificate, Standard Reference Material 931b, Liquid

Absorbance Standards for Ultraviolet and Visible Spectrophotometry 101

Appendix IX, NBS Certificate, Standard Reference Material 932, Quartz Cuvette

for Spectrophotometry 105

Appendix X, NBS Certificate, Standard Reference Material 935, Crystalline

Potassium Dichromate for Use as an Ultraviolet Absorbance Standard 107

x
LIST OF TABLES

TABLE NO. PAGE

1. Standard Reference Materials for Spectrophotometry 6

2. Stability of an evaporated metal-on-fused silica filter exposed to radiations H

3. Comparison between the percent transmittance {% T) measured on three

metal-on-fused silica filters at NBS and NPL 12

4. Percent transmittance (% T) measured on a Schott neutral glass filter 2 mm

thick, and an inconel-on-fused silica filter 2 mm thick at 590 nm, for

normal incidence and for the angles of 3° 6° and 9° 23

5. Transmittance of some inconel-on-fused silica filters obtained using the

Center for Analytical Chemistry (CAC) and the Center for Padiation

Research (CRR) high-accuracy spectrophotometers 24

6. Transmittance of a set of SRM 930 at two wavelengths obtained using the

Center for Analytical Chemistry (CAC) and the Center for Radiation

Research (CRR) high-accuracy spectrophotometers 25

7. Comparative measurements of transmittance on inconel-on-fused silica filters

in the ultraviolet and visible from 250 nm to 635 nm at NBS and at Naval

Weapons Center (NWC) 26

8. Measurement of scatter, homogeneity, and polarization on three

inconel-on-fused silica filters 27

9. Measurement of scatter, homogeneity, and polarization on three Schott NG4

neutral glass filters 28

10. Transmittance measurements on inconel-on-fused silica filters in the

ultraviolet and visible from 250 to 635 nm performed at NBS and at the Tennessee Eastman Co. (TEC) 30

11. Transmittance (T) - spectral bandpass (SBP) dependence of an evaporated

chromium metal-on- fused silica filter at two wavelengths (a) 32

12. Transmittance of two sets of chromium-on-fused silica filters measured over

a period of two years 36

13A. Relative differences in transmittance, AT%, observed over the time interval:

February 1977 to March 1979 on 19 sets of evaporated chromium-on-fused

silica filters . V 37*'

13B. Absolute differences in transmittance, AT, observed over the time interval:

February 1977 to March 1979 on 19 sets of evaporated chromium-on-fused

silica filters 38

14. Summary of the percent transmittance difference, AT%, measured after a time

interval of two years on 19 sets of chromium-on-fused silica filters

with nominal transmittances of 10, 30, and 90 percent 40

xi
LIST OF FIGURES

FIGURE NO. PAGE

1. Illustration of several publications issued by NBS in the field of

spectrophotometry 2

2. Illustration of SRM 930D, Glass Filters for Spectrophotometry, showing the

black-anodized aluminum alloy container for storing three filters in their holders and an empty filter holder 3

3. Illustration of SRM 2030, Glass Filter for Spectrophotometry, showing the

glass filter, Schott NG4, in its metal holder with front and rear removable shutters 4

4. Illustration of SRM 931a, Liquid Absorbance Standard for Ultraviolet and

Visible Spectrometry; SRM 935, Crystalline Potassium Dichromate for

Use as an Ultraviolet Absorbance Standard; and SRM 932, Quartz

Cuvette for Spectrophotometry 5

5. Spectral characteristics for five transparent materials from 200 nm

to 700 nm 9

6. Transmittance of a clear fused silica plate, of two clear fused silica

plates assembled with the Sylgard resin, and of two fused silica

plates coated with a semi-transparent layer of evaporated inconel

metal and assembled with Sylgard resin 10

7. Disposition of the radiation sources used with the high-accuracy

spectrophotometer ' 13

8. Microcomputer and accessories 15

9. Aluminum alloy holder used to hold the evaporated metal -on-fused

silica filters 18

10. Details of the black anodized aluminum alloy 2024 filter holder body 19

11. Details of the retaining spring used with the unit from Figure 10 20

12. Details of the front and rear shutters used with the unit from Figure 10 . . . 21

13. Filter holder with shutters 22

14. Comparative spectral transmittances of an evaporated inconel-on-fused

silica filter and an evaporated chromium-on-fused silica filter 33

15. Instrumentation used to examine the chromium-on-fused silica filters 35

16. Illustration of SRM 2031, Metal-on-Quartz Filters for Spectrophotometry. ... 42

xii
Abstract

This publication describes in detail the selection, production,

testing, and certification of the semi-transparent evaporated metal-onfused silica filters as a Standard Reference Material for spectrophotometry. Particular attention is given to various factors that can affect the accuracy of the transmittance values established for these

filters, such as: reflections, polarization, angle of incidence, and

stability in time. Adequate procedures were devised and are described,

for the protection of the exposed metal layer surface using optical

contact, and for the protection of the filter aggregate, by the use of

metal holders provided with shutters and of metal storage containers.

The data from several interlaboratory tests are also discussed,

together with the results obtained on transmittance measurements on 19 sets of chromium-on-fused silica filters, protected by optical contact,

which were studied over a period of two years. An Appendix contains

the reproduction of several publications relevant, to the subject

discussed in this work.

Key Words: Filters for spectrophotometry; metal-on-fused silica

filters; optical transmittance accuracy; polarization;

reflections; spectral bandpass; Standard Reference Materials

in Spectrophotometry; stray radiation; transmittance

stabi 1 ity.

Disclaimer

In order to adequately describe materials and experimental procedures,

it was occasionally necessary to identify commercial products by

manufacturer's name or label. In no instance does such identification

imply endorsement by the National Bureau of Standards nor does it imply

that the particular products or equipment is necessarily the best

available for that purpose.

xiii

1. General

Molecular absorption spectrometry, or spectrophotometry2

, is a measuring technique

widely used in science and technology for the determination of a large number of inorganic

and organic chemical species in a wide variety of matrices. These determinations are based

on measuring the optical transmittance of the samples, and the accurate measurement of this

parameter is an essential requirement, especially when used to establish the optical

characteristics of the material.

Due to the importance of this parameter and as a result of numerous requests from the

users of spectrophotometers, and in particular from the clinical chemists, the National

Bureau of Standards (NBS) had initiated in 1969 a project to design and develop means for

defining and measuring optical transmittances with a well determined accuracy, and to provide certified materials that can be used to verify the transmittance accuracy of

conventional spectrophotometers and to monitor their stability and reproducibility.

As a result of this project, implemented in the Center for Analytical Chemistry at

NBS, a special high-accuracy transmittance spectrometer was designed and constructed that measures the optical transmittance of solid or liquid samples with an accuracy of one part

I in 10 4 transmittance units [2].

This instrument constitutes the primary transmittance standard, and was used to measure and certify the transmittance of a variety of selected solid and liquid materials.

These certified materials constitute secondary standards that provide a practical means

for transferring to the user accurate optical transmittance values with a well established

uncertainty. These, in turn, can be used to verify the accuracy of the transmittance and

absorbance scales of conventional spectrophotometers.

The certified materials, known as Standard Reference Materials (SRM's), are produced at NBS and are available to the user. A general discussion of the SRM's for spectrophotometry

will be given here. For a more detailed description, the reader is referred to the material

reproduced in the Appendix and the publications mentioned in the references at the end of

this publication. Some of the publications issued by NBS in the field of high-accuracy

spectrophotometry are illustrated in Figure 1. Standard Reference Materials 930 and 2030

I are illustrated in Figures 2 and 3, and SRM's 931, 935, and 932 in Figure 4.

1.1 Standard Reference Materials for Spectrophotometry Issued by the

National Bureau of Standards

The use of solid materials, in the form of glass or quartz (fused silica), crystals,

and of liquids and solutions of inorganic and sometimes organic compounds, to test the

correct function of spectrophotometers is a common practice [3]. After a careful review, a number of materials were selected to serve as SRM's and were issued during the last few

years. Table 1 gives a general summary of these materials, indicating their use and some of

their limitations. Except for SRM's 2009, 2010, 2013, and 2014, they were all certified in

the Center for Analytical Chemistry of the National Measurement Laboratory at NBS. From

this table it can be concluded that all the SRM's, solid or liquid, issued as transmittance

standards require the use of spectrophotometers capable of producing narrow spectral bandpasses from about 0.8 to 6.5 nm throughout the spectral range where the measurements are performed. The use of the glass filters (SRM 930 and SRM 2030) is limited to the visible

spectral region due to the intrinsic transmittance properties of this material. SRM 935

can be used only in the ultraviolet and requires the accurate preparation of several solutions together with the use of accurate cuvettes (SRM 932). SRM 931 provides only one certified value in the ultraviolet and requires the use of accurate cuvettes (SRM 932). It

is also a "one measurement at a time" type of material. The limitations enumerated above

were evident from the early efforts cited to provide SRM's for spectrophotometry. However,

at the time of the preparation and issuance of the SRM's described in Table 1, they were

the best materials available for use as transfer standards for transmittance.

2 Note: The terminology used throughout this publication follows the nomenclature defined

by K. D. Mielenz [l] 3

. 3 Figures in brackets indicate the literature references at the end of this paper.

1

Figure 2. Illustration of SRM 930D, Glass Filters for Spectrophotometry, showing the blackanodized aluminum alloy container for storing three filters in their holders and

an empty filter holder. One of the three glass filters, Schott NG4 and NG5, is shown with the retaining spring, nylon screw, washer, and the filter holder with

the front and rear shutters.

3
Figure 3. Illustration of SRM 2030, Glass Filter for Spectrophotometry, showing the glass

filter, Schott NG4, in its metal holder with front and rear removable shutters

The wood- like plastic box is used to store the glass filter in its holder and an empty filter holder.

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1.2 Conditions to be Satisfied by SRM's for Spectrophotometry

A material which can be used as a transmittance standard should satisfy the following

major conditions:

- Be transparent over the entire spectral range of interest, usually between 200 nm and 800 nm;

- Have a transmittance independent of wavelength (optically neutral), hence be usable

with spectrophotometers having wide spectral bandpasses;

- Be free of optical disturbances such as fluorescence, polarization, reflections;

- Have a transmittance independent of temperature;

- Be stable, homogeneous, and free of strain and scatter;

- Be a solid material with good mechanical stability for the size used (thickness,

length, width);

- Be simple for use in conjunction with conventional spectrophotometers currently

available in analytical laboratories;

- Be readily available at an acceptable cost.

From the numerous substances examined since 1969, which can be considered as acceptable

material, the semi-transparent metal layers deposited by evaporation or sputtering on a fused silica substrate satisfied the conditions cited above.

Indeed, as can be seen from Figure 5, this material satisfies particularly well the

conditions of optical transmittance and neutrality throughout the spectral range from 200 nm

to 700 nm for which the standard was intended. Furthermore, an extension of this range,

especially toward the long wavelengths, and within the transmittance limits of fused silica

(about 190 nm to over 3000 nm) seems possible. However, at the time, the problem of

protecting the exposed surface of the metal layer from the environmental conditions was a real limitation that could not be solved satisfactorily. The possibility that isochromatic

stray light as well as multiple reflections, might be generated at a prohibitive level in

the sample compartment of the spectrophotometer, due to reflections at the semi-transparent

metal layer, were also considered as potentially disturbing factors.

8
CORNING

8364

<

o

CO

CD<

CHANCE

ON-10

SCHOTT

NG-4

EVAPORATED METAL ON QUARTZ

[INCONEL]

RONCHI RULING ON GLASS

ICOLINES/in.

200 300 400 500

X, NANOMETERS

600 700

Figure 5. Spectral characteristics for five transparent

materials from 200 nm to 700 nm.

Satisfactory solutions have been found, however, during the last several years to these limitations, and, as a result, a new material was developed and it is now being

issued as SRM 2031, Metal -on-Quartz-Fi 1 ters for Spectrophotometry.

The object of this work is to describe in some detail the properties, preparation,

and certification of this new standard reference material.

The first studies on the optical characteristics of evaporated metals-on-fused silica

substrates were initiated in 1969 [8], when a series of filters were prepared by the

Optical Shop of NBS. A number of non-fluorescent fused silica plates of optical quality

10 mm x 25 mm and 1 mm thick were cut and polished to a flatness of two fringes (mercury

green, A546.07 nm) and a parallelism of ±0.02 mm. An inconel alloy was evaporated on the

surface to produce nominal transmittances of 25, 50, and 75 percent. The surface carrying

the semi-transparent evaporated metal was coated with a thin layer of optical cement

Sylgard, made by Dow Chemicals, which has good transmittance to ultraviolet radiation to about A.230 nm. A clear plate of the same material was used to cover and protect the metal

j layer. Each filter was then marked at one corner to allow positioning of the filter in a

; black-anodized aluminum alloy holder of conventional size (12.5 mm square x 57 mm) fitting

! the cuvette holder in the sample compartment of conventional spectrophotometers.

The transmittance characteristics of these filters from A.200 nm to 800 nm were

determined with a Cary 14 recording spectrophotometer. Figure 6 illustrates the transmittance of a clear fused silica plate, followed by that of an assembly made of two clear

plates assembled with the optical cement, and finally, that of the assembly of a metalcoated plate and a clear plate, cemented together with the Sylgard resin.

2. Introduction

2.1 Initial Studies

2.2 Characteristics of the Metal -on-Fused Silica Filters

9
10
From these data, it can be seen that the assembled metal-on-fused silica filter

exhibits a good transmittance throughout the spectral range from A.240 nm in the ultraviolet

to A.800 nm in the visible where the recording was stopped. Furthermore, the dependence of

transmittance on wavelength i-s small over this entire and wide spectral range. Hence, this

material satisfies the two major conditions of spectral range coverage and optical

neutrality.

The stability of this type of material to radiation was then studied by exposing the

metal-on-fused silica filter to an accelerated test in which the radiation had the same

spectral distribution as the fluorescent lighting of the laboratory, except that it was

1000 times more intense. The filter was exposed for an equivalent of 36,000 hours of

continuous normal laboratory exposure. This test was performed in the Center for Building

Technology at NBS. The transmittance of the filter was measured before and after the

exposure, using a Cary 16 spectrophotometer. The results are given in Table 2.

Table 2. Stability of an evaporated metal-on-fused silica

filter exposed to radiations.

Transmittance, percent

Wavelength ^\nm 250 380 500 650

Exposure ^v\v^

Before

Exposure 44.48 51.35 48.90 47.41

After

Exposure .44.11 51.34 48.92 47.47

The differences in transmittance observed at A.380, 500, 650 nm, as a result of this

severe test, are small, and in the range of the uncertainty of the instrumental measurements. The larger difference observed at \250 nm is probably due to some radiation

instability of the organic resin which absorbs the ultraviolet radiations from this

wavelength down.

Due to the severity of the radiation test and to the magnitude of the transmittance

differences observed, it was decided to disregard this instability and to resume the study

of this material by submitting these filters to a comparative test.

2.3 Comparison of Transmittance Measurements at the National Bureau of Standards

and the National Physical Laboratory

Several sets of these filters were made and calibrated at three selected wavelengths:

450 nm, 550 nm, and 650 nm, using the high-accuracy spectrophotometer. The same filters

were forwarded to the National Physical Laboratory (NPL), Teddington, Middlesex, U. K. where their transmittance was determined by F. J. J. Clarke using his high-accuracy

spectrophotometer. The results are given in Table 3.

The agreement between the measurements performed at NBS and NPL on the nominal 25 and

72 percent filters is good, with the NBS data showing a small negative bias. The average

difference of -0.21 percent is close to that found in a similar comparison performed on glass filters (ref. 2, p. 420, Table 9).

The values obtained for the nominal 50 percent transmittance filter indicate, however,

larger negative differences. F. J. J. Clarke observed that this particular filter had a flaw in the form of a crack which was at times visible and other times invisible. The

larger differences found in the measurements of this filter were attributed to this flaw.

11
Table 3. Comparison between the percent transmittance (% T) measured

on three metal-on-fused silica filters at NBS and NPL.

Laboratory

Wavel enoth^^^^

nm

NBS, % T NPL, % T NBS-NPL, % Difference

450.0 24 87 24. 93 -0.24

550.0 23 80 23 86 -0.25

650. 0 23 38 23 46 -0.34

450.0 49 34 49 56 -0.45

550.0 47 60 47 81 "-0.44

650.0 46 85 47 14 -0.62

450.0 72 18 72 30 -0.17

550.0 72 08 72 20 -0.17

650.0 72 27 72 33 -0.08

This observation prevented the selection of this material as an SRM for spectrophotometry. However, since the evaporated metal-on-fused silica filters possess unique optical

qualities, it was decided to continue the investigation of their properties and try to find ways to remedy these limitations.

3. Further Studies

Before discussing the results obtained from further studies concerning the transmittance characteristics of the evaporated metal-on-fused silica filters, it was considered

appropriate to describe briefly some of the new instrumental features of the high-accuracy

spectrophotometer used to perform these measurements.

3.1 The High-Accuracy Spectrophotometer at the National Bureau of Standards,

Center for Analytical Chemistry

Unless otherwise specified, all transmittance measurements discussed in this work were performed with this high-accuracy spectrophotometer that, as mentioned earlier, was designed

and constructed in the Center for Analytical Chemistry. A detailed description of this

instrument and of its characteristics is given in Reference 2, which is included in the

Appendix as a convenience to the reader. During the last few years certain improvements

have been made to the initial design. A description will be given here of these changes.

Figure 7 illustrates the optical disposition of the radiation sources used for measurements in the ultraviolet and visible spectral regions. A 1.2 m lathe-bed type optical

bench is positioned in front of the 1 m Czerny-Turner grating spectrometer, which is provided with a predi sperser. The position of the bench is adjustable through four screws,

and is maintained in the proper position by the aluminum angle supports that can be

tightened to the top of the vibration isolation table provided with tapped holes. On the

optical bench are placed, from the rear to the front: A low-power laser used for alignment

and for verifying its stability; a deuterium discharge lamp which is used when transmittance

measurements are performed in the spectral range from about A.190 nm to A400 nm; the power

supply for the deuterium lamp; a circular front- face mirror; a quartz circular neutral wedge

with its quartz lens; and a rectangular front-face mirror, which reflects the image of the

12
13
light sources on the entrance slit of the predi sperser-spectrometer assembly. Left of the

optical bench is the tungsten incandescent lamp used when transmittance measurements are performed in the near-ultraviolet and visible spectral region, from about A.360 nm to \800 nm. Every optical element can be rotated and adjusted in the x-y-z axes. The circular

mirror, in front of the deuterium lamp can be moved "in" or "out" of the optical path

through a slide moved across the optical axis by a pneumatically operated piston. When the

mirror is in the "out" position the radiation from the deuterium arc lamp enters the

spectrometer to permit measurements in the ultraviolet. When the mirror is in the "in"

position, the radiation from the tungsten incandescent lamp is deflected into the spectrometer for measurements in the visible spectral region. The circular and rectangular mirrors

are attached to their main holders through a spring-loaded three-point supporting plate

that is used to adjust both mirrors to the desired tilting. The position of the circular

neutral wedge can be selected by rotation to provide the desired radiation intensity

through an electric slow rotating reversible motor. The wavelength scanning shaft of the

spectrometer is provided with a stepping motor and a shaft encoder. This system is interfaced with a microcomputer. The microcomputer can be seen in Figure 8 atop the metal

cabinet which contains in addition the power supply of the photomultipl ier; the null meter

indicator for the potentiometer (partially visible at the right); the amplifier for the

photomultipl ier ; the digital voltmeter, and the computer operation panel. The teletypewriter is visible in front of the cabinet.

A further modification to the circular sample-carrier platform provides for 15 filter

or cuvette holders for blanks and samples, instead of the initial seven holders. This

modification was made to permit an empty filter holder to be placed in the eight reference

beam positions of the spectrophotometer, so that approximately equivalent conditions of

stray radiations are achieved for both beams. When measurements are made in the ultraviolet, the usual integrating sphere is replaced by a highly efficient averaging sphere

provided with a fluorescent wavelength converter [9].

The spectrophotometer data acquisition and presentation are automated through interface

with a microcomputer. The computer system consists of: an Altair 8800A computer with 20K

of RAM memory, a Dual North Star Computer floppy disc drive, and an ASR 33 Teletype. The

computer is programmed in BASIC with some assembly language subroutines.

The computer can control the following experimental variables:

1. The spectrophotometer wavelength drive position,

2. Position of an incremental sample changer,

3. Light source, either visible or U.V. , and

4. A neutral wedge light attenuator.

The computer can sense the following experimental variables:

1. The spectrophotometer wavelength drive position,

2. The position of the sample changer, and

3. The amount of light passing through the spectrometer.

The software written in BASIC performs the following sequence of events to control

the experiment:

1. Sets measurement wavelength,

2. Takes measurements of a blank and adjusts the neutral wedge in the light

beam to produce a voltmeter reading of 2.000 volts,

3. Advances sample changer to next position and acquires the data for a sample,

14

4. Reads blanks and samples alternately, and

5. Changes measurement wavelength and repeats steps (2), (3), and (4).

If the wavelength needed to acquire data extends into the U.V., then the computer can change from the visible to the U.V. light source.

Initial data reduction consists of taking the average and standard deviation of 50

individual measurements of both the sample (I) and reference beam (I

q

) intensities. This

is followed by computing the ratio of I/I

0

» followed by the correction for linearity.

Finally, the transmittance density ( _ log 10 (I/I

q

)) is computed and displayed. A tabular

form is provided for final data presentation.

A microcomputer system was selected because of its low cost and its suitability to perform a rather simple function. The programming of the entire system was made by

J. F. Barkley and the interfacing of the microcomputer with the high-accuracy spectrophotometer was performed by F. C. Ruegg, both of the Center for Analytical Chemistry at NBS [11].

The high-accuracy spectrophotometer measures the ratio of two radiation intensities:

that attenuated by the sample (I) to that passing through air or through a blank sample

^o air or*o blank^' * n ^ rst case, the measurements produce a transmittance

T = y which includes the reflection losses occurring at the surface of the transparent

o air

material. The measured transmittance, T, has a corresponding calculated transmittance

density, defined as -log 10 T.

When the measurements are performed against a blank sample in the reference beam, the

reflection losses are eliminated, and the parameter measured under these circumstances is

the internal transmittance T. = - . From this parameter, the absorbance, A, is

o blank

calculated by the expression A = -1og 10 T.

Since transmittance is the ratio of two radiant flux 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. Transmittances are measured accurately when there is a 1 inear relation

between the two radiant fluxes and the resulting photoelectric signals. Hence, linear

photoelectric measurements result in accurate transmittance values.

The linearity of the photodetector used in the high-accuracy spectrophotometer is established by the radiation addition principle used in conjunction with a double aperture

system described in detail in reference 2. This is similar to the double aperture unit

developed and used at the National Physical Laboratory for many years to establish the

accuracy of their reference spectrophotometers [17]. A critical study of the double

aperture device was made recently at NBS by K. D. Mielenz and K. L. Eckerle and is published

in reference 6 of the Certificate for SRM 2031 in paragraph 4.6.

An intrinsic property of the evaporated metal-on-fused silica filters is that of

attenuating the incident radiation by reflecting part of it, rather than absorbing. As mentioned earlier, the consequence of this property is that this type of filter is sus- ceptible to the generation of isochromatic stray radiations and multiple reflections in the

sample compartment of some conventional spectrophotometers, and in particular in those

instruments with lens optics. It could also have a position dependence transmittance, and

could be sensitive to polarized radiations.

A number of tests were designed to determine the magnitude of these characteristics.

16
3.2 Description of the Metal -on-Fused Silica Filters Made at the

National Bureau of Standards

Several sets of evaporated inconel-on-fused silica were prepared in the NBS Optical

Shop, using a single non-protected plate as a support for the semi-transparent metal layer.

Each set was composed from three filters and one blank, using non-fluorescent fused silica

i of optical quality as a substrate on which semi-transparent layers of inconel were deposited

by evaporation. Inconel was selected since it has a relatively good optical neutrality in

the spectral range from 250 nm to 700 nm. All fused silica filters were ground and polished

! at the same time and on the same polishing wheel to a parallelism of 0.02 mm and a flatness

| of less than two fringes (mercury green, A546.1 nm). The dimensions of these 2 mm thick

! filters are given in Figure 9 and the nominal transmittances are 1, 20, 30, and 90 percent

(clear fused silica plate).

Each filter was mounted in a special black anodized (flat) aluminum alloy holder

I provided with front and rear sliding shutters as illustrated in Figures 10, 11, 12, and 13.

' These holders were made in the NBS Instrument Shops and were designed to provide two

] functions: one to protect the filters from "fingerprinting" and exposure to environmental

I contamination; and the other to detect and measure the amount of isochromatic stray radiation that could be generated in the sample compartment of the spectrophotometer due to reflections produced at the surface of the semi-transparent metal layer of the filter.

3.3 Measurement of Isochromatic Stray Light

Isochromatic stray light may be assessed as follows: A background signal may be

I measured when the instrument shutter at the photomultipl ier is closed; this represents the

| dark current of the instrument. Another slightly higher background signal may be detected

with the photomultiplier open and with the filter holder placed in the sample compartment

with both sliding shutters closed to intercept the incident radiation beam. This signal

might be produced by reflections at the black (flat) surface of the shutters. Both of

these signals should be very small. A third signal may be detected when the front sliding

shutter at the filter holder is removed and the rear sliding shutter is closed. If, under

I these circumstances, a signal is detected, it is very likely due to the isochromatic stray

radiation produced in the sample compartment of the spectrophotometer by reflections at the

j semi-transparent mirror-like surface of the evaporated metal of the filter. This should

also be of low magnitude.

The transmittance of the filter is measured when both sliding shutters of the filter

holder are removed.

3.4 Multiple Reflections

The problem of multiple reflections, which may be produced when the filter is inserted

in the sample compartment of spectrophotometers, and particularly by those instruments

which are equipped with lens optics, was discussed in a previous publication [12]. This

phenomenon constitutes a source of systematic bias in transmittance measurements which can

be removed by applying a numerical correction obtained from measurements on tilted samples

in polarized light. This parameter was determined for the high-accuracy spectrophotometer

used in this work and is of the order of one or two 10" 4 transmittance units, independent

i of polarization, but slightly dependent of wavelength. It is also, for a given material,

I instrument dependent. For the evaporated metal -on-fused silica filter, having a transmit-

! tance of 0.4 at 474 nm, the value is about -1.4xl0~3 transmittance difference. This was

used to correct the measurement.

17
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19
<2 (,.\Vl^

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VI.71

Figure 11. Details of the retaining spring used with the

unit from Figure 10. Material: phosphorbronze or beryllium-copper. Finish: black

oxide. Tolerance: ±0.1 mm. Dimensions in

parentheses are in inches, all others are

in mm.

20
IP -4

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21
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22
3.5 Influence of Positioning of the Filters on Transmittances

The transmittance error resulting from positioning the inconel-on-fused silica filter

at various angles to the incident radiation beam, was measured by placing the filter in the

high-accuracy spectrophotometer on a vertical holder attached to a 10 cm diameter rotary

horizontal table. This table had a scale divided into 360° with a vernier reading to

5 min, and was held on the single sample carriage of the spectrophotometer with a vertical

mounting rod (see Ref. 2, Figure 2). This remote-controlled carriage has a smooth transversal movement, and the sample positioning in and out of the beam can be reproduced to within 0.025 mm.

Table 4 presents the comparative results for transmittance measurements made at

A.590 nm on an inconel-on-fused silica filter and a Schott NG4 neutral glass filter 2 mm thick having a nominal transmittance of 30 percent, for normal incidence and for angles of

3°, 6°, and 9°.

Table 4. Percent transmittance (% T) measured on a Schott

neutral glass filter 2 mm thick, and an inconel-onfused silica filter 2 mm thick at 590 nm, for normal

incidence and for angles of 3°, 6°, and 9°.

% T ^^^^

Filter Schott Inconel-on- fused silica

NG4

Angle Glass Front Back

0° 28.13 29.91 29.87

3° 28.10 29.98 29.87

6° 28.03 29.84 29.82

go 27.98 29.92 29.90

The inconel filter transmittance was respectively measured with both the metal coated

plate (front) facing the incident radiation beam and with the clear fused silica plate

(back) facing the beam. From these results it can be concluded that a deviation of up to

3° from a normal incidence position can be tolerated. Since this parameter is instrument

dependent, the results from Table 4 are valid only for the high-accuracy spectrophotometer

used to perform these measurements.

3.6 Intercomparison of Transmittance Measurements at the

National Bureau of Standards

The transmittance characteristics of these filters were further studied by performing

intercomparison measurements on the high-accuracy spectrophotometer in the Center for

Analytical Chemistry (CAC) and on a similar instrument available at NBS in the Center for

Radiation Research (CRR) [13,14,15]. The latter measurements were performed by

K. L. Eckerle. Table 5 contains the results of the transmittance measurements and includes

the differences obtained at the Center for Radiation Research by measuring the transmittances in polarized radiations having the electric vector both horizontal and vertical.

23
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cn o o CNl CNJ co o CM CM CM o o o o CNJ CNJ CM o cn cn cn o o O o d o

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24
The transmittances were measured in the Center for Analytical Chemistry in September

1975 and in October 1976. Those in the Center for Radiation Research were performed in

August 1976.

Several observations can be made after examining the data from this table.

- The inconel-on-fused silica filters exhibit good stability over a period of one

year.

- Good agreement was obtained between the two high-accuracy spectrophotometers

available at NBS, although one instrument uses achromatic lens optics (CAC) while

the other uses off-axis mirrors.

- The transmittance values are practically unaffected when measurements are made in

polarized radiations with the electric vector horizontal and vertical.

Comparable results were obtained in a similar test performed on three Schott NG4

neutral glasses as seen from Table 6.

Table 6. Transmittance of a set of SRM 930 at two wavelengths obtained using the Center for

Analytical Chemistry (CAC) and the Center for Radiation Research (CRR) highaccuracy spectrophotometers. The difference, A(CRR), is the difference in

transmittance measured by illuminating the filters with polarized radiation having

the electric vector in either the horizontal or vertical directions.

Transmittance

Nominal 30% Filter Nominal 20% Filter Nominal 10% Filter

^^^Wavel ength ^\nm

Instrument

465 546 465 546.465 546.

CAC

CRR

A(CRR)

0.3427

0.3430

-0.00002

0.3280

0.3284

-0.00007

0.1980

0. 1983

-0.00008

0. 1840

0. 1844

-0.00005

0. 1120

0. 1121

-0.00001

0. 1014

0. 1015

-0.00002

3.7 Intercompari son of Transmittance Measurements Performed at the National Bureau

of Standards and the Michel son Laboratory of the Naval Weapons Center

The intercompari son study was extended to include the special optical measuring capabilities available at the Michelson Laboratory, Physical Optics Branch of the Naval Weapons

Center (NWC), China Lake, California. The transmittance measurements were performed by

Harold Bennett and Jean M. Bennett.

Table 7 contains the results of these measurements performed on four inconel-on-fused

silica filters having nominal transmittances of 1, 20, 30, and 90 percent.

The agreement between these measurements is excellent, even for the one percent

filter.

25
Table 7. Comparative measurements of transmittance on inconel -on-fused silica filters in

the ultraviolet and visible from 250 nm to 635 nm at NBS and at Naval Weapons

Center (NWC)

Transmittance, Percent

r i i ter

NO. L8.D

250 300 340 400 440 465 DUU 546.

590 0-30

NBS 1 .492 1 844 1 899 1 973 2 083 2 171 2.318 2. 540 2 750 2.956

At; to

INWO 1 .501 1 .837 1 884 1 953 2 070 2 158 9 "? 1 7 2. 552 2 755

NBS 21 .49 21 39 20 46 19 05 18 45 18 20 18.01 17. 94 17 98 18.07

A C

21 .50 21 37 20 39 19 04 18 38 18 19 I o. UO 17. 99 17 92

a

lo. 1 U

NBS 28 .73 29 96 30 24 29 99 29 70 29 56 29.44 29. 41 29 45 29.55

47

NWC 28 .80 29 89 30 23 29 95 29 68 29 55 29.45 29 42 29 43 29.48

NBS 91 .25 92 28 92 64 92 91 93 00 93 04 93. 10 93. 15 93 19 93.22

48

NWC 91 .21 92 22 92 59 92 90 92 98 93 01 93. 14 93. 19 93 21 93.24

Value corrected as indicated in J. M. Bennett communication.

3.8 Scatter, Homogeneity, and Polarization of Inconel-on-Fused Silica Filters

and Schott NG4 Glass Filters

The conditions required for a material to be useful as a reference standard were enumerated at the beginning of this work. Among those mentioned were: freedom of scatter

and homogeneity. The presence of pinholes and dust on the metal layer of the inconel-onfused silica filters could result in scatter and produce apparent i nhomogeneity and reduce

the usefulness of this material. Hence, a set of three inconel-on-fused silica filters,

identified as No. 20 (1, 2, and 3) with nominal transmittances of 10, 30, and 90 percent

were subjected to measurements to assess these parameters. They were compared with a set

of glass filters subjected to the same measurements.

Scatter and homogeneity measurements in polarized radiation were performed by

K. L. Eckerle in the Center for Radiation Research at NBS using the instrument described

in reference 15. All measurements were performed at A546.1 nm and are given in Table 8. The quantities (x,y) are the coordinates of the center of the illuminating spot with

respect to the center of the filter in millimeters when looking in the direction of the

source. The parameter, Q, is the approximate solid angle in steradians. The quantities

T(0°), T(90°), and T(AVE) are the transmittances for the electric vector of the illuminating

radiation horizontal, vertical, and average of horizontal and vertical, respectively,

measured at A546.1 nm. Their respective standard errors are ATp(0°), ATp(90°), and

ATp(AVE). The radiation scatter measurements were made by determining the transmittance of

the filters successively at distances from the averaging sphere of 210 mm, 370 mm, and 715 mm corresponding to angles of about 0.014, 0.0045, and 0.0012 steradians, respectively.

The diameter of the circular entrance aperture at the averaging sphere was 28 mm. The

transmittance measurements to assess the uniformity of the filters were made at a constant

angle of 0.0045 steradians.

26
Table 8. Measurement of scatter, homogeneity, and polarization on three inconel-on-fused

silica filters.

(x, y) Q T(0°) AT

R (0°) T(90°) AT

R (90°) T(AVE) AT

R (AVE)

#20 Filter 1

(+1,0) 0 0045 0.10807 0.000018

(-1,0) 0 0045 0.10798 0.000009

(0,0) 0 0045 0. 10805 0.000013 0.10841 0.000020 0.10823 0.000012

(0,0) 0 0012 0.10801 0.000027 0.10840 0.000020 0.10821 0.000017

(0,0) 0 014 0.10799 0.000010 0.10831 0.000013 0.10815 0.000008

#20 Filter 2

(+1,0) 0. 0045 0 32089 0 000035

(-1,0) 0. 0045 0 32050 0 000037

(0,0) 0. 0045 0. 32066 0 000046 0 32092 0. 000030 0 32081 0 (0,0) 0. 0012 0. 32069 0 000039 0 32079 0. 000045 0 32073 0 (0,0) 0. 0014 0. 32045 0 000047 0. 32050 0. 000057 0. 32047 0

#20 Filter 3

(+1,0) 0 0045 0 93139 0 000118

(-1,0) 0 0045 0 93160 0. 000096

(0,0) 0 0045 0 93131 0 000067 0 93125 0 000064 0 93128 0 000046

(0,0) 0 0012 0 93107 0. 000122 0 93095 0. 000039 0 93101 0 000064

(0,0) 0 014 0 93089 0. 000041 0 93076 0. 000080 0 93083 0 000045

From these data, it can be concluded that within the experimental measuring conditions

there is no significant change of transmittance caused by scatter. The material has a good

homogeneity, and no significant polarization effect exists. Identical measurements performed on a set of three Schott NG4 neutral glass filters serial number 537 (1, 2, and 3)

have produced similar results, as shown in the data in Table 9.

27
Table 9. Measurement of scatter, homogeneity, and polarization on three Schott NG4 neutral

glass filters.

(x, y) Q T(0°) AT

R (0°) T(90°) ATR (90°) T(AVE) AT

R (AVE)

#537 Filter 1

(+1,0) 0. 0045 0. 10038 0. 000014 —

n 0(145 nu

1 0047 nu

UUUU

D (0,0) 0. 0045 0. 10042 0. 000012 0.10042 0. 000012 0. 10042 0. 000008

(0,0) 0. 0012 0. 10046 0. 000013 0.10042 0. 000026 0. 10044 0. 000015

(0,0) 0. 014 0. 10039 0. 000013 0.10035 0. 000020 0. 10037 0. 000012

#537 Filter 2

(+1,0) 0. 0045 0. 19240 0. 000023

f-i rh n\J 0045 n 11 Q?l J £ 1 U n 000031

(0,0) 0. 0045 0. 19254 0 000022 0.19250 0 000028 0. 19252 0 000018

(0,0) 0. 0012 0. 19252 0. 000026 0.19249 0 000021 0. 19251 0 000017

(0,0) 0 0014 0. 19237 0 000024 0. 19239 0 000027 0 19238 0 000018

#537 Filter 3

(+1,0) 0 0045 0. 30875 0 000024

(-1,0) 0 0045 0 30872 0 000019

(0,0) 0 0045 0 30877 0 000020 0.30874 0 000028 0 30875 0 000017

(0,0) 0 0012 0 30875 0 000042 0.30874 0 000049 0 30874 0 000032

(0,0) 0 014 0 30848 0 000028 0.30847 0 000035 0 30847 0 000023

28
3.9 Further Evaluation of the Metal-on-Fused Silica Filters

A further evaluation of the evaporated metal-on-fused silica filters as a potential

SRM in spectrophotometry was initiated by inviting several manufacturers of spectrophotometers to express their opinion on this matter. For this purpose, a number of sets

of inconel-on-f used silica filters were distributed to the participating companies and

research laboratories listed here in alphabetical order:

Companies

- Bausch and Lomb, Rochester, New York 14625; - Beckman Instruments Inc., Irvine, California 92664; - Cary-Varian Instruments, Palo Alto, California 94303; - Coleman, a Division of Perkin-Elmer Co., Oak Brook, Illinois 60521; - Gilford Instruments Laboratories, Inc., Oberlin, Ohio 44074; - McPherson Instruments, Acton, Massachusetts 01720;

Laboratories

- Eastman Kodak Co. , Rochester, New York 14650; - Purdue University, Department of Chemistry, West Lafayette, Indiana 47907; - Tennessee Eastman Co., Kingsport, Tennessee 37662.

The data from Table 10 were selected to illustrate the results obtained from such an interlaboratory comparison. The transmittance measurements were performed at the Tennessee

Eastman Co. by J. H. Lady, using a computerized Cary 14 spectrophotometer.

The general opinions expressed by the participants in these interlaboratory comparisons

were favorable to the use of the evaporated metal-on-fused silica filters as a reference

standard in spectrophotometry.

V

29
Ec

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0) _c

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ni

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03

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30
3.10 Metal-on-Fused Silica Filters at the National Physical Laboratory

A discussion concerning the transmission characteristics of metal-on-fused silica

filters, using a particular variety of nichrome alloy developed at the National Physical

Laboratory (NPL), Teddington, U. K. , was given recently by F. J. J. Clarke and associates

[16]. The good optical neutrality of this material associated with a good adherence to the

i fused silica substrate has determined the certification of such filters as transfer standards for transmittance by NPL. The filters conrist of single fused silica plates coated

on one side with a semi-transparent layer of the nichrome alloy, and each filter is placed

in a special holder. The exposed surface of the metallic film is not protected from

exposure to environmental conditions and mishandling. Hence, for accurate use, it is

recommended that these filters be recalibrated every year to insure that no degradation

of the initial calibration has occurred, as indicated by F. J. J. Clarke in a private

communication.

A high-accuracy spectrophotometer developed at NPL is used for the certification;

its transmittance accuracy is verified with the double aperture system described in

reference 17.

3.11 Limitation in the Use of the Metal-on-Fused Silica Filters as SRM's in

Spectrophotometry and Removal of this Limitation

The only limitation that has prevented the metal -on-fused silica filters from being

! issued by NBS as a standard reference material was the fact that the surface of the metallic

layer was not protected from exposure to contamination from environmental factors and from

| careless handling. Under such circumstances, the certified transmittance values could not

be expected to retain their integrity for long periods of time.

However, a solution to this problem was found. It consisted of covering the fused

silica plate carrying the semi-transparent evaporated metal layer with a clear fused

silica plate. This clear plate is about 1.5 mm thick ground and polished in such a manner

as to achieve an optical contact with the evaporated semi-transparent metal layer. Both

fused silica plates are slightly beveled, and an optical cement is applied to protect the

edges from damage. Under these conditions, this assembly behaves optically as if it were

a single, solid, semi-transparent plate, and provides full protection to the metal layer.

The possibility to achieve an optical contact between an evaporated metal layer and a fused

silica plate was suggested as a protective means for the metal-on-fused silica filters by

H. E. Bennett, Michelson Laboratory, Naval Weapons Center, China Lake, California, in

1975.

The possibility of producing such filters was explored with H. Peters at the Cosmo

Optics, Inc. in Middletown, New York. After a number of experiments with materials prepared

at NBS, he achieved the desired optical contact between the metalized surface of a fused

silica plate and another clear fused silica plate. Additional experiments indicated that

this procedure could be economically used to produce such filters.

4. Semi-Transparent Metal-on-Fused Silica Filters Protected by a Clear Fused

Silica Plate Assembled by Optical Contact

The provision of a clear, fused silica plate attached by optical contact to the semi-

| transparent evaporated metal surface provides a satisfactory solution for the protection of

; such filters. Also, the method developed for producing such filters proved economical. The

use of metal filter holders with removable front and rear shutters, and the use of metal

containers for storage of these filters provides added protection against exposure to environmental conditions and mechanical damage. The only parameter that had yet to be

assessed was the stability of the transmittance of the filters over a long period of time.

31
4.1 Preparation of the Chromium-on-Fused Silica Filters

Twenty sets of filters were prepared according to NBS specifications by Cosmo Optics

Inc. Each set contained three filters with a nominal transmittance of 10, 30, and 90

percent. The metal used to produce the semi-transparent film was chromium, which was selected for its better adherence to the fused silica substrate than inconel, general

resistance to environmental exposure, and acceptable optical neutrality.

Each fused silica plate was 1.5 mm thick and 30.5 mm long and 10.4 mm wide. It was ground and polished to produce the optical contact. The parallelism of each plate was better

than 0.01 mm and the fused silica material was of optical quality and non-fluorescent. The

semi-transparent chromium layer deposited by evaporation was free of pinholes when examined

microscopically under a x30 magnification. The purity of the chromium metal used was 99.998

percent as indicated by its manufacturer, the Atomergic Chemetals Corp., Plainview, NY, and

was specially produced by the van Arkel method for the evaporation process. The filters were placed in the holders described in Figures 9-13 as mentioned previously, and each set was kept in the same laboratory as the high-accuracy spectrophotometer at a temperature of

23.5 °C and a humidity of 40 percent.

4.2 Optical Transmittance of the Filters and Spectral Bandpass Dependence

The wavelength dependence of the optical transmittance of the semi-transparent layer

of the chromium metal on the fused silica substrate as compared with inconel is illustrated

in Figure 14. These measurements were made with a Cary 14 spectrophotometer. The dependence

of the transmittance on the spectral bandpass was determined with a Cary 16 spectrophotometer at A.300 nm and A.600 nm. The results are given in Table 11. From these data it can

be seen that the material has good optical neutrality. Under these circumstances the

accuracy of the wavelength scale of conventional spectrophotometers and their spectral

bandpass characteristics are much less critical when using the chromium-on-fused silica

filters, than the glass or liquid filters (SRM's 930, 931, and 935), as shown in their

certificates and the NBS Special Publications 260-51 [4] and 260-54 [7]. However, for

meaningful results it is recommended that SRM 2031 be used in conjunction with spectrophotometers that have an accurate wavelength scale, and adequate spectral bandpass, as shown in Table 11, and for which the heterochromatic and isochromatic stray radiations are

at a low level at which they will not affect the measurements (about one order of magnitude

lower than the smallest signal measured).

Table 11. Transmittance (T) - spectral bandpass (SBP) dependence of an evaporated chromium

metal-on-fused silica filter at two wavelengths (A.).

T, percent

\ SBP

\nm

k

nm

9 0 7 5 6. 0 4 5 3 0 1 5 0 3

300 11 00 11 00 10 95 11 00 11 00 10 95 11 00

\ SBP

\nm

\ X.

nm \^

81 0 67 5 54 0 40 5 27 0 13 5 2 7

600 11 42 11 53 11 55 11 65 11 68. 11 65 11 60

NOTE: The transmittance measurements were performed with a Cary 16 spectrophotometer.

32
CD

CD CO CD "3- CS) CD CNI CD CD CD CD

30Nvaaosav

33
4.3 Inspection of the Chromi um-on-Fused Silica Filters

Each filter was examined visually in monochromatic light to determine that a faultless

optical contact was achieved and maintained, using for this purpose the light source illustrated in Figure 15. This source consists of two low-pressure mercury discharge tubes

mounted in a desk type lamp replacing the conventional fluorescent tubes. A green filter,

made from a colored plastic sheet, contained between two glass plates, was attached to the

lamp with springs. The transmittance maximum of the filter permitted the isolation of the

green radiation of mercury at A.546.1 nm.

The surfaces of each filter and the quality of the evaporated metal layer were examined

with the Bausch and Lomb microscope illustrated in Figure 15. This i nstrument provides both

transmitted and reflected light examination capabilities. Under these conditions the

filters were found to be free of visible defects over the period of two years, during which

the transmittance stability tests were performed.

4.4 Tests to Determine the Transmittance Stability, in Time, for the

Chromi um-on-Fused Silica Filters

Stability transmittance tests were made at certain time intervals on 19 sets. The

first measurements were made in February 1977, and were repeated in August 1977, December

1977, and July 1978. The last transmittance measurements were performed in March 1979.

Each transmittance value is the average of three repetitive measurements and each mea-

. . 21 surement consists of determining the ratio p

- = T; I being the photodetector signal

o o produced by the attenuated radiation passing through the filter, and 1^ and 1^' the signals

produced by the non-attenuated radiation passing through air and bracketing the I measurements. Each I and I are the average of 50 individual digital voltmeter readings [2]. The

transmittance measurements were made at 23.5 °C and a humidity of 40 percent in a room where the particulate matter is controlled through special filters that rates it in the

100,000 class.

The transmi ttances were measured at three wavelengths in the ultraviolet: A250, 300,

and 340 nm, and at seven wavelengths in the visible spectral regions: A.400, 440, 465,

500, 546. 1

, 590, and 635 nm.

The spectral bandpass for the ultraviolet measurements was 1.6 nm and 0.3 nm for the

measurements performed in the visible.

A tabulation of all measurements performed on the 19 sets would contain 2850 transmittance values, and would take too much space to be all included here. Therefore, two representative examples were selected to illustrate the results obtained in this test. Table 12 shows the transmi ttances for sets 4 and 6. Each was made from three filters having nominal

transmittances of 10, 30, and 90 percent. Set No. 4 was selected as a representative

example for filters that have shown a change in transmittance over the two years time

period of less than ±0.5 percent. Set No. 6 illustrates the behavior of filters that

exhibited a change in transmittance of over ±0.5 percent, but less than ±1.0 percent. The

differences, expressed as transmittance percents, are those between the February 1977

measurements and the March 1979 measurements and are referred to the initial measurements

performed in February 1977. The corresponding relative transmittance differences obtained

for all 19 sets are recorded in Table 13A. The absolute transmittance differences are recorded in Table 13B. A summary of the data from Table 13A is presented in Table 14.

34
cuCL O

ol <LI o r <u +J

dj +j o CU

co _Q o

4_) CD (_) ri ro

-i_> e Q o

co

1 +J

-Q ro en

fO

( <~ CL o (_> o

(/) +-> cu

fD r— CO 4_> o t|_ c o

-!_> cj q_ o

CD ^ o

_1 o

4-> "O

</) c;o

<D

CD

, c~

<-l_ 4->

m CU >,

+_)

1•r— j ro CO

ri cu "O ro CD t~i o

CO ro -l-J 13 4_ "O

1 HI o to o :3

1 -(-> E CO ^ C

E E +-> O ro S- X

jz: CU V

+J e'

<U -£= c

.c: CO

4—

<uc o(11 LO E +-• c< >>

fO O

X CU +J -O

CD ro E

4^ cu o CU 00

+-> f~ o 00

ro "O "D +-> cd ro to

(/) ro 0) 3 T3 +-> ro ro c cu S- o -P CL

4-> c 4-J CU (0

fO E CU O

4—

to S- c CT

OJ ro E S- >i to

rs 4-> s- s- =3 P -C u cu

c/l +-> s- to C cu i—

3 E 4-

LO

cu
Table 12. Transmittance of two sets of chromium-on-fused silica filters measured over a period of two years.

Transmittance, percent

\Wavelength \ nm

Filter

# and Date x.

250 300 340 400 440 465 500 546.1 590 635

2/77

4 8/77

12/77

10% 7/78

3/79

12.94 11.86 11.29

12.90 11.86 11.31

12.90 11.84 11.29

12.92 11.85 11.30

12.88 11.82 11.28

11.08 10.47 10.27 10.38 11.14 12.11 13.08

11.09 10.51 10.28 10.40 11.17 12.13 13.12

11.09 10.43 10.24 10.34 11.13 12.10 13.13

11.09 10.45 10.26 10.35 11.15 12.10 13.11

11.03 10.45 10.25 10.36 11.15 12.11 13.12

Difference, %T -0.46 -0.34 -0.09 -0.45 -0.19 -0.19 -0.19 +0.09 0.00 +0.31

2/77

4 8/77

12/77

30% 7/78

3/79

43.59 40.90 38.28

43.51 40.76 38.46

43.62 40.91 38.29

43.53 40.83 38.42

43.54 40.77 38.41

36.39 34.35 33.14 32.38 32.46 33.05 34.39

36.43 34.29 33.05 32.39 32.55 33.02 34.50

36.38 34.17 33.11 32.12 32.24 33.37 34.35

36.39 34.34 33.12 32.36 32.50 33.06 34.47

36.41 34.31 33.18 32.30 32.41 33.17 34.38

Difference, %T -0.11 -0.32 +0.34 +0.05 -0.12 +0.12 -0.25 -0.15 +0.36 -0.03

2/77

4 8/77

12/77

90% 7/78

3/79

91.49 92.32 92.65

91.39 92.25 92.61

91.63 92.30 92.60

91.48 92.40 92.75

91.48 92.42 92.73

92.93 93.02 93.07 93.14 93.18 93.27 93.27

92.89 93.02 93.04 93.12 93.20 93.23 93.29

92.89 93.02 93.08 93.14 93.20 93.24 93.32

93.06 93.20 93.24 93.32 93.37 93.42 93.45

93.05 93.16 93.21 93.28 93.30 93.37 93.38

Difference, %T -0.01 +0.11 +0.09 +0.13 +0.15 +0.15 +0.15 +0.13 +0.11 +0.12

2/77

6 8/77

12/77

10% 7/78

3/79

12.54 11.55 11.04

12.50 11.54 11.03

12.56 11.55 11.04

12.56 11.56 11.06

12.51 11.55 11.04

10.88 10.24 10.05 10.21 10.93 11.94 12.89

10.88 10.28 10.09 10.26 11.00 12.02 12.96

10.88 10.29 10.11 10.22 10.99 11.99 12.96

10.90 10.30 10.12 10.25 11.00 12.02 12.98

10.90 10.29 10.12 10.23 11.00 12.01 12.97

Difference, %T -0.24 0.00 0.00 +0.18 +0.49 +0.70 +0.20 +0.64 +0.59 +0.62

2/77

6 8/77

12/77

30% 7/78

3/79

43.29 40.57 38.21

43.40 40.68 38.32

43.49 40.66 38.28

43.41 40.67 38.30

43.40 40.68 38.31

36.05 33.91 32.85 31.95 32.15 33.25 34.48

36.13 33.99 32.92 32.01 32.27 33.35 34.61

36.11 34.00 32.90 32.02 32.26 33.30 34.56

36.10 34.00 32.92 32.03 32.23 33.31 34.56

36.12 33.99 32.91 32.01 32.27 33.35 34.62

Difference, %T +0.25 +0.27 +0.26 +0.19 +0.24 +0.18 +0.19 +0.37 +0.30 +0.41

2/77

6 8/77

12/77

90% 7/78

3/79

91.28 92.24 92.64

91.16 92.18 92.57

91.45 92.22 92.59

91.31 92.30 92.71

91.31 92.31 92.72

92.90 93.06 93.08 93.18 93.20 93.21 93.30

92.88 93.03 93.10 93.17 93.21 93.24 93.29

92.89 93.00 93.04 93.11 93.17 93.23 93.27

93.06 93.20 93.26 93.33 93.41 93.49 93.49

93.04 93.13 93.19 93.25 93.33 93.37 93.42

Difference, %T +0.03 +0.08 +0.09 +0.15 +0.08 +0.12 +0.08 +0.14 +0.17 +0.13

36
Table 13A. Relative differences in transmittance, AT%, observed over the time interval

February 1977 to March 1979 on 19 sets of evaporated chromium-on-fused

silica filters. aT% is obtained from: T%

( 19^~g ^( 1979 ^ x 100.

Set Number

and Filter

Identification

AT%

Ultraviolet

250 300 340

Visible

400 440 465 500 546.1 590 635

10

2 30

90

10

3 30

90

10

4 30

90

10

5 30

90

10

6 30

90

10

7 30

90

10

8 30

90

10

9 30

90

10

10 30

90

10

11 30

90

10

12 30

90

10

13 30

90

10

14 30

90

-0.24

-0.39

-0.09

-0.68

-0.30

+0.12

-0.46

-0.11

-0.01

-0.47

-0.21

+0.15

-0.24

+0.25

+0.03

-0.48

-0.16

+0.09

-0.55

-0.28

-0.88

•0.64

-0.39

-0.30

-0.10

-0.41

-0.18

+0.13

+0.74

-0.20

-0.02

-0.54

-0.32

-0.20

+0.08

-0.29

-0.03

-0.17

-0.07

+0.04

-0.57

-0.19

+0.07

-0.34

-0.32

+0.11

-0.43

-0.02

+0.11

0.00

+0.27

+0.08

-0.35

+0.07

+0.13

-0.17

0.00

+0.01 +0.09

-0.56

+0.07

+0.13 +0.10

-0.08

-0.10

+0.03

+0.09

+0.12

+0.14

+0.36

-0.24

+0.10

-0.33

+0.05

-0.09

+0.17

-0.24

0.00

+0.09

-0.13

+0.03

-0.51

-0.05

+0.15

-0.09

+0.34

+0.09

-0.36

+0.31

+0.14

0.00

+0.26

+0.09

-0.09

+0.25

+0.15

-0.09

0.00

+0.10

-0.34

+0.10

+0.14

-0.09

-0.10

+0.11

+0.19

-0.05

+0.11

+0.66

+0.05

+0.11

-0.53

-0.03

+0.03

+0.28

-0.08

+0.09

0.00

+0.36

+0.07

-0.26

-0.03

+0.14

-0.45

+0.05

+0.13

-0.09

+0.50

+0.10

+0.18

+0.19

+0.15

-0.09

+0.19

+0.16

+0.19

+0.11

+0.10

-0.17

+0.05

+0.14

+0.34

+0.11

+0.13

+0.19

-0.14

+0.14

+0.28

+0.08

+0.09

-0.18

+0.25

+0.04

+0.09

+0.03

+0.10

0.00

-0.47

+0.01

-0.37

-0.20

+0.19

-0.19

-0.12

+0.15

+0.29

+0.94

+0.17

+0.49

+0.24

+0.08

+0.19

+0.11

+0.18

0.00

+0.18

+0.12

-0.37

-0.18

+0.22

+1.68

+0.03

+0.13

+0.30

-0.38

+0.11

0.00

-0.52

+0.10

-0.38

0.00

+0.13

+0.20

+0.17

+0.11

0.00

+0.28

-0.02

-0.38

-0.15

+0.24

-0.19

+0.12

+0.15

+0.10

+1.07

+0.17

+0.70

+0.18

+0.12

-0.39

+0.12

+0.11

-0.20

+0.21

+0.15

-0.46

+0.58

+0.26

-0.49

-0.03

+0.08

+0.92

-0.39

+0.17

+0.31

-0.36

+0.13

-0.77

0.00

+0.15

+0.10

-0.50

+0.11

+0.10

-0.47

-0.05

+0.18 +0.25

-0.09 +0.40

-0.05 0.00

-0.19 -0.17

-0.03 0.00

+0.16 +0.21

0.00

-0.06

+0.18

-0.19 +0.09 0.00

-0.25 -0.15 +0.36

+0.15 +0.13 +0.11

0.00 -0.27 +0.17

+0.88 -0.93 +0.88

+0.16 +0.14 +0.15

+0.20 +0.64 +0.59

+0.19 +0.37 +0.30

+0.08 +0.14 +0.17

+0.59 +0.73 +1.17

+0.15 +0.24 +0.20

+0.17 +0.15 +0.14

-0.39 0.00 +0.43

-0.09 -0.37 +0.09

+0.07 +0.11 +0.11

-0.46 -0.09 -0.32

-0.44 +0.37 +0.57

+0.19 +0.24 +0.18

+1.09 +0.55 +1.00

+0.03 +0.06 +0.09

+0.10 +0.07 +0.06

-0.30 +0.19 +0.77

-0.19 +0.19 -0.33

+0.18 +0.13 +0.20

+0.10 +0.28 +0.60

-0.71 +0.12 -0.45

+0.08 +0.11 +0.15

-0.29 -0.18 +0.16

-0.03 -0.06 -0.06

+0.08 +0.17 +0.14

+0.20

0.00

+0.11

+0.19

-0.03

+0.11

+0.17

+0.12

+0.14

+0.46

-0.09

-0.01

+0.15

+0.12

+0.25

+0.31

-0.03

+0.12

-0.23

-0.55

+0.16

+0.62

+0.41

+0.13

+0.23

+0.25

+0.13

+0.15

+0.41

+0.11

-0.22

+0.64

+0.19

+ 1.55

+0.23

+0.11

+0.87

0.00

+0.18

+0.24

+0.67

+0.12

-0.53

+0.23

+0.10

+0.32

+0.28

+0.11

37

continued
Table 13A continued.

Set Number aT%

and Filter

Identifi- Ultraviolet Visible

cation 250 300 340 400 440 465 500 546.1 590 635

10 -0.39 -0.08 +0.18 +0.36 +0.29 +0.29 +0. 38 +1.08 -0. 08 +0.15

1 5 30 -0.25 +0.05 -0.05 -0.08 -0.03 -0.12 -0 06 +0.24 +0. 03 +0.11

90 +0.16 +0.16 +0.15 +0.06 +0.17 +0. 18 +0 12 +0. 20 +0. 20 +0.16

10 -0.48 -0.26 -0.83 -0.09 -0.39 +0.41 -0 19 +0.92 +0.85 +0.94

16 30 -0.20 +0.14 +0.05 +0.30 -0.14 0.00 -0 15 +0.06 -0. 03 +0.08

90 -0.01 +0.05 +0.06 +0. 07 +0. 08 +0.10 +0 12 +0.03 +0. 07 +0.07

10 -0.49 -0.18 -0.19 +0.47 0.00 -0.20 -0 20 +0.09 -0. 34 +1 .66

17 30 -0.04 +0.10 +0.13 +0.16 +0.06 +0.18 +0 15 +0.28 +0. 27 +0.14

90 +0.14 +0.12 +0.13 +0.12 +0.16 +0. 07 +0 22 +0.15 +0. 19 +0' W • 16IU

10 0.00 +0.17 +0.37 +0.37 +0.49 +0.30 +0 39 +0.46 +0. 46 +0.70

18 30 0.00 +0.02 0.00 +0.14 +0.09 +0.03 o 00 +0.06 +0. 12 +0.15

90 +0.12 +0. 13 +0.14 +0. 1

+0. 1

+0. 17 +0 13 +0.06 +0. 06 +0.13

10 -0.75 -0.24 0.00 +0.17 -0.09 +0.19 -1 36 +0.94 +0. 08 -1.37

19 30 -0.31 +0.38 +1.09 0.00 -0.25 -0.50 -0 71 -0.53 +0. 23 +0.31

90 +0.10 +0.11 +0.09 +0.10 +0.11 +0.15 +0 16 +0.18 +0. 15 +0.18

10 -0.40 -0.26 -0.18 +0.18 -0.88 -1.20 -0 69 -0.28 -0. 59 -0.15

20 30 +0.35 +0.45 +0.37 +0.42 +0.24 +0.18 +0 25 +0.16 +0. 33 +0.29

90 -0.10 -0.01 +0.01 +0.15 +0.07 +0.08 +0 04 +0.07 +0. 06 +0.04

Table 13B. Absolute differences in transmittance, AT, observed over the time interval:

February 1977 to March 1979 on 19 sets of evaporated chromium-on-fused

silica filters. AT is obtained from T% (1977) - T% (1979).

AT Ultraviolet

250 300 340

Visible

400 440 465 500 546.1 590 635

10

30

90

-0.03

-0.17

-0.08

-0.09

-0.13

+0.11

-0.06

-0.05

-0.01

-0.06

-0.09

+0.14

-0.03

+0.11

+0.03

-0.02

-0.03

+0.04

-0.07

-0.08

+0.07

-0.04

-0.13

+0.10

-0.05

-0.01

+0.10

0.00

+0.11

+0.07

+0.01

-0.05

+0.03

-0.06

-0.02

+0.14

-0.01

+0.13

+0.08

-0.04

+0.12

+0.13

0.00

+0.10

+0.08

0.00

-0.13

+0.07

-0.03

-0.01

+0.13

-0.05

+0.02

+0.12

-0.01

+0.18

+0.09

+0.02

+0.07

+0.14

0.00

-0.16

+0.01

-0.04

-0.07

+0.18

-0.02

-0.04

+0.14

+0.03

+0.32

+0.16

+0.05

+0.08

+0.07

0.00

+0.09

-0.02

-0.04

-0.05

+0.22

-0.02

+0.04

+0.14

+0.01

+0.35

+0.16

+0.07

+0.06

+0.11

+0.01 +0.02 +0.03

-0.15 -0.03 +0.13

-0.05 -0.05 0.00

-0.02 -0.02 0.00

-0.01 0.00 -0.02

+0.15 +0.20 +0.17

-0.02 +0.01 0.00

-0.08 -0.05 +0.12

+0.14 +0.12 +0.10

0.00 -0.03 +0.02

+0.28 -0.30 +0.29

+0.15 +0.13 +0.14

+0.02

+0.06

+0.07

+0.07

+0.12

+0.13

+0.07

+0.10

+0.16

+0.06

-0.03

-0.01

+0.02

+0.04

+0.23

+0.04

-0.01

+0.11

-0.03

-0.19

+0.15

+0.08

+0.14

+0.12

38

continued
Table 13B continued.

Set Number AT

and Filter

IdentifiUltraviolet Visible

cation 250 300 340 400 440 465 500 546.1 590 635

10 -0.06 -0. 04 -0.01 -0.01 +0.02 -0.04 +0.06 +0 08 +0 14 +0.03

7 30 -0.07 +0. 07 +0.10 +0. 07 +0. 04 +0. 04 • a Ar +0.05 +0.08 +0 09 +0.09

90 +0.08 +0. 12 +0.14 +0.15 +0.17 +0.10 +0.16 +0 14 +0.13 +0.12

10 -0.07 -0. 02 -0.01 +0.02 0.00 -0.02 -0.04 0 00 J-A +U Ub +0.02

8 30 -0.12 0. 00 0.00 +0. 04 +0. 06 +0.07 -0. 03 -0.12 _lA +0 AOU3 +0.14

90 +0.01 +0.09 +0.09 +0.09 +0.11 +0.14 +0.07 +0 10 +0 10 +0.10

10 -0.12 -U. u/ -0.04 -0.02 -0.04 -0.05 -0.05 A-u U 1 A-u

A/1 -0.03

9 30 -0. 28 +u. uo +0. 04 +0. 02 -0. 06 +0.19 -0.14 +0 12 +U i A OA +0. 22

90 +0.12 +0. 09 +0.13 +0.13 +0.21 +0.24 r0.18 +0 22 +0 17 +0.18

10 -0.04 -0. 01 -0.01 +0.04 +0.17 -0.05 +0.11 +0 06 +uj.n

1 C +0.20

10 30 -0.13 -0. 04 -0.04 +0. 04 +0. 01 -0.01 i a ai +0. 01 +0 02 +0. 08

90 -0.09 +0. 03 +0.10 +0.12 +0.12 +0.08 +0.09 +0 07 +0 06 +0.10

10 -0.05 +0. 01 +0.02 +0.02 +0.03 +0.09 -0.03 +0. 02 +0.11

11 30 a AO -0. 08 +0. 05 a no -0. Oil a ac -0. 0b n no -0. 13 A TO -0. Ij A AC -0. Ob +0 06 A-u

,

A A A 0.00

90 +0.12 +0. 13 +0.10 +0.13 +0.10 +0.16 +0.17 +0 12 +0 19 +0.17

10 +0.09 +0. 04 +0.07 +0.03 0.00 +0.03 +0.01 +0 03 +fl u / +0.03

12 30 a An-u. uy -0. 10 _i_n no +U. Uc _i_n no +1). (Jo A 1 O -U . lo A TO -U . \c A OO -U. c6 +0 04 u

,

1 0

_i_n o o +U . c.6 90 -0.02 +0. 09 +0.10 +0.08 +0.09 +0.12 +0.08 +0. 10 +n 1 4 +0.11

10 -0.07 -0. 04 -0.06 -0.02 -0.04 -0.08 -0.03 -0. 02 +0 02 -0.07

13 30 A 1/1 -0. 14 +0. 02 -0.01 a m +0. 09 n An U. OU A AA 0. 00 A Al -0.01 -0. 02 -0 02 _i_A AO +0 . 08

90 -0.18 -0. 08 +0.03 +0.04 +0.12 +0.14 +0.08 +0.16 +0 13 +0.09

10 +0.01 +0.02 +0.03 +0.01 +0.02 +0.01 +0.02 +0 02 +0. 02 +0.04

14 30 -0.13 -0. 10 -0.03 +0.01 +0.06 -0.17 0.00 -0. 01 +0. 04 +0.10

90 -0.03 0. 00 +0.08 +0.09 +0.10 +0.10 +0.10 +0. 10 +0 13 +0.10

'2 1 -0.05 -0. 01 +0.02 +0.04 +0.03 +0.03 +0.04 +0. 12 -0 01 +0.02

15 30 j -0.11 +0.02 -0.02 -0.03 -0.01 -0.04 -0.02 +0 08 +0 01 +0.04

90 +0.15 +0. 15 +0.14 +0.06 +0.16 +0.17 +0.11 +0. 19 +0 19 +0.15

10

16 30

90

-0.06

-0.09

-0.01

-0.03

+0.06

+0.05

-0.09

+0.02

+0.06

-0.01

+0.11

+0.07

-0.04

-0.05

+0.08

+0.04

0.00

+0.09

-0.02

-0.05

+0.11

+0.10

+0.02

+0.03

+0.10

-0.01

+0.07

+0.12

+0.03

+0.07

10

17 30

90

-0.06

-0.02

+0.13

-0.02

+0.04

+0.11

-0.02

+0.05

+0.12

+0.05

+0.06

+0.11

0.00

+0.02

+0.15

-0.02

+0.06

+0.07

-0.02

+0.05

+0.21

+0.01

+0.09

+0.14

-0.04

+0.09

+0.18

+0.21

+0.05

+0.15

10

18 30

90

10

19 30

90

10 20 30

90

0.00

0.00

+0.11

-0.10

-0.14

+0.09

-0.05

+0.15

-0.09

+0.02

+0.01

+0.12

-0.03

+0.16

+0.10

-0.03

+0.18

-0.01

+0.04

0.00

+0.13

0.00

+0.43

+0.08

-0.02

+0.14

+0.01

+0.04

+0.05

+0.10

+0.02

0.00

+0.09

+0.03

+0.15

+0.14

+0.05

+0.03

+0.14

-0.01

-0.09

+0.10

-0.09

+0.08

+0.07

+0.03

+0.01

+0.16

+0. 02

-0.17

+0.14

-0.12

+0.06

+0.08

+0.04

0.00

+0.12

-0.15

-0.24

+0.15

-0.07

+0.08

+0.04

+0.05

+0.02

+0.06

+0.11

-0.18

+0.17

-0.03

+0.05

+0.07

+0.05

+0.04

+0.06

+0.01

+0.08

+0.14

-0.07

+0.11

+0.06

+0.09

+0.05

+0.12

-0.19

+0.11

+0.17

-0.02

+0.10

+0.04

39
Table 14. Summary of the percent transmittance difference, AT%, measured after a time

interval of two years on 19 sets of chromium-on-fused silica filters with

nominal transmittances (T) of 10, 30, and 90 percent.

Ultraviolet, A250 nm to A400 nm

Filters^v.

U . JU 1 . UU UVci 1 . UU Increase

f -

Decrease

ilii

No Change

T

10% 12 7 None 12 (21.05%)41 (71.93%)

4 (7.02%)

30% 17 1 1 26

(45.61%)

27 (47.37%)4 (7.02%)

90% 19 None None 46

(80.70%)

10 (17.54%)1 (1.75%)

Visible, A400 nm to \635 nm

10% 7 12 7 * 88

(66.16%)

36

(27.07%)

9 (6.77%)

30% 15 4 1 * 80

(60.15%)

45 8 (33.83%) (6.02%)

90% 19 None None 127

(95.49%)

5 1 (3.76%) (0.75%)

*In this case, several filters exhibited transmittance differences between 0.5 and 1.0 and over 1.0 as well. This accounts for a total larger than 19 filters.

NOTE: Each of the 19 sets contains 3 filters (57 filters), and the transmittance of each

filter was measured at 3 wavelengths in the ultraviolet (171 measurements) and 7 wavelengths in the visible (399 measurements) for a total of 570 measurements

(171 + 399 = 570).

4.5 Conclusions from the Time Stability Tests

From these data it can be inferred that, in general:

- The relative transmittance differences are smaller for the nominal 90 and 30 percent

filters (Table 13A), while the opposite is true when the transmittance differences are expressed on an absolute basis (Table 13B).

- The changes in transmittances are larger in the visible region than in the ultraviolet. This was a surprising result because it was believed that the measurements in

the ultraviolet would be more likely to be unstable.

- All of the nominal 90 percent filters showed only small changes in transmittance,

well below ±0.5 percent, with a tendency toward increasing transmittance, especially

in the visible (Table 13A).

- The nominal 30 percent filters (Table 13A) showed good stability in the ultraviolet. Only two measurements indicated a transmittance change greater than ±0.5

percent. The same 30 percent filters exhibited a larger change in the visible. Five

measurements showed a transmittance difference of more than ±0.5 percent. About an equal number showed both a decrease and an increase of transmittance in the ultraviolet, while in the visible, the majority of changes are toward an increase of

transmittance.

40
- The nominal 10 percent filters showed a larger change in transmittance (Table 13A).

Seven measurements showed a transmittance difference larger than ±0.5 percent in the

ultraviolet, nineteen measurements showed in the visible differences in excess of

±0.5 percent. For these filters, the tendency is to show a decrease of transmittance

in the ultraviolet. In the visible, the majority of changes show an increase in

transmittance.

4.6 Issuance of SRM 2031, Description of the Standard Reference Material,

and Certificate

Based on the results just described, it was decided to issue the evaporated chromiumon-fused silica filters as a Standard Reference Material for transmittance measurements in

the ultraviolet and visible spectral regions.

Each set will be "aged" in the laboratory for at least six months. The transmittance

will be measured at the beginning and end of the aging period. Only those filters that

exhibit satisfactory stability will be issued as SRM 2031. The transmittance data will be

certified at the time of the measurements with an uncertainty of ±1.0 percent (relative)

for the nominal 10 percent filters, and ±0.5 percent (relative) for the nominal 30 percent

and 90 percent filters. Because of only two years of experience with this material, it is

suggested that SRM 2031 be remeasured on a yearly basis, and more often if the user has a reason to believe that a loss in transmittance integrity has occurred.

In addition it must be also stated that, after two years, none of the filters showed

deterioration of the optical contact when stored and used as described in this work.

To reduce further the possibilities of undue changes in the certified transmittance

values, accidental "fingerprinting", and mechanical damages, each filter is mounted in an individual filter holder made from black-anodized aluminum alloy provided with removable

shutters as illustrated in Figures 9 to 13 and 16 . The same figure also shows the

aluminum alloy container with four cylindrical cavities for the three filters in their

holders and an empty filter holder. The container is closed with an aluminum alloy screw

cap. The empty filter holder is used in the reference beam of the sample compartment of

the spectrophotometer to produce an equivalent stray radiation condition.

All aluminum parts of the filter holders are black-anodized to reduce isochromatic

stray light during the measurements. The cylindrical container is also anodized black

since it was found that the outgassing of this coating over the aluminum alloy is negligible

as demonstrated by T. Heslin and R. Hunkeler [18]. They showed that there is little con- tribution to outgassing from the dyes used to produce the black anodization, and that the

evolution of the material which is condensible at 25 °C is minimal.

Under these conditions, little or no chance exists that some volatile products from

the filter holders and their container will contaminate the filters. The metallic storage

container provides not only protection from mechanical damages, but also minimizes contamination of the filters from particulate matter due to static charges. The cylindrical cavity

also prevents any contact between the filter surface and the walls of the storage container.

To prevent damage to the filters during transportation, four flat leaf springs stabilize the filter holders in their cylindrical cavities. These springs should be removed

upon receipt of the material. Further information on SRM 2031 can be found in the Certificate for this material which is reproduced at the conclusion of this work.

41
Figure 16. Illustration of SRM 2031, Metal -on-Quartz Filters for Spectrophotometry. This

same figure illustrates the certificate for SRM 2031. See text for details.

NOTE: The filter is shown here with its front surface facing up and the back surface facing

down. The filter, in its holder, should be placed in the cuvette compartment of the

spectrophotometer with its front surface facing the incident light beam and the back

surface facing the photodetector.

42
I U.S. Department of Commerce

.luanita M. Kreps

Secretary

|

N'£pr' Rational JBu«au of jltandartfe

Certificate

Standard Reference Material 2031

M et al-o n-Q uartz Filters f or S pectrophotometry

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended for use in the verification of the transmittance and absorbance

scales of spectrophotometers in the ultraviolet and visible spectral regions. It consists of three individual filters in their metal holders and one empty filter holder. Each filter is made from two non-fluorescent fused silica plates, one of which carries a semi-transparent

evaporated chromium metal layer while the other is clear. The two plates comprising the filter are assembled by

optical contact. The chromium coating on two of the filters produces nominal transmittances of 10 percent and

30 percent. The third filter is made from assembling two clear fused silica plates by optical contact, and has a nominal transmittance of 90 percent. The metal holders for these filters are provided with shutters to protect the

filters when not in use. These shutters must be removed at the time of measurement and be replaced after the measurements have been completed. Each filter bears an identification number.

1 TRANSMITTANCE (T)

Filter & Set

Ident. Number

Wavelength, nm

250.0 280.0 340.0 360.0 400.0 465.0 500.0 546.1 590.0 635.0

Filter & Set

Ident. Number

TRANSMITTANCE DENSITY (-logioT)

Wavelength, nm

250.0 280.0 340.0 360.0 400.0 465.0 500.0 546.1 590.0 635.0

Date of Certification:

Washington, D.C. 20234 George A. Uriano, Chief June I, 1979 Office of Standard Reference Materials

(over)

43
The transmittance values (T) can be converted to percent transmittance (%T) by multiplying by 100. The

transmittance densities are calculated from the measured transmittance (T). The transmittance densities should

be indicated by the absorbance scale of the spectrophotometer if the filters are measured against air. The

transmittance values given were measured against air at an ambient temperature of 23.5 °C. The transmittance values for the filter with a nominal transmittance of 10 percent are estimated to be accurate to within ± 1.0 percent at the time of certification. The transmittance values for the filters having a nominal

transmittance of 30 percent and 90 percent are estimated to be accurate to within ±0.5 percent at the time of

certification. This uncertainty includes the effects of random and systematic errors of the calibration procedure,

as well as possible transmittance changes of the filters during the period of calibration. Aging of the material may cause some filters to change transmittance with time. Improper storage or accidental touching of the filter surface may also cause changes.

When not in use, the filters should be stored in their holders with the shutters in place and in the metal container provided for this purpose. Extended exposure to laboratory atmosphere and dirty surroundings should be

avoided. In cases where verification is desirable, the filters should be returned to the National Bureau of Standards for cleaning and recalibration. It is advisable to perform the verification on a yearly basis. The transmittance measurements were made using the high-accuracy spectrophotometer designed and built at the NBS Center for Analytical Chemistry [

1]. This instrument represents the primary transmittance standard;

its transmittance accuracy was established using the double-aperture method of linearity testing [1,3,5,6]. The

estimated standard deviation obtained from a set of 20 measurements of the high-accuracy spectrophotometer

used for transmittance measurements on filters of the SRM 2031 type is 5 x 10

5 transmittance units for transmittance between 0. 1 and 0.3. The bounds of the systematic error of the instrument are estimated to be ± 10 transmittance units. The transmittance, T, was measured against air in the reference beam; hence it includes the reflection losses that occur at the air-filter interface. Under these circumstances the measured transmittance, T, has a corresponding

transmittance density, defined as -logio T. The internal transmittance, Tj, of a material is defined as the transmittance of the material corrected for reflection losses. This is obtained experimentally when the measure- ments are made against a blank sample in the reference beam. The absorbance, A, of a material is related to this internal transmittance, T, by the expression, A = -logioT. The transmittance measurements are made by

producing the vertical image of the slit (about 8 mm by 0.5 mm), using a convergent beam geometry with an

aperture ratio f: 1 0, in the middle of the entrance face of the filter. The filters were measured in the spectrophotometer in a position perpendicular to the incident light beam.

The metal-on-quartz filters which constitute this SRM were produced according to NBS specifications by

Cosmo Optics, Inc., Middletown, New York. The quartz plates were produced from non-fluorescent fused silica material of optical quality, ground and polished to produce an optical contact. The metal used to produce, by

evaporation,the semi-transparent metal coating was chromium, obtained through the van Arkel process, and

has an indicated purity of 99.998 percent. It was provided by The Atomergic Chemetals Co., Plainview, New

York. The fused silica plates were ground and polished at the same time and on the same grinding wheel, each to the nominal dimensions: 1.5 mm thick, 30.5 mm long, and 10.4 mm wide. The exposed surface of each filter is approximately 29 x 8 mm, measuring from a point 1.5 mm above the base of

the filter holder (see figure). The empty filter holder provided is to be used in the reference beam of the spectrophotometer so that approximately equivalent conditions of stray radiation are achieved for both beams. The transmittance of the filters depends upon the intrinsic properties of the material, wavelength, spectral

bandpass, geometry of the optical beam, temperature, and positioning of the filter. A change of ambient

temperature of I or 2 °C from 23.5 °C will not significantly affect the calibration [5]. Changes in the transmittance may be caused by surface conditions, aging of the material, exposure to a harmful atmosphere, or

careless handling [2,3,4,5]. SRM 2031 is stored in a black anodized aluminum container provided with a threaded cap made of the same

metal. Each filter is placed in a cylindrical cavity to prevent any contact between the filter face and the walls of

the storage container. Contamination of the filter surface with particulate matter due to static charges is minimized through the metallic nature of the container. A flat leaf spring is inserted into the cylindrical cavity with each filter holder to minimize damage during transportation. These springs can be removed during normal

use in the laboratory.

44
The filter is shown in the assembled unit with its front surface facing up. The filter, in its filter holder, should be

placed in the cuvette compartment of the spectrophotometer with its front surface facing the incident light beam

and the rear surface facing the photodetector.

Top: Cylindrical container with its screw cap, both made of black-anodized aluminum alloy. Four filter holders can be stored in the cylindrical container. Bottom (from left to right): Aluminum alloy filter holder, 12.5 mm

square and 58 mm high; Metal-on-quartz filter; Retaining spring of beryllium-copper with nylon screw and

washer; Assembled unit; and Two Delrin shutters. All metal and plastic parts are fiat black.

45
Prior to certification measurements, each filter was examined for surface defects and the condition of the optical contact [5]. Should the surface of the filter become contaminated, no attempt should be made to clean it unless the user has the facilities to demonstrate that the cleaning treatment will not alter the surface or degrade the accuracy of the certified values. As SRM 2031 is a transfer standard, the only means available to verify its integrity is to remeasure its transmittance with a primary standard instrument similar to that used in this certification [ 1 ,5]. In most cases, where verification or recertification of the transmittance values is desirable, it will be most expeditious to return the filters to the National Bureau of Standards for measurement. Prior to shipment, the NBS Office of Standard Reference Materials should be contacted regarding the conditions and

cost of verification and /or recertification.

Further information concerning the selection, preparation and properties of SRM 2031 will be found in reference 5.

This Standard Reference Material was issued with the assistance and advice of K.D. Mielenz and I.L. Barnes. 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 R.W. Seward.

We wish to acknowledge the cooperation of George N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, Connecticut; Royden N. Rand, Ph.D., of the Eastman Kodak Co. Research Laboratories, Rochester, New

York; and Donald S. Young, M.D., Ph.D., of the Mayo Clinic, Rochester, Minnesota.

The contributions of H.E. Bennett and J.M. Bennett of the Michelson Laboratory, Physical Optics Branch,

Naval Weapons Center, China Lake, California, who helped in the production of SRM 2031 are also gratefully

acknowledged.

References

1. R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and 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, O. Menis and J.I. Shultz, Eds., 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. R. Mavrodineanu, Considerations for the Use of Semi-transparent Metallic Thin Films as Potential Transmittance Standards in Spectrophotometry. NBS Journal of Research 80A, No. 4, 637-641 (1976).

5. R. Mavrodineanu and J.R. Baldwin, Metal-on-Quartz Filters as a Standard Reference Material for Spectrophotometry, SRM 2031. NBS Special Publication 260-68, U.S. Government Printing Office,

Washington, D.C. 20402, 1979.

6. K.D. Mielenz and K.L. Eckerle, Spectrophotometer Linearity Testing Using the Double-Aperture Method,

Appl. Optics 11, 2294-2303 (1972).

46
5. Summarizing Remarks

To achieve meaningful measurements by spectrophotometry requires that every step of

the chemical processes involved in the analytical procedure, and every parameter of the

instrumental spectrophotometry technique be well-known and under the complete control of

the analytical chemist. He must assess the importance of these two factors in a quantitative manner, and evaluate their individual contribution to the uncertainty of the final

measurement.

Instrumental stability, wavelength accuracy, adequate spectral bandpass, freedom of

stray light, and transmittance accuracy are basic parameters that play a determinant role

in the accurate transmittance measurements of a transparent material in the spectral range

of interest. Without achieving these conditions, no accurate transmittance measurements

can be performed. SRM 2031 appears to be the means for achieving instrumental accuracy

in the ultraviolet and visible spectral regions with little spectral bandpass limitations.

While the use of SRM 2031 will not solve all the problems in spectrophotometry, its

availability should contribute to the production of more accurate and more meaningful

measurements in all areas where spectrophotometry is applied.

The production of accurate chemical analytical data, that is, the capability to determine accurately the amount of a particular chemical element i

a given matrix , requires

spectrophotometric precision, as determined by the instrumental stability and sensitivity.

The accuracy of the analytical measurements is determined by the accuracy achieved in the

preparation of the analytical samples and in the establishment of the reference curve. These depend on the stability of the chemical reactions involved in the spectrophotometric

process, and on the ability to prepare accurate reference solutions that have accurately

known concentrations of the chemical element of interest in a matrix identical to that of

the analytical sample. Under these conditions, analytical accuracy can be achieved with

spectrophotometers that exhibit 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 can affect the sensitivity and linearity, and in some cases, the specificity of spectrophotochemical analyses, rather than their chemical accuracy.

Standard Reference Material SRM 2031, Metal -on-Quartz Filters for Spectrophotometry,

is a transfer standard that is calibrated for transmittance in the spectral range of 250 nm

to 635.0 nm, with a high-accuracy spectrophotometer which represents the primary transmittance standard. The transmittance values are certified with an uncertainty of 1.0 and 0.5

percent. The validity of these transmittance values with time will depend on the stability

of the material and on the conditions under which this SRM is used. The measurements from

Table 12 verify that the stability of the certified transmittance values for set No. 4 are reproducible, within the uncertainty indicated, over a period of two 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 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 in the metal container provided for the purpose. Should the surface of the filters

become accidentally contaminated, no attempts should be made to wash them, if the user does

not have means to determine whether the cleaning treatment has not altered the properties of

the material and degraded the accuracy of the certified transmittance values. Since SRM

2031 is a transfer standard, the only means available for determining its integrity is by

remeasuring its transmittance with a primary standard instrument. 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, SRM 2031 should be returned to

NBS for verification, and for cleaning and recal ibration, if necessary. As the transmittance stability of SRM 2031 was studied over a time period of two years only, it is

recommended that this verification be performed on a yearly basis. Before sending SRM 2031 to NBS, proper arrangements should be made by writing the Office of Standard Reference

Materials for instructions.

47
SRM 2031 is a delicate and costly unit that has required a considerable amount of

careful work in its preparation and certification. This material should be handled care- fully to insure accurate measurements and long life. This is particularly true when the

working conditions require frequent use of SRM 2031. To protect this reference material,

it might be desirable to develop "in house" secondary standards based on SRM 2031, and use these secondary standards for daily control of the spectrophotometers. When these standards

indicate a significant change in the measurements, only then should SRM 2031 be used as a final verification of the data.

Secondary metal -on-quartz standards can be produced by acquiring the material used at NBS for the production of SRM 2031, cut to size as described in this publication and placed

in holders similar to those illustrated in detail in Figures 9, 13, and 16. The transmittance of these filters should be established at the desired wavelengths with the best

spectrophotometer available in the laboratory after its transmittance scale is verified

with SRM 2031.

The certification of SRM 2031 was performed at 10 predetermined wavelengths: 250,

280, 340, 360, 400, 465, 500, 546.1, 590, and 635 nm. Certification at other wavelengths,

within the spectral range of A.200 nm to A800 nm, can be provided. Requests for additional

wavelength certification should be addressed to the Office of Standard Reference Materials,

National Bureau of Standards, Washington, D.C. 20234.

The authors gratefully acknowledge the assistance of the following persons and

organizations:

- I. L. Barnes, Chief, K. D. Mielenz and R. W. Burke, Inorganic Analytical Research

Division; K. L. Eckerle, Radiometric Physics Division; J. F. Barkley and F. C. Ruegg,

Instrumentation Development Group, J. R. DeVoe, Leader; R. W. Seward, Office of

Standard Reference Materials; E. P. Muth, and R. E. Lach of the Instrument Shops

Division, at NBS.

- H. E. and J. M. Bennett of the Michel son Laboratory, Physical Optics Branch, Naval

Weapons Center, China Lake, California.

- J. I. Shultz, NBS/ASTM Research Associate, National Bureau of Standards, Washington,

D. C.

- F. J. J. Clarke, National Physical Laboratory, Teddington, Middlesex, U. K.

- J. H. Lady, Tennessee Eastman Co. , Kingsport, Tennessee.

- R. N. Rand, Eastman Kodak Co., Rochester, New York.

- Bausch and Lomb; Beckman Instruments; Cary-Varian; Coleman, a Division of PerkinElmer; Gilford Instruments Labs, and McPherson Instruments.

- The National Institute of General Medical Sciences for supporting, in part, the

work reported in this publication.

- The Center for Analytical Chemistry Text Editing Facility for typing the

manuscript.

48
6. References

[I] Mielenz, K. D. , Anal. Chem. 48, 1093-1094 (1976), included in Appendix I.

[2] Mavrodineanu, R. , in Tech. Note 584, 9-21 (1970-1971), and J. Res. National Bureau of

Standards, (U.S.) 76A (Phys and Chem.), No. 5, 405-425 (Sept. -Oct. 1972), included in

Appendix II.

[3] Rand, R. N. , Clin. Chem. 15, 839-863 (1969).

[4] Mavrodineanu, R. and Baldwin, J. R. , NBS Spec. Publ. 260-51, 118 pgs. , 1975.

[5] Burke, R. W. , Deardorff, E. R. , and Menis, 0., J. Res. National Bureau of Standards

(U.S.) 76A (Phys. and Chem.), 469-482 (Sept. - Oct. 1972).

[6] Mavrodineanu, R. and Lazar, J. W. , NBS Spec. Publ. 260-32, 27 pgs., 1973.

[7] Burke, R. W. and Mavrodineanu, R. , NBS Spec. Publ. 260-54, 157 pgs., 1977.

[8] Mavrodineanu, R. , in NBS Tech. Note 544, 12-17 (Sept. 1970).

[9] Mielenz, K. D. , Mavrodineanu, R. , and Cehelnik, E. D. , J. Res. National Bureau of

Standards (U.S.) 78A (Phys. and Chem.) No. 5, 631-635 (Sept. - Oct. 1974), included

in Appendix III. See also: Applied Optics, 14, 1940-1947 (1975); and U.S. Patent

3,968,363, July 6, 1976.

[10] Mavrodineanu, R. , J. Res. National Bureau of Standards (U.S.) 80A (Phys. and Chem.)

No. 4, 637-641 (July - Aug. 1976), included in Appendix IV.

[II] Barkley, J. F. , Ruegg, F. C. , and Mavrodineanu, R. , paper No. 452 presented at the

29th Pittsburgh-Cleveland Conference of Analytical Chemistry and Applied Spectroscopy,

Feb. 27 to Mar. 3, 1978.

[12] Mielenz, K. D. and Mavrodineanu, R. , J. Res. National Bureau of Standards (U.S.) 77A , No. 6, 699-703 (Nov. - Dec, 1973), included in Appendix V.

[13] Mielenz, K. D. and Eckerle, K. L. , NBS Tech. Note 729, 60 pgs., 1972.

[14] Mielenz, K. D. , Eckerle, K. L. , Madden, R. P., and Reader, J., Applied Optics 12,

No. 7, 1630-1641 (1973).

[15] Eckerle, K. L. , NBS Tech. Note 913, 43 pgs. , 1976.

[16] Clarke, F. J. J., Downs, M. J. and McGivern, W. , UV Spectrometry Group Bulletin No. 5,

104-108 (1977).

[17] Clarke, F. J. J. , NBS Spec. Publ. 378, 1-29 (1973).

[18] Heslin, T. and Hunkeler, R. , Material Technology Report MTR-No. 755-011, 11 pgs.,

Apr. 1975, Goddard Space Flight Center.

7. Appendices

These appendices contain reproductions from five publications in the field of highaccuracy spectrophotometry that are considered relevant to the topics discussed in this

work. These publications should provide further and more detailed information on specific

subjects and are reproduced here as a convenience to the interested reader.

Also reproduced in these appendices are the certificates issued by NBS for the SRM's

930D, 931b, 2030, 932, and 935.

49

APPENDIX I

Reprinted from ANALYTICAL CHEMISTRY, Vol. 48, Page 1093, June 1976

Comments on Spectrometry Nomenclature

Sir: As an optical physicist who collaborates with analytical chemists in the development and use of absorption and

luminescence spectrometers, I have become increasingly aware of the fact that our vocabularies are not always the same. Although some of these differences have been point- ed out before (i) it seems worthwhile to familiarize the

readers of this journal with the currently prevailing termi- nology of physical optics. Perhaps the following comments may be helpful to editors, authors, and nomenclature committees in establishing a common and consistent terminol- ogy that can be used in all branches of spectrometry and

would save readers a good deal of puzzlement and reading between lines. The main purpose of these comments is to emphasize the need for interdisciplinary efforts in defining spectrometry nomenclature without perpetuating the in- consistencies that exist at present. They do not represent an official position of the National Bureau of Standards. The basic concept of modern optical terminology (2, 3) is to combine nouns and adjectives in order to describe quantities and properties as explicitly as necessary, rather than

using glossaries of short names for them. This "Chinese

Restaurant" method of nomenclature (4) offers flexibility in introducing new terms, and has recently been extended

to include the photon quantities used in photochemistry and similar disciplines dealing with the interaction of light and matter (5). The basic list of nouns describing the transport of energy

according to the laws of geometrical optics is: Energy, Q. Energy density, u = dQ/dV. Energy per unit volume.

Flux, 4> = dQ/dt. Time rate of energy flow. Flux density, W = d$/dA. Flux per unit area. Intensity, I = d<f>/dfi. Flux per unit solid angle. Sterance, L = d

2 */d(Acos9)dfi. Flux per unit projected area and unit solid angle. (A number of alternative and additional terms have been

proposed. For example: pointance to replace the continually misused term intensity; incidance and exitance, or inci- dent areance and (transmitted, emitted, etc.) areance for the flux densities arriving at and leaving a surface; steri- sent for the sterance generated per unit path length by

emission or scattering into the direction of propagation; and fluence for the surface energy density incident upon a volume element irradiated from within a large solid angle

of rays (F. E. Nicodemus, private communication). As these have not yet been accepted generally, they were not includ- ed here. However, the new term sterance was included as it avoids the misnomer photon radiance that has appeared in papers on luminescence spectrometry.) These nouns are modified by the adjective radiant, and

the above symbols are written with a subscript 'e' (for energy), when radiometric units are used. The modifiers lumi- nous and photon are used in conjunction with subscripts

V (for visual) and 'p' (for photon) to indicate the use of photometric and photon units, respectively. For example: Radiant energy density, u e [J m~3

]. Luminous intensity, /„[lm sr-1

J.

Photon sterance, LP [E m

-2

sr _1

s-1

]. (The basic unit for photon energy used here is the einstein

[E], defined as one mole of photons. It is not an SI unit, but

is used so extensively in photochemistry and photobiology

that its acceptance as a supplementary SI unit may be desirable.) The additional modifier spectral and subscripts X and a

are used to denote derivatives of radiometric and photon

quantities with respect to wavelength and wavenumber, re- spectively. Thus:

Spectral radiant energy density, ue ,x = du e/dX [J m

-4

]. Radiant energy density per unit wavelength interval. Spectral photon intensity, Ipa = dlplda [E sr _1

s _l m]. Photon intensity per unit wavenumber interval. Any of these can be modified further; such as: fluorescence photon flux, transmitted spectral radiant flux density, or absorbed luminous energy. Usually, it is possible to drop most of the modifying adjectives as well as the subscripts e, v, or p, once the context has been clearly established or whenever a distinction is not necessary. The same method of nomenclature also provides a simple and logical way of specifying the precise meaning of the

quantities and material properties commonly measured in analytical spectrometry. Thus radiant absorptance ae and

photon absorptance ap should be used for the ratios of the

radiant or photon fluxes absorbed by a sample to those in- cident upon it, when measured with a large bandwidth so that these two ratios are not the same. On the other hand,

spectral absorptance a(X) is sufficiently accurate in the

case of measurements made with a small bandwidth. Similarly, luminescence yields (the ratios of the radiant or photon fluxes emitted by a sample to those absorbed by it) should be designated as radiant yield 7j e or photon yield tj p (not energy yield or quantum yield). In this case, spectral radiant yield, r)e< „ = driJda, and spectral photon yield, ij Pi „ = d7)p/der, are different quantities even in the limit of infi- nitely narrow bandwidths, and thus should be referred to by these names.

[The dependence on wavelength or wavenumber is indi- cated by a subscript (t) p

,<,) when the spectral distribution is defined as a derivative, but in functional form [a(X)] in the

case of spectral distributions that are not derivatives. This

is an important distinction that must also be borne in mind

in the presentation of spectra. The positions of the peaks and valleys in derivative spectra depend on the units used,

so that these should not be published in the form of a single graph with dual scales (such as wavelength and wavenumber, or spectral radiant and photon yield).] In addition to this general description of current optical terminology, the following comments are made in direct reference to the nomenclature list in the December 1975

issue of Analytical Chemistry (6).

1) This list cautions not to use optical density instead of absorbance, but defines the latter as "the logarithm to the base ten of the reciprocal of transmittance"—which is the

definition of optical density. Internal transmittance

should have been substituted for transmittance in this defANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976 • 1093

51
inition of absorbance. The International Lighting Vocabulary suggests transmission density and internal transmission density instead of optical density and absorbance. This is more determinative, and also fits into a general scheme where reflection density is defined as the negative

logarithm to the base ten of reflectance. On the other hand,

transmission and reflection density are operational quantities that merely express measured data on a logarithmic

scale, whereas absorbance is directly related to molecular

constants through the Lambert-Beer and Strickler-Berg

equations. Thus, in my opinion, it should be retained as a separate term. I believe that all confusion would be avoided by defining: Transmittance, t. Ratio of the flux transmitted by a sample to the flux incident upon it. Internal transmittance, t\. Transmittance exclusive of

losses at boundary surfaces and effects of interreflec- tions between them. Transmittance density, D = —logior. Negative loga- rithm to base ten of transmittance. Absorbance (Internal transmission density), A = —logioTj. Negative logarithm to base ten of internal transmittance.

2) The definitions of absorptivity in chemistry and physics are not the same. In chemistry, it means absorbance per

unit path length and unit concentration (A/bc), whereas

the International Lighting Vocabulary (2) defines it as in- ternal absorptance per unit path length (AaJ&b). Similarly,

it specifies transmissivity as internal transmittance per

unit path length and reflectivity as the reflectance of a thick layer (so that a further increase in thickness will no

longer change its value). Generally, terms ending in -ance

represent sample properties, whereas terms ending in -ivity denote material properties that are independent of sample

geometry. The quantity A/bc does not fall in this latter cat- egory as it is also independent of sample concentration. The International Union for Pure and Applied Chemistry,

apparently aware of this discrepancy, has suggested calling

it absorption coefficient. However, as this name has been

given different and mutually inconsistent meanings in the

past, a less ambiguous word—perhaps specific absorbance—

would have been better. Therefore it is proposed to define: Absorptivity, a = da;/d6 [m-1

]. Internal absorptance per

unit path length. Specific absorbance, e = A/bc [kg-1 m2

]. Absorbance per

unit path length and unit concentration.

Specific molar absorbance, tm = A/bc m [mol-1 m2

]. Absorbance per unit path length and unit molar concentration.

3) The December 1975 nomenclature list also implies that the Beer-Lambert laws are the same. The correct defi- nitions are: Beer's law: Absorbance is proportional to concentration. Lambert's law: Absorbance is proportional to path

length. Also called Bouguer's law.

4) The definition of the angstrom unit in terms of the red line of cadmium has been abrogated several years ago.

It is now defined as 10-10 m, exactly. However, the ang- strom is not an SI unit, and has been sanctioned by the In- ternational Committee on Weights and Measures only as a supplementary unit that will eventually be abandoned (7). Therefore, authors should be encouraged to use micrometers or nanometers.

5) The December 1975 list defines spectrometry as the "measurement of spectra", but restricts the meaning of spectrometer to "instrument with an entrance slit, a dis- persing device, and with one or more exit slits . . .". As this excludes non-dispersive and slit-less instruments, such as Fourier and Girard spectrometers, it would be more consis- tent to call any instrument used for spectrometry a spectrometer. Therefore: Spectrometer: Instrument for the measurement of spectra. The general term for instruments that measure spectral distributions of radiometric quantities is spectroradiometer. However, as this implies measurements in absolute

units, spectrometer is a better term for the simpler, usually ratio-forming instruments used in analytical spectrometry. Thus, a spectrophotometer could also be called absorption

spectrometer, and fluoroescence spectrometer would end

the controversy of spectrofluor imeter vs. spectrofluor ometer.(Strictly speaking, the word spectrophotometer is a mis- nomer. A photometer is an instrument that measures lumi- nous flux in lumens. Since the adjective luminous implies the integral effect of visual radiation as perceived by the human eye, the spectral analysis of luminous flux has no

physical meaning. However, in view of the firmly established meaning of spectrophotometer, it is not suggested to change it, although a scanning of the recent literature shows an increasing usage of absorption spectrometer.)

LITERATURE CITED

(1) G. A. W. Rutgers, in "Accuracy in Spectrophotometry and Luminescence

Measurements." Nat. Bur. Stand. (U.S.), Spec. Publ. 378, 53 (1973).

(2) "International Lighting Vocabulary", 3rd ed.. Publication CIE No. 17,

Paris, 1970.

(3) Editor's Page, J. Opt. Soc. Am., 57, 854 (1967).

(4) J. Geist and E. Zalewski, Appl. Opt., 12, 435 (1973).

(5) J. J. Murray, F. E. Nicodemus, and I. Wunderman, Appt. Opt., 10, 1465

(1971).

(6) "Spectrometry Nomenclature", Anal. Chem., 47, 2526 (1975).

(7) "The International System of Units (SI)", Nat. Bur. Stand. (U.S.), Spec.

Publ. 330 (1971). Klaus D. Mielenz

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

RECEIVED for review September 26, 1975. Accepted De- cember 23, 1975.

1094 • ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976

52
APPENDIX II

JOURNAL OF RESEARCH of the National 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 fee 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-photomultiplier 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.

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 is defined as the transmittance of the material 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 collimated radiation the reflectance R, for a material with an index of refraction, n, and an absorptivity, a, at wavelength, A, is given through:

R _ (n,-l)' + nl aj

* (ru+D' + nj al For a nonabsorbing material and collimated radiation:

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 noncollimated radiation is used:_ sin* (a-/3)» R*~ sin" (a + 0)»

for perpendicular polarized radiation, and

tan1 (o-fl)> «"

k = 5 — tarf (a + <3)»

for parallel polarized radiation, where a and 0 are the angles of incidence and refraction, respectively.

In coUimated radiation and in air, a = $ = 0 and R1 = f?" = RTransmittance 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 collimated 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

2 Figures in brackets indicate the literature references at the end of this paper. 405

53
of the spectrophotometer is calibrated in meaningful

units, using a series of refr*"pnp» 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 spectrophotometric 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 1 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

1 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 1

sections.

a. The Radiation Source. Since the instrument is a single-beam type, it is essential that the radiation source be constant and homogeneous. Additional

desirable conditions are: capability of monitoring the current supplied to the source and radiation similar i 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- I 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

1 voltage modes. To achieve the constant current mode

an external sensing resistor of 0.1 0 and 50 A and 1 a current control circuit are placed in series with the power supply. A feedback voltage across this resistoi

is connected to the sensing system. The character

3 The commercial instruments and parts used in the construction of the spectrophotom

eter are identified in the addendum.

406

54
Precision Optical Bench

in sims mi

iiii-igcviict sriCTiifitTiaiTii

1m Grating Monochromator (Czerny-Turner)

Precision Optical Bench in Lightproof Enclosure

Figure 1. Principle of the single beam high accuracy spectrophotometer. The aperture unit is placed on the optical 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 nominal current regulation

obtained is better than 0.01 percent, and the stability over an 8 hour period, at constant load temperature,

is better than 0.02 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 (O.Olfl 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 S1O2)

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 fight 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. General 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. 407

55
Figure 3. Close view of the tungsten 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 Littrow-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 spectrophotometer 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 f-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 spectrophotometers. 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).

408

56
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 photometric 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 Geitel 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]. A novel approach to the problem of accurate

Figure 6. The tungsten ribbon filament lamp in the newly designed

adjustable holder. 1 he 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 60° 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 Gibson 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] in conjunction with a single beam spectrophotometer, using a

five aperture screen. One year later, Buchmuller and

Kbnig [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] at NPL

described a two aperture system. Multiapertures were

used by Hoppmann [21], Bischoff [23], Sanders [26] and Nonaka and Kashima [27]. Finally, Clarke f30] 409

57
Figure 7. Same as figure 2 except tor 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 15

position switches (7 sample positions and 8 blank positions) are

visible along with the two quartz lenses. The exit slit of the monochromator 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 NPL. 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 monochromator.

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 monochromator 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

58
4MK Ml

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 closed.

should be placed between the aperture plate and the monochromator. 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 diff erence 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 monochromator 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 BaS04 ; the outside is painted black. Under these circumstances the sensitive surface of the photodetector receives the radiations originating

from the exit slit of the monochromator 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 108H. 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.

411

59
r

CLOSED SHIELDED 101 r

L_.

| RANGE |

I IC'JISI|

101 NOISE CABLE

ill PHENOL

21 - Si? > input >-e

INSULATED FROM

CHASSIS

1

X

S3

n i rm

I a

0U1PUT

II FN IIDIUtEO

FULL SCALE

NOTES

* SELECTED FOR 2 333 I MTIO 10 tITNIN 0.1%

til RESISTORS SETTER TH1I ;.\ lOlElUCE HAVE TC

OR LESS

1 ! 1 < S i 1 I 9 10

SEMICONDUCTOR

CIRCUITS 2-15-50

FIGURE 13. Schematic of the current-to-voltage circuitry. Courtesy of K. W. Ye

The optical components located after the exit slit of the monochromator, 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 alignment 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, \ 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

412

60
CAL IBKAT I ON OF SRM930 AT UKO NM SAMPLE NUMBER AND POSITION:

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

checks 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.

1-70 IN 2; 1-79 IN Hi 2 -79 in 6; reference in b; 3-79 IN 10; 1-91 IN 12 ; 3-91 IN 1 4

POS AV s S/AV PERCENT T

1 • 200 I 700E 0 1 •4388E-03 • 21 92 E -03

2 •64 1 344 1

E

00 • 325 6E -0 3 •5077F-03

3 • 200 1 230E 0 1 • 4 320F -0 3 .21 59 F - 0 3 32 .044

4 • 6528346E 00 • 4804E -03 • 73 59E -03

5 •200 1 040E 0 1 • 42 54E -03 .2 126F-03 32 • 623

6 • 422 1 1 1 OE 00 •2293E -03 • 5432 E-03

7 •2000995F 0 1 . 304 1 E -03 • 1520E-03 2 1 • 095

8 •65601 42E 00 • 3 1 89E -03 .486 1 E-03

9 .200 1402F 0 1 • 4476E-0 3 .e236F-03 32 • 781

10 •2358762E 00 • 1 42 5E-03 •6039E-03

1 1 •200 1 539E 0 1 •41 1 4F -03 • 2055E-03 1 I 785

12 •6739426E 00 •2848E-03 •4226E-03

13 •£00 1205E 0 1 •4678E-03 • 2338E -03 33 • 6 74

14 .238676 1

E

00 . 1 064E -03 •4456E-03

1 5 •2001 1 0 1

E

0 I •4 1 50E-03 • 20 74E-03 1 1 • 927

1 .200 1 622E 0 1 • 3 1 87E-03 • 1 592 F-03

2 •6412983E 00 • 1 32 1 E-03 • 2060F-0 3

3 •2002043E 01 • 6 1 39E-03 • 3067E-03 32 .036

4 •6526482E 00 •2474E-03 • 3790E- 03

5 •200 1 55 1

E

0 1 •4756E-03 •2376F-03 32 .603

6 •4222282E 00 •2 1 74E -03 • 5 1 49F-03

7 •2001 507E 0 1 •4355F -03 •2 1 76F -03 2 1 .095

6 •6562004E 00 • 3344E -03 • 509 5F - 03

9 •2001 529E 0 1 • 533 7E-03 - 2667K -03 32 785

10 •2359933E 00 . 1886E-03 . 7992E-03

1 1 .2001 634E 0 1 •4532E-03 • 22 64 E-0 3 1 1 790

12 .674 1099E 00 .341 8F-03 • 5070F-03

13 .200 1 589F 0 1 -6368E-03 . 3 1 62F-0 3 33 6 78

14 •23868P8F 00 • 1 604E -03 • 6720F-03

15 •200 1829E 0 1 •3388E-03 • 1 693E-03 1 1 924

1 .20022 14E 01 •2R26E -03 • 1 4 1 2E-03

2 •64 1 3472E 00 •3518E-03 54R6E-03

3 •200 1 882E 0 1 . 388PF-03 . 1 939E-0 3 32 035

4 .65261 77E 00 •251 3E-03 • 385 1 E-03

5 •200 1 74 1

E

01 • 384 1 F-03 • 1 9 1 9F-03 32 601

6 •42234 1 8F 00 •2957F-03 . 7000E-03

7 •2002075E 01 •4892F-03 • 2443E-03 2

)

097

8 •65620R3F 00 • 1 365E-03 . 208 1 E-03

9 •200 1 871

F

0 1 • 3708E -03 • 1 852F-03 32 778

10 .2358786F 00 . 1 596F-03 .6767E-03

1 1 .20021 30E 0 I . 5036E-03 •25 1 5E-03 1 1 782

12 •6740563E 00 •3022E-03 .4484F-03

13 -2001997F 01 -4545F-03 •2270F-O3 33 668

14 •23883S4E 00 •1 704E-03 . 71 33E-03

•2p024 78F 0 1 •2975E-03 . 1485F-03 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 indi- 413

61
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-voltage converter alone measured in three groups of 15 replications each

Average of 50 individual Percent standard

lication measurements; volts deviation

1 1.003494 0.001

3

6 2 1.003496 0.00129 3 1.003482 0.0014s 4 1.003507 0.0011, 5 1.003515 0.00134 6 1.003508 0.00136 7 1.003497 0.0011 8 8 1.003498 0.00133 9 1.003505 O.OOllo

10 1.003510 0.00142

11 1.003518 0.00133 12 1.003521 0.00120 13 1.003522 0.0013,

14 1.003507 0.00126 15 1.003527 0.0013,

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

1.003507

0.0012

1.003535

0.0015

1.003545

0.0010

First group

Second group

Third group

Average of replications Percent standard

deviation

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 constant 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 0.0225 for single

measurements with 2.00 V at the photomultiplier tube

anode.

62
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 1.012329 0.027,

2 1.012342 0.0488 3 1.012322 0.0272 4 1.012320 0.0338 5 1.012394 0.0435 6 1.012421 0.0153 7 1.012404 O.OI84 8 1.012406 0.029e 9 1.012365 0.0196 10 1.012402 0.019,

11 1.012465 0.0252 12 1.012412 O.O6I5

13 1.012451 0.023s 14 1.012417 0.0298 15 1.012481 0.0248

Second group

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

1.012395

0.0050

1.012467

0.0033

1.012510

0.0035

First group

Third group

I 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 105 during a series of 15 consecutive measI urements (5 min).

Following the four stability tests discussed earlier, a consecutive series 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

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

Mfi 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:

T - ^2 rp _ U rp _ h

l2 ~ /o+ /o ; l4 ~ /o_l /o ; Yfi— /o + ;o; etc - • •

• '1 ' '3 3 5 5 7 2 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

415

63
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

i 1 536345 0.0275 9 1 536326 0.0302 oo 1 5361 96 0.0228

4- 1 536289 0.0235 0 1 536106 0.0309 0 1 ^361 1

7

X • JOU X X J 0.0314 7 1X . Jt)J" ^359161 u 0.025t

QO 1 536065 0.023s Q 1 5361 79 0.0293 1U 1 536003 0.0233

1

1

1 536083 0.02U

12 1.535961 O.O263 13 1.536052 0.031 3 14 1.536095 0.0266 15 1.536092 O.O262

Average of replications Percent standard

deviation

1.536122

0.0082

First group

Average of replications Percent standard

deviation

1.535768

0.0095 Second group

Average of replications 1.535522

j Percent standard \ 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 (SRE) 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 A. 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 10s

. 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

416

64
Table 4. Stability of the current-to-voltage converter, the photomultiplier tube at 850 V, and the tungsten ribbon

filament lamp

Replication

1234

56

78

9

10

111213

14

15

Average of 50 individual measurements; volts

2.002395

2.001356

2.002145

2.000975 2.001944

2.000925

2.001832

2.000825

2.001551 2.000960 2.001739

2.000851 2.001729

2.000825

2.001557

Percent standard

deviation

0.0380 0.0226 0.024, 0.0262 0.0207 0.028,

0.026, 0.0235 0.0263 0.021 2 0.023,

0.024., 0.0282 0.023o

0.024,

Average of replications Percent standard

deviation

2.001441

0.026

First group

Average of replications Percent standard

deviation

2.001517

0.012 Second group

Average of replications Percent standard

deviation

2.000826

0.025 Third group

Average of replications Percent standard

deviation

2.001268

0.027 Fourth group

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 <j

2 4 6 8 10 12 14

33.327 21 711 12.236 50.990 33.377 20.906 13.473 33.325 21 710 12.237 50.983 33.377 20.903 13.471 33.321 21 711 12.241 50.992 33.383 20.900 13.474 33.320 21 708 12.240 50.988 33.375 20.901 13.470 33.323 21 710 12.239 50.983 33.379 20.901 13.474 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 <j 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

417

65
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 earlier. 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 Mil 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! 2.00181 2.00233- = -0.026

(A + B)

1.00049 1.00078 1.00066 1.000%

1.00071 1.00112 - = -0.041

(A + B)

0.50062

B

0.9864, 0.9864o

0.9862s

1.01596 1.01599 1.01609 1.01598 + 0.98635 = 2.00233

2.00181 =+0.00052

0.5102o 0.50996 0.51004 0.51007

1.00071 =

A

0.25657

B

0.49108 0.4909, 0.4911s

(- 0.49105=1.00112

0.00041

B

0.24430

0.50056 0.5007o 0.50062

0.25673 0.2566s 0.2443s 0.24432 0.25665 + 0.24433 = 0.50098

Av.

Diff. % Corr.

Step 4

Av.

Diff. % Corr.

0.50063

0.50098 - 0.50063 = = -0.069

0.00035

(A + B)

0.2502„

0.25028 0.25019 0.25023 0.25023

0.25031 - = -0.031

A

0.12872 0.12856 0.12854 0.12861 + 0.12170 = 0.25031

B

0.1217s 0.12168 0.1216e

0.25023 = + 0.00008

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 of % T

2.00

1.00 0.50

0.25 0.125

100

5025

12.5 6.25

0.0 0.026

0.067

0.14 0.167 When transmittance measurements are performed,

the h reading is initially set with the circular neutral wedge to a value near 2.0020 V. The 7 value is then

measured. If the initial 70 = 2.00214 V and final 70 = 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-(

27- Q81

io

X

o Q072)=27.06L

418

66
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 earlier (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. Transmittance 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 A. = 440 nm, at three angles of incidence

Angle of Filter Filter Filter incidence 1.0 mm 1.5 mm 2.0 mm

Normal incidence 32.91 5 19.83 8 11.606 1° 32.899 19.83 3 II.6O4

2° 32.89, 19.81 9 11.59„

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 niters A, B, C, D, E, F, and G at A = 465.0 nm

Run No. A B C D E F G

%T 27.08 16.44 46.39 26.12 15.22 37.34 23.23

I St. No. 2 4 6 8 10 12' 14 %T 27.07 16.40 46.39 26.11 15.21 37.34 23.23

II St. No. 14 2 4 6 8 10 12 %T 27.08 16.43 46.39

III St. No. 10 12 14

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 measurements were made using the radiations produced by

the spectrophotometer. Column two shows the results

26.11 15.21 37.33 23.23

2 4 6 8

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 hav- ing 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

419

67
Table 8. Effect of polarization on percent transmittance (%T) measured at four wavelengths on a Schott NG-4

glass filter %T

Wavelength nm

Produced by

spectro- photometer

Polarizer, plane of

vibration horizontal

Polarizer, plane of vibration

vertical

% Difference

%r, - %t3

440.0

465.0

590.0 635.0

1

19.81s 22.59, 19.178 20.61,

19.80„ 22.600 19.17„

20.60,

19.788 22.569 19.099 20.54,

-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 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 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 tc 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

tvelength NBS, %T NBS, %T NBS, %T NPL, %T % Diff. nm March 12, May 18, average February NBS to N

1971 1971 1971

440.0 12.92 12.91 12.915 12.93 - 0.11 465.0 14.965 14.98 14.973 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 I9.6O3 19.62 - 0.09 465.0 22.38s 22.35 22.367 22.43 - 0.28 590.0 19.06 18.95 I9.OO5 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 31.13 3 31.21 - 0.25 635.0 32.56s 32.52 32.543 32.62 - 0.24

Average difference between NBS and NPL percent T values^ — 0.19 percent.

filters available, Schott NG-4 "neutral glass" was by NBS as a means to check the photometric scale of selected, prepared and characterized. It is now offered spectrophotometers.

420

68
Table 10. Comparison between the percent transmittances (%T) measured on three inconel-on-silica filters at NPL and NBS

Wavelength NBS, 9!

ft u NBS, %T % Diff. nm 1 2 average NPL, %T NBS to NPL

450.0 24.87 24.88 24.87s 24.93 - 0.18 550.0 23.78 23.82 23.80 23.86 - 0.25 650.0 23.38 23.39 23.38, 23.46 - 0.32

450.0 49.35 49.33 49.34 49.56 - 0.44* 550.0 47.60 47.60 47.60 47.81 - 0.44 650.0 46.85 46.85 46.85 47.14 - 0.64

450.0 72.17 72.20 72.185 72.30 - 0.16 550.0 72.05 72.11 72.08 72.20 - 0.17 650.0 72.20 72.34 72.27 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,

69
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 photometric 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]. For instance,

a well-known phenomenon called "blooming" of the

glass is due to the formation of an SiO 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. Generally speaking, several years are needed for a fresh surface to reach equilibrium. This, and similar

422

70
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 and 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, percent

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 absorbing. 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.

423

71
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 equipment 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

424

72
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.

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[20] Reule, A., Checking the photometer scale of absorption instru- ments, Zeiss-Mitt. 1, 283-299 (1959).

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[22] Gibson, G. L., Hammond, H. K., Ill, Holford, W. L., and

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143-147 (1961).

[24] Jones, O. C, and Clarke, F. J. J., A new photometric technique

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[26] Sanders, C. L., A photocell linearity tester, Appl. Optics 1, 207-271 (1962).

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791 (1963).

[28] Hbfert, H. J., and Loof, H., Calibration of photometric scales of

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[37] Mielenz, K. D.. and Eckerle, K. Accuracy of polarization attenuators (to be published), Appl. Optics (1972).

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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)

425

73

APPENDIX III

JOURNAL OF RESEARCH of the National 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; fluorescent 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],1 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 1-meter, f/8.7 Czerny-

* This paper describes a cooperative project on the part of the three authors. The basic design concepts and the accomplishment of a functioning instrument are due to K. D. Mielenz and R. Mavrodineanu. The optimization of the fluorescent wavelength converter for the averaging sphere was achieved by E.D. Cehelnik.

1 Figures in brackets indicate the literature references at the end 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 photomidtiplier 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 -7 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-compartment lenses by fused-silica, lithium-fluoride achromats.

631

75
2. Modification of Spectrophotometer

2.1. Deuterium Arc Lamp

To a significant extent, the sharp drop of ultraviolet signal level shown by curve (a) in figure 1 is attrib200 300 400 500 WAVELENGTH, nm

FIGURE 1. Successive improvement of clear-space signal level of

original spectrophotometer (curve a) obtained by:

(6) deuterium arc lamp, (c) improved averaging sphere, and [d) new grating.

utable to the tungsten lamp of the original spectrophotometer. 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 spectrophotometer. 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 clear-space 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

2.00

OPERATING HOURS

FIGURE 2. Long-term signal drift observed

with deuterium lamp.

632

76
measurements with a precision better than 10 -4 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

FIGURE 3. Geometry of (a) original averaging sphere and (6) improved

sphere.

A: entrance port. A': exit port. D: detector, F: fluorescent dye cell, S: 9phere wall, SH:

shutter, T: target.

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,

cpDl4> = [r2

/ ( 1 - r ) ] ( 1 - eld) ePj 16s 2 (1

)

where

<f> = radiant flux into sphere,

(po = 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 (1-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 fluores- 633

77
r

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 U = Qh, (2a)

where Q is the quantum efficiency of the dye and/a is the intensity absorbed. The latter is given by Beer's law as

/o = /o (l-10-«») (2b)

where 10 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 10~7 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] 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 1 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 Ao, the diameter of the blur circle in the sample plane for a wavelength X < \o can easily be shown to be equal to

8 = D[2f(k0 )lf(K)-l], (3)

I 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, j 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 compartment 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

78
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 (Phys. and Chem.), No. 5, 405-425 (Sept.-Oct. 1972).

[2] Mielenz, K. D., Eckerle, K. L., Madden, R. P., and Reader, J., Appl. Optics 12, 1630-1641 (1973); Mielenz, K. D., and

Eckerle, K. L., Nat. Bur. Stand. (U.S.), Tech. Note 729, 60

pages (June 1972).

[3] The systematic error due to interreflection between the samplecompartment lenses is accounted for in all measurements made with this spectrophotometer. See Mielenz, 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. Mielenz, 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 ultra- violet 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 nm. 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] Grum, F., and Luckey, G. 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 AI2O3 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] Mielenz, K. D., Mavrodineanu, R., and Cehelnik, E. D., Appl. Optics (to be published).

[9] Mielenz, K. D. and Eckerle, K. L., Appl. Optics 11, 2294-2303

(1972). (Paper 78A5-838)

635

79

APPENDIX IV

JOURNAL OF RESEARCH of the National Bureau of Standards—A. Physics and Chemistry

Vol. 80A, No. 4, July-August 1976 Considerations for the Use of Semi-Transparent Metallic Thin Films

as Potential Transmittance Standards in Spectrophotometry*

R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 20, 1976)

Various characteristics of evaporated metal-on-fused silica filters are discussed in relation to their optical transmission properties. Special metal holders provided with shutters were designed to be used with these filters, and are described in detail. Transmittance

measurements, performed in various conditions, are reported and indicate that the evaporated metal-on-fused silica filters might present an acceptable material as transfer standards

in spectrophotometry.

Key words: Evaporated metal-on-quartz; filters, transmittance; neutral filters; standard

reference materials; transmittance characteristics; ultraviolet-visible filters.

The use of solid materials, in the form of glass

filters and of solutions of inorganic and sometime

organic compounds, to test the correct functioning

of spectrophotometers is a common practice [l].

1 A

variety of materials are available from the National

Bureau of Standards (NBS) which can be used as

transfer standards to verify the accuracy of the

transmittance scale and the short and long term

stabilities of conventional spectrophotometers. Two

Standard Reference Materials (SRM's) have been

issued by NBS for the verification of the accuracy

of the transmittance scale; these are SRM 930 and

SRM 931. SRM 930 is a solid material which consists of a

set of three glass filters having nominal transmittance of 10; 20; and 30 percent. They are certified

for transmittance in the visible spectral range from

400 nm to 635 nm. A detailed description of this SRM is given in NBS Special Publication 260-51

[2]. SRM 931 is a liquid standard which consists

of a solution of cobalt and nickel in dilute perchloric

acid contained in glass ampoules [3]. The trans- mittance of these solutions is certified from 302 nm

to 678 nm and should be used in conjunction with

curvettes having a known light path; such curvettes

are available from NBS as SRM 932 [4]. Both SRM's 930 (the glass filters) and 931 (the

liquids) are limited in their spectral transmittance

range and require the use of spectral bandpasses,

from 2.2 nm to 6.5 nm for the glass filters and from

1.0 nm to 6.5 nm for the liquid filters depending

of wavelength, when accurate transmittance values

are sought.

•This work was supported in part by the National Institute of General

Medical Sciences.

1 Figures in brackets indicate the literature references at the end of this paper.

The need to provide similar transfer standards, but

with expanded spectral range to the ultraviolet and

with less stringent -spectral bandpass requirements,

has resulted in an investigation to find adequate

materials for this purpose. 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 nonfluorescent; (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. Thus, various solid materials were examined and the final choice was

the evaporated metal-on- (non-fluorescent) fused

silica type filter. The transmission characteristics of such filters are illustrated in figure 1 and are compared with that

of three glass filters and Ronchi ruling on glass.

The major limitation of the evaporated metal-onquartz filters results from their intrinsic property of attenuating the incident radiation by reflecting

rather than absorbing part of it. As a consequence

of this property, this type of filter could generate

stray radiations in the sample compartment of conventional spectrophotometers and is susceptible to produce interreflections when used with instruments

equipped with lenses. To determine the practical value of such filters as a

transfer transmittance standard, comparative meas637

81
CORNING 8364

CHANCE

ON -10

ujo

O

m evaporated metal on quartz < [inconel]

VRONCHI RULING ON GLASS

100 LINES/in.

i i i

I I

1 1 1 1 1— 200 300 400 500 600 700

X. NANOMETERS

Figure 1. Spectral characteristics for five transparent

materials from 200 nm to 700 nm.

urements were performed with the National Physical

Laboratory in England (NPL), and the results are given in table I. In this case the inconel-on-fused

silica filter was protected by a clean fused silica plate held in place with an organic cement. Except

for a filter which showed some mechanical flaws in its structure, the reproducibility of transmittance measurements was as good as that obtained for the

absorbing glass filters (table II). The error which

would result from positioning the inconel-on-fused

silica was measured by rotating from 0° angle to

3°—6°—and 9°. From the results obtained (table

III) it can be concluded that in the case of the highaccuracy spectrophotometer used at NBS a positioning error within 3° can be tolerated. The interreflection error affecting the measure- ments when a filter is inserted in the radiation path

between the two lenses of a spectrophotometer was examined in detail [5]. The measured error for a glass filter established for the high-accuracy spec- trophotometer was one or two 10 -4 transmittance

units; a value which is about four times larger was

found for an evaporated metal-on-fused silica filter. It should be mentioned here that both the positioning

and interreflection errors are, for a given material,

instrument dependent.

As a result of these considerations a decision was made to produce a limited number of sets of evaporated inconel-on-fused silica and to test these filters in actual measurements performed on the con- ventional spectrophotometers manufactured in the

United States. One of the filters from a set is shown in figure 2. From left to right one can see the main body of the

filter holder made from an aluminum alloy anodized

black. This body is provided with dove-tail grooves

which can accept a front and rear shutter made

Table I. Comparison between the percent transmittances (% T) measured on three inconel-on-silica filters at NPL and NBS

Wavelength nm

NBS

1

%T

2

NBS, %T

average NPL, %T

*ko u. u OA Q7 Z'±. Of 41. 88 <s4. 875 24. 93

550. 0 23. 78 23. 82 23. 80 23. 86

650. 0 23. 38 23. 39 23. 385 23. 46

450. 0 49. 35 49. 33 49. 34 • 49. 56

550. 0 47. 60 47. 60 47. 60 47. 81

650. 0 46. 85 46. 85 46. 85 47. 14

450. 0 72. 17 72. 20 72. 185 72. 30

550. 0 72. 05 72. 11 72. 08 72. 20

650. 0 72. 20 72. 34 72. 27 72. 33

Average difference between NBS and NPL percent T

values = —0.30 percent

a This filter had a flaw in the form of a crack which was

sometimes visible and other times invisible. The larger differ- ences found in the measurements of this filter may be due to

this flaw.

Table II. Comparison between the percent transmittances (%T) measured on three Schott NG-4 glass filters at NPL

and NBS

Wavelength nmNBS, %T

March 12, 1971

NBS, %T

May 18, 1971

NBS, %T

average NPL, %TFebruary

1971

440. 0 12. 92 12. 91 12. 91 5 12. 93

465. 0 14. 965 14. 98 14. 973 15. 01

590. 0 11. 70 11. 64 11. 67 11. 67

635. 0 12. 72 12. 68 12. 70 12. 72

440. 0 19. 625 19. 58 19. 603 19. 62

465. 0 22. 385 22. 35 22. 367 22. 43

590. 0 19. 06 18. 95 19. 005 19. 01

635. 0 20. 455 20. 37 20. 413 20. 47

440. 0 32. 89 32. 86 32. 875 32. 98

465. 0 35. 52 35. 54 35. 53 35. 66

590. 0 31. 165 31. 10 31. 133 31. 21

635. 0 32. 565 32. 52 32. 543 32. 62

Average difference between NBS and NPL percent T

values= — 0.19 percent

Table III. Percent transmittance (%T) measured on a Schott neutral glass filter 2 mm thick, and an inconel-on-fused silica

filter 2 mm thick at 690 nm, for normal incidence and for the angle of 3°; 6°; and 9° %T

>v Filter Schott Inconel-on-fused silica NG-4

Angle Glass Front Back

0° 28. 13 29. 91 29. 87

3° 28. 10 29. 98 29. 87

6° 28. 03 29. 84 29. 82

9° 27. 98 29. 92 29. 90

638

82
Figure 2. Filter holder with shutters. From left to right: front and rear view of the holder body provided with dove-ta view of the filter holder with front shutter; rear view of the 1

from a black plastic (Dehin 2 ). The last three units

illustrate the holder with these shutters on. A more

detailed dimensional drawing of the filter holder is shown in figure 3. The shutters provide two functions

:

one is to protect the filter from contamination, the

other to detect if stray radiant energy (SRE) is produced in the spectrophotometer compartment

as a result of reflections generated by the incident

beam on the semi-transparent metal layer. The

determination of stray radiant energy generated

in the sample compartment of the spectrophotometer,

due to the reflecting propreties of these filters, may

be assessed as follows: A background signal may be

measurable when the instrument shutter at the

photomultiplier tube is closed. Another slightly

higher background signal may be detectable with

the instrument shutter open and with the filter placed in the beam, in the sample compartment, with

both sliding shutters closed. Both of these signals

should be very small. A third signal may be detected

when the front sliding shutter at the filter holder is removed and the rear sliding shutter is closed. If under these circumstances a signal is detected, it is very likely due to SRE produced by reflections re- sulting from the semi-transparent mirror which

j scatter from the walls of the cell compartment. This

should also be low in magnitude. The transmittance

! of the filter is measured where both sliding shutters

| at the filter holder are removed.

2 In order to describe materials and experimental procedures adequately, it was

occasionally necessary to identify commercial products by manufacturer's name

or label. In no instance does such identification imply endorsement by the National Bureau of Standards, nor does it imply that the particular product or equipment is necessarily the best available for that purpose.

grooves; retaining spring with nylon screw and washer; filter; two shutters; front

ter with rear shutter; filter holder with both shutters closed.

Under these circumstances a source of systematic

error can be from multiple reflections between the

lenses in the instrument and the filter surfaces [5]. An indication of the existence and magnitude of such

interreflection phenomena can be obtained by comparing the differences between the transmittance

measurements and the certified values for SRM 930

and 931 to the corresponding differences for the

inconel-on-fused silica filters. Each set is made from three filters and one blank

placed in individual metal holders which are pro- vided with the front and rear sliding shutters. The

selection of inconel as the semi-transparent metal

layer was made on the basis of its relatively good

optical neutrality in the spectral range from 250 nm

to 700 nm. The fused silica substrate is of optical

quality and non-fluorescent. All the silica filters, including the blank, were ground and polished at the same time and together to a 'parallelism of 0.02 mm

and a flatness of less than 2 fringes (mercury 546.1 nm). The nominal dimensions are 30.5 mm by 10.4 mm by 2 mm thick, and the nominal transmittances

of these filters are 1 percent; 20 percent; 30 percent;

and 90 percent.3 The transmittances of each set were measured at 250 nm, 300 nm, 340 nm, 400 nm, 440

nm, 465 nm, 500 nm, 546.1 nm, 590 nm, and 635 nm.

Sets of these filters were sent to the manufacturers

of spectrophotometers for evaluation and tests on

the individual instruments. The results of these

J The filter holders and the evaporated inconel-on-fused silica filters were made

in the NBS Instrument Shops.

639

83
Table IV. Percent transmittance (%T) measured on four inconel-on-fused silica filters at 10 wavelengths and on the NBS-IMR

high-accuracy spectrophotometer and on two conventional instruments A and B

%T

^^Wavelength nm

Instruments^

250 300 340 400 440 465 500 546 590 635

NBS-IMR 1. 45 1. 81 1. 86 1. 94 2. 04 2. 13 2. 27 2. 49 2. 70 2. 90 High-accuracy 21. 39 21. 27 20. 33 18. 93 18. 33 18. 09 17. 90 17. 83 17. 86 17. 95 Spectrophotom- 28. 68 29. 92 30. 20 29. 96 29. 70 29. 55 29. 44 29. 44 29. 49 29. 57

eter 91. 42 92. 34 92. 66 92. 90 93. 01 93. 05 93. 11 93. 16 93. 19 93. 23 Spectrophotom- "1. 53 1. 84 1. 93 2. 00 2. 12 2. 20 2. 34 2. 55 2. 75 2. 95

eter A 21. 37 21. 24 20. 31 18. 91 18. 29 18. 07 17. 89 17. 82 17. 84 17. 92

28. 67 29. 87 30. 19 29. 95 29. 70 29. 58 29. 49 29. 46 29. 52 29. 60

91. 59 92. 60 93. 32 93. 20 93. 30 93. 30 93. 42 93. 46 93. 50 93. 58 Spectrophotom- 1. 49 1. 85 1. 90 1. 97 2. 07 2. 16 2. 31 2. 53 2. 83 3. 06

eter B 21. 6 21. 5 20. 5 19. 1 18. 5 18. 3 18. 1 18. 0 18. 3 18. 5

28. 8 30. 0 30. 2 30. 0 29. 9 29. 7 29. 5 29. 5 29. 9 30. 0

91. 4 92. 6 92. 6 93. 1 93. 3 93. 3 93. 3 93. 3 93. 9 93. 9

tests will permit to establish the usefulness of these

filters as SRM's in spectrophotometry. In the mean time comparative measurements were

performed on two conventional spectrophotometers

available at NBS, and the results are assembled in table IV. These results are preliminary measurements

and should be considered with caution. The data seem to indicate, however, that the evaporated

metal-on-fused silica filters may be an acceptable

material for use as transfer standards in spectrophotometry.

Note.—The semi-transparent metallic surface of the filters described in this paper is not protected from environmental

exposure and careless handling. A solution to this problem was found recently. It consists in placing on top of the fused

silica plate carrying the semi-transparent evaporated metal

layer a clear fused silica plate, about 1.5 mm thick, adequately

ground and polished, to achieve an optical contact with the evaporated semi-transparent metal layer surface. The two

fused silica plates are slightly beveled, and an optical cement

is applied to protect the edges. Under these conditions the two fused silica plate assembly, with the semi-transparent

evaporated metal layer in between, behaves optically as if it were a single semi-transparent plate, and provides full protection of the metal layer. Several sets of these new filters were made for evaluation in actual working conditions.

640

84
References

[1] Rand, R. N., Practical spectrophotometric standards,

Clin. Chem, 15, 839-863 (1969).

[2] Mavrodineanu, R., and Baldwin, J. R., Glass filters as a standard reference material for spectrophotometry

selection, preparation, certification, use, SRM 930, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-51, 118

pages (Nov. 1975).

i [3] Burke, R. W., Deardorff, E. R., and Menis, O., Liquid absorbance standards, J. Res. Nat. Bur. Stand.

(U.S.), 76A, (Phys. and Chem.), No. 5, 469-482

(Sept.-Oct. 1972).

[4] Mavrodineanu, R., and Lazar, J. W., Standard quartz

cuvettes for high accuracy spectrophotometry, Nat.

Bur. Stand. (U.S.), Spec. Publ. 260-32, 26 pages (Dec.

1973) and Clin. Chem. 19, (9) 1053-1057 (1973).

[5] Mielenz, K. D., and Mavrodineanu, R., Reflection

correction for high-accuracy transmittance measure- ments on filter glasses, J. Res. Nat. Bur. Stand. (U.S.), 77A (Phys. and Chem.), No. 6, 699-703 (Nov.-Dec.

1973). (Paper 80A4-909)

641

APPENDIX V

JOURNAL OF RESEARCH of the National 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 lO"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, TV,

into the correct valuet=tv+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.

MONOCHROMATOR

EXIT SLIT

FIGURE 1. Sample-compartment optics of a focused-beam spectrophotometer.

87
The purpose of the measurement is assumed to be

the determination of the normal-incidence trans- mittance,

T =

Ti(l-r) 2

1 - Hi-,-2 (2a)

of a glass filter surrounded by air. The filter is characterized by its thickness t and complex refractive index 7i(l + i/c), so that for each wavelength k the normal-incidence internal transmittance t, and

Fresnel reflectance r appearing in eq (2a) are given by = g-4irn«(/X

r= (fri)-

(2b)

(2c)

This quantity t can be measured accurately with

specially designed instrumentation in which the sample

is placed normal to a collimated beam of fight, 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 r 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

</>o = 7

,

I 7'2 (l + 7? 1 K2 + . . .), (3a)

where the initial flux is taken as unity, and where 7\

,

R\, T2 , 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

<P = T1 TT2 (l + R l R2 T

2 + RR2 + R 1 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

TM=<f>l<t>0 = t[i + (r 1 +r2 )r-r1 r2 (i-d]. (3 C)

The correction which must be applied to Tm to eliminate the reflection bias is T-TM = AT+AT (4a)

(a)

ii

|

!

!

(b) T.TT,

T,TR 2 TR|TT 2

T| TR2 RT 2

T, RR, TT 2

FIGURE 2. Reflected beams in a spectrophotometer with lenses. AT=-(R l + R2 )RT,

AT' =R l R2 T{\-Ti ).

(4b)

(4c)

The first of these two error terms, A71

, 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

AT" 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

r=T[l+ (l/2n2

) (1iit,)(0?.+ |0§)

where ± (4r/n) 6%+. . .], (5)

700

88
where T is the measured transmittance (corrected for

reflection errors),- t, t,, and r are the normal-incidence

values of transmittance, internal transmittance, and

Fresnel reflectance as given by eqs (2a, b, c); 6t and

do 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 IS 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 20o 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 transmittance 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 transmittance 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- mittance 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 reflection 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- mittance.

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 Ri and R2 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

6t, 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),

(AT) m=-n (l/2»») ln(y^)±(4r/n)]<?*, (6)

.557

.556

.555

.554

.553

.557

.556

-x x

where the approximate values TV and TV/(1 — 2r) were substituted for t and Tj 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

FIGURE 3. (a) Measured transmittance TM versus tilt angle 6T for 55 percent filter in S and P polarized light at K = 574 nm.

(b) The same data after application of tilt correction.

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 transmittance. From the measurements

performed on all of the filters one may then determine

the dependence of A T 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 transmittance

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 fight. 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 singlesample 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

701

89
0 0.5 1.0

Figure 4. Reflection correction AT versus T for X — 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 spectrophotometer 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 transmittance 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 ///o of the attenuated to the unattenuated radiation value, each I and 70 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

A = 574 nm. This yielded the result represented by the

straight line in figure 4; namely,

A77r=-8.9X 10-4

, for\ = 574 nm. (7a):

Each of the individual values of AT could be measured i to a precision better than 10~4 transmittance units. ; There was no evidence suggesting a dependence of AT on polarization.

Secondly, the measurements on the clear glass

plate were repeated for \ = 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 AT/T=- 1.26 X10"6 (X -574 nm), for 7=0.92.

(7b)

j These results were then combined into the final empirical expression.

A77r=-8.9x 10-4 [1 + 1.4X 10-3 (\-574 nm)],

'

(8)

for the correction AT to be applied for different trans- mittances T and wavelengths A.. This formula agrees

with all individual measurements of AT to within

± 5 X 10~5 transmittance 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,

\ (/?i + /?2 ) ~ 6X10-3

. (9a)

Provided that this arithmetic average of R t and R2 is not substantially different from their geometric average

(#i/?2 )

1/2

, it follows from eq (4b) that

AT' ~ 3.6X 10-5 r(l-r2 ), (9b)

1.2- -(AT/T)XI0~3

400 500 600 700

FIGURE 5. Reflection correction AT versus K for 1 = 0.92

702

90
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

I that

./ . 8r(/?i + Rz)T , rf(Af)=

(„ + !)»

dn- m

| For ra=1.5, r=0.04, 7=0.5, and the above value

I (9a)for (A?! + r? 2 ), this is reduced to d{AT) -3X10"4

I dn, showing that AT will not be affected by more than

j 10 -4 transmittance units as long as dn < 0.3, or 1.2

j < 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

j than indicated by figure 3 for uncoated samples.

5. Conclusion

Based upon this work, all transmittance measureI ments on glass filters with this high-accuracy spectrophotometer 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

(AT) f =-(TI3n*)(lnj^£)d$;

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 t 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)

703

91

U.S.. Department of Commerce

Juanita M. Kreps

Secretary

National Bureau of Standards

Ernest Ambler, Acting Director

APPENDIX VI

^attmtal ^ttram of jstfanrlartte

Certificate

Standard Reference Material 930D

Glass Filters for Spectrophotometry

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for the verification of the transmittance and absorbance scales of spectrophotometers. It consists of three individual glass filters in their metal holders and one empty filter holder. The holders are provided with shutters to protect the glass filters when not in use. These shutters must be removed at the time of measurements and be replaced after the measurements have been

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

Filter & Set

Ident. Number

TRANSMITTANCE (T) TRANSMISSION DENSITY (-log,0T)

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)

440.00

(2.2)

465.0

(2.7)

546.1

(6.5)

590.0

(5.4)

635.0

(6.0)

Date of Certification: The transmittance values (T) can be converted to percent transmittance (%T) by multiplying by 100. The

transmission densities are calculated from the measured transmittance (T). These values should be indicated by

the absorbance scale of the spectrophotometer if the filters are measured against air. The transmittance values given were measured against air at an ambient temperature of 23.5 °C. The transmittance values are estimated to be accurate to within ±0.5 percent at the time of certification. This

uncertainty includes the effects of the random and systematic errors of the calibration procedure, as well as possible transmittance changes of the filters during the period of calibration. Aging of the glass may cause some filters to change transmittance by about ± 1 percent over a period of approximately one year from the date of calibration. Improper storage or handling of the filters may also cause changes

[5]. It is recommended that the filters in the holders be handled only by the edges with soft plastic (polyethylene)

gloves and optical lens tissue. When not in use they should be stored in their holders and in the container provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should be avoided. In cases where verification is desirable, the filters should be returned to the National Bureau of Standards for cleaning and recalibration.

This Standard Reference Material was issued with the assistance and advice of K. D. Mielenz and J. R. DeVoe. 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 and

R. W. Seward.

Washington, D.C. August I. 1977

20234

i over)

J. Paul Cali, Chief

Office of Standard Reference Materials

93
The transmittance measurements were made using the high-accuracy spectrophotometer designed and built at the NBS Institute for Materials Research [1]. This instrument represents the primary transmittance standard;

its transmittance accuracy was established using the double-aperture method of linearity testing [1,3,5,6]. The

estimated standard deviation obtained from a set of 20 measurements of the high-accuracy spectrometer used

for transmittance measurements on glass filters of the SRM 930 type is 5 x 10 transmittance units for transmit- tance between 0. 1 and 0.3. The bounds of the systematic error of the instrument are estimated to be + 10 trans- mittance units. The neutral glasses for the filters were provided by Schott of Mainz, Germany, and are designated as "Jena Color and Filter Glass." The glass material was prepared by the manufacturer especially for SRM 930. It was

fine-annealed and was selected for best homogeneity and minimum of inclusions and striae. The glass filters were aged at NBS for about one year prior to certification. They are of the type NG-4 and NG-5, and their nominal transmittances are 10, 20, and 30 percent [2,5]. The glasses were selected to provide a means to verify the transmittance scale of conventional spectrophotometers at three levels. The exposed surface of the glass is approximately 29 x 8 mm, measuring from a point 1.5 mm above the base of the filter holder (see figure). The

empty filter holder is provided to be used in the reference beam of the spectrophotometer so that approximately

equivalent conditions of stray radiations are achieved for both beams. The transmittance of the filters depends upon the intrinsic properties of the material, wavelength, spectral bandpass, geometry of the optical beam, temperature, and positioning of the filter. A change of ambient temperature

of 1 or 2 °C from 23.5 °C will not significantly affect the calibration [5]. Changes in the transmittance may be caused by surface conditions, aging of the glass, exposure to a harmful atmosphere, or careless handling as indicated on the face of this certificate [2,3,4,5]. The effective spectral bandpasses used to determine the certified values are given on the face of the certificate and the transmittance measurements are made by producting the

vertical image of the slit (about 8 mm by 0. 5 mm), using a convergent beam geometry with an aperture ratio f: 1 0, in the middle of the entrance face of the glass filter. The filters were measured in the spectrophotometer in a position perpendicular to the incident light beam as shown in the attached figure.

SRM 930D is stored in a black anodized aluminum container provided with a threaded cap made of the same

metal. Each filter is placed in a cylindrical cavity to prevent any contact between the filter face and the walls of

the storage container. Contamination of the glass filter surface with particulate matter due to static charges is minimized through the metallic nature of the container. Each filter holder is provided with a flat leaf spring

which is inserted into the cylindrical cavity to minimize damage during transportation. These springs can be removed during normal use in the laboratory.

Prior to certification measurements, each filter was examined for surface defects and thorougly cleaned [5]. Should the surface of the glass filter become contaminated, no attempt should be made to clean it unless the user has the facilities to demonstrate that the cleaning treatment has not altered the glass surface or degraded the accuracy of the certified values. As the Standard Reference Material is a transfer standard, the only means

available to verify its integrity is remeasuring its transmittance with a primary standard instrument similar to

that used in this certification [1,4,5]. In most cases where verification, or recertification, of the transmittance

values is desirable, it will be most expeditious to return the filters to the National Bureau of Standards for measurement. Prior to shipment, the NBS Office of Standard Reference Materials should be contacted regarding the conditions and cost of verification and /or recertification. We wish to acknowledge the cooperation of George N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, Connecticut; Royden N. Rand, Ph.D., of the Eastman Kodak Co. Research Laboratories, Rochester, New

York; and Donald S. Young, M.D., Ph.D., of the National Institutes of Health, Bethesda, Maryland.

94
REFERENCES

1. R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and 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, O. Menis and J. I. Shultz, Eds., 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. Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence Measurements, NBS Journal of Research 76A, No. 5, 375-510 (1972).

5. R. Mavrodineanu and J. R. Baldwin, Glass Filters as a Standard Reference Material for SpectrophotometrySelection, Preparation, Certification, Use, SRM 930, NBS Special Publication 260-51, U.S. Government

Printing Office, Washington, D.C. 20402, 1975.

6. K. D. Mielenz and K. L. Eckerle, Spectrophotometer Linearity Testing Using the Double-Aperture Method,

Appl. Optics 11, 2294-2303 (1972). METAL HOLDER FOR THE COLORED GLASS FILTERS*

FRONT TOP

METAL HOLDER

57 X 13 X 13

EXPOSED FILTER SURFACE

29 X 8

30.5 .X II X 2.0 (Or 1.5) COLORED GLASS FILTER

1.5

dimensions in mm

•Shutters not illustrated.

95

U.S. Department of Commerce

Elliot L. Richardson,

Secretary

APPENDIX VII

National Bureau of Standards

Ernest Ambler, Acting Director Rational ^nttnn of ji»tattrtartte

Certificate

Standard Reference Material 2030

Glass Filter for Transmittance Measurement

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for one-point verification of the transmit- tance and absorbance scales of spectrophotometers at the given wavelength and measured transmittance. It consists of one glass filter in its holder and one empty filter holder. The filter bears an identification number. For protection, the metal holder is provided with two shutters that should be removed during measurements. The transmittance, T, at A 465.0 nm and for a spectral bandpass of 2.7 nm is: The cor- responding transmission density, —logioT, is: .

Date of Certification: . The transmittance value (T) can be converted to percent transmittance (%T) by multiplying by 100. The trans- mission density is calculated from the measured transmittance (T). This value should be indicated by the absorbance scale of the spectrophotometer if the filter is measured against air. The transmittance value given was measured against air at an ambient temperature of 23.5 °C, and is estimated

to be accurate to within ±0.5 percent at the time of certification. This uncertainty includes the random and

systematic errors of the calibration procedure, as well as transmittance changes of the filter during the period of

calibration.

It is possible that aging of the glass may cause some filters to change transmittance by about ±1 percent over a period of approximately 1 year from the date of calibration. Improper storage or handling of the filter may also cause changes [5]. In cases where verification is desirable, the filter should be returned to the National Bureau of Standards for cleaning and recalibration.

It is recommended that the filter in the holder be handled only by the edges and with soft plastic (polyethylene)

gloves and optical lens tissue. When not in use, it should be stored in its holder, with the shutters on, and in the box provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should be avoided.

This Standard Reference Material was issued with the assistance and advice of K. D. Mielenz and J. R. DeVoe. The technical and support aspect involved in the preparation, certification, and issuance of this Standard

Reference Material were coordinated through" the Office of Standard Reference Materials by R. W. Seward.

Washington, D.C. 20234 J. Paul Cali, Chief

September 23, 1976 Office of Standard Reference Materials

(over)

97
The transmittance measurements were made using the high-accuracy spectrophotometer designed and built at the Institute for Materials Research, National Bureau of Standards [1]. This instrument represents the primary

transmittance standard; its transmittance accuracy was established using the double-aperture method of

linearity testing [1,3,5,6]. The reproducibility (relative standard deviation obtained from a set of 20 measurements) of this high-accuracy spectrometer for transmittance measurements on glass filters similar to the one

used to produce this SRM, is approximately 0.02 percent.

The combined uncorrected systematic error of the instrument is estimated to be not more than ±10" transmit- tance units. As indicated on the face of this certificate, SRM 2030 is intended to be used as a one-point verification of the transmittance (absorbance) scales of spectrophotometers. If these scales require verification at other wavelengths and transmittances, "Glass Filters for Spectrophotometry," SRM 930c or subsequent issues, should be

used. The neutral glass for the filter was provided by Schott of Mainz, Germany, and is designated as "Jena Color and

Filter Glass "

[2,5]. The exposed surface of the glass is approximately 29 X 8 mm, measuring from a point 1.5 mm above the base of

the filter holder (see figure). The empty filter holder is provided to be used in the reference beam of the spectro- photometer so that approximately equivalent conditions of stray radiations are achieved for both beams. The transmittance of the filter depends upon the intrinsic properties of the material, spectral bandpass, wavelength, geometry of the optical beam, temperature, and positioning of the filter. A change of ambient temperature of 1 or 2

0 C from 23.5 ° C will not significantly affect the calibration [5].Changes in the transmittance maybe

caused by surface conditions, aging of the glass, exposure to a harmful atmosphere, or careless handling as indicated on the face of this certificate [2,3,4,5]. The effective spectral bandpass used to determine the certified value is given on the face of the certificate and the transmittance measurements are made by producing the

vertical image of the slit (about 8 mm by 0.5 mm), using a convergent beam geometry with an aperture ratio f: 10, in the middle of the entrance face of the glass filter. The filter was measured in the spectrophotometer in a position perpendicular to the incident light beam as shown in the attached figure.

Prior to certification measurements, the filter was examined for surface defects and thoroughly cleaned [5]. Should the surface of the glass filter become contaminated, no attempt should be made to clean it unless the user has the facilities to demonstrate that the cleaning treatment has not altered the glass surface or degraded the accuracy of the certified value. Because the Standard Reference Material is a transfer standard, the only means

available to verify its integrity is remeasuring its transmittance with a primary standard instrument similar to that used in this certification [1,4,5]. In most cases where verification, or recertification, of the transmittance value is desirable, it will be most expeditious to return the filter to the National Bureau of Standards for measurement. Prior to shipment the NBS Office of Standard Reference Materials should be contacted regarding the conditions and cost of verification and /or recertification. REFERENCES

[1] R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and 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, O. Menis and J. I. 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] Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence Measurements, NBS Journal of Research 76A, No. 5, 375-510 (1972).

[5] R. Mavrodineanu and J. R. Baldwin, Glass Filters as a Standard Reference Material for Spectrophotometry-Selection, Preparation, Certification, Use SRM 930, NBS Special Publication 260-51, U.S. Govern- ment Printing Office, Washington, D.C. 20402, (1975).

[6] K. D. Mielenz and K. L. Eckerle, Spectrophotometric Linearity Testing Using Double-Beam-Aperture

Method, Appl. Optics, 11, 2294-2303 (1972).

98
METAL HOLDER FOR THE COLORED GLASS FILTERS

FRONT

Of

TOP

METAL HOLDER

57 X 13 X 13

EXPOSED FILTER SURFACE

29 x 8

COLORED GLASS FILTER

30.5 X II X 2.0 (Of 1.5) BASE

1.5

DIMENSIONS IN mm

99

IIO n ^

. r APPENDIX VIII

U.S. Department of Commerce

Juanita 1W. Kreps

Secretary

National B;jfetw of Standards

Ernest Ambler, Acting Director Rational ^mt&n of j^tattdartte

Certificate

Standard Reference Material 931b

Liquid Absorbance Standards for

Ultraviolet and Visible Spectrophotometry

R. W. Burke and E. R. Deardorff

This Standard Reference Material is certified as solutions of known net absorbance at specific spectral wavelengths. It is intended primarily for use in the calibration and checking of accuracy of the photometric scale of narrow bandpass spectrophotometers employed in clincial analysis and for routine critical evaluation of daily

working standards used in clinical spectrophotometry. This Standard Reference Material is applicable for cali- brating those instruments that provide an effective spectral bandpass of 1 .5 nm or less at 302 nm, 2.0 nm or less at 395 nm, 3.3 nm or less at 512 nm and 8.5 nm or less at 678 nm [1]. Net Absorbance

3

Wavelength and (Bandpass), nm

Filter 302(1.0) 395(1.7) 512(2.0) 678(6.5)

"I" -"Blank" 0.306 ± 0.003 0.302 ± 0.003 0.300 ± 0.003 0.115±0.002

"II" -"Blank" .607 ± .004 .605 ± .004 .602 ± .004 .230 ± .003

"III" - "Blank" .891 ± .005 .902 ± .005 .899 ± .005 .342± .003

Net absorbances (T - "Blank", "II" - "Blank", and "III" - "Blank") were determined using 10.00 mm cuvettes (SRM 932) at 25 °C. See

Instructions for Use. Absorbance measurements were performed on a high precision double-beam spectrophotometer equipped with

a double monochromator. The accuracy of the photometric scale of this instrument was established with the NBS high-accuracy spectrophotometer described by Mavrodineanu [2]. The uncertainties of the certified values include all known sources of possible systematic error and the 95 percent confidence level for the mean. While no long-term stability studies have been made on this lot (931b), studies on previous lots (931 and 931a) over a three-year period showed no degradation of the material when stored in the original sealed ampoules.

Nevertheless, until additional information is forthcoming, it is recommended that this material not be used after three years from the date of purchase.

The overall direction and coordination of technical measurements leading to certification were performed under

the chairmanship of I. L. Barnes. 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 W. P. Reed.

Washington, D.C. 20234 J. Paul Cali, Chief

November 10, 1977 Office of Standard Reference Materials

(over)

101
Temperature Dependence

Absorbances at various temperatures (17 to 35 °C) may be calculated using the equation

At =A2 5[l+CA(t-25)],

where: At = Absorbance at temperature t (°C)

A25 = Absorbance certified at 25.0 °C

Ca = Fractional change in absorbance per

0 C

The values of Ca, at the four wavelengths, are given below. [NOTE: At wavelength 302 nm, absorbance

decreases with increasing temperature; at the other wavelengths, absorbance increases with increasing temperature.]

Wavelength, nm Ca

302 -0.0014

395 +0.0014

512 +0.0018

678 +O.0014

Preparation of Filters

The filters were prepared by dissolving high-purity cobalt and nickel in a mixture of nitric and perchloric acids. The absorbance spectrum of the resulting solution is shown in the following figure. The maxima at 302 and 512 nm are due to absorbance by NCb" and Co(H20)es* respectively. The maximum at 395 nm and the plateau at 650-700 nm are due to Ni(H20)6t The pH of these solutions is about 1.

ABsanaucE

302 395 5C

"Sfe 3&5 5*5 6*5 too

VWVELENGTH. nm

Warning

This Standard Reference Material is intended for "in vitro" diagnostic use only.

Instructions for Use

This material is for use as a spectrophotometric absorbance standard.

1. Select two clean 10.00 mm cuvettes free of scratches. At least one should be fitted with a ground glass or Teflon stopper to minimize evaporation. Reserve it for all sample measurements.

2. Mark each of the cuvettes to assure the same orientation in the spectrophotometer.

3. Place the cuvettes in their respective holders and fill with distilled water. (Borosilicate Pasteur-type pipettes

fitted with rubber bulbs are recommended for transferring all solutions to and from the cuvettes. Soft glass

pipettes, which are available commercially, contain residual amounts of ultraviolet absorbing material, but may be used after proper cleaning. Several rinses, first with isopropyl alcohol and then with distilled water,

are generally adequate.)

4. Obtain the optical mismatch of the cuvettes at 302, 395, 512, and 678 nm, using the spectral bandpass limita- tions given on the face of the certificate.

5. Empty the cuvettes by suction without removing them from their holders, refill with distilled water and measure the absorbances again at each of the above wavelengths.

102
6. Repeat the emptying and refilling operation until constant absorbance readings are obtained.

7. Using the liquid filters provided, measure, in turn, the absorbance of the "Blank", "I", "11", and "III" against

distilled water. Shake each ampoule before opening to remix any condensate which may have collected in the neck. (The ampoules have been prescored directly below the gold band to facilitate opening.)

8. Subtract the appropriate "Blank" reading from the absorbances obtained for"I", "II", and "III". These net absorbances should agree with the certified values within the uncertainties specified. Consult the manufacturer of the instrument if they do not. The above instructions are for use with the standard 10-mm rectangular cuvette. For calibration of the several spectrophotometric systems used in various automated instruments, the user is referred to the instruction manual for the particular instrument. The absorbances of these liquid absorbance standards will depend not only on the accuracy of the photometric

scale, but also on the wavelength accuracy and the spectral bandpass. A mercury lamp is recommended for checking the wavelength scale. In addition, for those spectrophotometers having a hydrogen (H) or deuterium

(D) source, the two emission lines at 486. 1 and 656.3 nm (H) or 486.0 and 656. 1 nm (D) may provide a convenient

check at these wavelengths.

To insure that the measured absorbances are not significantly different from the certified values, the following

restrictions are placed on the size of the spectral bandpass selected: To obtain ±0. 1 percent of the true value, the

effective spectral bandpass should not exceed 1.5, 2.0, 3.3, and 8.5 nmat 302, 395, 512, and 678 nm, respectively. For ±0.2 percent, the respective bandpasses should not exceed 2.2, 2.9, 4.8, and 12.3 nm. Additional information on the effect of spectral bandpass on the absorbances of these filters is given in the figure below. These

curves are not to be used, however, to correct the measured absorbances. + 2.00

CHANGE,

%

bob

o

o

o

678 nm

UJ^ -2.00

<

GO

oc -3.00

o

302 nm\

(f) 2 -4.00 \ 395 nm

-5.00 i i i i i

1 1 \l l 0 2 4 6 8 10 12 14 16 18 EFFECTIVE SPECTRAL BANDPASS, nm

This Standard Reference Material should be kept in the original sealed ampoules. Once opened, the material should be used immediately. No attempt should be made to reseal the ampoule. In addition, it is recommended

that this Standard Reference Material not be used after three years from the purchase date.

References

[1] R. W. Burke, E. R. Deardorff, and O. Menis, J. Research. Nat. Bur. Stand. 76A, 469-482 (1972).

[2] R. Mavrodineanu, J. Research.Nat. Bur. Stand. 76A, 405-425 (1972).

Note: The above papers are also published in NBS Special Publication 378, Accuracy in Spectrophotometry and Luminescence Measurements, R. Mavrodineanu, J. I. Shultz, and O. Menis, editors, U.S. Government Printing Office, Washington, D.C. 20402, 1973.

103

APPENDIX IX

U.S. Department of Commerce

Juanita Kreps

Secretary

•

Certificate

Standard Reference Material 932

Quartz Cuvette for Spectrophotometry

R. Mavrodineanu and J. W. Lazar

This Standard Reference Material consists of a single, accurately calibrated cuvette that is issued for use in the production of accurate spectrophotometry data on liquids. The design and nominal dimensions of the all- quartz cuvette are shown in Figure 1 . The pat hlengt h of the cuvette is defined by the distances between the two

optically transparent windows taken at several heights within the cuvette. Cuvettes issued as Standard Refer- ence Material 932 have a nominal pat hlengt h of 10 mm. The pat hlengt h and parallelism are certified with an uncertainty of ±0.0005 mm as determined by measurements (at 20° C) taken at the positions indicated below.

Cuvette number is issued with this certificate. For this cuvette the following measurements were

The cuvette must be handled with great care and should be held only by the frosted-quartz side windows. When

not in use, it should be stored in the container provided for this purpose. Extended exposure to laboratory

atmosphere and dusty surroundings is to be avoided.

The overall direction and coordination of the technical measurements leading to certification were performed

under the chairmanship of O. Menis and J. A. Simpson.

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, D.C. 20234 J. Paul Cali, Chief

August 2, 1978 Office of Standard Reference Materials (Revision of certificates

obtained.

Height Pathlength

mm mm

dated 11-5-73 and 6-16-77) (over)

105
The original cuvettes were designed and produced at the National Bureau of Standards using special techniques

and non-fluorescent optical-quality fused silica as described in NBS SP 260-32 [1]. The transparent windows are attached to the body of the cuvette by direct fusion, and the unit was stress-relieved by annealing. The overall

flatness of each surface of a transparent window is within two fringes (546 nm Hg line). The radiation pathlength measurements of the cuvette were performed using electronic feeler-gauge type instruments capable of a resolu- tion of 5 parts in 10 . The development and production of SRM 932 is a result of the combined efforts of the Center for Analytical Chemistry, the Center for Mechanical Engineering and Process Technology, and the Instrument Shops Division. E. P. Muth and E. I. Klein designed and assembled the cuvettes, respectively.

Since January 1977, the cuvettes issued as SRM 932 have been prepared by Starna, Ltd., Dayenham, Essex, U. K., in accordance with NBS techniques and specifications described in NBS SP 260-32. The radiation pathlength measurements of all of these cuvettes, regardless of manufacturer, have been performed by E. G. Erber of

the Mechanical Processes Division.

Reference:

[1] Mavrodineanu, R., and Lazar, J. W., Standard Reference Materials: Standard Quartz Cuvettes for High

Accuracy Spectrophotometry, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-32 (December 1973). Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402, 55 cents.

RADIATION PATHLENGTH"

1.25-

10-

1 T

j

10

6 TOP

SIDE.

48

4- —

(FROSTED)

FRONT

(TRANSPARENT)

BOTTOM

TOP MEASUREMENT

LEVEL

1.25

__1

12.5 10 -

12.5 T

BOTTOM

MEASURMENT

LEVEL

T

12.5 2.

INCIDENT RADIATION BEAM

106
U.S. Department of Commerce APPENDIX X

Juanita M. Kreps

Secretary

National Bureau of Standards

Ernest Ambler, Acting Director^National ^nrtnn of jirtanrlartte

Certificate

Standard Reference Material 935

Crystalline Potassium Dichromate

for Use as an Ultraviolet Absorbance Standard

R. W. Burke and R. Mavrodineanu

This Standard Reference Material consists of crystalline potassium dichromate of established purity. Solutions

of known concentrations of this SRM in 0.001 N perchloric acid are certified for their apparent* specific

absorbances**, €a , at 23.5 °C.

This SRM is intended to be used as a reference standard for the verification of the accuracy and linearity of the absorbance scale of absorption spectrometers that can provide an effective spectral bandpass of 1.2 nm or less at 235 nm, and 0.8 nm or less at 257, 313, 345, and 350 nm. Such verification is accomplished by comparing the measured apparent absorbances, Aa , to the Aa calculated from the certified 6a values as described under

"Instructions for Use."

Table 1 gives the certified values of Ca in kgg ' cm

1 for five concentrations of the SRM 935 potassium

dichromate in 0.001 N perchloric acid at 23.5 °C and the indicated wavelengths and spectral bandpasses for a 1-cm internal pathlength.

Table 1. €a , Apparent Specific Absorbance,

3 kg g '-cm1

Nominal Wavelength and (Bandpass) nm

Concentration

gkg"' 235.0(1.2) 257.0(0.8) 313.0(0.8) 345.0(0.8) Uncertaii

b 350.0(0.8)

0.020 12.260 14.262 4.805 10.604 10.672 ± 0.034

.040 12.304 14.318 4.811 10.603 10.682 ± .020

d

.060 12.347 14.374 4.816 10.602 10.692 ± .020

d

.080 12.390 14.430 4.821 10.601 10.701 ± .020

d

.100 12.434 14.486 4.827 10.600 10.711 ± .020

d

6a values are given to the third decimal place to preserve the smooth variation of the data with concentration, although the uncertainties are in the second decimal place. Wavelength 345.0 nm is near one of the two isosbestic points in HCrOJ/CnO? spectra. Because it is on the slope of the composite

spectrum, reproduction of the Ca values is dependent on wavelength accuracy. Measurements at this wavelength should be made only for verification of the linearity of the absorbance scale. 6a values are not corrected for the effects of internal multiple reflections within the cuvette, nor have the weights been corrected to vacuum. With these two exceptions, the uncertainties given include all known systematic errors and the 95 percent confidence

interval of the mean. At wavelength 313.0 nm, the uncertainty is reduced to ±0.010.

*The term "apparent" is used because no corrections have been applied to the data for the effects of internal multiple reflections within the cuvette or for buoyancy, i.e., the weights used to express concentrations have not been corrected to vacuum. These combined corrections do not exceed 0.2 percent. The specific absorbances are given in reference 1. **The nomenclature used in this certificate is that recommended by K. D. Mielenz, Anal. Chem. 48, 1 093- 1 094 ( 1976), which is reproduced

in the Appendix of NBS Special Publication 260-54.

Washington, D.C. 20234 J. Paul Cali, Chief June 1, 1977 Office of Standard Reference Materials

(over)

107
The overall direction and coordination of the technical measurements leading to this certificate were per- formed under the joint chairmanship of I. L. Barnes and J. R. DeVoe. 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 R. W. Seward.

We wish to acknowledge: The contributions of K. D. Mielenz, NBS Analytical Chemistry Division, for his many

valuable discussions and suggestions, and H. H. Ku, NBS Applied Mathematics Division, for his statistical treatment of the data; the cooperation and early support of the National Institute of General Medical Sciences in the research leading to this SRM; and the encouragement of George N. Bowers, Jr., M.D., Hartford Hospital,

Hartford, Conn., and Royden N. Rand, Ph.D., Eastman Kodak Co., Research Laboratories, Rochester, N.Y.

PREPARATION AND CERTIFICATION

The details of the preparation and certification of SRM 935 are provided in NBS Special Publication 260-54,

Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance Standard [2]. This publication should be referred to every time SRM 935 is to be used. Briefly, the transmittances, T, of the

solutions prepared from the undried, as received, material were measured with the NBS Institute for Materials Research high-accuracy transmission spectrometer [3]. The 6a values were calculated for each wavelength using the relation: _ Ds - Db _ Aa ,,, ea "Trr - bTr (1)

where: 6a = apparent specific absorbance

Aa = apparent absorbance

Ds = transmittance density of the sample solution, -logioTs Db= transmittance density of the blank solution, -logioTb b = internal cuvette pathlength, cm

c = concentration, by weight, of K2Cr207 solution, g kg

The crystalline potassium dichromate used for SRM 935 is a special lot of analytical reagent grade material obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J.

Assay: A coulometric assay of the purity of the undried material was performed by G. Marinenko of the NBS

Analytical Chemistry Division. The purity of SRM 935, expressed as an oxidant, was found to be 99.972 ± 0.005 percent where the uncertainty figure represents the 95 percent confidence interval for the mean based on 1

1

degrees of freedom. In addition, the material was examind by optical emission spectrometry for trace elemental

impurities by J. A. Norris of the NBS Analytical Chemistry Division. The only significant impurities detected were sodium and rubidium. Their concentrations were estimated to be in the range of 0.02 and 0.03 percent,

respectively. Drying at 105 °C for 12 hours showed that the surface moisture of this material was less than

0.01 percent.

Stability: Solutions prepared from SRM 935 in the concentration range indicated in table 1 and made according

to the instructions given in NBS SP 260-54 have been found to be stable within the uncertainties given in table 1 for at least six months when stored at room temperature and protected from evaporation and exposure to light.

INSTRUCTIONS FOR USE

The use of SRM 935 as an absorbance standard requires the careful preparation of a series of solutions of known concentrations, c, of the potassium dichromate in 0.001 N perchloric acid. These solutions are trans- ferred to a quartz cuvette of known pathlength, b, and their apparent absorbances measured at wavelengths

235, 257, 313, and 350 nm, using the spectral bandpass requirements given in table 1. The preparation and measurement of these solutions are described in detail in Section 5 of NBS SP 260-54.

108
The accuracy of the absorbance scale of the spectrometer being tested is ascertained by comparing the measured

apparent absorbances, Aa> of a series of 0.001 N perchloric acid solutions containing 0.020 to 0.100 gram

K2Cr207/kg to the Aa values calculated from the certified €a values. Although the €a data in table 1 are given for nominal concentrations of 0.020, 0.040, 0.060, 0.080, and 0. 100 g K2O2O7/ kg, the Ca values for concentrations between these nominal concentrations can be determined by linear interpolation. Using the appropriate €a values, the calculated Aa values at 23.5 °C are obtained from the expression:

Aa = Ca x b x c (2)

Calculations: An example of the calculation of Aa for one concentration of K^C^O under a specified set of conditions is shown below. Calculations of Aa for other concentrations and wavelengths are performed in a similar manner.

Conditions: Wavelength = 350 nm, spectral bandpass 0.8 nm or less b = 0.9982 cm

c = 0.04375 g kg"

1

t = 23.5 °C

From column 6, table 1, the 6a for concentrations of 0.040 and 0.060 g kg"

1 are 10.682 and 10.692, respectively.

The corresponding 6a for c = 0.04375 g-kg is:

6a = 10-682 ^^-y(10.692 -10.682)

Ca = 10.682 + 0.0019

Ca = 10.684

The calculated apparent absorbance, Aa , from equation 2, is: Aa = 10.684 x 0.9982 x 0.04375 Aa = 0.4666 The uncertainty, AAa, in the calculated Aa is determined from the combined uncertainties in €a , b, and c in equation 2, provided no other systematic errors are present. Thus:

AAa = be

I A€a I + €acI Ab I + €a b

I Ac I (3) To evaluate AAa, A€a is taken from column 7 of table 1 and the Ab and Ac values must be determined experimentally.

In the experiments performed to obtain the €a values in table 1 , the uncertainties for b and c did not exceed 1 part

in 10 and 2 parts in 10 , respectively.

The solution of equation 3 gives:

AAa = 1(0.044) (0.020) + 10.7(0.044) (0.0001) + 10.7(1) (0.0000088) = 0.0010 Thus, the uncertainty of Aa, for the above set of conditions, is ±0.0010. The correction of the absorbance scale of the absorption spectrometer under test is determined by plotting the

differences between Aa measured and Aa calculated as a function of absorbance. A typical plot of such a graph

is shown in figure 1. The apparent absorbances measured on this instrument at 350 nm are accurate when the indicated correction is subtracted from the corresponding absorbance scale reading, provided that the conditions of wavelength accuracy, spectral bandpass, and absence of stray light are fulfilled as specified in NBS SP 260-54. Correction curves for wavelengths 235, 257, and 313 nm are obtained in a similar manner.

109
8 iFigure 1. Correction curve for the absorbance

scale of a precision spectrometer. The error bars

are the sum of the errors arising from the uncertainties in the certified apparent specific absorbances,

6a , cuvette pathlength, b, and concentration, c.

0 I 1 li I I I I I I I

i i 0 0.2 0.4 0.6 0.8 1.0 Absorbance Temperature Correction: Although €a values in table 1 are certified at 23.5 °C, SRM 935 can be used as an absorbance standard at other temperatures in the range 20 to 30 °C provided corrections are made to the €a values. Over this range the apparent specific absorbances decrease linearly with increasing temperature for all the wavelengths given in table 1. The corresponding temperature coefficients, k, for these wavelengths are given in table 2.

Table 2. Variation of 6a with Temperature Over the Range 20 to 30 °C.

A, nm Temperature Coefficient, k Percent per degree Celsius

235 -0.05 257 -0.05

313 -0.02

345 -0.08 I 350 -0.05 ! The value of €a at any temperature in the range 20 to 30 °C can be calculated from the certified value and the appropriate temperature coefficient using the relation:

el = el

3- 5 ri+JL<t- 23.5)1

where: Ca = apparent specific absorbance at temperature t (°C)

23 5 6a

' = apparent specific absorbance certified at 23.5 °C. k = temperature coefficient, percent per °C.

REFERENCES

1. Burke, R. W., and Mavrodineanu, R., Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance

Standards, J. Res. Nat. Bur. Stand. (U.S.), 80A (Phys. and Chem.), No. 4, 631-636 (July-Aug. 1976).

2. Burke, R. W., and Mavrodineanu, R., Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance Standard, NBS Spec. Publ. 260-54 (1977). Copies may be obtained from the Office of Standard Reference Materials, National Bureau of Standards, Washington, D.C. 20234.

3. Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials, J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No. 5, 405^25 (1972).

110
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. SP 260-68

2. Gov't Accession No, 3. Recipient's Accession No,

4 TITI F AND SIIRTITI F Standard Reference Materials:

Me ta I -un-ijuar lz m iters as a otanaara KcTerence

Material for Spectrophotometry-SRM 2031

5. Publication Date April 1980

6. Performing Organization Code

7. AUTHOR(S)

Mavrodineanu, R. and Baldwin, J. R.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) National Institute for General Medical Sciences

National Institutes of Health

9000 Rockville Pike

Bethesda, MD 20205

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 79-600192

I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

! 16. ABSTRACT (A 200-word or less [actual summary of most si grii ficant information. If document includes a s i gni ficant bibliography or

literature survey, mention it here.) This publication describes in detail the selection, production, testing, and

certification of the semi-transparent evaporated metal -on-fused silica filters as a Standard Reference Material for spectrophotometry. Particular attention is given to

various factors that can affect the accuracy of the transmi ttance values established

I for these filters, such as: reflections, polarization, angle of incidence, and

stability in time. Adequate procedures were devised and are described for the protection of the exposed metal layer surface using optical contact, and for the

protection of the filter aggregate, by the use of metal holders provided with shutters

and of metal storage containers.

The data from several interlaboratory tests are also discussed, together with the

results obtained on transmi ttance measurements on 19 sets of chromium-on-fused silica

filters, protected by optical contact, which were studied over a period of two years.

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 fir3t letter of the first key word unless a proper nam&;

separated by semicolon*) Filters for spectrophotometry ; metal -on-fused silica filters;

optical transmi ttance accuracy, polarization; reflections; spectral bandpass; Standard

Reference Materials in Spectrophotometry; stray radiation; transmittance stability.

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