A11107 31D731

NBSIR 84-2889

NBS Response to the Fourth

CORM Report on Pressing

Problems and Projected National

Needs in Optical Radiation

Measurements

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Radiometric Physics Division

Gaithersburg, MD 20899

May 1 984

Prepared for U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

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NBSIR 84-2889

NBS RESPONSE TO THE FOURTH

CORM REPORT ON PRESSING

PROBLEMS AND PROJECTED NATIONAL

NEEDS IN OPTICAL RADIATION

MEASUREMENTS

K. D. Mielenz

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Radiometric Physics Division

Gaithersburg, MD 20899

May 1984

Prepared for U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

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U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

PREFACE

The Council for Optical Radiation Measurements (CORM) was founded

in 1972 as a non-profit organization of professionals and

institutions engaged in optical radiometry and spectrophotometry.

The aims and purposes of CORM include:

o Assessment of national requirements for standards,

calibrations, and interlaboratory collaborations.

o Establishment of national consensus on priorities for

these requirements

.

o Liason with the National Bureau of Standards in order to

advise NBS of requirements and priorities.

o Dissemination of information on standards activities in

optical radiometry and spectrophotometry.

In fulfilment of its statutory obligations, CORM has periodically

issued technical reports on measurement problems and needed

standards. In its past reports, CORM listed many requests for

specific services from NBS over such a wide range that it was

difficult for the Bureau to make a significant response. To allow

more expeditious solutions, the CORM Directors chose a new

approach in preparing their Fourth Report and have now classified

the perceived standards needs into three categories: those that

must be addressed by NBS in its role as national laboratory,

those that can be solved by collaborative efforts of industry and

NBS, and those that can be solved by the users themselves with

guidance from the Bureau. NBS agrees that such a classification

of needs is realistic.

This publication constitutes the NBS Response to the Fourth CORM

Report. It describes NBS policies for radiometry and

spectrophotometry, the current status of projects suggested by

CORM, and future plans. It also contains specific proposals for

collaborative CORM/NBS efforts to provide needed standards and

measurement services. With permission by CORM, the Fourth CORM

Report itself is included as an appendix.

Klaus D. Mielenz, Chief

Radiometric Physics Division

National Bureau of Standards

CONTENTS

Page

I. GENERAL COMMENTS 1

II. RADIOMETRY 4

AK-1: Speetral-Irradiance Standard to 2500nm 4 ES-2: Develop a Sustained Source of Radiometric

Standards with Rapid Accessibility 5

WS-1: Develop Low-Light-Level Calibration Standards.. 6

ES - 3 : Evaluate Silicon Detectors as Reliable

Standards for Radiometry 6

WS-2: Near-Infrared Detectors 8

AK-2: Spectral-Radiance Standard to 50ym 8

WP-2: Computerized Array Radiometry 9

III. SPECTROPHOTOMETRY ..................................... 11

1. NBS and Private Calibration Laboratories 11

2. Requirements for Calibration Laboratories 13

3. Evaluation of Measurement Capabilities 14

3. Establishment of Needs 16

REFERENCES 18

APPENDIX: FOURTH CORM REPORT 19

NBS RESPONSE TO THE FOURTH CORM REPORT ON PRESSING PROBLEMS AND

PROJECTED NATIONAL NEEDS IN OPTICAL RADIATION MEASUREMENTS

I . GENERAL COMMENTS

Like its predecessors, the Fourth Report on "Pressing Problems

and Projected National Needs" of the Council for Optical

Radiation Measurements (CORM) offers sound technical advice to

the management and staff of the National Bureau of Standards. The

information contained in it is important for evaluating the

relevance and effectiveness of our ongoing programs in optical

radiometry and for planning our future activities in these areas.

Without such input from its constituents, NBS could not be

responsive to user requirements for physical standards and

measurement services.

The importance of the Fourth CORM Report has been enhanced by the

fact that major portions of it were incorporated into the 1982

National Conference of Standards Laboratories (NCSL) National

Measurement Requirements Survey. The NCSL survey augments the

findings of CORM, inasmuch as it addresses some of the

radiometric measurement needs of user groups, notably the defense

and aerospace communities, which are not as strongly represented

in CORM. However, these needs were not ranked by priority as in

the CORM Report.

NBS is indebted to CORM for the considerable degree of thought

and effort that went into the preparation of the Report. In

particular the Bureau appreciated the new approach taken by CORM

in classifying the measurement needs in radiometry and

spectrophotometry , not only by their technical significance and

industrial impact, but also in terms of ways of achieving them in

the most efficient manner. This approach has led CORM to conclude

that it is impossible for the National Bureau of Standards to

supply every standard, but that the voluntary cooperation of

industry is needed if all requirements are to be met. The Bureau

endorses this view as a realistic basis for progress in a climate

of growing industrial needs and limited public resources, and

gladly accepts CORM's proposal of a collaborative i ndustr y/NBS

program to improve and extend existing radiometric standards. We

believe that the Fourth CORM Report constitutes an excellent

basis for such a program, and will cooperate to the fullest

extent possible in the achievement of mutually established goals.

1
NBS management assigns a high priority to providing measurement

services in optical radiometry and has allocated a fair share of

Bureau resources to this area. Even so, the resources available

to us are too small to achieve the CORM goals in a reasonable

time unless the burden is appropriately shared. In our opinion,

there are three areas in which active participation by CORM is

needed:

° Deleg at ion o f Industrial Research Assoc iate and Guest

S cie n tists. During the past several years, the programs

of the Radiometric Physics Division of NBS have attracted

a substantial number of foreign guest scientists who have

joined our staff for extended periods of time. In our

opinion, the same mechanism should be used to effect a better leverage of NBS resources in the transfer of

federally developed technology to the radiometric

community of the U.S. The Bureau has several programs

under which our facilities can be made available to guest

scientists. For the purposes addressed by CORM, the

most appropriate of these is the NBS Industrial Research

Associate Program, as described in available brochures. 3)

We invite the industries that comprise CORM to sponsor

research associates to work at NBS on the priority items

identified in the Fourth CORM Report as suitable for

industry/NBS cooperation. Likewise, we invite CORM

members affiliated with academic institutions to work

with our staff on radiometric problems of mutual

interest, or to assign graduate students to the Bureau

for thesis work on research required for radiometric

standardization.

o Certification o f Inter m ediate Calibration Laboratories.

As suggested in the Fourth CORM Report, intermediate

calibration laboratories linked to NBS through

appropriate measurement assurance programs could

supplement the range of calibrations and standards

provided by the Bureau. The formal accreditation of such

laboratories is possible through the NBS National

Voluntary Laboratory Accreditation Program (NAVLAP).

However, formal accreditation may not be necessary in the

instances cited in the Fourth CORM Report where

documentation of traceability to standards maintained at

NBS is not a legal requirement. We believe that it would

be more cost-effective in such cases if CORM, rather than

NBS, would accredit laboratories. Suggested procedures

for accreditations by CORM are given in Section III. 3*

below.

- 2 -
o Techn i cal Task Forces , Workshops , and Intercomparisons . In a few cases, the requirements stated in the Fourth

CORM Report are more stringent than those identified by

other sources. When questions of this nature arise, a CORM/NBS task force should be set up to make a reassessment of needs. We believe that this approach

would quickly produce positive results, as is

demonstrated by the Fourth CORM Report itself and by the

recent establishment of a CORM Array Radiometer Users’

Group. One of the jobs of a users’ group or task force

should be to sponsor workshops in which experts discuss

specific measurement problems, assess needs and

priorities, and suggest solutions. In cases where the

state of the art is in doubt, intercomparisons should be

conducted to diagnose the problem. Specific needs for

task forces will be identified wherever appropriate in

this Response.

- 3 -
II. RADIOMETRY

Section II of the Fourth CORM Report has been especially useful

to NBS because it focuses on a few, carefully chosen priority

proposals for R&D projects with a good probability of succeeding

within the framework of available resources. The ranking of these

projects confirms information received from other segments of the

user community, and is generally consistent with NBS priorities

and long-range plans in radiometry. As may be seen from the

following progress reports, the Radiometric Physics Division of

NBS has already begun to implement several of the CORM radiometry

proposals

.

AK-1: Spectral Irradiance Standard to 2500nm

In 1973, a new NBS scale of spectral irradiance was realized with

a three-fold increase in accuracy over the previous scale. The

detection system that was chosen comprises a cooled

photomultiplier tube for the 250-800nm spectral region and a cooled lead-sulfide detector for the near infrared. Because of

poor signal- to-noise ratios at longer wavelengths, the infrared

limit of the scale was set at I 600nm.

We have just implemented an improved method of spectralirradiance scale realization. The new method is simpler and

potentially more accurate than that used before, because the

measurement chain from blackbody standards to spectral-irradiance

lamps has been shortened through increased reliance on the

verified linearity of photomultiplier tubes rather than on

numerous temperature settings of transfer lamps. A 1984 rerealization of the NBS spectral-irradiance scale between 250 and

800nm showed no departures from the 1 973 scale outside of

experimental uncertainties.

Work is under way to characterize the linearity of the leadsulfide detector, so that the new method can also be applied in

the near infrared. However, this in itself will not allow an

extension of the scale to longer wavelengths. We have tested the

spectral response of our equipment in the infrared and have found

that the signal levels obtained in straight lamp-to-lamp

comparisons drop so rapidly that the s i gna 1 - t o - no i s e ratios

beyond I600nm are excessively low. This indicates that a new

approach may be needed for extending the spectral-irradiance

scale to 2500nm, requiring a larger effort than previously

anticipated.

- 4 -
ES-2: Develop a_ Sustained Source of Radiometric Standards with

Rapid Accessibility

We agree that this proposal is timely and should be given a high

priority. To explore the problems associated with the establishment of an intermediate laboratory program, a measurement area

with a well-defined user community was chosen.

For industrial measurements of geometrically total flux, the only

NBS standards needed thus far were incandescent 1 um i nous- f 1 ux

lamp standards. Secondary standard lamps of other types were

developed independently by the U.S. lighting industry, and

industry-wide measurement quality assurance was provided through

i n t e r c o m pa r i s o ns conducted by the Lamp Testing Engineers'

Conference (LTEC). However, the development of new, energyefficient lamp types that have numerous emission lines

superimposed on a continuous spectrum now requires improved

standards for spectral measurements of total flux.

The U.S. lamp companies and NBS, working through LTEC, have

formed a task force to study these needs and to formulate a plan

to meet them. The task force met several times during 1983 and

1984, and agreed that there is a need for NBS to provide

incandescent spectral flux standards and at least one spectrally

rich type. A 400W clear high-pressure sodium lamp was deemed most

appropriate for the latter. A detailed list of control parameters

and measurement conditions for this lamp was formulated.

This additional effort on the part of NBS is considered

sufficient to build reduncancy into the measurement system. It

will allow the establishment of measurement assurance procedures

for an intermediate calibration laboratory to issue additional

incandescent, m e t a 1 - ha 1 i d e , and fluorescent lamp standards as

required. The measurement capability of the intermediate

laboratory is to be verified by LTEC i nt er com par i sons with NBS

participation.

The project is now in an equipment procurement phase. The major

items are: A spectroradiometer with associated microprocessor, a 3kVA ac line conditioner, and a high-accuracy, true-rms

wattmeter. It is presently hoped that laboratory work on the

incandescent-lamp phase of the project can begin in 1985.

- 5 -
WS-1 : Develop Low-Light-Level Cal ibrat ion Standards

At present, NBS has no plans to produce low-level standard

sources. In the past we have performed special tests on LED's and

t r i t i u m - a c t i v a t e d self-luminous sources, but have not found

enough demand for a regular issue of low-level calibration

sources by NBS.

Because of the demonstrated linearity of silicon photodiodes, it

is now possible to extend the use of absolute detector standards

to lower levels. It seems, therefore, that a detector/amplifier

package with verified linearity over several decades may have

greater versatility than specific low-level sources. Would such a package be useful to a broad community of users? What spectral

region is needed most? What is the most useful range of radiant

power or irradiance?

We suggest that a CORM/NBS task force be established to determine

the standards needs in this area.

ES-3: Evaluate Silicon Detectors as Reliable Standards for

Radiometry

The application of solid-state physics as a new theoretical and

practical basis for absolute radiometry has been the most

important research activity of the Radiometric Physics Division

for the past several years. As a result, the feasibility of using

silicon photodiodes as reliable radiometric standards for the

visible spectrum is now firmly established.

For example, the Division participated in several

intercomparisons that have demonstrated full agreement of selfcalibrated silicon diodes with other radiometric standards:

o An electrically calibrated, cryogenic cavity radiometer

of the National Physical Laboratory in Teddington,

England, was compared at 568nm with an NBS selfcalibrated silicon photodiode. Agreement was achieved to

within +/- 0 . 2 %

.

o The spectral irradiance of a tungsten-halogen standard

lamp was measured with a spectroradiometer that was

calibrated at 633nm with a HeNe laser and a selfcalibrated silicon photodiode. The ratio of the

b 1 a c k b o d

y - ba s e d irradiance to detector-based irradiance

was found to be 1.011 with a combined uncertainty of

1 . 1 $.

- 6 -
o The three-way intercompar ison of a s i 1 i c o

n

- pho t o d i o d

e

filter radiometer, a tungsten-halogen spectral-irradiance

standard lamp, and synchrotron radiation at 600nm

demonstrated the mutual consistency of these three scales

within 1 %

.

The Radiometric Physics Division has also devoted substantial

efforts to facilitate the practical application of silicon

photodiodes

o Collaborating with a domestic detector manufacturer,

staff members of the Radiometric Physics Division have

developed 100$ quant urn- eff i c i ent detector packages for

measurements of incoherent sources in the visible

spectral region. This was achieved by forming several

large-area silicon photodiodes into a t i gh t 1 y- pa c k e d

,

m u 1 t i p 1 e- r e f 1 e c t i on light trap in which all of the

incident radiation is absorbed within an f/7 cone, thus

producing one electron in the external circuit for each

incident photon. These detector packages are now

commercially available.

o We have developed a self-contained electronics package

for the self-calibration of silicon photodiodes in user

laboratories. The package contains channels for highaccuracy dc current-to-voltage conversion and

conditioning of diodes, divider circuits, and power

sources for forward and reverse biasing. It was developed

for the Army Calibration Laboratory at Redstone Arsenal

in Huntsville, Alabama, where it is now used for primary

radiometric calibrations below lOmW.

o A collaborative project with the National Standards

Institute (NIS) of Egypt is under way to generate a spectral-irradiance scale in the visible spectral region

by means of i n t e r f e r e n c

e - f i 1 1 e r e d , s e 1 f - c a 1 i b r a t e

d

silicon photodiodes. This scale will in turn be used to

generate and maintain absolute photometric scales at

NIS. The required optical and electronic equipment has

been specified, procured, characterized, and shipped to

Cairo with the help of two Egyptian guest scientists. At

the present time, the actual realization of absolute

photometric scales at NIS is dependent on the resolution

of customs delays.

- 7 -
Silicon-diode radiometry is rapidly finding new applications in

foreign and domestic standards laboratories and academic

institutions. We believe that this technology also offers unique

opportunities for industrial applications and, therefore, urge

the appointment of research associates to gain hands-on

experience by participating in our R&D activities in this field.

WS-2: Near-Infrared Detectors

For the past two years, NBS has provided research grants to the

Photovoltaic Laboratory of Purdue University to develop germanium

detectors for high-accuracy radiometry in the 600-l600nm spectral

region. The program consists of iterative computer modeling,

construction, and testing of germanium diodes in an attempt to

develop a viable procedure for the fabrication of near-infrared

detectors that meet the needs of accurate radiometry. We have

received two prototype germanium photodiodes from Purdue

University and will evaluate them when the construction of a new,

monochromator-based detector characterization facility has been

completed.

The quan t urn - e f f i c i en c y model used to interpolate the spectral

response of silicon photodiodes was modified to improve its

accuracy at long wavelengths. Reflectance measurements at

infrared laser lines and our model of the reflectance of an airSiC> 2 -Si interface were used to determine the loss at the back of

the diode for infrared radiation that passes through the diode

without being fully absorbed. The term in the collectionefficiency model that describes recombination in the region

between the depletion region and the rear of the diode was

replaced by a more accurate expression. The net result is that

the interpolation procedure is now accurate to + / — 1 % out to

9 60nm

.

AK-2: Spectral Radiance Standard to 50pm

Long-wave infrared (LWIR) radiometry is highly relevant to needs

of the Department of Defense (DoD), and consequently most of the

LWIR work at NBS is in response to these needs. As may be seen

from the NCSL requirements study 1 * 2 ) the importance of this area

is becoming more widely recognized. Low-level ir sources and

attenuator performance are of prime interest. We are seeking DoD

funding for a major LWIR program at NBS, but have had only

limited success so far.

- 8 -
During the past fiscal year, we have built and characterized a highly precise room- temperature blackbody for the Air Force. Its

design was based on a blackbody that had been designed about ten

years ago at NBS. The blackbody will be used to calibrate a reference r o o m - t e m pe r a t ur e blackbody for FLIR calibrations.

Another calibration involved a blackbody in support of the DARPA

Teale-Ruby project.

In the past, we have also performed special calibrations of

var iable- temperature (200-400K) blackbodies in a cryogenic, highvacuum environment. Because the cryogenic chamber used for this

work was not originally designed for routine testing, a decision

was made to build a new chamber, making various improvements such

as enlarging the working volume and increasing the detectivity.

A new capacitance thermometer and high-frequency signal generator

have been ordered with DoD funds, and will be installed in the

new chamber.

As a component of our pyrometry program, we have developed a new

gold-point blackbody, using a heat pipe to achieve a very uniform

temperature distribution. This technology appears to be easily

adaptable to variable, 1 o w er - t e m p e r a t ur e (down to about 300°C)

blackbodies, and DoD has expressed interest in supporting their

further development. One of the intended applications of these

blackbodies is their use as spectral-radiance standards for the

infrared

.

WP-2: Computer ized Array Radi ome try

In response to the CORM proposal that we evaluate the arraydetector approach to radiometry, NBS management authorized the

procurement of an array radiometer. We examined two commercial

instruments and purchased one during Fiscal Year 1983.

We have studied the linearity of our array radiometer and found

that the instrument is capable of radiometric measurements to an

accuracy of \ % without applying corrections. This is a significant improvement over an earlier model that we had

examined. Below the 1 t level, we observed two distinctly

different response functions for the odd and even numbered diodes

of the array. This can be attributed to the fact that two

amplifiers, having different characteristics, are used in this

design in order to increase the measurement speed of the array.

One amplifier services the odd diodes, while the other is

simultaneously servicing the even ones.

- 9 -
As an alternative to the spatial-dispersion devices, we are

currently evaluating the feasibility of Four i er- transform array

radiometers. The optical design employed in our study is a

1 ateral- shear Michelson interferometer, producing a Fizeau fringe

pattern that is detected by a 1024-pixel, linear diode array and

optical multichannel analyzer interfaced with a microprocessor

for fast Fourier transforms. Initial results are encouraging, and

we expect to complete the feasibility study next year. The most

promising final design is a compact interferometer consisting of

two right-angle prisms joined into a cube, so that the common

hypothenuse forms the beam splitter. The throughput of this cube

interferometer should be fully c h a r a c t e r i z a b 1 e by simple

measurements and calculations.

- 10 -
III. SPECTROPHOTOMETRY

NBS welcomes the CORM proposal of a joint effort to establish a measurement support system for intermediate calibration

laboratories in spectrophotometry. Because the Fourth CORM Report

does not recommend specific steps to achieve this goal, we urge

the formation of a CORM/NBS task force to define such action

steps and to implement them expeditiously. In our opinion, the

task force should:

1. Define the respective roles of NBS and intermediate

calibration laboratories;

2. Identify candidate laboratories;

3. Evaluate their measurement capabilities;

4. Specify the standards or calibrations that are needed.

As a basis for discussion, our views on these topics are

summarized as follows. Pertinent comments may also be found in a

1 9 7 7 NBS study of the National Measurement System for

Spectrophotometry.^)

1 . NBS and Private Cal ibrat ion Laboratories

S p e c t r o p h o t o m e t r i c standards can be classified into four broad

categories, as listed below.

Absolute Standards . Absolute accuracy in spectrophotometry

requires direct measurements of radiant flux in relative units,

as well as calibrations of wavelength and bandwidth. The

standards for such measurements are procedural and documentary in

form, and are provided by NBS and other national laboratories.

They include:

o Critically evaluated data of intrinsic properties of

physical standards (e.g., wavelengths of emission and

absorption lines).

o Published methodologies for making absolute measurements

(e.g., of diffuse reflectance for specified geometries).

- 11 -
o Recommended procedures for instrument calibrations based

on first principles (e.g., linearity testing by beamaddition methods.)

In principle anyone can use these "absolute standards" for

spectrophotometry, but in fact few laboratories have the

facilities to do so on a large scale. During the last decade, a major effort was made at NBS to develop the high-accuracy

instruments and techniques that are required for absolute

measurements of a broad range of spectrophotometr ic parameters.

This capability now constitutes the absolute basis for

spectrophotometry standardization in the U.S.

M aster Standards . Because of the complexities of absolute

spectrophotometr i c measurements, a need exists for standards that

have been compared to absolute standards but are themselves

similar to test objects. The calibration of a spectrophotometer

then becomes a straightforward matter of comparing like objects

under like conditions. NBS supplies a large number of "master

standards" for this purpose:

o Standard Reference Materials to ensure the accuracy of

spectrophotometric measurements.

o Reference Data for other materials that are suitable for

instrument calibrations.

o Measurement Assurance Programs for establishing quality

control procedures.

o Special Calibrations of materials or objects for highaccuracy measurements.

The common denominator for this class of standards is that they

serve the spectrophotometric user community as a whole. Most of

them allow the calibration of highly precise and versatile

commercial spectrophotometers that are used in different fields

such as analytical chemistry and measurements of color and

appearance

.

ZHiLHiL f. ££. In routine applications of

spectrophotometry, it is desirable that standards meet certain

convenience criteria. For example, solution standards are best

for use in analytical chemistry while glass filters are preferred

for color measurements. Many of the specific standards needs

- 12 -
cited in the Fourth and earlier CORM Reports fall into this

category of "transfer standards." Although we recognize the need

for these standards, we believe that their development at NBS

would lead to duplication of efforts and would overextend the

resources available to us. It is more appropriate that these

standards be issued by intermediate calibration laboratories

which have a broad measurement capability in spectrophotometry

and receive measurement assistance from NBS.

W__o_ r_ k__i_ n__g .S _ta.n_ dar _d s_

.

There are dozens of specialized

applications in which spectrophotometry is simply one of several

test methods. In these applications, the operator cannot pay

attention to details and, therefore, needs "working standards"

which are specific to his purpose. Examples of these are crossed

filters to assess the wavelength accuracy of abridged

spectrophotometers, colored ceramic tiles used as standards of

c hr o m a t i c i t y , or polished glass used as a gauge for gloss

measurements. In general, these working standards are so

specialized that only a small subset of users is served by their

issuance. Their development should be left to private

laboratories that specialize in the respective subdisciplines of

spectrophotometry.

The main conclusion that can be drawn from this classification of

s p e c t r o p h o t o m e t r i c standards is that the priorities stated in

Section III.C of the Fourth CORM Report need to be reconsidered.

We agree that NBS leadership is required in the development of

absolute standards and master standards, but suggest a stronger

industry involvement in the areas of transfer and working

standards

.

2 . Requirements for Calibration Labor ator i es

At present, the main obstacle to a workable NBS/industry program

in spectrophotometry standardization is the lack of private

calibration laboratories that can supply transfer and working

standards to end users. We believe that the requirements for such

laboratories are as follows.

The greatest need exists for laboratories capabable of developing

transfer standards with a broad applicability

spectrophotometry. The internal quality control of then*:

laboratories should be based on absolute standards to the

greatest extent possible. A few private laboratories in the U. .

- 13
have already developed their own high-accuracy spectrophotometers

for absolute measurements of selected parameters, such as regular

transmittance or diffuse reflectance for one of the standard CIE

geometries. Calibration laboratories that wish to issue

spectrophotometr i c transfer standards should be encouraged to do

so also. At a minimum, they should employ master standards from

NBS to ensure consistency of their calibration equipment and

procedures with national standards. Where master standards are

not provided by NBS, it may be possible to obtain them from other

national laboratories under the auspices of reciprocity

agreements that assure the compatibility of the national

standards of different countries. A committee of CIE Division 2

is currently compiling a list of spectrophotometric standards

that are available from different standardizing laboratories.

Working standards for spectrophotometry could be supplied by the

same private laboratories that issue transfer standards. An

alternative source would be through specialized laboratories,

such as instrument manufacturers and others who adapt transfer

standards for applications within a given field. Because the

accuracy requirements for working standards are less, the quality

control for their development can be based on transfer standards

and, therefore, does not require direct measurement assistance

from NBS. However, standards organizations should establish

control procedures to ensure that the working standards used in

different fields are properly correlated with each other and are

consistent with national standards. In many cases, NBS staff

members are engaged in the work of these standards organizations

and will therefore be able to assist in this endeavor.

3 . Evaluation of Measurement Capabilities

The Radiometric Physics Division of NBS has developed several

Measurement Assurance Programs (MAPs) that can be used to

evaluate the measurement capabilities of private calibration

laboratories for spectrophotometry. These MAP packages contain

artifact standards which are fully characterized by NBS before

being sent to a customer’s laboratory. The customer measures the

artifacts also, and returns them to NBS along with his data. NBS

then repeats the measurements, performs a diagnostic analysis of

all data, and issues a test report which contains estimates of

the random and systematic uncertainties of the customer's

measurements. NBS charges a set fee for these services.

- 14
The spectrophotoinetric MAPs that are or will soon be available

from NBS are:

o Spectral Transmittance MAP. A package containing one

didymium glass filter for checking wavelength scales from

^00 to 750nm, and seven neutral- dens i ty filters (0.001 to

0.92 T) for checking transmittance accuracy.

o Diffuse Reflectance MAPs. One package contains a white

ceramic tile (0.85 RF) and a black porcelain tile (0.07

RF) for 6°/hem i spher i cal measurements. The other contains

five gray porcelain enamel tiles (0.02 - 0.87 RF) and one

black glass (0.001 RF) for 4 5 ° /

0

0 measurements.

o Retroref lectance MAP for Luminous-Intensity Coefficients.

A package containing one high-intensity specimen and one

e n g i n e e r i n g- gr a d e specimen of bead sheeting, and one

cube-corner r e t r or e f 1 e c t or . Also included are seven

filters for spectral tests.

The first two of these are suitable for measurement assurance in

the calibration of broadly applicable transfer standards for

transmission and reflection spectrophotometry. The last should be

useful in the development of additional standards for

retroreflection.

Where MAP services are not provided by NBS, blind

intercomparisons among calibration laboratories can be used as an

alternative method of verifying the performance of

laboratories. In our opinion, the following steps are necessary

for conducting such i nt er com par i sons

o The C0RM/NBS task force determines the objects to be

i n t e r c o m pa r e d and establishes control parameters. This

can be done in cooperation with voluntary standards

organizations, such as ASTM or ANSI.

o CORM convenes the intercompar ison. NBS participates as as

a reference laboratory on a fee basis, thus providing a tie to national standards.

o CORM performs the data analysis. If the i n ter com par i son

is done for accreditation purposes (see below), CORM

documents the results by publication in the archival

literature or in the CORM Newsletter.

- 15 -
The issue of a MAP report by NBS or its participation in an

intercomparison cannot be construed as Bureau endorsement or

accreditation of laboratories. As already mentioned, a formal

accreditation of private laboratories is possible through the NBS

NAVLAP program but might not be necessary if the demonstrated

competence of a laboratory is sufficient for user acceptance of

standards. In these cases the accreditation of laboratories by

CORM, based on the results of MAPs or i n t er com par i s ons , may be

possible. We suggest that criteria and guidelines for such

accreditations be established by the CORM/NBS task force. The

requirements for accreditation should include:

o Definitions of quantities measured, physical principles

of measurements, instruments used, etc.

o Requirements for measurement assurance (MAPs,

i n t e r c o m p a r i s o n s , statistical controls, operator

requirements, etc.)

o Procedures for accreditation, renewals, and terminations.

4 . Establishment of Needs

The extensive list of "Specific Needs" in Section III.D of the

Fourth CORM Report emphasizes the urgency of establishing private

calibration laboratories for spectrophotometry. However, the list

is apparently not prioritized and, in a few cases, states

requirements that seem beyond the state of the art in

spectrophotometry. For example, the required absolute accuracy

for spectral reflectance scale standards is given as 0.1$ from

200 to 2500nm whereas the current NBS capability is only 0.15% in

the visible and increases to 1 % at the stated uv and ir limits.

These and other questions regarding the establishment of a national standardization system for spectrophotometry should be

resolved by the CORM/NBS task force. Specifically, the task force

should

o £ N_e_e_d_s_. The need for spectrophotometric

transfer and working standards should be ranked in the

same way as outlined in the Introduction to the Fourth

CORM Report; i.e., in terms of technical impact as well

as availability of resources. The availability of

services from NBS should be considered together with the

capabilities of private calibration laboratories. Those

areas should be identified where urgently needed

standards can be developed most expeditiously.

16 -
o Conduct Intercomparisons . The list of "Specific Needs" in

the Fourth CORM Report is likely to overwhelm even the

combined efforts of NBS and CORM. It appears that the

list can still be narrowed. In order to decide which

standards and SRMs are most needed, carefully organized

i nter com par i sons could be conducted. NBS can serve as a reference laboratory in these intercomparisons.

o Identify M easure m ent Assistance Needs . NBS needs to be

informed if presently offered master standards are

insufficient to support the development of transfer

standards by private laboratories. We offer the use of

our facilities to industrial research associates for the

development of needed standards or measurement assurance

procedures, if these cannot be provided by NBS staff.

o Assist i n Transfer o f NBS Techology . NBS currently

offers services for specialized applications of

spectrophotometry, such as measurements of haze, gloss,

and optical density. Our long-term goal is to gradually

reduce the number of these services, allowing them to be

done by private laboratories. Proper procedures for

effecting this transfer of technology must be

establ ished

.

- 17
REFERENCES

National Conference of Standards Laboratories (NCSL)

1 982 National Measurement Requirements Survey, NMRC 8

3

— 0 1

,

May 1 983

2

)

NBS Response to the 1982 National Measurement Requirements

Survey of the NCSL, NBSIR 84-28117, March 1984

3) Inquiries regarding the NBS Industrial Research Program may be

directed to:

Mr. P. R. deBruyn, Industrial Liaison

Office of Research and Technology Applications

National Bureau of Standards

Gaithersburg, MD 20899

(301) 921-3591

4 )’ Inquiries regarding the NBS National Voluntary Laboratory

Accrediation Program may be directed to:

Mr. J. W. Locke, Manager

Laboratory Acceditation

National Bureau of Standards

Gaithersburg, MD 20899

(301) 921-3431

s 1 The National Measurement System for Spectrophotometry,

NBSIR 75-940, November 1977

s

- 18 -
COUNCIL FOR OPTICAL RADIATION MEASUREMENTS

FOURTH REPORT TO THE NATIONAL BUREAU OF STANDARDS

PRESSING PROBLEMS AND PROJECTED

NATIONAL NEEDS IN OPTICAL

RADIATION MEASUREMENTS

August, 1982
I. INTRODUCTION

In preparing its Fourth Report on Pressing Problems and Projected National

Needs in Optical Radiation Measurements, the Council for Optical Radiation

Measurements (CORM) has revised its previous approach to the subject in an effort to produce a more realistic assessment, not of the problems themselves, but

of the means for their practical and expeditious solutions.

In its past reports, CORM listed at length the needs it perceived in the

optical radiation community, identifying many specific needs for standards and

procedures, all of which were presented to the Bureau. The objective, of course,

was to define those major needs so that the NBS in its planning would have the

opportunity to develop new programs or incorporate the requested needs into their

existing project scopes. Subsequently the Bureau advised CORM that the very wide

range of requests diluted the planning effort and created uncertainty when those

requests from CORM were applied to the NBS programs in the planning process.

Even so, the NBS response to those requests was highly significant.

To avoid this difficulty, and with the realization that no one organization

can possible respond to all needs at all levels, the CORM Directors undertook for

the Fourth Report two steps not incorporated in previous reports. The first dealt

with the assignment of priorities. Here, areas of technology were first studied

to identify the most significant fields in terms of standards needs and industrial

implications. Second, once those areas were identified the most significant needs

were defined, and finally those needs were ranked in order of priority.

Second, the CORM Directors studied the identified national needs in terms of

ways of achieving them in the most effective and efficient manner. The Directors,

after many meetings with the Bureau program managers and scientists, suggested

that some of the requests for standards could only be acted on effectively by the

Bureau in its role as the national laboratory. Other needs could be achieved most

effectively through the coordinated efforts of the NBS and segments of industry,

and there were still others which the users themselves could solve through programmed efforts, asking the Bureau only for guidance or consultation.

The Directors prepared a listing of standards needs and found that the most

essential projects were in either of two technical categories, spectroradiometry

or spectrophotometry. Having made this division, CORM established two Task Forces,

one in each of the technical areas, and charged them with the responsibility of

identifying only the most essential standards needs within their respective areas. These standards were then studied and statements of the specifications and technical implications were prepared for each one. These standards needs were ranked

in priority and recommendations were prepared as to which of the three paths would

be the most effective and realistic in achieving the goal.

Details of the specifications and technical implications of standards needs

in each of these areas form the body of this report. The report reflects, to

the best of our understanding, the consensus of those who use optical standards

in the measurement of radiant energy. Before preparing the report, CORM established

criteria to assure that the users' needs stated in the report, needs involving

specific standards and their specifications, were the most essential to industry

in the conduct of their business.

- 20 -
CORM has as its purpose establishing and defining a consensus of the

industrial and academic requirements for physical standards and calibration

sources in the field of optical radiation measurements. The Directors urge

that this report be seriously considered by the Bureau as well as by industrial and academic standards users in order to improve the adequacy of the

required standards. CORM and NBS should work together to improve the present

state of the art of optical radiation measurements by providing needed standards and by increasing national expertise through technological advances.

Council for Optical Radiation Measurements

E. S. Steeb, President

William E. Schneider, Vice-President

Fred W. Billmeyer, Jr., Secretary-Treasurer

Directors

Franc Grum

Henry Hemmendinger

Alton R. Karol

Thomas A. Lumenello

Calvin S. McCamy

William D. Parti ow

- 21
II. CURRENT AND FUTURE NEEDS IN RADIOMETRY

A. Selection of Needs and Assignment of Priorities

We report the findings of the CORM Radiometry Task Force, which was appointed to determine and assign priorities to the most critical needs of

the nation's radiometry-photometry community. The task force gathered its

data and assembled proposals for programs which, if carried out, would each

satisfy a particular area of need. Each proposal had a specific technical

objective, and approaches were suggested for meeting it. Documentation was

provided on which user groups would benefit from the programs and on their

technical, economic, and strategic impact. We also stated whether CORM

feels that N3S should carry out the program, whether it is a responsibility

of the user community represented by CORM, or whether it should be a ioint

effort.

A formal scoring procedure was followed to evaluate the proposals.

The Task Force then met to review the scores and debate the merits of the

proposals in order to arrive at a group consensus of priorities of the proposals. The decisions were made based on the following factors.

1. Degree of need; breadth and depth of need;

2. Appropriateness; compatibility of the proposed program with CORMNBS objectives;

3. Probability of succeeding in the proposal's objective within the

framework of reasonably possible CORM-NBS resources;

4. Immediacy of urgency of need.

Seven of the proposals submitted were placed on the priority list. They

were grouped in three classifications:

Priority 1: CORM feels that immediate action is necessary;

Priority 2: These are also important but not as immediate as Priority

1 items;

Priority 3: These are back-burner items; they will be important in

the future.

We present a synopsis of the results of the study in Table I. The priority

proposals are summarized in section B.

B. Priority Proposals

AK-1: Spectral Irradiance Standard to 2.5 Micrometers (Priority 1)

The objective of this proposal is to provide a lamp standard of spectral irradiance to at least 2.5 micrometers. The 1973 NBS scale improved

the accuracy of the 1963 NBS scale over the range 0.25 to 1.6 micrometers,

but did not cover the region from 1.6 to 2.5 micrometers. The two scales

differ by several percent at the 1.6 micrometer point, so an attempt to pro- vide a calibration that covers both regions results in a discontinuity at

1.6 micrometers which has no real basis. Alternatively, the 1963 scale may

be used entirely, but it is known to be in error below 1.6 micrometers.

- 22 -
TABLE I. CORN Radiometry Proposal and Priorities

Proposal

Identification

Priority Proposal title

AK-1 1 Spectral irradiance standard to 2.5

micrometers

ES-2 1 Develop a sustained source of radiometric standards with rapid accessibility

WS-1 2 Develop low-light-level calibration

standards

ES-3 2 Evaluate silicon detectors as reliable

standards for radiometry

WS-2 2 Near-infrared detectors

AK-2 3 Spectral radiance standard to 50

micrometers

WP-2 3 Computerized-array radiometry

- 23 -
Users of irradiance standards in this region of the spectrum include almost every industry, agency, and university deeply involved in radiometric

measurements. A customer list from a major calibration laboratory covering

the last two years includes 27 industries, 11 universities, 8 government

laboratories, and 6 foreign countries. 80% of customers purchased the 1.6

to 2.5 micrometer calibration range which is an extra cost option.

The economic impact of the confusion resulting from the scale inconsistency is difficult to determine, but it is not unreasonable to estimate that

it bears on the quality of products representing several millions of dollars . in sales. Research which relies on near-infrared calibrations can impact

on large markets. There has recently been increased activity in this region

of the spectrum, for example, with the push toward the infrared in fiber-optic communications.

We feel that this work would not require a large effort and should be

performed at the NBS as part of their continuing improvements and extensions

of measurement scales.

ES-2: Develop a Sustained Source of Radiometric Standards with Rapid Accessibility (Priority 1)

The proposed technical objective is to develop an organization or system

from which the standards users can quickly and easily obtain radiometric standards of total flux, ultraviolet and visible spectral irradiance, and geometric

total flux for a number of lamps.

Beneficiaries would include all those users of radiometric standards who

often urgently need lamp standards but who now must wait long periods of time

before calibrated sources are available.

All areas of source-based radiometry would be impacted by the proposed

development. The lamp industry, for example, would benefit substantially.

This industry is beginning to be a factor in international trade in the 1980's

as several foreign lamp manufacturers are moving into the United States markets

and local companies are entering foreign markets. Our ability to market abroad

depends on the quality of our standards.

One approach to this problem that has received favorable acclaim is to

adopt an intermediate standards laboratory program, monitored by the NBS,

which has proven itself in other technical diciplines to be capable of providing standards with the authenticity of the national laboratory. Users of lamp

standards who have been contacted are almost unanimously in favor of such a system.

Such a program should be carried out as a cooperative effort between the

NBS, CORM, the standard users, and the participating intermediate calibration

laboratories. The initial, costs of the system to these intermediate labs must

be carried by them on a venture basis, with the standard users ultimately paying for these costs and profits by the purchase of standards and calibrations.

The NBS would have the responsibility of accrediation of the intermediate laboratories by providing adequate guidance and conducting intercomparisons. The

- 24 -
role of CORM in this process would be to determine which standards and services

are most needed and to communicate this to NBS and the intermediate labs.

WS-1: Develop Low-Light-Level Calibration Standards (Priority 2)

Present irradiance standards have irradiance levels of 10" 5 W cm" 2 nm" 1 in

the visible region. It is impossible to use these standards to calibrate instrumentation measuring level 5 to 10 decades lower. Accordingly, a line of lowlight-level calibration sources which can be used as standards of spectral irradiance, spectral radiance, luminance, and illuminance is needed to satisfy these

requirements.

Beneficiaries would include about 40% of the users of calibrated radiometric

standards, representing industries, agencies, and academic institutions. Areas

which routinely involve low-light-level measurements include fluorescence, phosphorescence, atomic and molecular spectroscopy, flame and plasma diagnostics,

star tracking, LEDs and miniature lamps, display screens and arrays, astronomy,

optical countermeasures, photograph, television, bioluminescence, and more.

This development would require a medium sized effort at the NBS carried out

over a several year period.

ES-3: Evaluate Silicon Detectors as Reliable Standards for Radiometry (Priority 2)

The objective of this program is to evaluate the feasibility of using silicon

photodiodes with enhanced ultraviolet response as standards in spectroradiometric

measurements. CORM realized that there will always be a need for source standards.

In many cases, however, detector-based radiometry can be used situations where

source-based radiometry was traditional, offering many advantages in cost, stability, portability, and ease of intercomparison and measurement assurance program

(MAP) services. The program for the' development of self-calibrated silicon-detector radiometry at the NBS has made great strides in advancing detector-based

radiometry. Its usefulness for measuring wel 1 -controll ed monochromatic sources

such as lasers is a reality. It remains to adapt this technique to the broadband, wide-angle applications most often encountered in applications of spectroradiometry

.

The impact of the achievement of broadly applicable detector radiometry

would be felt throughout the entire field of radiometry. The achievement of this

goal can be accomplished by a long-term cooperative effort between the NBS and

the radiation measurement community. CORM gives its full support to the continuing commitment that NBS has made to detector-based radiometry, and points out

that several CORM members have begun using sel f-cal ibrated silicon detectors in

practical applications of spectroradiometry . This foothold of acceptance is an opportunity for NBS to broaden the impact of its accomplishments by structuring

the detector programs so as to support and complement these early users. A

e

feel that it is the role of CORM to encourage the interaction of the NBS and users by providing a forum for communication and interchange at our CORM conferences .

- 25 -
WS-2: Near- Infrared Detectors (Priority 2)

The objective of this proposal is to improve detector performance in the

0.9 to 1.7 micrometer range, evaluating germanium and other detectors for this

purpose. As mentioned earlier, the near-infrared is the focus of much recent

activity, but stable, efficient detectors are not available beyond the I micrometer limit of silicon photodiodes. We feel that it is important to evaluate

the alternative detectors that have been used or suggested for this region of

the spectrum, to identify the major technical obstacles to their use in detector radiometry, and to determine approaches to overcoming these obstacles.

The fiber-optic industry would benefit from this development, and its com- petitive position against Japan and Europe could be improved by our technical

leadership in this area.

Most of this work should be carried out at the NBS, but detector manu- facturers can provide a valuable contribution to this investigation by supplying the NBS with detectors, especially large-area devices made to custom specifications.

AK-2: Spectral Radiance Standard to 50 Micrometers (Priority 3)

The objective is to provide a blackbody-type source for calibrating infrared detection systems in the 2.5 to 50 micrometer range, possibly approached

in steps of 15, 25, then 50 micrometers.

This would impact on earth-resources data collection, military surveillance,

meteorology, and remote sensing of extraterrestrial bodies, providing benefits

to the world's food supply, long-range weather forecasting, and defence and

astronomical capability.

A joint effort is suggested between the NBS and other agency users of

far-infrared radiation, such as the Department of Defense.

WP-2: Computerized Array Radiometry (Priority 3)

The objective of this proposal is to bring the NBS into array-radiometry

technology. This technology has developed over a period of years mainly as a research tool, but it is beginning to be used in calibrations and commercial

radiometric measurements. In the future the quality and credibility of measurements made with these devices will have increasing impact on business and trade.

Involvement of the Bureau in this field wiTl bring to the surface various difficult and unanticipated problems that may accompany the introduction of these

techniques to standard-quality measurements. We feel that this objective need

not be an end in itself, as if for example the NBS acquired instruments from

several manufacturers and evaluated them. Rather, we think that the Bureau

should use the array-detector approach in the instrumentation needed to accomplish its other objectives in the radiometric area.

The resources for accomplishing this objective must come from the capital

budget of the NBS, utilizing array technology in the new instruments rather

than conventional approaches. Although initial costs might be higher, they

would be offset to some extent later by the benefits of efficiency, speed, con- venience, and flexibility that can be obtained with array devices.

- 26 -
III. A NATIONAL MEASUREMENT STANDARDIZATION SYSTEM FOR SPECTROPHOTOMETRY

A. Introduction

By spectrophotometry we mean the quantitative application of measurements

of spectral transmittance, reflectance, and luminescence, and related quantities, in the near-ultraviolet, visible, and near-infrared wavelength regions.

This type of metrology is of vital importance to many material s-producing and

using industries for research and development and for quality control; to many

operations involving clinical (health) analysis; and to the analysis of many

processes and materials related to energy conservation, to name only a few

appl ications.

Aspects of standardization related to spectrophotometry have long been

neglected for various reasons. Foremost among these may be a misunderstanding

of the complexities associated with this type of metrology; the opinion that

spectrophotometry is no more than a simple ratio measurement is far from realistic. Two examples may be given of the support in depth of the standardization needs in spectrophotometry:

1. The United States Army wishes to have spectrophotometric methods of

setting color tolerances for uniforms. The Army has been accepting shipments

of textiles to be made into uniforms on a visual basis. The inspector is provided with a standard sample and eight limit samples representing maximum allowable deviations from the standard in each of eight directions in color space.

Armed with these nine samp! es , the inspector examines the submitted sample and

issues a pass-or-fail verdict. This visual method has proved troublesome, as evidenced by frequent disagreements between the Army and the textile mills submitting the samples. The Army wishes to change its acceptance procedure to one

using instrumental measurement and valid tolerance limits for acceptance.

This example is typical and represents several unsolved problems that need

attention. All three aspects of standardization (calibration, systems tests,

and diagnosis) must be carried out with great care for this application. As

documented in the Third CORM Report (items* 1 and 5), adequate standard reference materials (SRM's) for the calibration step, needed by both the instrument

manufacturers and the users (the Army laboratories in this example) do not exist.

In this case the systems check standards must be colorimetric in nature since

it is necessary to correlate instrumental results to visual judgments. Such

material standards were called for in the Third CORM Report (item 6). In their

absence, reliance has been placed on equivalent standard materials obtained

from foreign national standardizing laboratories. Delays result from United

States Customs restrictions, costs are high, continuing supplies cannot be

assured, and the available standards do not meet the specific need for checking color differences. Finally there are no adequate diagnostic standards for

spectrophotometry available in the United States, the last ones having disappeared when the supply of SRM 2101-2105 was exhausted and the NBS decided not

to replace them.

* Numbers refer to items in Section IIID of this report, repeated with abridgement and revision from Section B of the Third CORM Report.

- 27 -
2. A large United States industrial firm wishes to replace material

standards for an important product line with stored numerical standards in

computer-controlled spectrophotometers . Because of the difficulty in maintaining for long periods of time relatively fragile product standards, subject to day-to-day use in a production environment, it is felt that product

uniformity, critical in this application, can be improved by replacing product material standards with the stored numerical equivalent. The investment

in the necessary equipment has approached $500,000. Tests of the system over

several years have pinpointed inadequacies in the same three types of material

standards mentioned in the first example: calibration, systems check, and

diagnostic. Lacking these, the goal of the undertaking has not been satisfactorily achieved.

While the above examples refer to material goods, there is another area

of use of spectrophotometers that gives rise to even greater concern: its

increasing use as a diagnostic tool in many medical and health-hazard applications, where the precision and accuracy of these radiation-measuring devices

is, often literally, of vital importance.

A survey estimates that there are over 32,000 reflectance and fluorescence

spectrophotometers of various types in use in various installations. One can easily estimate that there are many more transmission (analytical) spectrophotometers in use. That only a small fraction of these instruments could meet

government specifications for accuracy and traceability is clear from existing

inconsistencies when results are compared from one spectrophotometer to another.

Usually, instrument calibrations and measurement quality-assurance tests are employed to achieve consistency. In the case of spectrophotometry and spectroradiometry, the proper use of reference materials can serve this purpose. The

current system of measurement standardization lacks the availability of key

calibration and measurement quality assurance tests that are needed for effective, equitable control of spectrophotometric metrology without unnecessary

economic burden.

B. Objectives

C0RM believes that a collaborative industry-NBS program should be set up

to improve the deficiencies that exist in spectrophotometer standardization.

The goal of such a program should be to improve services from NBS to the private, state, and federal sectors related to spectrophotometric measurement in

a manner to assure safety, compatibility, and traceabil ity, consistent with

the economic health of the nation.

The overall objective is to remove at least major deficiencies in the key areas of metrology and to develop and implement a national measurement support

system that will enable periodic demonstration of the adequacy of spectrophotometric measurements made routinely at the field level. When implemented, such

a system will assure adequate measurement quality by providing the mechanisms

needed to achieve consistency with national and international standards.

Secondary or intermediate standards calibration laboratories, certified

by the NBS, are badly needed to provide the day-to-day workings of measurement

consistency.

- 28 -
CORM feels that the proposed program should be carried out in cooperation

between intermediate laboratories and NBS. By doing so the burden will be

appropriately shared between industry and NBS. If properly executed such a program will:

1. Establish intermediate calibration laboratories;

2. Design and test SRM's that can be used as transfer standards;

3. Provide guidelines on methodologies for measurements;

4. Provide training programs.

C. ImplementationFirst of all, the most urgent areas of deficiency should be specified and

the role of NBS and that of the industry and academia in the program should be

established in each of the areas. The procedure to achieve the goals in a reasonable time must be established. The four essential components of the suggested

program are:

1. A MAP service in spectrophotometry

2. SRM's for instrument standardization for routine spectrophotometry

3. SRM's for special measurements

4. New methodology for spectrophotometry

In the first two items NBS leadership is required; item three would be a joint venture (NBS-CORM), and in the fourth one the NBS participation is sought.

The plans for implementation of this proposed program should be worked out jointly with NBS representatives and the CORM Board of Directors.

1. A MAP Service in Spectrophotometry (Priority 1)

NBS should provide and maintain- on a continuing basis a MAP service in the

area of spectrophotometry. It is anticipated that establishing such a service

will provide the basis for the national standardization program in spectrophotometry previously referred to. It should adhere to the concept of measurement

traceability promulgated by the NBS's Ionizing Radiation Section.

Establishment of this service will require the identification of laboratories providing accurate calibration services to users of spectrophotometry.

These laboratories will serve as intermediate standards laboratories in the measurement traceability chain. Their own measurements will be monitored by the NBS

through the MAP procedure, and they in turn will monitor the measurements performed in user laboratories by similar mechanisms.

It is anticipated that CORM can play a major role in the establishment of

these quality assurance programs by seeking out and identifying suitable candidate intermediate standards laboratories. These may come from several sector : government laboratories, for example the United States Army laboratory responsible for development of the program for color tolerances for uniforms cited aoo.'e;

industrial laboratories, for example those responsible for coordinating coloring

efforts in a series of manufacturing plants; academic 1 aboratories ; producers

spectrophotometric instruments; and private standardization laborator ies

.

- 29 -
What is required from NBS is the provision or at least primary calibration

of the necessary SRM's, the final selection and certification of the intermediate standards laboratories, the implementation of the necessary performance

tests, and oversight of the development of the relations between the intermediate standards laboratories and the user laboratories. Once the system is operating, it is anticipated that the NBS's effort would be minimal in comparison to

the overall benefits of the program.

It is anticipated that CORM could play a continuing role in monitoring and

promoting the national standardization system for spectrophotometry, relieving

the NBS of a considerable burden in this respect.

2. SRM's for Instrument Standardization for Routine Spectrophotometry (Priority 1)

The purpose of this objective is to insure the provision to both the intermediate standards laboratories and the user laboratories of the calibration, systems check, and diagnostic standards required for satisfactory operation of the

measurement traceability scheme. Unfortunately virtually none of these standards

now exist, either as SRM's or as identified materials. In achieving this objective more than any other, NBS leadership is required. CORM will assist as much

as possible in identifying needs and possible sources of supply. A partial listing of needs in this area follows, with references to the Third CORM Report where

appropriate.

a. Calibration Standards

(1) Wavelength. As described in the Third CORM Report (item 5), standards based on absorptance maxima of the didymium and holmium oxide filters, including SRM's 2009, 2010, 2013, and 2014, fulfill only a small part of the need.

Additional standards based on the linear-filter concept are also required, and

possibly others not yet identified. The goal of the national standardization

system should be to provide SRM's suitable for calibrating the wavelength scales

of all spectrophotometers -- analytical as well as color-measuring -- regardless

of spectral bandwidth, including those utilizing both conventional prism and

grating monochromators and those utilizing interference filters or interference

wedges as monochromators, array detectors, or other technologies.

(2) Photometric. Transmittance scale standards suitable for the calibration of the photometric scales of spectrophotometers were called for in the

Third CORM Report (item 2). The deficiencies of existing SRM's (e.g. 930) and

recommendations (e.g. dichromate solutions) were pointed out. Other SRM's that

might be suitable for this purpose have either been discontinued (2101-2105) or are under consideration for discontinuation (items 7.8B-E in NBS SP 250). A

few suitable reflectance-scale standards exist (three of twelve ceramic tiles

supplied by the National Physical Laboratory in England) but the supply is ex- pected to be exhausted in 1982 and it is not clear when suitable replacements

will be available. In any case, the applicability of reflectance-scale standards is limited rather than general. (This series of standards is discussed

further under item b.)

- 30 -
(3) Reflectance. For reflectance measurement additional calibration

standards are required to set the level of the photometric scale. Here the

need is not so much for photometric linearity standards, since this can usually be accomplished by transmittance-scale standards. Rather, what are required

are suitable transfer standards for the calibration of user working standards

in terms of the international primary standard of reflectance, the perfect re- flecting diffuser. These are required for the four internationally recomnended

measuring geometries. While some progress has been made in establishing international agreement for some of these geometries, this work needs to be brought

to completion and the necessary SRM's developed.

b. Systems Check Standards

Since all the types of systems check standards that would be required

in the national standardization system have not been identified (e.g. clinical,

analytical, densitometric , colorimetric, etc.), one class only will be discussed:

colorimetric systems-check standards. The need for these was also pointed out

in the Third CORM Report (item 6). As noted there, it is extremely regrettable

that SRM 2101-2105, the only set of such standards available from NBS, was discontinued, particularly so shortly after the excellent recalibration study that

significantly enhanced their utility. At the very least a replacement set of

SRM's, free from the defects known to exist in the previous set, should be provided .

Because of the differences in geometry, an additional set of systemscheck SRM's for reflectance is required. In the past, leadership in producing

such standards has been taken by the NPL in England, but as mentioned in item

a ( 2 ) the supply of these standards is near exhaustion. A replacement set is

being developed which should ultimately be even more useful, but full details

are not yet available. Procurement will no doubt continue to be hampered by

customs restrictions that, we are informed, effectively prevent these standards

being imported for some important us-es. The possibility of NBS working in

cooperation with NPL to make the new standards available as SRM's should be ex- plored.

Systems check standards for color densitometry were also called for

in the Third CORM Report (item 4).

c. Diagnostic Standards

Diagnostic standards are the least developed of the Priority 1 items

required for a national measurement system for spectrophotometry. In some cases

systems check standards can also serve as diagnostic standards. Set 2101-2105

was originally designed with this use in mind, for example. Nevertheless, it

is not certain that all of the needs for diagnostics have been identified, and

the required standards and methodology developed. CORM should play an active

role in supplying this information to the NBS.

Among the defects- in spectrophotometers for which diagnostic standard:

are needed are: stray light, spectral bandpass and slit-function shape, unc

(for integrating-sphere reflectance geometry) condition of specular-inclusion

ports.

- 31
3. SRM's for Special Measurements (Priority 2)

The usefulness of a national standardization system for spectrophotometry

should not be limited to the routine determination of transmittance, reflectance,

and related quantities of specimens that pose no special measurement problems.

It should be obvious that the greatest need for the system will be in the solution of difficult rather than simple measurement problems. The reason that the

development of SRM's for these special measurements is assigned Priority 2 is

merely that the establishment of the system and its implementation for the simpler cases must of necessity come first. Among the measurements that require

special consideration and special SRM's are: fluorescence, retroreflection,

transl ucency, specular reflectance including gloss, and extremely low transmittance.

a. Fluorescence

There are two aspects of the measurement of fluorescent specimens to

be considered: the spectrofluorimetry of fluorescent liquids and the spectrophotometry and colorimetry by reflectance of fluorescent solids. The latter is

discussed here by way of example. The topic was the subject of a symposium at

the CORM 81 International Conference on Optical Radiation Measurement of Fluorescent and Retroreflective Materials, at which one paper dealt specifically with

fluorescent standards. International activity in the development of methods

for the spectrophotometry and colorimetry of reflecting fluorescent materials

continues and is closely monitored by CORM. Specific recommendations for SRM's,

supplementing the request in the Third CORM Report (items 7 and 8), will be developed in the near future.

b. Retroref! ection

The situation for the measurement of retroref! ection is not unlike that

for fluorescence in that rapid progress in the establishment of the necessary

techniques is being made at both the national and international levels. This

work was also discussed at a symposium at CORM 81, and is being closely monitored

by CORM. More specific needs for SRM's, probably in the form of systems check

and diagnostic standards, supplementing the request in the Third CORM Report

(items 9-11), will be developed in the near future.

c. Transl ucency

One of the most difficult series of measurements by spectrophotometry

is that of diffuse transmittance, required together with the determination of

diffuse reflectance for the characterization of the transl ucency, opacity, and

related light-scattering properties of translucent specimens. In part new standard methodology is required, and to this extent the problem falls under objective

4. The need for SRM's when the necessary methodology is developed remains to be

addressed in the future, supplementing item B6 in the Third CORM Report.

d. Specular Reflection and Gloss

The recent development of SRM's for specular reflection (in response

to item Bll in the Third CORM Report) has addressed part of the need in this

area, but more needs to be done, particularly in the transfer of these results

- 32 -
to the calibration of scales of gloss. Although it may be argued that this is

not directly an application of spectrophotometry, nevertheless experience dictates that measurement and control of gloss is essential for the accurate spectrophotometry of reflecting materials, so the two cannot be fully separated.

Additional requests for SRM's can be anticipated in the future.

e. Extremely Low Transmittance (High Optical Density)

The need for SRM's of high optical density was addressed in the Third

CORM Report (item 3). The need remains, and should be addressed consistent

with the Priority 2 level assigned.

4. Methodology for Spectrophotometry (Priority 2)

It should be clear from the above discussion that the development of new methodology for spectrophotometry and in related areas is required for the solution of several difficult measurement problems. In addition to those already

mentioned, three already identified are mentioned here.

a. Evaluation of Metameric Pairs

Evidence from the field clearly shows that there is a highly significant difference in the performance of spectrophotometers for the colorimetric

evaluation (in terms of color differences) of nonmetameric and metameric pairs

of specimens. The uncertainty of measurement of the color difference between

the members of a metameric pair is approximately an order of magnitude greater

than for a nonmetameric pair. Although it now appears that this reflects poor

practice in the calibration of the instruments, further study, with the possible

development of new methodology, is required. CORM is prepared to take the initiative in this work.

b. Definition of Geometry of Measurements

The precise definitions of the geometric conditions for reflectance

and transmittance measurements have never been made. The advantages of doing

so can be inferred from the significant improvements achieved in the field of

densitometry when the corresponding definitions were made. International standards for these geometries are inadequate, but there is no indication that the

problem will be addressed in the near future at the international level. CORM

is prepared to take the initiative in this work. The related problem of the inclusion or exclusion of the specular component of reflectance will also be addressed.

c. Advancement of the State of the Art by Work on Objectives Not Yet

Identified

CORM wishes to be informed of additional objectives in all aspects o f the field of spectrophotometry for which new methodology or new standards

required.

- 33
D. Specific Needs

Specific needs and problems in spectrophotometry were clearly identified

in the Third CORM Report (June 1979). Seventeen items were listed in that

report but only two of them have so far been made available. The NBS has provided the SRM of absolute white spectral reflectance (item No. 2 in 1979 CORM

Report) and also the SRM for regular (specular) spectral reflectance (item

No. 11 in 1979 Report). The needs for the remaining items have neither changed

nor diminished in the interim, as has been indicated in the foregoing discussion.

For the convenience of those wishing to refer to thse items, they are summarized

below with emphasis on the intended use and technical specifications of the

proposed material standards.

1. Spectral Reflectance Scale Standards

a. Description

Physical standards of diffuse spectral reflectance at various reflectance levels are needed to establish a scale. These standards are required for

checking the linearity of the photometri

meters.

b. Specifications

Reflectance Levels

Spectral Character

Geometry of Calibration'

Wavelength Range

Uncertainty

Physical Size

Diffusing Characteristics

scales of reflectance spectrophotoInitially, 0.10, 0.50, and 0.90

nominal (± 5%). Other levels to

be provided later

As near neutral as possible

0/45 and d/0

360-740 nm; extended to

0.2-2. 5 ym later

0.1% absolute

50 x 50 nm

Either approximately Lambertian

from 0° to 60°, with no specular

peak, or highly glossy (polished

glass) with approximately

Lambertian diffuse reflectance

and narrow specular peak

- 34 -
2. Regular Spectral Transmittance Scale Standards

a. Description

Physical standards of regular spectral transmittance at various

transmittance levels are needed to establish a scale. These standards are required for checking the linearity of the photometric scales of transmittance

spectrophotometers, including analytical instruments. The dichromate solutions

recommended as transmission standards (NBS SP378) are neither convenient nor

stable. Permanent solid standards are required. Both the manufacturers of

spectrophotometers as well as the users need these regular spectral transmittance standards. The majority are not happy with presently available SRM 930

standards. Glass must be more carefully

reduced.

b. Specifications

Transmittance Levels

Spectral Character

Geometry of Calibration

Wavel ength Range

Uncertainty

Physical Size

Diffusing Characteristics

selected, and uncertainties must be

Initially, 0.1, 1, 10, 50, and

90% nominal. Others to be provided later.

Nominally flat for at least

2 nm on either side of the calibration wavelengths

0/0

350-1100 nm; 0.2-2. 5 pm later

0.1% of val ue

Both 50 x 50 mm and 8 x 29 mm

Fully transparent and haze free

3. Visual Diffuse Transmission and Reflection Density Standards

a. Description

Visual diffuse transmission and reflection density standards are needed

with the following specifications. These standards are used by the photographic

industry to calibrate densitometers which are in turn used to evaluate and control the production of photographic film. They are also used to calibrate other

modulators or attenuators which are used in the exposure of film prior to densitometry. At present NBS provides a transmission density standard of limited un- certainty as a standard reference material. More accurate special calibrations

are also available. However, NBS now has essentially no capability to provide

reliable visual diffuse reflection density standards or calibrations. The need

to develop some capability in the reflection density area is especially critical

35 -
In practical transmission densitometry an opal glass is often used

in the optical system in place of an integrating sphere. Densitometry measurements made with an opal glass do not agree with those made with an integrating

sphere, principally at low densities. To resolve the differences between the

two methods, it is recommended that

(1) The practice of densitometry be reviewed regarding usage of integrating spheres and opal glasses, the significance of the measurements made

by these two methods to the user, the reproducibility which may be achieved

by these two methods, and any reconvnendations being made by ANSI committees.

(2) Using a variety of samples, agreement be explored between the

currently used photoelectric densitometer method and the inverse-square-law

procedures; and, where necessary, the measurements from these two techniques

be brought into agreement.

Measurement laboratories using NBS calibrated samples measured by different methods now experience difficulty in making measurements that agree.

Since most densitometers routinely employ an opal glass and since opal glass

density measurement correlates well with the density seen by many photographic

printing operations, a standard calibrated by an opal glass method would serve

a real need.

b. Specifications

Ref! ection Density:

Range

Accuracy

Spectral

Geometry

Source

Material

s

Transmission Density:

Range

Accuracy

Spectral

Geometry

Source

Material

s

0 to 2.5 density

0.5% of density

response detector

45/0

3000 K ± 100 K

Selected ceramic tiles (spectrally neutral, nonfluorescent)

0 to 6 density

0.5% of density

response detector

0/integrating sphere and 0/opal

glass

3000 K ± 100 K

Silver photographic step tablet

and/or carbon step tablet

- 36
4. Standards for Calibration of Transmission and Reflection Density for

Color Materials

a. Description

Standards should be provided for the calibration and check procedures

for densitometers used in three-color densitometry. These standards are used

in photographic printing and graphic arts industries to calibrate densitometers

that are used in quality control of these products. There are many such densitometers in the field yet there is no SRM available for validation of these

instruments. This results in large variations in measured values from laboratory to laboratory and densitometer to densitometer. Leadership is required

in the realization of ANSI standards and to suggest possible alternatives to

the documentary standard if this standard is not readily realizable.

Samples of neutral areas of three-layer color reversal and color negative film should be calibrated for red, green and blue densities as specified

in ANSI PH 2.1-1952, and also using specified color filters, such as Kodak

certified AA and MM filters for the measurement of Status A and Status M density.

b. Specifications

Reflection Density:

Range

Accuracy

Spectral

Geometry

Source

Material

s

Transmission Density:

Range

Accuracy

Spectral

Geometry

Source

Material

s

0-2.5 density

0.5% of density

Status A, Status M, PH 2:1-1952

45/0, 0/45

3000 K ± 100 K

Suitable color step tablets

0 to 6 density

0.5% of density

Status A, Status M, PH 2:1-1952

0/integrating sphere and 0/opal

glass

3000 K ± 100 K

Suitable color step tablets

37
5. Wavelength Scale Standards

a. Description

Standards are required for the calibration of the wavelength scales

of spectrophotometers. These standards are needed for checking and calibrating the wavelength scales of all spectrophotometers operating in the wavelength

region of interest. Present SRM's using absorption filters should be supplemented

by other types of standards. In particular, standards are required with sufficient calibration wavelengths to provide point-by-point error correction curves

for instruments utilizing interference wedges; and standards are required to pro- vide information on the centroid wavelength, spectral bandpass, and shape of the

slit function for all broad-bandpass spectrophotometers. The problem is par- ticularly acute for abridged spectrophotometers, for which no standards are readily available. The availability of SRM 2009, 2010, 2013, and 2014, based on

the didymium absorption filter, answers only a small part of this need. New

methodology suitable for the calibration of array detectors, as well as new

types of standards, may be required.

b. Specifications

Spectral Range 200-760 nm; extend to 2500 nm later

Spectral Bandwidth 1-10 nm

Type (a) Absorption Filter

(b) Other

Uncertainty ± 0.5 nm or better

Appl icabil ity

Information Required

Dispersive spectrophotometers,

instruments using interference

wedges as monochromators, and

abridged spectrophotometers

(a) Effective or centroid wavelength; (b) Spectral bandwidth

and some indication of slit

function shape

6. Colorimetric Reflectance and Transmittance Standards

a. Description

Physical standards of reflectance and transmittance, calibrated in

colorimetric terms, are required as systems check standards. These standards

would meet the needs for systems testing and diagnostic standards in this application. NBS SRM 2101-2105, now no longer available, provided some examples

of the type of standard required, but had some defects. Better selected transmittance standards and a variety of reflectance standards are both required.

They should be selected to provide diagnostic information on sources of error,

as was done in past with SRM 2101-2105.

- 38 -
b. Specifications

Chromaticity High purity, spaced around

the CIE locus

Spectral Range 380-760 nm

Sources CIE standard illuminants A, C,

D 65’ D 50

Observers CIE 1931 2° and 1964 10° standard observers

Uncertainty 0.0005 in x and y

± 0.2% in Y

Physical Size 50 x 50 mm

Geometry:

Reflectance 45/0 and /0

Transmittance d/0

7. Standards of Total Radiance Factor of Luminescent Materials

a. Description

Physical standards of total radiance factor are required. Total

radiance factor is the sum of the reflected and fluoresced flux from a luminescent material. It is measured by irradiating the sample with heterochromatic

light, with controlled spectral distribution, and detecting the radiance monochromatically.

b. Specifications

Spectral Excitation Range

Spectral Emission Range

Source Spectral Distribution

Geometry

Physical Size

Stabil ity

Uncertainty

300-400 nm

400-700 nm

3000 K, D 65

d/0, 45/0

50 x 50 mm

Fully stable to conditions

of use

± 1% of value

- 39
8. Spectrofl uorimeter Calibration Standards

a. Description

At least three standard materials are required for checking and

calibrating spectrofl uorimeters. These standards should luminesce in the

blue 450 nm), green ('u 550 nm.), and red 620 nm) spectral regions.

Having such standards, properly calibrated, it will be possible to compute

from the apparent luminescence spectra the relative efficiencies of luminescent materials and intercompare the results. Nearly all analytical laboratories are now equipped with some fluorimetric equipment to measure apparent

luminescence as a part of their material characterization tests. Apparent

spectra greatly depend on instrumental parameters, hence are not comparable

because of the differences in instrumental parameters. The absolute fl uorimeters capable of providing corrected (absolute) luminescence spectra are not

readily available. There are now many clinical laboratories that are using

fluorescence measurements as a tool in clinical diagnostics. These measurements are often made in turbid media (dispersions, solid plates and webs).

It is imperative for these laboratories to have reliable SRM’s through which

their measurement can be kept under control. They also need procedural directives for making correct measurements and evaluations.

b. Specifications

1. Samples should preferably be solid non-diffusing and fit a standard cuvette holder. Also useful would be a set of liquid standards.

2. Should have moderate to high quantum efficiency.

3. Should be calibrated for 0/45 and 0/90 geometry.

4. Should exhibit low absorptance (< 10%) in the region of luminescence emission (and also at the wavelength of excitation if intended for 0/90

geometry)

.

5. Samples should be stable to light and heat, easy to handle.

6. The following calibrations are required:

Excitation X , max

Emission Xm . max Absorptance (a) at Xmax

of excitation.

Quantum Yield,

Refractive Index.

7. Suitable samples are glass such as Corning 3750 or B&L uranium

glass and similar samples such as Rhodamine and other stable fluors imbedded

in plastic chips.

- 40 -
9. Retroreflective Standards of Nighttime Brightness

a. Description

Physical standards of brightness (in units of candelas per lux per

square meter for nighttime observation conditions) are required for retroreflective materials. These standards, in retroreflective materials, are for

use in substitutional testing against government specifications of the nighttime brightness of retroreflective sheeting.

b. Specifications

Brightness Levels

Colors

Geometry of Calibration

Source

Uncertainty

Physical Size

Material

Midpoints of ranges of

commonly used Federal and

State government specifications

Traffic colors as called for

in above specifications, including white, yellow, red,

blue, green, and orange

Observation and entrance

angles commonly used in above

specifications

CIE Source A

± 1 % of val ue

300 x 300 mm

Retroreflective

10. Retroreflective Standards of Daytime Color

a. Description

Physical colorimetric standards are required for the daytime observation conditions for retroreflective materials. These reference standards, in

retroreflective materials, are for use in substitutional testing against govern

ment specifications of the daytime color of retroreflective materials. Since

these materials must be tested using 45/0 geometry, and 45/0 spectrophotometers

are not widely available, most testing is carried out using 45/0 colorimeters.

It is particularly urgent that suitable calibrated standards, spectrally and

geometrically similar to the material being tested, are available for local call

bration of these instruments.

- 41
b. Specifications

Calibration Method

Wavelength Range

Illuminants

Observers

Colors

Geometry of Calibration

Uncertainty

Physical Size

Material

Spectrophotometr i

c

380-760 nm

CIE standard illuminants C

and D

65

CIE 1931 2° and 1964 10°

standard observers

Traffic colors as called

for in commonly used Federal

and State government specifications, including white,

yellow, red, blue, green,

brown, and orange

45/0

± 0.001 in x,y; ± 0.2 in Y

100 x 100 mm

Retroreflective

11. Retroreflective Standards of Nighttime Chromaticity

a. Description

Physical chromaticity standards are needed for the nighttime observation

conditions for retroreflective materials. These reference standards, in retroreflective materials, are for use in substitutional testing of the nighttime chromaticity of retroreflective materials.

b. Specifications

Calibration Method

Wavelength Range

Illuminant

Observer

Spectroradiometric

380-760 nm

CIE standard illuminant A

CIE 1931 2° standard observer

Colors Traffic colors as called for in

commonly used Federal and State

government specifications, including white, yellow, red,

blue, green, and orange

Geometry of Calibration

- 42 -

Initially, 0.2° observation

angle and 1° entrance angle;

later, other sets of angles
b. Specifications (continued)

Uncertainty

Physical Size

Material

± 0.002 in x,y

300 x 300 mm

Retrorefl ective

- 43
NBS-1 14A (REV. 2-80

U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

NBSIR 84-2889

SHEET (See /nstruct/ons) NBSIR 84-2889 June 1984

4. TITLE AND SUBTITLE

NBS Response to the Fourth CORM Report on Pressing Problems and Projected National

Needs in Optical Radiation Measurements

5. AUTHOR(S)

K. D. Mielenz

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

NBS

10. SUPPLEMENTARY NOTES

LJ Document describes a computer program; SF-185, FIPS Software Summary, is attached. u* ABSTRACT f A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here)

This publication constitutes the NBS Response to the Fourth CORM Report. It

describes NBS policies for radiometry and spectrophotometry, the current status

of projects suggested by CORM, and future plans. It also contains specific

proposals for collaborative CORM/NBS efforts to provide needed standards and

measurement services. With permission by CORM, the Fourth CORM Report itself

is included as an appendix.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Council for Optical Radiation Measurements; measurement services; NBS/industry

collaboration; private calibration laboratories; radiometry; spectrophotometry; standards

13. AVAILABILITY

[jp Unlimited For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. DC Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

48

15. Price

$8.50

USCOMM-OC 6043-P80