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PUBLICATIONS

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NIST SPECIAL PUBLICATION 260-140

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials:

Technical Specifications for Certification of

Spectrophotometry NTRMs

John C. Travis, Melody V. Smith,

Stanley D. Rasberry, and Gary W. Kramer

QC

100

157

.260-110

000
rhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology . . . needed to improve product quality, to modernize manufacturing processes,

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2 Some elements at Boulder, CO.
NIST Special Publication 260-140

Standard Reference Materials:

Technical Specifications for Certification of

Spectrophotometric NTRMs

John C. Travis and Melody V. Smith

Analytical Chemistry Division

Chemical Science and Technology Laboratory

Stanley D. Rasberry (retired)

Office of Measurement Services

Technology Services

and

Gary W. Kramer

Analytical Chemistry Division

Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director

Issued February 2000
National Institute of Standards and Technology Special Publication 260-140

Natl. Inst. Stand. Technol. Spec. Publ. 260-140, 59 pages (Feb. 2000)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2000

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Abstract

This document provides the technical requirements for operating laboratories that prepare,

measure, and certify visible absorbance spectrophotometric NIST Traceable Reference Materials

(NTRMs) with quality assessment administered by the National Voluntary Laboratory

Accreditation Program (NVLAP) of the National Institute of Standards and Technology (NIST).

Specifically, descriptions are given of the processes involved in qualifying a potential NTRM

certifier, detailed specifications and construction of NTRM filters, the spectrophotometric

certification and recertification process, and maintaining measurement traceability to the

reference spectrophotometer in the NIST Analytical Chemistry Division. Measurement

traceability is assured through direct and indirect proficiency testing, including annual

comparison measurements, control charts using certified NIST Standard Reference Materials and

in-house working standards, and blind testing.

NIST Commercial Disclaimer

Certain commercial equipment, instruments, and materials are identified to describe adequately

the work presented herein. Such identification does not imply recommendation or endorsement

by NIST, nor does it imply that the equipment, instruments, or materials are necessarily the best

available for the purpose.

Copyright

This article is a U.S. Government work and, as such, is in the public domain in the United States

of America.

Keywords

absorbance, neutral density, optical filter, reference material, spectrophotometry, traceability,

transmittance.

111

Foreword

The National Institute of Standards and Technology (NIST), formerly the National Bureau of

Standards, was established by the U.S. Congress in 1901 and charged with the responsibility for

establishing a measurement foundation to facilitate both U.S. and international commerce. This

charge was purposely stated in broad terms to provide NIST with the ability to establish and

implement its programs in response to changing national needs and priorities.

Increased requirements for quality systems documentation for trade and effective decisionmaking regarding the health and safety of the U.S. population have increased the need for

demonstrating "traceability-to-NIST" and establishing a more formal means for documenting

measurement comparability with standards laboratories of other nations and/or regions. Standard

Reference Materials (SRMs) are certified reference materials (CRMs) issued under the NIST

trademark that are well-characterized using state-of-the-art measurement methods and/or

technologies for chemical composition and/or physical properties. Traditionally, SRMs have

been the primary tools that NIST provides to the user community for achieving chemical

measurement quality assurance and traceability to national standards. Currently, NIST catalogs

nearly 1300 different types of SRMs covering 29 technical categories. Since it has the world's

leading, most mature, and most comprehensive reference materials program, most of the world

looks to NIST as the de facto source for high quality CRMs for chemical measurements.

NIST has met the reference materials needs of U.S. industry and commerce for nearly 100 years.

While our reference materials program has focused primarily on U.S. requirements, it is clear

that these materials address international measurement needs as well. As the demonstration of

quality and "traceability" for chemical measurements have become increasingly global issues,

the need for internationally recognized and accepted CRMs for chemical composition has

increased correspondingly. Their use is now often mandated in measurement/quality protocols

for analytical testing laboratories. The fast pace of technological change coupled with increased

demands on quality, traceability, and SRM types have required NIST to devise new strategies for

customers to obtain measurement linkage to NIST. With a shift in paradigm NIST will be able

to more effectively address future needs for reference materials, both nationally and

internationally.

The NIST Traceable Reference Materials (NTRM) program was created to address the problem

of increasing needs for reference materials with a well-defined linkage to national standards. An

NTRM is a commercially produced reference material with a well-defined traceability linkage to existing NIST chemical measurement standards. This traceability linkage is established via

criteria and protocols defined by NIST and tailored to meet the needs of the metrological

community to be served. The NTRM concept was implemented initially in the gas standards area

to allow NIST to respond to increasing demands for high quality reference materials needed to implement the "Emissions Trading" provisions of the Clean Air Act of 1990. The program has

been highly successful in providing over 400,000 NIST traceable gas standards to end users at a

cost benefit ratio of 1 :70.

This document provides the technical requirements for operating laboratories that prepare,

measure, and certify visible absorbance spectrophotometric NIST Traceable Reference Materials

v
(NTRMs). The operational aspects of this NTRM program that pertain to NTRM producers'

laboratory accreditation through the NIST National Voluntary Laboratory Accreditation Prog

(NVLAP) are detailed elsewhere in NIST Handbook 150-21

.

Willie E. May, Chief

Analytical Chemistry Division

Chemical Science and Technology Laboratory

Thomas E. Gills, Director

Office of Measurement Services

Technology Services

vi
Technical Specifications for Certification of

Spectrophotometric NTRMs

Table of Contents

1. INTRODUCTION 1

1.1 NIST Traceable Reference Materials 4

1 .2 NIST Spectrophotometric Standard Reference Materials 4

1 .3 Neutral Density Glass NTRMs 5

2. CERTIFIER QUALIFICATION 6

2. 1 Coordination with NVLAP 7

2.2 Facilities 7

2.2.1 Transfer Spectrophotometer 7 2.2.2 Means of Verifying Optical Specifications 9

2.3 Prototype Batch 10

2.4 Assignment of Uncertainties 1

0

2.5 Rights and Terms of Accreditation 1

1

3. PRODUCTION OF SPECTROPHOTOMETRIC NTRMS 1

1

3.1 Filter Glass 12

3.1.1 Optical Specifications 12 3.1.2 Rejection of Out-of-Specification Materials 14

3.2 Filter Holders 14

3.3 Set Containers 16

3 .4 Identification Marking 1

6

3.5 Assembly of Sets 1

7

4. CERTIFICATION AND RECERTIFICATION OF NTRMS 18

4. 1 Rejection and Replacement of Filter Materials 1

8

4.1.1 Uniformity Testing 1

8

4.2 Spectrophotometric Certification Measurements 19

4.2.1 Environmental Control 20

4.2.2 Certification Wavelengths and Bandwidths 20

vii
4.2.3 Instrumental Parameters and Measurement Protocol 21

4.2.4 Measurement Data 27

4.2.5 Revision of Measurement Parameters or Protocols 27

4.3 Aging Protocol 27

4.3.1 Precertification Measurements 28

4.3.2 Stability Approval and Certification 28

4.4 Certification Measurement 29

4.5 Recertification 29

4.6 Documentation 31

5. TRACEABILITY TO NIST 32

5.1 The Traceability Chain 32

5.2 NIST SRMs 32

5.2.1 Storage and Measurement Frequency 33

5.2.2 Wavelength Calibration 34

5.3 NTRM Certifier Standards 34

5.4 Control Charts 34

5.5 Annual Comparison of Transmittance Scales 36

5.6 Lot Samples and Blind Testing by NIST 36

5.7 Reporting Measurement Data to NIST 36

5.7. 1 Data to be Reported 37

5.7.2 Metadata to be Reported with Each Data Record 37

5.7.3 Structure of a Data Record 38

5.7.4 Frequency of Reporting Data to NIST 38

5.7.5 Mechanism for Reporting Data to NIST 39

5.8 Accreditation Review 39

6. FEES AND REVIEW OF FEE STRUCTURE 39

REFERENCES 41

APPENDIX A. WORKSHOPS ON NIST TRACEABILITY IN CHEMICAL

SPECTROPHOTOMETRY Al

APPENDIX B. DESIGN OF HOLDERS FOR OPTICAL FILTERS SRMS. Bl

APPENDIX C. DESIGN OF SHIPPING AND STORAGE CANISTERS AND SPRINGS

FOR OPTICAL FILTERS SRMS. CI

vin
Technical Specifications for Certification of

Spectrophotometric NTRMs

1. INTRODUCTION

This document provides the technical requirements for operating laboratories that

prepare, measure, and certify visible absorbance spectrophotometric NIST Traceable

Reference Materials (NTRMs). NIST traceability for these materials is assured through

the National Voluntary Laboratory Accreditation Program (NVLAP) of NIST, as described in NIST Handbook 150, NVLAP Procedures and General Requirements, and

NIST Handbook 150-21, Chemical Calibration: Certifiers ofSpectrophotometric

NTRMs. These two documents present the specific requirements for accreditation of

laboratories that produce spectrophotometric filters.

The spectrophotometric neutral density glass filter NTRMs discussed in this document

and Handbook 150-21 may be referred to by the short title "filter NTRMs." The filter NTRMs referred to in this document are neutral density glass filters certified for visible

absorbance. A glossary of terms as used in this document is given in Table 1 , for the

convenience of the reader. Certain terms have been defined to provide for consistency in

operating this program, realizing that they may have additional usages within the field.

Table 1 Glossary of Terms Used in SP 260-140

Absorbance

ACD

Blank

Certifier

Negative logarithm of the transmittance; used interchangeably with

"transmittance density" in this document, but always for an air blank.

Normally associated with the "internal transmittance" in chemical usage.

The NIST Analytical Chemistry Division.

In the present context, the blank is taken to be an empty filter holder, so

that the reference beam passes through air but is subject to an identical

geometry to the sample beam at the periphery. In more typical chemical

usage, the blank may be a cuvette containing a matrix fluid appropriate to the sample and yielding an "internal transmittance" and traditional

absorbance corrected for external reflections.

The term used to denote a company that has been approved by the ACD

and accredited by NVLAP to prepare, certify, market, and recertify filter NTRMs.

1
Cuvette A cell for containing liquid samples and blanks for measurements in

chemical spectrophotometers. The dimensions are highly standardized,

and the exterior dimensions are identical to the NTRMs.

Double-beam

Dual Aperture

Filter

Filter NTRM

Internal

Transmittance

NTRM

Optical Filter

Optical Wedge

PMT

Reference Beam

Reference

Spectrophotometer

Referred to...

Regular

Transmittance

Regular

Transmittance Scale

Resolution

A descriptor of a spectrophotometer in which the blank and the sample are measured virtually simultaneously, in two nominally equivalent beams.

The implicit standard methodology used to calibrate the detector linearity

of national reference spectrophotometers.

As used herein, refers to a partially transmitting optically finished solid

artifact or sealed liquid-containing cuvette that may be conveniently

inserted in the sample beam of a chemical spectrophotometer.

Short form for neutral density glass filter NIST Traceable Reference

Material.

Special case of transmittance corrected for reflective losses at the entry

and exit interfaces of a liquid-containing cuvette or a solid glass absorbing

filter. NIST Traceable Reference Material.

Used synonymously with "Filter" in the present document.

The angular measure of the departure from perfect parallelism of the entry

and exit interfaces of filter NTRMs and SRMs.

Photomultiplier tube; the detector commonly used in spectrophotometers.

The optical path in a chemical spectrophotometer in which the blank

would normally be placed, especially in a double-beam instrument

The custom-built ACD spectrophotometer used to maintain the regular

transmittance scale for chemical spectrophotometry in the US.

Taken to indicate the nature of the blank. For the present purposes, all absorbance measurements are "referred to" air, and thus are entirely

equivalent to "transmittance density" measurements.

A special case of transmittance (see below) in which the transmitted

radiant flux is not deflected upon passage through the sample.

The primary standard for regular transmittance as maintained by a National Measurement Institute (NMI) by means of a reference

spectrophotometer. NIST maintains a scale in the ACD for chemical

spectrophotometry and a scale in the Optical Technology Division for

physical measurements.

Often used interchangeably with bandwidth, but current usage - and usage

in this document - involves measurement of a particular peak-to-valley

ratio of transmittance in a specified sample.

2
Sample Beam

SED

Single-beam

Slew-scan

Spectrophotometer

Spectrum

SSW

SRM

STP

Stray light

Transfer

Spectrophotometer

Transmittance

Transmittance

Density

The optical path in a spectrophotometer in which the sample would

normally be placed, especially in a double-beam instrument.

The NIST Statistical Engineering Division.

A descriptor of a spectrophotometer in which the blank and the sample are measured sequentially, and the two "single-channel" spectra are divided to obtain the transmittance.

The method of operation of a scanning spectrophotometer to yield

measured values of transmittance and/or absorbance at a few discreet

wavelengths, as for the certification of filter NTRMs.

An instrument designed to measure the transmittance — and hence by

calculation the absorbance ~ of partially transmitting materials at designated wavelengths. The expression "chemical spectrophotometer" or "chemical spectrophotometry" is used throughout this document to signify

the class of instruments used primarily for chemical measurements and for

which traceability to the regular transmittance scale maintained in the

ACD is desired.

The ordered display of transmittance or absorbance readings at consecutive wavelengths.

Spectral slit width, also known as the spectral bandpass, spectral

bandwidth, and instrument function; a measure of the monochromaticity

of the light of a given spectrophotometer. Typically determined by the

grating ruling density, the focal length, and the mechanical slit widths of

the wavelength dispersion component of the instrument.

Standard Reference Material®; a certified reference material issued only

by the NIST.

Standard temperature and pressure; 273 K and 101.325 kPa.

Detected light that is inappropriate to the correct transmittance

measurement. May be light of any wavelength that did not pass through

the sample, or may be light of the wrong wavelength (whether or not it passed through the sample).

A qualified high-resolution spectrophotometer that is systematically

validated with respect to the ACD reference spectrophotometer and is used to certify and recertify filter NTRMs.

The ratio of the radiant flux transmitted through a sample to the radiant

flux incident upon the sample, at a specified wavelength of light.

The absorbance when "referred to" an air "blank." Synonymous with

"optical density" as used in optical metrology. Used interchangeably with

"absorbance" in this document, but always referred to an air blank.

3
Wavelength The metric by which the energy of UV and visible light is characterized,

given by the distance in air at STP between consecutive maxima or minima in the electromagnetic wave.

1.1 NIST Traceable Reference Materials

The program described in this document is for neutral density glass filters that are

certified as visible absorbance spectrophotometric NTRMs and extends the concept of the

NTRM to a new type of material. The original definition of an NTRM anticipated the

extension of the concept to cover a variety of reference materials and is still quite valid:

A NIST Traceable Reference Material (NTRM) is a reference material produced

by a commercial supplier with a well-defined traceability . . .to the National

Institute of Standards and Technology (NIST). This traceability is established via

criteria and protocols defined by NIST that are tailored to meet the needs of the

metrological community to be served. The NTRM concept was established to allow NIST to respond to the increasing needs for high quality reference materials

with constant human and financial resources. Reference material certifiers

adhering to these requirements are allowed to use the "NTRM" trademark to identify their product.

1

1.2 NIST Spectrophotometric Standard Reference Materials

Several Standard Reference Materials® (SRMs®) offered through the NIST Standard

Reference Materials Program may be used for verifying the accuracy of the absorbance

and transmittance scales of chemical spectrophotometers.2 These materials are certified

for absorbance at a number of wavelengths in the UV and visible spectral regions, using

the National Reference Spectrophotometer built and maintained by the Analytical

Chemistry Division (ACD) of NIST.3 These SRMs are compatible with the sample holder

commonly found in UV/visible chemical spectrophotometers that is designed to hold 1 0-

mm pathlength sample cuvettes with a 12.5 mm square footprint. A liquid wavelength

standard permanently sealed into a cuvette is also available. The spectrophotometric

UV/visible SRMs are summarized in Table 2.

In Table 2 and throughout the text of this document, SRMs are referred to by number and

without regard to a "series" or "lot" designation, which is usually given by a letter following the number. For instance, SRM 930 was first issued in 1970, and the series

designation was incremented annually from SRM 930a to SRM 930d over the next few

years. The designation was frozen at SRM 93 Od for a number of years, inasmuch as the

production was seen to be essentially continuous, with no distinctive changes in

specification. The latest series was incremented to SRM 930e, because the specification on optical wedge was tightened relative to the prior series. The series designation will not

4
be changed again until such time as a change is made in the specifications for the

material.

SRM 93 1 is batch certified, by contrast, and each batch has slightly different assigned

values than prior batches. The series designation is incremented each time the standard is issued, and the current issue is SRM 93 If.

Table 2

UV/Visible Spectrophotometric Standard Reference Materials

SRM Material Certified

Property

3 Levels waveiengin/nm

930 Neutral density glass TD, T 3 440 to 635 (5)

931 Ni/Co nitrate solution A 3 302 to 678 (4)

935 Potassium Dichromate A 10 235 to 350 (4)

1930 Neutral density glass TD, T 3 440 to 635 (5)

2030 Neutral density glass TD, T 1 465 (1)

2031 Metal on Fused Silica TD, T 3 250 to 635 (10)

2032 Potassium Iodide T NA 240 to 275 (8)

2034 Holmium-oxide Solution Peak

Positions/nm

NA 241 to 265 (14)

aT = transmittance; TD = transmittance density; A = (internal) absor 3ance.

''Number of wavelengths for which certified values are given is in parentheses.

SRMs 930 and 1930 are the relevant NTRMs for the production of filter NTRMs, and

there is little reason to prefer SRM 93 Oe over prior series, for certifiers who already own

sets of SRMs. Indeed, older filters may be more stable than newer ones due to the

completion of surface reactions, and hence may be superior control materials.

1.3 Neutral Density Glass NTRMs

A series of workshops was held at NIST to formulate the concepts of the first spectrophotometric NTRM program, as detailed in this document. Appendix A describes

the first of these workshops, whose attendees were technically knowledgeable

representatives of spectrophotometer manufacturers, manufacturers of optical

filters/cuvettes/liquid reference materials, relevant measurement services laboratories, and

end users of spectrophotometric reference materials from such user communities as

clinical laboratories, pharmaceutical manufacturers, chemical manufacturers, etc. From

the beginning, the workshop attendees supported the NIST suggestion to develop NTRM

neutral density glass absorbance filters first and then eventually to expand the NTRM

spectrophotometric filters into other areas. The neutral density glass NTRMs (referred to as filter NTRMs throughout this document) are to be closely related to SRMs 930, 1930,

and 2030 shown in Table 2 and are traceable to NIST through SRMs 930 and 1930 (since

SRM 2030 is a special case filter of SRM 930). Initially, filter NTRMs will share the

5
nominal transmittance levels and certification wavelengths of SRM filters. This

restriction may eventually be eased to permit the production of filter NTRMs at other

nominal transmittances and wavelengths within the ranges spanned by SRM filters. Within the current framework, a given certifier may choose to offer fewer transmittance

levels and/or certification wavelengths than NIST offers, but may not offer more of

either.

For any prospective filter NTRM certifier, the program as described in this document

involves two phases. The first phase is a startup phase, involving qualification and

approval, and the second phase is one of production, certification, and recertification of

filter NTRMs with periodic renewal of accreditation and NIST approval. Over-riding

both phases is the necessity of assuring all parties involved - from the end user to the

filter NTRM certifier to the NIST ACD - that NIST traceability of filter absorbance

measurements to the primary absorbance measurements made at NIST is maintained.

Section 2 below describes the processes involved in qualifying a potential NTRM

certifier. The second phase, continuous production, is described in Sections 3-4, dealing

separately with the detailed specifications and construction of NTRM and SRM filters in

Section 3, and the spectrophotometric certification and recertification process in Section

4. Finally, Section 5 is devoted to the subject of NIST traceability, detailing the

conducting of comparison measurements between transfer spectrophotometers and the

NIST ACD reference instrument, NIST access to certification data and control data, and

blind testing.

2. CERTIFIER QUALIFICATION

To be accredited under this program an NTRM certifier must fulfill the requirements

found in NIST Handbooks 1504 and 150-21. 5 In general, accredited certifiers must:

1 . oversee the manufacture and certify the absorbance values of filter NTRMs;

2. be accredited by NVLAP as a certifier of filter NTRMs; and

3. have their technical proficiency as a certifier of filter NTRMs monitored and

approved by NIST ACD.

Accreditation by NIST's National Voluntary Laboratory Accreditation Program

(NVLAP®) is for the specific laboratory tasks of preparing and certifying NTRM neutral

density glass filters. The NVLAP program will provide a structure for maintaining

traceability to the regular transmittance scale maintained by the NIST ACD, while the

ACD will provide the technical measurement services involved. Certifier qualification

will be coordinated through the NVLAP program, with the technical participation of the

ACD. The following sub-sections will treat the NVLAP/ACD coordination as well as

6
technical qualifications that will be required to achieve and maintain NVLAP

accreditation for the preparation and certification of filter NTRMs.

2.1 Coordination with NVLAP

The NIST Handbook 150; Procedures and General Requirements 4 (HB 150) contains

the general description of the NVLAP program and the basic requirements for

accreditation. Handbook 150-21 5 and NIST Special Publication 260-140 (this document),

amplify HB 150 and contain specific technical requirements which must be met for

accreditation.

NVLAP accreditation, and the required annual renewal of that accreditation, includes

biennial onsite assessment of certifier facilities as well as proficiency testing. Technical

issues peculiar to qualifying to produce neutral density glass spectrophotometric NTRM

filters are discussed in the following paragraphs.

2.2 Facilities

Filter NTRM certifiers will be responsible for maintaining all of the specifications

described in Section 4 below, although some of the production and measurement needs

may be sub-contracted to properly equipped and accredited vendors. At a minimum, the

filter NTRM certifier will be expected to provide the actual value assignment of the

certified transmittance and/or absorbance values. Spectrophotometric measurements

must be carried out in a well ventilated, class 1 00,000 laboratory environment, to assure minimal surface contamination of reference filters and samples. The laboratory

temperature must be controlled between 20 °C and 22 °C, and the relative humidity must

not exceed 70 %. Provisions must be made to protect laboratory instruments and filter NTRMs from manipulation by unauthorized personnel.

2.2.1 Transfer Spectrophotometer

A "Transfer Spectrophotometer" is a high quality, commercial or custom built

spectrophotometer that has been "qualified" to maintain a "disseminated regular

transmittance scale" relative to the national regular transmittance scale maintained by the

reference spectrophotometer at NIST. Weekly measurements of NIST-certified Standard

Reference Material (SRM) filters and annual comparison measurements with NIST assure

the relationship of the disseminated scale to the national scale. A requirement for

qualification to produce the full range of filter NTRMs is the ownership of at least one

currently certified set of SRM 930 (nominal transmittances of 0.1, 0.2, and 0.3), one

currently certified set of SRM 1930 (nominal transmittances of 0.01, 0.03 and 0.5), and one SRM 2034 holmium oxide wavelength standard.

A transfer spectrophotometer must, at a minimum, meet the specifications shown in

Table 3, which also lists appropriate written standards or reference materials to test the

7
specifications. The values given are typical of modern, high resolution, scanning

laboratory spectrophotometers. Testing the specifications of the candidate transfer

spectrophotometer is a necessary part of the qualification process, and the NVLAP

assessor will consider documentation of the instrument qualification process and results.

The instrument qualification process also includes the first of the annual comparisons

with the NIST reference spectrophotometer. These comparisons constitute the "Direct

Proficiency Testing" discussed in Handbook 150-21. The process is described in Section

5.5 below.

Table 3

Minimal Specifications for Transfer Spectrophotometers

Specification Value T T ' j. Units Reference Material Written Standard

Wavelength Range 200 - 700 nm NA XT A NA

Photometric Range 0-4.0 AU NA NA

Spectral Slit Width 0.5-2.0 nm NA ASTM E 958-93

(I993)[a]

Beam Convergence < 6 degrees XT A NA XT A NA

Cone Angle

Resolution >1.5 None Toluene in Hexane USP[b], EP[c]

Wavelength 1 nm SRM 2034 ASTM E 925-83

Uncertainty (I994)[d]

Wavelength 0.1 nm SRM 2034 or 656.1 ASTM E 275-93

Repeatability nm D2 line (I993)[e]

Photometric 0.0022 AU SRM 930-10 ASTM E 925-83

Uncertainty 0.005 AU SRM 1930-1 (1994)

Photometric 0.001 AU SRM 930-10 ASTM E 275-93

Repeatability 0.002 AU SRM 1930-1 (1993)

Stray Light Ratio <0.005 % None SRM 2032 @ 240

nm

ASTM E 925-83

(1994); ASTM E387-

84(l995)[f]

Stability O.0005 AU/h NA NA

NA = not applicable.

a. "Standard Practice for Measuring Practical Spectral Bandwidth of Ultraviolet-Visible

Spectrophotometers," ASTM E 958-93 (1993), Annual Book ofASTMStandards, 03.06,

786-790 (1997), West Conshohocken, PA.6

b. "Spectrophotometry and Light-Scattering," United States Pharmacopoeia

XXIII/National Formulary XVIII, General Chapter 851, 1 830 (1996), The United States

Pharmacopoeia Convention, Inc., Rockville, MD.7

8
c. "Absorption Spectrophotometry, Ultraviolet and Visible," European Pharmacopoeia,

2.2.25, 28-29 (1997), Strasbourg, France.

8

d. "Standard Practice for the Periodic Calibration of Narrow Band-Pass

Spectrophotometers," ASTM E 925-83 (1994), Annual Book ofASTMStandards, 03.06,

775-780 (1997), West Conshohocken, PA.9

e. "Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible,

and Near-Infrared Spectrophotometers," ASTM E 275-93 (1993), Annual Book ofASTM

Standards, 03.06, 708-718 (1997), West Conshohocken, PA. 10

f. "Standard Test Method for Estimating Stray Radiant Power Ratio of

Spectrophotometers by the Opaque Filter Method," ASTM E387-84 (1995), Annual Book

ofASTMStandards, 03.06, 740-749 (1997), West Conshohocken, PA."

2.2.2 Means of Verifying Optical Specifications

SRM and NTRM optical filter CRMs are required to meet more rigorous optical

specifications than routinely produced optical filters. Specifically, flatness, opposite face

parallelism (optical wedge), and transmittance uniformity (optical homogeneity) are specifications that may require the individual testing of filters.

The surface flatness of SRM filters was originally specified as "two fringes of the Hg

546.1 nm line" and has more recently evolved to 1 wavelength of the 633 nm HeNe laser over the central 5 mm by 20 mm area of the filter. Since Fabry-Perot fringes occur at half-wavelength intervals, this is a minor adjustment representing the ascendancy of the

laser over the traditional light source. Also, the restriction to the central area of the filter has been found to be both reasonable and practical, since the filters are smaller and

thinner than most substrates ground to such flatness, and since the edges are most

vulnerable to grinding error. NTRM certifiers may rely upon a supplier for ground and

polished filters, with the instrumentation to verify the optical flatness of the filters residing with the supplier as well. Nevertheless, the certifier is responsible for the result,

by assuring that the testing is done and the specifications are met.

The NIST ACD has tested for opposite face parallelism (optical wedge) for only the last two years, using a pair of autocollimators set up as a collimator/telescope pair.

12 A

description for building such a device has also been described by the UV Spectrometry

Group13 of the United Kingdom. (A single autocollimator may be used for highly

transmitting filters (T > 20 %), but the reflection from the second surface is rapidly lost

for more absorbing filters.) Although it would be advantageous for NTRM certifiers to have in-house capability for measuring optical wedge, it may be sufficient to control

wedge mechanically in the production process. NTRM certifiers will not be required to have instrumentation for measuring opposite face parallelism, so long as they or their

supplier(s) can assure parallelism through optical or mechanical means. ACD will test for

9
parallelism on the NTRM filters for the initial approval process (see Section 2.3 below),

and will monitor samples from production lots of NTRMs against specifications.

The experience of the ACD is that parallelism is readily achieved with double-sided

grinding apparatus, but not with the older single-sided methods. In the double-sided

methodology, the parallelism may be checked and assured with periodic measurements

over a lever arm of tens of centimeters, giving more than adequate control at the 0.1 mrad

level. However, with one-sided grinding, the hot wax used to secure the pieces to a base

plate may lift the piece unevenly by capillary action.

Transmittance uniformity testing involves spectrophotometric measurement at least at two positions on the filter, as described in Section 4.1.1. Therefore, the transfer

instrument may be used, or a separate spectrophotometer not qualified as a transfer

instrument may be used. The certifier must specify the equipment to be used.

2.3 Prototype Batch

As a part of the NVLAP accreditation process, the certifier will prepare a prototype batch,

following the methodology described in Sections 3 and 4 below on production and

certification. The size of the batch is left to the certifier's discretion, with a minimum of

five sets. Once the candidate transfer spectrophotometer has been qualified, initial transmittance density measurements are to be made on all filters of the batch. A sample of

at least five sets is then to be sent to NIST, along with all transmittance density data, for

testing and possible comparative measurements. These tests will form an integral part of

the NVLAP assessment, but the results will not be made public.

The prototype batch may be used as the initial production batch, pending NVLAP

accreditation, completion of aging, and successful testing for temporal drift (see Section

4.3 below on aging and drift testing).

2.4 Assignment of Uncertainties

The NVLAP accreditation certificate will include an assignment of uncertainties

approved by the NIST Statistical Engineering Division (SED), working with the certifier

and the ACD. The major contributions to the total uncertainty will be "Type B"

[determined by non-statistical means] components that are well known through the SRM

optical filters program. The certifier will submit estimates of Type A [determined by

statistical means] uncertainty components.

The estimation of Type A uncertainties requires replicate measurements under the

conditions to be used for certification. The certifier is to acquire data with at least 30

degrees of freedom (acquired over several days) for each nominal transmittance

(absorbance) level and each certification wavelength of the proposed NTRM. In addition

to using the same time constants, gain settings, etc., as proposed for certification, the

sample handling sequence must also follow the certification process.

10
For each nominal transmittance and wavelength, the number of degrees of freedom is given by

where vis the number of degrees of freedom, nrep is the number of replicate

measurements made on each day, nf'xs the number of sets used (or filters at each of the

nominal transmittances), and nrf is the number of days on which replicate measurements

are made. Thus, 36 degrees of freedom are obtained for the five replicate measurements on three filters at each level for each of three days.

Data from different filters of the same nominal transmittance and data taken on different

days may be pooled to yield a single uncertainty. At the same time, analysis of variance

may be employed to determine whether a "day effect" exists (i.e., whether the

measurement is statistically distinguishable from one day to another).

2.5 Rights and Terms of Accreditation

The certificate of accreditation indicates not only successful completion of laboratory

assessment, but also that performance evaluation and NIST traceability have been

demonstrated to the satisfaction of the ACD. The certificate permits the recipient to prepare, certify, and recertify filter NTRMs under the conditions set forth in this

document. The certificate will specify the approved nominal absorbance levels,

certification wavelengths, and uncertainties for filters to be produced as NTRMs.

NVLAP accreditation is subject to on-going proficiency testing, annual renewal, and a

biennial onsite audit.

3. PRODUCTION OF SPECTROPHOTOMETRY NTRMS

The functional component of a neutral density glass spectrophotometric NTRM is a small

glass filter doped in the melt with chemical species such that the sum of their natural

spectra yield a net spectrum which is nominally neutral, or wavelength independent

(although containing many shallow local maxima and minima). Each NTRM filter glass

is supported by a metal filter holder whose exterior dimensions are the same as those of

the common 10-mm pathlength glass and fused-silica cuvettes used in UV/visible

spectrophotometry. The cuvette-style holder is equipped with shutters, to protect the glass

element when the filter is not in use. Several optical filters, having different transmittance

levels, are grouped together to form a set. The filter set is stored in a container

appropriate for shipping and between-use storage.

11
3.1 Filter Glass

The original study of neutral density glasses for use as visible absorption reference

materials is described in NIST SP 260-1 16.

14 The family of glasses chosen by both

NBS/NIST and by the National Physical Laboratory (NPL) in the UK15 are members of

the "NG" series of glasses manufactured by Schott Glass of Mainz, Germany.* The

glasses are designated as "NG-X", where "X" is a single- or double-digit number. The

glasses in the series have congruent spectra.

3.1.1 Optical Specifications

Table 4 shows the optical specifications common to all of the solid neutral density glass

filters (SRMs 930, 1930, and 2030). While the dimensions may be modified to suit

variations in the design of the filter holder, this document contains the specifications and

mechanical drawings for the certifier to produce exact duplicates of the SRMs, if so

desired. A chamfer has been included on all SRM filters, so that the filter may be

removed for cleaning and replaced in the original orientation. The chamfer is unnecessary

if the filter is otherwise marked for identification and/or orientation.

Specifications unique to the nominal transmittance level are given in Table 5. The

tolerances listed for nominal transmittance are new to the SRM filters program and

represent a contribution of the NTRM workshops to the quality of the SRM filters. Filters are now ground to a nominal transmittance rather than being ground to a target

thickness. This change reflects the fact that a given glass type may vary enough from

melt-to-melt that grinding to a target thickness results in an uncomfortably large variation

in the nominal transmittance. NBS/NIST had not addressed the problem earlier because

the actual transmittance was certified, making the nominal transmittance largely

irrelevant. However, the NTRM/SRM program will be both technically and aesthetically

strengthened by reasonable tolerances on the nominal transmittance levels.

Grinding to nominal transmittance involves making a test filter from a new melt or block

of glass, determining the internal absorbance per unit length for the glass, and computing

the appropriate thickness to attain the desired nominal transmittance. The tolerance range

for nominal transmittance in Table 5 is based on a thickness tolerance of ±0.05 mm.

Thus, once the target thickness is measured for a given block of glass and nominal

* To describe experimental procedures adequately, it is occasionally necessary to identify

commercial products by manufacturer's name or label. In no instance does such

identification imply endorsement by the National Institute of Standards and Technology,

nor does it imply that the particular products or equipment are necessarily the best

available for that purpose.

12
transmittance filter, grinding filters to the target thickness ±0.05 mm will result in filters within the nominal transmittance tolerance.

Table 4

Optical Specifications Common to All Neutral Density Glass SRM Filters

Item Specification

Width3 10.4 mm ±0.2 mm

Height3 30.3 mm ± 0.2 mm

Chamferb 2 mm x 2 mm x 45°, one corner

Flatness < 1A. (633 nm) over central 5-mm x 20-mm clear aperture

Parallelism < 0.1 mrad

Quality Scratch/dig (MIL-O-13830A) 80/50, no visible inclusions or

straie, Standard bubble/inclusion class 2

aOptional. The certifier may choose other dimensions consistent with the flatness

specification below, and with the size constraints of the filter holder.

bUsed to orient the filter, so that it may be cleaned and remounted in the same orientation.

If the filter is otherwise marked, a chamfer may be unnecessary.

Table 5

Neutral Density Glass SRM Optical Specifications Unique to Transmittance Level at 546 nm

TD 1 Tnom/%

2 Range ofT/%3 Glass4 UTM5

2.0

6

1 0.88-1.12 NG-3 2.0

1.5

6 3 2.64-3.36 NG-3 1.5 1.0

7 10 9.3-10.7 NG-4 1.9

0.7

7 20 18.6-21.4 NG-4 1.3 0.5

7 ' 8 30 29.1 -30.9 NG-5 1.9

0.3

6 50 49-51 NG-11 2.3

'Nominal transmittance density, or absorbance referred to air, at 546. 1 nm.

2Nominal transmittance referred to air at 546.1 nm.

Permissible range for the certified transmittance at 546.1 nm, based on an uncertainty of

0.05 mm in thickness. The range is computed as 12 % relative for NG-3 glass, 7%

relative for NG-4 glass, 3 % relative for NG-5 glass, and 1 .6 % relative for NG-1 1

.

designation of glass made by Schott Glass.

5Nominal thickness in mm for the filter, as calculated from Schott catalog values for the

internal absorbance per unit thickness. The actual thickness required to achieve the

specified transmittance tolerance may vary from block to block of glass material.

6 One of three filters included in the set for SRM 1930.

7 One of three filters included in the set for SRM 930.

8The only filter included in SRM 2030.

13
3.1.2 Rejection of Out-of-Specification Materials

Verification measurements for the flatness and parallelism specifications were discussed

earlier, in Section 2.2.2. Verification of these specifications is ultimately the

responsibility of the NTRM certifier, but may be performed by adequately equipped and

qualified supplier(s). The NTRM certifier must maintain records of the equipment and

methods used by external suppliers of ground and polished filters. Filters not meeting the

specification for flatness or parallelism must be rejected. The tolerance range for nominal

transmittance is to be checked with the earliest spectrophotometric measurements made

on a given filter; out-of-tolerance filters again must be rejected.

3.2 Filter Holders

Figure 1 shows an assembled SRM 930 filter with the two shutters partially open, along

with front and back views of two filter holders, a shutter, a retaining clip and screw, and a

glass filter. Machine drawings for all of the component parts of the filter holder are given

in Appendix B. The default optical tolerances on the reduced machine drawings are given

in a separate table in Appendix B. Full size drawings are available from NIST upon

request. Each optical filter consists of a glass filter, a filter holder, a retaining clip and

screw, and two black Delrin shutters. The shutters, retained by beveled grooves in the

filter holder, are completely removed from the filter for use and are replaced for storage.

For many years, the NBS/NIST optical filter SRM holders were machined from 0.5-inch

square profile bar stock, to an exterior dimension determined by the stock. The holders

are now machined from plate stock by a numerically controlled (NC) mill, and the

nominal exterior dimension has been reduced to 12.5 mm in agreement with the common

exterior dimensions of modern spectrophotometric cuvettes. Some of the older filters fail to fit into modern instrument filter holders. The holder and retaining clip are black

anodized aluminum.

The same filter holder design is used for all neutral density glass filters of SRM 930,

SRM 1930, and SRM 2030, as well as for the metal-on-fused silica filters of SRM 2031.

The retaining clip is compatible with filter thickness from 1.3 mm to 3 mm. Individual

filters are distinguished by engraving, as discussed below.

14
(Ecrttfinttc

Standard Reference Material* 930c

Cilass FiJlers roi SpecfropkoiQm'eltry

dwtiilHi Values of

de:erraieedi for each fill spectmm pvieo lt. Ta

'i

-.!

- Cfiftinfcd irar^rrjlianccs { the >pcct:cpaa .omcccr 1: i iiansjoiisancc values of Tabtt i a; for a panicuU? isise in tabic 1 i* 0J Waned by mitkiptyjgj

aacffltaiaiy :Jp?n: iasle 3&i. mi dividing.^

as 1 hbk 2 are ghncn in the pvsrs ajsifcipate p#s feadam-emai srawriiis effects. Sp«B

and Transmlttstke Dsiifj

! *C isii.fi at tlvc wavclj

traBsnuttan-cc densit T. These values s&oulcfBB'mdks

ire aieaiUifeiJ sigauLsi sir, Us

2"indkned u Tamils Ja. lit relative |>efe

c Mid iiha StSOi lh?se train i«!uaj g.a<s !npo$c4 aur:^c oi :h« glass is

filter holder (afcpigsire 1 1.

20. ur 30), j^B HKiJDond

930e diffaTiijJB MrTir series, AStruritrnl yfl ¥ Warning!.

cs iridependcrv.ly sienroiraane-iic

s calculated from

ti'^ltiCC • \' Mills ,'f creioues !or the certified

rails iiijs lance u ac e ttataty

1tun& by the relative ssKeat

for lie jnuisnwantie density values

; Alj) m tabic Jb. The usieertaiaitues due to surface cciaimination asd

in paiwtthesss beneath ifce- wavelengths of

i value

Figure 1 . SRM 930 component parts, showing a side view of a filter holder with partially open

shutters, front and back views of an empty filter holder, filter glass, retaining spring and screw, and shutter.

15
3.3 Set Containers

NIST optical filter SRMs are shipped in cylindrical canisters with screw-top lids,

machined from solid aluminum and anodized. Four cylindrical holes machined into the

solid canister serve as storage positions for the three filters and single empty filter holder

(blank) that comes with each set of SRM 930 or SRM 1930. Four beryllium-copper

springs are used to secure the holders into the four positions for shipping and should be

retained for transporting the filters for recertification. Machine drawings for the

canisters, lids, and shipping springs are given in Appendix C for four-position canisters.

NIST canisters for the single filter and blank holder of SRM 2030 have only two holes

drilled and two shipping springs per set. The aluminum is prepared free of machining oil to reduce the potential for filter contamination.

Although they are not airtight, the metal canisters provide a rugged container for

shipping, and reasonable protection from contamination of the filter surfaces. The all- metal construction is intended to reduce the electrostatic attraction of dust particles to the

surface of the filters. NTRM certifiers may adopt the NIST design for shipping and

storage canisters, but are not constrained to use this design.

NTRM certifiers must avoid the use of packaging materials that will deteriorate with

time, and could contaminate the filters by outgassing or producing particulate matter.

3.4 Identification Marking

Upon admission to the NTRM program, each certifier will be issued a unique code to designate its filters. This code will be two or three alphanumeric characters. Eachfilter

type will further be identified by a five-position (maximum) alphanumeric character field. Eachfilter set will be identified by a unique five-position (maximum) alphanumeric serial

code. Within each set, individualfilters will be encoded with two digits representing the

nominal percent transmittance.

Cert. Filter Set Norn

Code Type S/N %T

yyyyy zzzzz ##

SRM 930e 124 30

SD VISF AF334 30

NIST example

Super-Duper Filter Co. example

The outside of the set container must be indelibly marked with at least the manufacturer's

code and filter type. Other information such as the NTRM name, the set serial code, the

manufacturer's name, and the NTRM logo may also be present. Neither the NIST logo nor the NVLAP logo may be displayed here. NIST SRMs have the NIST logo, the SRM

number (filter type), and SRM name engraved on the canister lid—the set number (serial

code) is not marked on the canister.

16
Each filter holder is to be engraved on one side near the top with the set serial code (set

number) and the nominal transmittance (%) and on the other side with the manufacturer's

code and filter type. No marks are to be made on the top side of the filter holder. (NIST

is reserving this space for a future barcode to provide filter tracking during the

certification process.) Engraving is to be done before the filter is mounted in its holder.

This encoding scheme will allow each filter and its associated data to be uniquely

identified and tracked. NIST is also preparing to mark each piece of glass used in the

filters near its top with a unique serial number. This will permit better data handling,

preparation tracking, and inventory control. It will permit positive identification of the

glass used to replace damaged glass in SRM sets. Furthermore, it can be used in lieu of

the chamfer as an orientation mark. NTRM certifiers may also wish to serialize their

filter glasses.

3.5 Assembly of Sets

At NIST, numbered sets of SRM filters are assembled before any of the

spectrophotometric measurements are made. This protocol provides identification and

accurate data logging since the filter holders are uniquely identified, but the glass is not

currently being serialized. Uniformity testing and drift testing, described in Section 4

below, may result in the need to replace individual filters in a set, or to an entire set being

held back from circulation until a single filter meets the drift rate specification. Certifiers

may find it advantageous to serialize the glass and use temporary holders for preliminary

measurements. Sets could then be assembled from qualified filters just prior to the final

certification measurements. The NIST procedure for assembling sets from freshly

prepared filter glass and holder parts is described below.

All fully prepared filter glass and holder components must be thoroughly cleaned and air- dried before assembly. This is especially important for the glass filters, as particulate

matter may be left behind from the polishing rouge used. Filters are to be rinsed with an

appropriate solvent to remove grinding and polishing materials. Holder parts will have

been cleaned of machine oil before anodizing, but must be carefully examined for loose

contaminants after anodizing, and any buffing or polishing materials must be fully

removed.

Filters must be assembled on a clean surface with good lighting. The filter glass is to be

handled by the edges and with powder-free gloves. Vigorous wiping with lint-free lens

tissue is to be used to clean the filter, after first blowing off loose dust with a manual air- bulb. Care must be taken to avoid transferring body oil to the filter via the lens tissue -

parts of the tissue that have been in contact with skin must not contact the glass. (Tissue

should be used and discarded liberally!) Before mounting, the filter is to be examined

under bright, uniform light to check for residual surface dirt and smears or defects in the

glass itself.

17
The filter is to be carefully placed into the proper side of the holder using plastic forceps

or plastic-tipped tweezers to avoid scratching or smudging the filter. The retaining clip is then to be attached with the black plastic screw and the shutters slid into place. The

filters constituting a given set are placed in the set container, along with an empty (and

not engraved) filter holder to be used for blank measurements.

4. CERTIFICATION AND RECERTIFICATION OF NTRMS

The spectrophotometric measurements that lead to certification are virtually identical to those for recertification of filter sets which are returned at the end of their certification

period, although some differences may be identified in the overall processes. The

subsections below will concentrate on the certification process, following the filter cleaning and assembly discussed above. Subsection 4.5 is devoted to the differences

between the recertification process and the certification process.

4.1 Rejection and Replacement of Filter Materials

Filters will be rejected on the basis of excessive optical wedge (lack of sufficient

parallelism), inadequate flatness, low optical quality, and poor transmittance uniformity.

The rejection criteria for the first three of these were given in Table 4 and Section 3.1.1,

and instrumentation and tests for flatness and parallelism were described in Section 2.2.2.

The "Optical Quality" specifications of Table 4 are somewhat subjective, but are familiar

to sufficiently experienced optical technicians, and are judged simply by visual

inspection.

Two additional acceptance criteria - based on spectrophotometric measurements - are given in Table 6 and described below. Transmittance uniformity and transmittance drift

represent known properties of neutral density glass filters that are included in the

uncertainty statement and in the acceptance criteria for filters. That is, a certain level of

non-uniformity and a certain amount of drift are considered to be inevitable in filters and

are accounted for in the uncertainty evaluation. However, filters are also individually

examined for each of these properties to eliminate filters in the tails of the distribution.

Testing of the uniformity property is described below. Testing of the drift property will

be covered in Section 4.3 on the aging of filters.

4.1.1 Uniformity Testing

The transmittance uniformity of optical filters used as transfer standards is important

because the certifying instrument and the instrument under test may project different sizes

and shapes of incident light onto the filter at different positions on the filter.

16 The spectrophotometer used for uniformity testing must be capable of measuring the

transmittance density of the filter at the center of the filter, which is nominally 15 mm

above the base of the filter. The second measurement is then made by raising the filter 7

mm in the beam, so that the light is incident at a point nominally 8 mm above the base of

18
the filter. The absolute value of the difference of these two absorbance readings, at 546.

1

nm, must be equal to or less than the tolerance given in Table 6 to accept a given filter. (An effective method of displaying the absorbance difference on the instrument is to define one of the two readings as a "blank" and the other as a "sample." The absorbance

then displayed is in fact the absorbance difference between the two positions.)

Table 6

Rejection Limits for Transmittance Uniformity and Drift

Tnom/% su

a 5d

b

1 2 0.0045 0.0025

3 1.5 0.0040 0.0025

10 1 0.0015 0.0015

20 0.7 0.0015 0.0015

30 0.5 0.0015 0.0015

50 0.3 0.0010 0.0015

a Permissible absorbance difference for two positions on a filter,

separated by 7 mm.

b Permissible absorbance drift for a six-month aging period.

To avoid "false positive" rejections, for which the filter is sufficiently uniform but

random uncertainty in the two readings results in a difference exceeding the rejection

criterion, the random uncertainty must be accommodated. Filters that "fail" the

uniformity test by a margin of less than 0.0005 AU may be tested four additional times

and re-evaluated on the basis of the average of the five results. To minimize the random

uncertainty, the measurement may be made at an arbitrarily large - but consistent -

spectral slit width (SSW). It may be advisable to employ the largest SSW available for

sufficient precision for low transmittance samples.

4.2 Spectrophotometric Certification Measurements

The transfer spectrophotometer used for certification measurements has been described in

Table 3 and the qualification process for the spectrophotometer in Section 2.2.1 . The

qualification process is to determine the optimal instrumental settings and measurement

protocol to be used for certification. The following sub-sections discuss the protocol for a single spectrophotometric measurement, including environmental and warm-up

conditions, instrumental parameters fixed by the certification requirements, and

instrumental parameters and sequencing that are to be optimized for particular

instruments.

A logbook must be kept near each operational transfer spectrophotometer, in which to record temperatures, humidities, and times at which the instrument is turned on and off, as noted in the paragraphs below.

19
4.2.1 Environmental Control

For many years, the neutral density glass SRM filters have been certified at a laboratory

temperature of 21 °C ± 1 °C, and the accompanying certificate has approved their use over a temperature range of 21 °C ± 2 °C. It is obviously necessary that NTRM filters be

certified within the temperature range of 21 °C ± 1 °C for ready comparison of

measurements with NIST. Temperature control and monitoring of the sample may be

especially important for commercial instruments with compact sample compartments. It

is therefore necessary that certifiers monitor and/or control the temperature of their

transfer spectrophotometer sample chamber to ensure that measurements are made at the

proper temperature.

Humidity has not been tightly controlled in the ACD laboratory containing the reference

spectrophotometer. The relative humidity of the laboratory has varied between

approximately 30 % and 65 % over the last two years. The most constant humidity

performance has been during air conditioning season, when the relative humidity rarely

strays more than two or three percentage points from 45 %. NTRM certifiers are to have

at least a simple humidity measurement device in the spectrophotometric measurement

laboratory, and record the reading daily. Even better, a calibrated electronic hygrometer

could be used to record the humidity automatically with each instrumental run.

Temperature and humidity readings (and the date and time of day) are to be entered into

the instrument logbook at least twice per operational day - once when the instrument is turned on for warm-up and once when the instrument is turned off.

4.2.2 Certification Wavelengths and Bandwidths

The five wavelengths chosen as certification positions for the neutral density glass SRMs

and NTRMs were selected to give a reasonably comprehensive coverage of the visible

spectrum with a manageable amount of data. The wavelengths were also chosen to correspond to local maxima and minima in the gently undulating spectra, as may be seen

in Figure 2. (Coincidentally, the only extremum wavelength specified to the nearest tenth

of a nm, 546.1 nm, is that of the widely used atomic mercury green line.) The use of a

local extremum, or position of zero slope, reduces the effect of wavelength calibration

errors on the accuracy of the measured transmittance density. Certificates for neutral

density glass NTRMs will require that the wavelength scale of the user's instrument have

an uncertainty of less than 1 nm. A minor problem caused by the spectral undulations and certification at local extrema is that large instrumental bandwidths will affect the apparent absorbance. Thus, maximum

permissible bandwidths have been established for these reference materials, as shown in

20
Certification Wavelengths/nm

2.5 <

£ 2 w

c

aQ

1.5

Figure 2 and listed in Table 7. These bandwidths are determined by a combination of

thelocal curvature (second derivative) at each position and the uncertainty component

included for

bandwidth effects.

Thus, the

maximum

permissible

bandwidth could be

increased at the

expense of

expanding the

uncertainties.

Computing

correction

coefficients for

larger bandwidths

and including a

table of such

coefficients on the

0.5

440 465 5416.1 590 6 J5

1 H Maximum Bandwidth s

1 h

SRM 1930-01

SRM 1930-03

SRM 930-10 SRM 930-20 SRM 930-30 SRM 1930-50

400 450 500 550 Wavelength/nm

600 650

Figure 2. Transmittance density spectra for SRMs 930 and

illustrating the wavelengths of certification and maximum

bandwidths for use.

I930

certificate could also increase it. Such changes may be taken into consideration later in

the NTRM program, since many modern utility instruments operate at large fixed

bandwidths.

Table 7

Certification Wavelengths and Permissible Spectral Slit Widths

Wavelength/nm 440 465 546.1 590 635

SSW/nm 2.2 2.7 6.5 5.4 6.0

4.2.3 Instrumental Parameters and Measurement Protocol

The most critical element in certifying reference filters is the optimal and consistent

operation of reference and transfer spectrophotometers. The qualification process

described in 2.2.1 will be used to establish the optimal instrumental parameters and

measurement protocol that will be used for a single "measurement." In particular, a

certification measurement may well be more time consuming than the recommendations

for a "typical" measurement by the manufacturer of the instrument. Table 8 lists some

instrumental operation recommendations that are explored further in the following

paragraphs. All uncertainties imply a "coverage factor" or "expansion factor" of two,

which would correspond to a 95 % confidence interval for >30 degrees of freedom.

Wavelength Accuracy. The use of local extrema for certification wavelengths gives a

certain leeway in the need for wavelength accuracy, but most of that leeway should be

allotted to the end user's spectrophotometer, not the certifier's. The certifier must thus

21
reduce wavelength uncertainty to 0.1 nm or less. This target uncertainty may be more

stringent than the instrument manufacturer will routinely support and thus requires

diligent calibration and verification by the NTRM certifier.

Table 8

Instrumental Operation Recommendations

Property Recommendation

Wavelength Uncertainty 0.1 nm

Warm-up Time 1 hour typical

Data Acquisition Time Resulting in <0.0003 AU repeatability at 1 AU

Replication >3x

Gain Invariant for each T, X; see text Spectral Slit Width 1 nm

Single-beam Equivalence Daily correction data; see text

Figure 3 illustrates the calibration of

the wavelength scale of the original

reference spectrophotometer in the

ACD. The "bias" plotted on the

ordinate is the difference in the

wavelength reading on the instrument

dial at the peak of a given atomic line

and the accepted wavelength of the

line. (Atomic line position

uncertainties are sufficiently small as

to be negligible for the present

purposes.) Thus, the instrument may

be seen to read high by slightly more

than 0.1 nm at 440 nm. Since we set the instrument to the nearest 0. 1 nm

for all certification measurements,

the instrument will be set to a wavelength reading of 440.1 nm for

certification at 440 nm, until such

time as the calibration is shown to change. Similarly, 590 nm certification is performed

at an indicated setting of 590.2 nm, and so forth. Many instruments are capable of using one or both of the well-known visible lines from the system deuterium lamp (used as the

UV continuum source) for re-calibration, using built-in software. Some systems may

accommodate the use of atomic pen lamps for wavelength calibration.

Atomic lines are the most highly recommended means of wavelength calibration, but

SRM 2034 is available for the calibration of instruments for which atomic sources cannot

be readily used. Regardless of the calibration method, full spectral scans of SRM 2034

22
must be acquired and saved on a monthly basis as documentation of wavelength accuracy

for NVLAP assessment. The certified wavelength uncertainty of SRM 2034 is 0.1 nm for

each of 14 certified band positions. The certified band positions of SRM 2034 are shown

in Table 9, and the spectrum is shown in Figure 4.

Table 9.

Certified Wavelengths (nm) of Minimum Transmittance

of 14 Bands for SRM 2034 at Six Spectral Slit Widths

SRM 2034

Band No.

Spectral Slit Width/nm*

A 1 0.1 0.25 A C 0.5 1 2 3

1 240.99 240.97 241.01 241.13 241.08 240.90

2

")/1 O 1Q Z4V. /o Z4^. ly T/IO on 1 A O OO z4V.yz

3 278.15 278.14 278.13 278.10 278.03 278.03

4 287.01 287.00 287.01 287.18 287.47 287.47

5 333.47 333.44 333.43 333.44 333.40 333.32

6 345.55 345.55 345.52 345.47 345.49 345.49

7 361.36 361.35 361.33 361.31 361.16 361.04

8 385.45 385.42 385.50 385.66 385.86 386.01

9 416.07 416.07 416.09 416.28 416.62 416.84

10 451.30 451.30 451.24

11 467.82 467.82 467.80 467.83 467.94 468.07

12 485.28 485.28 485.27 485.29 485.33 485.21

13 536.54 536.53 536.54 536.64 536.97 537.19

14 640.51 640.49 640.49 640.52 640.84 641.05

Certificate expanded uncertainty for all positions is ±0.1 nm.

Warm-up Time. During the process of instrument qualification, data will be taken to observe temporal effects in the returned measurement values and their repeatability

during the first hour or so after the instrument is turned on. Traditionally, such effects are mostly attributed to stabilization of the system lamps and the detector, especially for

photomultiplier detectors. A minimum stabilization time will be established for the

system as a whole, as well as for the lamp(s) and/or detector, if they are separately

controlled. For the ACD reference spectrophotometer, a stabilization time of one hour is required for the system electronics (including the high voltage to the PMT) and either of

the two system lamps.

23
The date, time, and action must be recorded in the instrument logbook each time the

instrument (or individually controlled component) is turned on and off.

Data Acquisition Time

digitally. Although a

single digital reading

requires only

milliseconds, the

result displayed as "a

reading" may

represent an average

of many digital

readings taken of an

analog voltage

appropriately timefiltered for the digital

sampling rate. The

operator is typically

presented with a

variety of possible

data acquisition times

from which to choose

in the operation of the

instrument. During

instrument qualification, replicate data are to be acquired to characterize the absorbance

repeatability (e.g., the standard deviation of ten replicate measurements) of the instrument

as a function of the acquisition time parameter, for a 1-AU filter. This data may be

acquired at all five certification wavelengths, or at the certification wavelength for which

the repeatability is the poorest for a given data acquisition time. This data may then be

used to establish the instrumental acquisition time parameter for certification

measurements, in accord with Table 8. That is, an acquisition time may be identified for

which the standard deviation of ten measurements is less than 0.0003 AU.

The single acquisition time parameter established on the basis of the above measurements

at 1 AU may be used for all wavelengths and for all transmittance values from 0 AU to 2

AU. However, it is also acceptable for the certifier to employ longer acquisition times for

transmittance densities above 1 AU. The experimental data and acquisition time choice(s)

for certification measurements must be clearly documented in the qualification report.

The total effective signal averaging time for the certification measurement is given by the

acquisition time per reading multiplied by the number of readings taken (for a given filter at a given wavelength) in a certification measurement, as discussed below.

A transfer spectrophotometer will acquire and store data

Kill

i0 I i 1 i

1 1 i i 1 i 200 250 300 350 400 450 500 550 600 650 700

Wavelength/nm

Figure 4. The transmittance spectrum of SRM 2034 holmium

oxide solution wavelength standard with the certified bands

indicated.

24
Measurement Sequence/Replication. Each replicate reading involving cycling of the

samples or the grating consumes extra time for the mechanical motion and introduces wear to the instrument mechanical systems. The number of such replicates is therefore

kept low, and is not used for "Type A" uncertainty evaluation on a filter-to-filter basis.

For a double-beam transfer instrument, it is acceptable for replication to be direct -

without cycling of the samples or grating - if the instrument software permits. A

minimum of three replicate readings is required in either event, though more could be

used for direct replication without excessive use of time.

The measurement sequence within an instrumental "run" must be clearly stated in the

measurement protocol, and used for all certification measurements. The sequence is the

description of the order in which replicate measurements are made at multiple

wavelengths for multiple filters (if an automated sample transport is used in the

instrument).

The data file(s) generated by the instrument may require post-processing to produce the

averages of the replicate measurements in the format for database entry. The certifier

must maintain the intermediate data, complete with replicate measurements, for at least one year. Such data files are considered to be "run files", which can be examined for

"outlier" values or instrument problems when questions arise about certification data.

The final measurement for a given filter and wavelength is the mean of the replicate

readings. The standard deviation of the mean of the replicate readings is to be computed

and compared to the expected repeatability as a quality check on the measurement.

Instrumental Gain. Any instrumental gain settings are to be chosen to minimize the noise

contribution of the electronics to the final measurements. As with all other instrumental

parameters used for certification, gain settings must be consistent throughout each

accreditation year, at minimum. Reference beam attenuation methods available for

dynamic range extension are not recommended for the range of the NTRM filters, and

should be used for the lower transmittance filters only after consultation with ACD.

Spectral Slit Width. Transfer spectrophotometers are to be operated at an instrumental

SSW of 1 nm or less for optimal agreement with the NIST reference instrument (operated

at a SSW of 0.8 nm). For bandwidths less than 1 nm, the bandwidth dependence of the

measured absorbance is negligible. Smaller spectral slit widths might be desirable to limit

the light on the photocathode of the detector and improve the linearity of the

photomultiplier tube. Once an operating bandwidth is chosen for certification

measurements, it must be used consistently, at least through a given accreditation year.

Single-Beam Equivalence. For a double-beam spectrophotometer, the baseline

transmittance may be defined as TB =I0JIo2 where IQl is the radiant flux of beam 1 and I02 is the radiant flux of beam 2 (with no

attenuation in either beam). If an arbitrary sample of true transmittance (with respect to

25
air) T is placed in the sample position and transmits a radiant flux IS] , then we may show

that

T Jsi = (^//,2 ) =

T' L {Ior 1 102) TB

where T may be seen to be the possibly inaccurate double-beam measurement of the

transmittance of the sample. Thus, the true value of transmittance is given by dividing the

measured value at each wavelength by the baseline transmittance at the same wavelength.

The equivalent treatment in absorbance is to subtract the baseline absorbance from the

double-beam absorbance, due to the logarithmic relationship of the two measures.

Transmittance and absorbance "baseline" spectra are shown in Figure 5 for a commercial

double-beam spectrophotometer used for utility work in the ACD. The baseline spectrum

is simply a scan over the

desired spectral region

with no sample or blank

in either beam. For

perfectly balanced

beams, the transmittance

should be within the

measurement

repeatability of 100 % at every wavelength and the

absorbance should be

within the measurement

repeatability of zero at every wavelength. The

observed departure from

perfect balance may be

attributed to a number of

factors including small

differences in the

reflecting surfaces in the

two beams, and in the aging of these surfaces. The discontinuity at 380 nm,

corresponding to a change of lamps, clearly reflects alignment differences between the

tungsten and deuterium lamps.

Candidate transfer spectrophotometers will typically be double-beam instruments with

the option of baseline correction to single-beam equivalence. This treatment combines the

best features of single-beam and double-beam spectrophotometry, at the cost of some

increase in noise. If automatic baseline correction is not available, the correction must

still be included in the data reduction, using suitable baseline measurements. Baseline

correction data must be acquired at the beginning of each day of data acquisition, and

may be acquired more frequently. If the transfer spectrophotometer is a true single-beam

^ 96.5

200 250 300 350 400 450 500 550 600 650 700

Wavelength/nm

Figure 5. Baseline transmittance and absorbance spectra for a commercial double-beam spectrophotometer with

automatic baseline correction.

26
instrument, or operated in true single-beam fashion, reference readings must be made

immediately before and after each sample reading.

4.2.4 Measurement Data

The final measurement data - one measured transmittance and/or absorbance for each

certification wavelength and each filter- must be saved in computer readable form and in

hard copy. Data detailing intermediate measurements and calculations involved in the

determination of each reported transmittance are to be saved either electronically or in

hard copy. Such "run data" may be used to look for instrumental problems and possible

measurement errors in a given set of data and are to be kept for a period of time equal to the certification period of the filter NTRM.

The final measurement data and run data must clearly contain the identity of the relevant

filter(s), and the final measurement data must be imported into an appropriate database to log all valid measurements on all of the certifier's filter NTRJVIs. Data determined by the

qualified operator to be invalid must not be imported into or retained in the database. The

certifier's quality system is to specify remedial action to be taken.

4.2.5 Revision of Measurement Parameters or Protocols

No instrument parameter or protocol for the certification or recertification of filter NTRMs is to be changed within a given accreditation period without the prior approval of

the ACD and NVLAP. The certifier may propose any desired changes in writing to ACD/NVLAP at the time of accreditation renewal. Routine transfer spectrophotometer

maintenance and repairs (such as lamp changes) must be noted in the instrument log

book. Alterations and repairs deemed significant must be reported to ACD/NVLAP and

may require a complete re-qualification of the transfer instrument.

4.3 Aging Protocol

Neutral density glass filters are subject to a slow but sometimes significant temporal drift

in the transmittance and/or absorbance. The predominant theories developed over the

years attribute drift to two possible sources. The more fundamental source is thought to

result from a reaction (such as oxidation, hydration, and/or dehydration) at the freshly

ground surface of a new filter which reduces the reflection of light at the entry and exit

faces of the filter, thereby increasing the transmittance with time.

17 The less fundamental

source is attributable to surface contamination from the storage environment of the filter,

leading to decreased transmittance with time. The more fundamental source is especially

troublesome for newly created filters, giving rise to the need for an aging protocol. The

surface contamination source is responsible for the need to continue recertifying filters annually, even after the surface chemistry has stabilized.

The protocol that has been used at NIST over the years, and seems to work reasonably

well, requires a minimum aging period of six months for all filters and requires the drift

27
at all certified wavelengths for all filters of a set to be less than the value quoted in Table

6 over a six-month period. The measurement protocol is discussed below.

4.3.1 Precertification Measurements

During the mandatory six-month aging process, periodic spectrophotometric

measurements are made on the filters using the transfer spectrophotometer. These

precertification measurements are to be permanently recorded to document the aging and

stability acceptance of the filters. An electronic record of each measurement is to be

entered into the master database discussed in 4.2.4, properly identified as a

precertification or stability measurement.

The frequency of precertification measurements is left to the discretion of the certifier. The tradeoffs between measurement frequency and aging time are discussed in the

following subsection. The certifier is not required to maintain a rigid measurement

schedule, as long as the stability acceptance protocol is observed.

4.3.2 Stability Approval and Certification

If a filter has been aged for six months, with spectrophotometric measurements at the

beginning and the end of the period, the stability is considered to be acceptable if the

absolute value of the change in absorbance at each certification wavelength is less than

Srf, the drift limit specified in Table 6. Alternatively, if the drift criterion is exceeded at only one of the five wavelengths, the stability is acceptable if the average of the absolute

values of absorbance change at the five wavelengths is less than the specification in

Table 6.

The measurement database used at NIST has a comparison feature, permitting the

comparison of any two selected data sets for a particular filter set. This feature is convenient for stability testing as well as for evaluating the performance of filter sets returned for recertification.

The frequency of precertification measurements determines the granularity of a six-month

sliding window. As an example, precertification measurements at NIST are normally

made at intervals of approximately two months. If a given filter fails to meet the stability

criterion, the next time at which a six-month period can be examined is six months after

the second precertification measurement. Thus, with the NIST protocol, a filter may fail the stability test after six months of aging but pass after eight months of aging, based

upon the drift between the measurement at the two-month point and at the eight-month

point.

At NIST, sets are assembled at the beginning of the aging process, and the failure of a

single filter to satisfy the drift criterion results in holding the entire set until the next

measurement is made. This approach may not be the most efficient use of materials, but

28
has been necessary to maintain the entire measurement history of each filter in the

absence of identification markings on the filter glass itself.

The NTRM certifier must have a documented protocol for the aging of filters, including

the precertification measurement intervals and the methodology for identifying and

tracking filters and comparing measurements at six-month intervals.

4.4 Certification Measurement

Once the stability criterion has been satisfied, as described in Section 4.3, the material is qualified for the final certification measurement. Indeed, the final aging measurement

may be used as the certification measurement.

However, since filter NTRJVls are certified for a period of one year from the date of

certification, the certifier may choose to stability-qualify filter sets and postpone the

official certification measurement until the set is sold to an end-user. The end user then

enjoys the full benefit of the certification period.

A permanent hard copy and computer-readable copy of the data acquired in the

certification measurement must be maintained as long as the filter is in use. The

electronic data must be imported into the database used to maintain the measurement

history of all NTRM filters. The data must be uniquely identified as the original

certification data for the filter set. In the NIST database, a "type" key identifies a

precertification measurement as "type 1 " and the original certification measurement as "type 2." Two other "types" are associated with recertification, as discussed below.

4.5 Recertification

Filter NTRMs are certified for a period of one year, permitting a smaller uncertainty

estimate for stability than for SRMs, and yielding overall uncertainties that can be

reasonably competitive with those for SRMs. Certifiers of filter NTRMs will maintain an

active recertification program for the materials that they produce and will accompany

each filter NTRM sold with adequate documentation of recertification procedures and

scheduling considerations.

End users are not required to have their filter NTRMs recertified by the original certifier,

but for the artifacts to retain their NTRM status, any recertifications must be done by a

currently accredited filter NTRM certifier. Filters to be recertified as NTRMs must be

NTRMs or SRMs. A producer may recertify other filters, but may not issue NTRM

certificates for these. If an SRM is recertified by an NTRM certifier instead of by NIST, it becomes an NTRM until such time as it is recertified by NIST.

The certifier should maintain a set turn-around time, so that customers may meet

recertification deadlines and not be deprived of the use of their reference materials for a prolonged period. The customer should be instructed to contact the certifier to schedule

29
recertification before sending filters. In times of excessive backlog, customers may then

retain their filters for use until notified by the certifier to ship them for recertification.

The NTRM certifier must maintain a recertification log, documenting customer contacts,

the arrival of filters for recertification, the location and status of filters in-house for

recertification, and the shipment of filters to their owners.

The recertification process involves two measurements on the filter set using the transfer

spectrophotometer and the full certification protocol as described in Section 4.2. The first measurement is made on the filter set in "as received" condition, and the measurement

data are imported into the filter history database and classified as a measurement "type"

known as "as received" (designated as "type 3" at NIST).

The filters in the set are then disassembled and cleaned one at a time (to assure that filters are not interchanged among holders) using lint-free lens tissue and the same procedures

as described for the original assembly in Section 3.5. After cleaning, each filter is carefully re-assembled, using the chamfer or identification mark on the filter to assure

that the orientation of the filter in the holder is the same as it was originally.

After cleaning, a second spectrophotometric measurement is made, and the results are imported into the filter history database as a "type" of measurement denoting

"recertification" (designated as "type 4" at NIST). The results are also incorporated into a new certificate to accompany the filter set and supersede the previously certified values.

The "as received" data are also to be sent to the end user, in an auxiliary report to the new

certificate. NIST has found that many end users need documentation of the discontinuous

change made in the filter transmittance by the cleaning and also need to re-evaluate the

accuracy of data taken on instruments being tested using the filters shortly before

recertification.

The certifier must compare the results of both types of measurements on the returned

filters to the previous year's certified values and note any "out-of-tolerance" results in the

end user report and in a record to be reviewed by the certifier and NVLAP during the

annual reassessment of assigned uncertainties. For this purpose, an "out-of-tolerance"

result is one for which the difference between the prior certification result and the

measurement in question exceeds the expanded uncertainty associated with the

certification.

Table 1 0 gives several of the more common outcomes of tolerance checking and the

interpretation that may be conveyed to the customer. For other types of results, the

certifier is to consult with the ACD technical advisor to the filter NTRM program. If the

filter set has not been recertified within the recommended interval, the customer is to be

advised to observe the schedule more rigorously in the future. If the drift and the cleaning

result indicate an alarming rate of surface contamination, the customer is to be advised to store and use the filters in a cleaner environment. If the drift of a cleaned and relatively

30
"young" filter indicates more rapid surface chemistry effects than accommodated by the

uncertainty estimate, the customer is to be advised to have the set recertified ahead of

schedule until the filter stabilizes.

Table 10

Out-of-tolerance Interpretations for Filter Recertifications

Asreceived1 Recert.

1 Time2 Interpretation

3

> > Mature filter; too much contamination; customer

advised to observe schedule. > < Contaminated mature filter; customer advised to store in

cleaner environment. ~ or > < > Immature filter with or without contamination; customer

advised to observe schedule. < < Immature filter with or without contamination; customer

advised to recertify early.

'For "As-Received" and "Recertification" data: > means that one or more measured

absorbance exceeds the previously certified value by more than the expanded uncertainty;

< means that one or more absorbance is low and out of tolerance; and ~ means that the

measurement is in tolerance.

2 Time between prior certification and recertification: > implies more that one year; and <

implies one year or less.

3A "mature" filter is one for which the surface chemistry has stabilized, and is generally

more than one or two years old; drift in absorbance is normally positive with time;

cleaning reveals observable decrease in absorbance. An "immature" filter is still showing

a negative absorbance drift with time after cleaning and is typically less than two years

old.

4.6 Documentation

Most of the relevant documentation issues associated with the certification process are covered in the companion documents Handbook 150 and Handbook 150-21. The most

critical element of concern from the technical point of view is that the measurement

history of a given filter be maintained as long as the filter is in service. Adequate

redundancy would include hardcopy of all certification and recertification measurements,

computer files of all such measurements, and frequent back-up of all computer files. Additional necessary documentation would include the logbook for each transfer

spectrophotometer and the logbook(s) for tracking production filters and recertification

filters.

31
One full set of certification and recertification data will be forwarded to and maintained at NIST. Certifiers will use electronic submission of data to NIST in a format specified by

the ACD and given in Subsection 5.7.

5. TRACEABILITY TO NIST

5.1 The Traceability Chain

Transmittance is a ratiometric measurement and hence is not ultimately traceable to any

of the seven fundamental SI units. Over the past three decades many of the National

Measurement Institutes (NMIs) worldwide - including NIST - have constructed

reference (or "high accuracy") spectrophotometers of closely related design. These

instruments are said to maintain the scale of regular transmittance for the nation. In the

US, two such instruments have been maintained; one in the Analytical Chemistry

Division and one in the Optical Technology Division (OTD). The "division of labor" at NIST has been that Standard Reference Materials for use in chemical spectrophotometry

are certified and recertified in the ACD, and all other applications of regular

transmittance are handled in the OTD.

The multiplicity of scales becomes moot to the extent that frequent comparisons among

the NMIs and between the NIST instruments shows the scales to be indistinguishable.

Indeed, comparison is one of the few ways to validate the accuracy of such instruments.

Although such comparisons have been infrequent to date, the current interest in

international comparability and equivalence should cause an increase in frequency.

Stable absorbing reference materials are routinely used by NMIs to validate the accuracy

of commercial spectrophotometers. Specifically, the concept of a "transfer

spectrophotometer" as introduced in Section 2.2.1 has been employed within NMI

laboratories to leverage the measurement capability of the reference instrument. The

transmittance scale maintained by a transfer spectrophotometer is referred to as a "disseminated transmittance" scale.

The traceability of disseminated transmittance measurements to the national regular

transmittance scale is assured by regular comparison of the transfer instrument and the

reference instrument, using stable reference materials. This process is described in the

following paragraphs.

5.2 NIST SRMs

The certifier must own a minimum of one set of SRM neutral density glass filters,

covering the nominal transmittances of the NTRM filters to be produced. The most

general coverage of approved nominal transmittances would be one set each of SRM 930

and SRM 1930. A somewhat preferable model would be to own two sets of each needed

SRM, with appropriately interleaved recertification schedule. In addition, the certifier

32
must own at least one NIST holmium oxide solution wavelength standard, SRM 2034,

which is certified for ten years and is not recertifiable.

The purchase of SRMs from NIST is no different for NTRM certifiers than for any other

NIST customer. SRMs may be purchased through the NIST Standard Reference Materials

Program.2

SRMs are normally certified for two years, but the filter NTRM traceability protocol

requires that the active-use SRMs have been recertified by NIST within one year. This

protocol may be observed by having the filters recertified at one-year intervals, but the

traceability chain will be broken for part of the time the filters are away from the certifier. Therefore, it is recommended that the certifier own two sets of SRMs, and have them

recertified alternately. The most recently certified set becomes the active reference set.

To obtain recertification of SRMs by NIST, contact ACD at 301-975-41 15. Typical

recertification turnaround time is three to four weeks for most months of the year. August

and September usually have longer turnaround times owing to vacations and the end of

the Government fiscal year.

5.2.1 Storage and Measurement Frequency

At least one set of SRMs representing transmittance levels being certified by the certifier

must be measured weekly (during periods of certification activity) on the transfer

spectrophotometer using the certification protocol. This regular measurement represents

the primary link between the transfer spectrophotometer and the ACD reference

spectrophotometer. The weekly measurement period represents a compromise between

keeping the filters closed up and protected in their container and having regular

comparison with NIST. The SRMs used for this purpose will have been recertified by

NIST within one year.

All measurements of these absorbance SRMs must be archived in both hard-copy and

electronic form. It may be advantageous to keep the data in the same database as the

production data, but identified as yet a fifth "type" of measurement ("control").

Otherwise, control data may be held in a separate database or archive file.

Recommendations for the storage and handling of NIST SRMs are given on the

certificates. The fundamental idea is that the SRMs are to be stored in their containers in a clean environment between uses and must be handled so as to avoid contaminating the

measurement surfaces, especially with fingerprints and dust.

In case of inadvertent contamination of the surface(s) of a neutral density glass filter with

substances other than dust, the set must be returned to NIST for cleaning and

recertification. This rather rigid policy regarding the cleaning of filters reflects the NIST

experience that cleaning is an inexact science - at best - and must be accompanied by

recertification for reliable results.

33
Clean air is to be used to blow dust from the surface of all filters, before they are loaded

into the instrument. The only reliable dusters we have found have been rubber bulb

blowers operated by hand. Pressurized, aerosol can dusters sold by optical companies are found to deposit a film that changes the measured transmittance of SRMs and must be

avoided.

5.2.2 Wavelength Calibration

Wavelength calibration with atomic lines and/or with SRM 2034 was discussed in

Section 4.2.3. For purposes of traceability, full spectral scans of SRM 2034 must be

acquired and saved at least monthly.

5.3 NTRM Certifier Standards

At least one full set of NTRM filters examined and measured by NIST as a part of the

original accreditation program is to be reserved by the certifier for use as local "Certifier

Standards." These filters must be measured at the same time as the weekly SRM

measurements (Section 5.2.1) and also are to be measured on every day of active NTRM

certification or recertification measurements. These filters will represent the daily control

samples used to chart instrument performance between SRM measurements.

Certifier standards are to be assigned a "certified value" for each level and wavelength,

just as for other filters. They must be cleaned and recertified at least once a year by the

certifier, with appropriate annotation of control charts and notebooks. These working

standards may be cleaned and recertified more frequently if required to maintain the

control chart.

The frequent control measurements of the certifier standards must be kept in electronic

form, from which hard copy charts may be generated, as discussed in Section 5.4. The

data may be kept in the same mode as selected for SRM filter data and discussed in 5.2.1.

This would be as either a "control" type of measurement in the production database or in a separate control database or archive file.

5.4 Control Charts

Although it is impractical to intercompare the transfer and reference instruments

frequently, it is relatively straightforward to document the consistent performance of the

transfer instrument between comparisons. This is done by means of utilizing the NTRM

certifier standards as daily control samples and the NIST SRMs as weekly control

samples.

The spectrophotometric certification of NTRM filters is a nearly ideal application of the

technique of control charting an instrument and may be implemented in a variety of ways

ranging from totally manual to totally automated. The process at NIST is semi34
automated, with control measurements automatically appended to a cumulative control

data file each time the instrument is run, and the data plotted in Excel with some manual

intervention. Figure 6 shows an example for the ACD reference spectrophotometer, using a nominal 10 % transmitting SRM 930 filter at 546.1 run.

The essence of operation is for the operator of the transfer spectrophotometer to know if the instrument is "in control" on each operational day. Otherwise, it is possible for

several days worth of data to be invalidated by an instrument drifting out of control

unnoticed. At the simplest level, the operator can verify that the instrument is in control

by comparing the present

measurements on the

control filters with ranges

defined as acceptable for

these filters. The

acceptable ranges are given by 95 % confidence

intervals about the mean

value for each filter for a

large number of prior

measurements. If a measurement lies outside

of the 95 % confidence

interval, but within a 99 %

confidence interval, the

instrument is not deemed

to be out of control unless

the behavior is repeated on

successive measurements.

If the instrument is judged to be out of control, certification and recertification

measurements must be suspended until the source of the problem is found, corrected, and

documented.

The exact methodology of control charting is left to the discretion of the certifier but must

yield charts that facilitate NVLAP assessment. At the very least, control data must be

archived in computer-readable form for control charting and/or review. An illustrative

semi-automated model would be as follows.

A hard-copy control chart for a single certification wavelength - such as shown in Figure

6 - could be generated for each nominal transmittance level once a month. (This process

could be synchronized with the monthly SRM 2034 run, for instance. See Section 5.2.2.)

The abscissa would represent a sliding three-month period of time, including the prior

two months and the coming month. The data for the prior two months would be shown,

and used to compute 95 % confidence control intervals. The chart would show the

computed mean for the two-month period and the 95 % control interval. The chart should

also show the certified value for the given filter and wavelength.

X

* *

«

*x

*x x

t *

* X

.

*

x **

* —1

8

2/1/98 2/11/98 2/21/98 3/3/98 3/13/98 3/23/98 4/2/98 4/12/98 4/22/98 5/2/98

Date

Figure 6. Control chart using SRM 930, filter 100-10 at 546.1

nm for the NIST ACD reference spectrophotometer.

35
The control charts for each transmittance level would be kept in a fixed location close to the reference instrument, and the control data for each new day could be plotted by hand

on the control charts. At the end of the month, fresh charts would be prepared using the

archived data, with room left for plotting the next month's data.

Control charts are to also be updated weekly for the SRM absorbance data, which is acquired weekly (Section 5.2.1). The control limits for each level and wavelength are to be given by the certified value and the expanded uncertainty. NIST statisticians may use such data to recommend bias corrections for transfer spectrophotometers.

5.5 Annual Comparison of Transmittance Scales

The NIST ACD will conduct annual comparison measurements with each NTRM

certifier, under the "Direct Proficiency Testing" provision of Handbook 150-21 (Sec.

285.22b2i). The ACD will send certified reference filters to the certifier, without

revealing the measured values. The certifier will measure the filters and return the filters and measurement results to NIST. The filters are measured once more at NIST, to assure

closure on the measurement, and a report is prepared for the certifier and the NVLAP

assessment.

5.6 Lot Samples and Blind Testing by NIST

The certifier is directed by the "Indirect Proficiency Testing" provision of Handbook 150-

21 (Sec. 285.22b2ii) to submit a portion of each lot of filters produced to the NIST ACD

for testing. Since optical filter production is neither entirely a batch process nor a continuous process, the exact number of sets required will be negotiated at the time of the

testing. These sets are to be accompanied by hard copy and/or electronic measurement

data. These samples will be subject to comparison measurements at the discretion of the

NIST ACD.

The NIST ACD also reserves the right to conduct blind testing on NTRM filters through

the use of third party intermediaries.

5.7 Reporting Measurement Data to NIST

To permit NIST to carry out its oversight of the NTRM certification process, certifiers

shall transmit certification, recertification, calibration, and other measurement results to NIST ACD in a prescribed electronic format. NIST will maintain the data from the filter NTRM program in a secure database. Initially, the data will be available only to NIST

personnel charged with producer performance monitoring; however, it is our intention to grant access eventually to selected portions of the database to NTRM producers so that

they can monitor their own filters. This access will be based upon the manufacturer's

code identification of the filter. ACD will use the database to "control chart" the

performance of producers, their filters, and their transfer spectrometers, to assist in

36
determining uncertainties of NTRM certification measurements, etc. NVLAP examiners

will have access to this database and will review a producer's past performance in the course of renewing its accreditation. The existence of and access to the NIST NTRM

database does not relieve producers of their own internal record keeping requirements as described elsewhere in this document.

It is NIST ACD's intention to make this data reporting requirement as painless as possible for NTRM producers to meet. The details of the data reporting scheme are still being worked out, and the data and metadata required to be reported, the structure of the

report, and the mechanism for reporting are still under development at NIST. The

following subsections are offered to suggest the scope and possible mechanisms for this

data reporting and should be considered preliminary and subject to change. NIST ACD

expects to work with the initial NTRM producers to develop the final data reporting

procedures. We expect that the reporting format and mechanism, once established, will

be the same for all producers.

5.7.1 Data to be Reported

The data reported to NIST shall include only the final measurement data for each

measurement made on a given filter set, as described in section 4.2.4. The producer is required to maintain the "run data" as prescribed in section 4.2.4, but this should not be

transmitted to NIST under the general data reporting requirement. If irregularities are noted in the final measurement data, NIST may request that run data be submitted as a

part of a trouble shooting and diagnostic process with the producer.

The following final measurement data must be reported to NIST ACD:

• Certification Measurement Data

• Precertification Measurement Data made during the filter aging process

• Recertification Measurement Data including the "As Received" and Recertified

Measurement Data

• Calibration/Control Measurement Data including the weekly control data from NIST

SRMs and the daily control data from the producer's "in house" standards

• Annual Comparison Measurement Data

5.7.2 Metadata to be Reported with Each Data Record

Measurement data by itself is not useful unless it is accompanied by ancillary information

relating to the measurement process. We refer to such ancillary data as metadata (data

about data). Table 1 1 lists such metadata along with format and field requirements.

37
Table 11

Metadata to be Reported with Each Data Record

Item Datatvne

[ividx neiu oizej

Format Rxamnle

Filter Identification character [3],character xxx,yyyyy,zzzzz,##,qq SKM,2U3Ua,123,3U,lN(j

mfg. code,filter type,set [5],character [5],integer qqqqqqqq 5U123

s/n,nom %T,glass s/n HI _ I x 1 1 Al [2],character[10]

Date & 1 ime integer [12| Y Y Y Y MMDDHHMM 199809101123

Operator character [15] r. ueKose

Sample Temperature, °C float [2.1] xx.x 22.1 Relative Humidity, % integer [2] XX AS45

Measurement Type Code character [1] K

Spectrometer Model No. character [15] IN1S1 ACL) HAS-11

Spectrometer S/N character [15] 00001

Reference Sample character [15] air Sample Position integer [2] 04

Effective Integration time, s integer [2]

->3

Spectral Slitwidth, nm float [2.3] xx.xxx 1.13 Replicates integer [2] 5 Measurement Mode character [25] single-beam; sphere

Spectrometer Qualification integer [12] Y Y Y Y MMDDHHMIV1 1 nnon^ 1 At sss 199502141555

Transmittance Proficiency integer [12] YYYYMMDDHHMM 199808111045

[Linearity Calibration]* integer [12] YYYYMMDDHHMM 199808111402

Wavelength Calibration integer (12] YYYYMMDDHHMM 199808121623

Wavelength Bias File character [12] xxxxxxxx.yyy 98wave.tbl

[Transmittance Linearity Bias character [12] xxxxxxxx.yyy 98trans.tbl

File]"" 'Double-aperture test date, if double-aperture device is supported and used.

"Transmittance bias correction curve, if measured by double-aperture device or assigned

by NIST to correct measured bias.

5.7.3 Structure of a Data Record

The metadata is prefixed to the measurement data as shown in Figure 7. Datafields are delimited with carriage return (ODH) and linefeed (OAH) characters (cr/lf). Subfields

within a datafield are delimited with comma (2CH) characters. A measurement datafield

consists of a nominal wavelength value, followed by a comma (2CH) delimiter and then a

transmittance value. The structure and format of the measurement datafields can be used

for isolated wavelength/transmittance value pairs as shown in Figure 7, as well as for a complete spectrum where wavelengths occur at the incremental wavelength interval used

to record the spectrum. Multiple data records may exist within a data file.

5.7.4 Frequency of Reporting Data to NIST

To ensure that NIST ACD can fulfill its oversight role, producers must transmit the

required data to NIST in a timely manner. However, the frequency of reporting will

depend on the number of filters being produced and recertified by the producer, among

38
other things. Accordingly, the reporting

frequency will be negotiated individually with

each NTRM producer during the accreditation

procedure and will remain constant through

the year, unless irregularities arise that

necessitate a change.

5.7.5 Mechanism for Reporting Data to NIST

To facilitate the data reporting to NIST, ACD

will set up an Internet web site to permit

electronic transmission of the data files. The

site will be password-protected and accessible

through the NIST firewall. The reception

mechanism will check the data upon receipt

and will return an appropriate

acknowledgment message to the producer.

5.8 Accreditation Review

The accreditation renewal process provides an

opportunity for fine tuning the mechanisms for

maintaining traceability. The review process

by the certifier and NVLAP assessor, with

assistance as needed from the NIST ACD and

SED, may well result in modifications to the

proficiency testing protocols, certification

protocols, and certification uncertainties.

SRM,2030a,123,30,NG501 23CR/LF

199809101 123CR/LF

P. DeRosecR/LF

22.1 CR/LF

45CR/LF

RCR/LF

NIST ACD HAS-IIcr/lf

00001CR/LF

aircR/LF

985CR/LF

1E-09CR/LF

04CR/LF

3CR/LF

1.13CR/LF

5CR/LF

single-beam, I-spherecR/LF

199802141555cr/lf

1998081 11045CR/LF

1998081 11402CR/LF

199808121623cr/lf

98wave.tblcR/LF

98trans.tblcR/LF

440,29.15 cr/lf

465,32.27 cr/lf

546.1,30.74 cr/lf

590,27.69 cr/lf

635,27.78 cr/lf

Figure 7. Example of a data record.

6. FEES AND REVIEW OF FEE STRUCTURE

The fees for NVLAP accreditation are covered elsewhere. These fees support the general

ISO-25 quality assessment process, including the cost of the biennial assessments and

annual renewal. In addition, the NIST/ACD will need to recover the costs associated with

the various comparison measurements associated with the program. For the initial year of

the program, ACD will evaluate a payment system based upon the rate schedule used for

the recertification of SRM filters. SRM 930 filters are currently recertified for $700 and

SRM 1930 filters will be recertified for the same cost as SRM 930 on the new reference

spectrophotometer. Each NTRM certifier will purchase the equivalent of four

recertifications per year from the ACD, broken down as follows:

• Recertification of SRM set; payable at the time of recertification.

• Direct proficiency test; payable near the anniversary of accreditation.

39
• Indirect proficiency test - one per year on submitted samples unless problems are encountered; payable at the time of the test. • Equivalent of one certification fee for the examination of all lot samples submitted to ACD for optical examination and testing; payable upon receipt of ACD report, near

the anniversary of accreditation.

It should be noted that the cost can vary by a factor of two, depending on the extent of

NTRM filters offered by the certifier, i.e., whether the range of the filters offered covers

that of both SRMs or just SRM 930. In the above list, the direct and indirect proficiency

tests involve the same number of measurements (two complete measurements) as a

recertification. At the present time, we have no data to compare the costs of examining lot sample filters with that of performing recertifications, nor do we have sufficient

information to assess the costs of the quality assurance monitoring of the NTRM data. It

is the intent of ACD to automate this process to minimize the expense, but the operation

of the filters NTRM program must become cost neutral to ACD after the initial year.

Accordingly, it will be necessary to re-assess the ACD charges as the program matures.

40
References

1 Franklin R. Guenther, William D. Dorko, Walter R. Miller, and George C. Rhoderick,

The NIST Traceable Reference Material Program for Gas Standards, NIST Spec. Pub.

260-126, U.S. Government Printing Office, Washington, D.C., 1996.

2 National Institute of Standards and Technology, Standard Reference Materials Program,

Room 204, Building 202, Gaithersburg, MD 20899-0001; Tel: (301) 975-6776; Fax:

(301) 948-3730; e-mail: SRMINFO@enh.nist.gov.

3 R. Mavrodineanu, "An Accurate Spectrophotometer for Measuring the Transmittance of

Solid and Liquid Materials", J. Res. NBS 76A, pp. 405-425 (1972).

4 NVLAP Procedures and General Requirements, NIST Handbook 150, U.S. Government

Printing Office, Washington, D.C. (1994).

5 G.W. Kramer, J.C. Travis, CD. Faison, and S.D. Rasberry, Chemical Calibration,

Certifiers of Spectrophotometric NTRMs, NIST Handbook 150-21, U.S. Government

Printing Office, Washington, D.C. (1999).

6 "Standard Practice for Measuring Practical Spectral Bandwidth of Ultraviolet-Visible

Spectrophotometers," ASTM E 958-93 (1993), Annual Book ofASTMStandards, 03.06,

786-790 (1997), West Conshohocken, PA.

7 "Spectrophotometry and Light-Scattering," United States Pharmacopoeia

XXIII/National Formulary XVIII, General Chapter 851, 1830 (1996), The United States

Pharmacopoeia Convention, Inc., Rockville, MD.

8 "Absorption Spectrophotometry, Ultraviolet and Visible," European Pharmacopoeia,

2.2.25, 28-29 (1997), Strasbourg, France.

9 "Standard Practice for the Periodic Calibration of Narrow Band-Pass

Spectrophotometers," ASTM E 925-83 (1994), Annual Book ofASTMStandards, 03.06,

775-780 (1997), West Conshohocken, PA.

10 "Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible,

and Near-Infrared Spectrophotometers," ASTM E 275-93 (1993), Annual Book ofASTM

Standards, 03.06, 708-718 (1997), West Conshohocken, PA.

11 "Standard Test Method for Estimating Stray Radiant Power Ratio of

Spectrophotometers by the Opaque Filter Method," ASTM E3 87-84 (1995), Annual Book

ofASTMStandards, 03.06, 740-749 (1997), West Conshohocken, PA.

12 J.C. Travis, M.V. Smith, and G. W. Kramer, "Optical Wedge Effects in Instruments

and Standards for Molecular Absorption Spectrophotometry", Appl. Spectrosc. 52, 1414

(1998).

41
13 C. Burgess and A. Knowles, Standards in Absorption Spectrometry, (Chapman and

Hall, London, 1981), pp. 128-9. The recent revision of this book, though not containing

the particular information cited, is nevertheless worth consulting: C. Burgess and T.

Frost, Standards and Best Practice in Absorption Spectrometry, (Blackwell Science Ltd.,

Oxford, 1999).

14 R. Mavrodineanu, R. W. Burke, J.R. Baldwin, M.V. Smith, J.D. Messman, J.C. Travis,

and J.C. Colbert, Standard Reference Materials: Glass Filters as a Standard Reference

Material for Spectrophotometry - Selection, Preparation, Certification, and Use of SRM

930 and SRM 1930, NIST Special Publication 260-1 16, (U.S. Government Printing

Office, Washington, 1994).

15 J.F. Verrill, in Advances in Standards & Methodology in Spectrophotometry, eds. C.

Burgess and K.D. Mielenz, Elsevier, Amsterdam, 1987, 1 1 1-124.

16 J.C. Travis, N.K. Winchester, and M.V. Smith, "Determination of the Transmittance

Uniformity of Optical Filter Standard Reference Materials," J. Res. NIST 100, 241

(1995).

17 Reference 6, p. 46.

42
APPENDIX A. WORKSHOPS ON NIST TRACEABILITY IN CHEMICAL

SPECTROPHOTOMETRY

Three workshops were held to assist in the design of the filters NTRM program. The

"NIST Highlight" report of the original workshop is reprinted below.

Workshop Sets the Stage for NIST Traceable

Reference Material (NTRMTM) Optical Filters

A "Workshop on NIST Traceability in Chemical Spectrophotometry" on September 1 1

,

1997, attracted 32 participants from pharmaceutical and chemical companies, instrument

and optical component manufacturers, government regulators, and NIST. The focus of

the program was to establish a recognized traceability link to the CSTL national reference

spectrophotometer through commercially produced reference materials for chemical

spectrophotometry. The demand for such reference materials has been accelerated by the

increase in both regulatory and voluntary quality control measures, exceeding NIST

production capacity. The consensus of the workshop was that leveraging of the NIST

measurement capability into the commercial sector was both feasible and desirable, as long as comparable quality and adequate traceability are assured and regulatory agencies

accept the standards. The primary outcome of the gathering was to endorse the concept

of NTRM standards for chemical spectrophotometry and to call for two additional

meetings for final implementation.

Contact: J. C. Travis (x-4117) or G.W. Kramer (x-4132)

Al

APPENDIX B. DESIGN OF HOLDERS FOR OPTICAL FILTERS SRMS.

Figures B1-B4 are the machine drawings for the NIST filter holders. Figure Bl is the

general assembly diagram, as an edge-on cross section showing the retaining clip holding

the filter into place. Three views of the holder body are shown in Figure B2, with the top

left view showing the holder body viewed from the side where the filter is to be mounted.

We refer to this as the "back" of the filter, or the side that faces away from the light source when mounted in a spectrophotometer. The bottom left view of Figure B2 is of one of the plain sides of the filter. NIST engraves the filter set number and nominal

transmittance (%) near the top of the filter on the side shown. The orientation of the NIST

reference spectrophotometer and all of our commercial spectrophotometers is such that

the filter is in the proper orientation if the thumb of a right-handed operator is covering

the engraving as the filter is inserted into the sample chamber of the instrument. In other

words, the beam of all of our instruments is incident from the left-hand side of an

operator facing the instrument sample chamber.

1,2 - Shutter

3 - Holder body

4,7 - Retainer spring

5 - 4-40 black nylon screw

6 - Filter

Figure Bl. Assembly diagram of filter.

A noteworthy feature shown in Figure B2 (in the top views) is a tiny protrusion at the

base of the holder, which keeps the filter from falling out of the holder. In the top right

view of Figure B2, the dovetail grooves that hold the shutters in place may be seen. The

shutters themselves are shown in Figure B3. They are made from black Delrin. Figure

B4 shows the retaining clip, which is made from 0.5-mm thick aluminum.

The aluminum parts are black anodized, and black plastic screws are used to hold the

retaining clip in place, so that all parts of the holder are black.

The default machining tolerances for the machine drawings are given in Table B 1

.

Bl
... . g

7 i

CD

"* in

^ ii

ii

1!

1

1

ii

1 " 1 !

I

in — k i i

t

. r

Part No. 101457

NOTES:

1 . MAT: AL 2024-T4 PER QQ-A225/60

3. REMOVE ALL SHARP EDGES

AND SCRATCHES

4. UNLESS OTHERWISE SPECIFIED

ALL SURFACES 1.6un

5. ALL DIM ARE BSC AT MMC

7. FINISH: BLK ANDZ TYP II PER

MIL-A-8625C

8. MUST BE INTRF FIT W/ PART

101458 (44.5 g)

I.-J

tz - z

Figure B2. Machine drawing of filter holder body.

B2
DO

I CD

LJ

LU

Part No. 101458

NOTES:

1 . MAT: DELRIN 1 57/507 PER L- P-392b TYP I CLASS 2 (BLK)

3. REMOVE ALL SHARP EDGES

AND SCRATCHES

4. UNLESS OTHERWISE

SPECIFIED ALL SURFACES 1.6un

5. ALL DIM ARE BSC AT MMC

7. FINISH: NON-REFLECTIVE

FLAT BEAT BLAST (OR EQUIV)

MAT PRIOR TO MFG

8. MUST BE INTRF FIT W/ PART

101457

CD GD

J

V

\

\

\

\

\

\

""J S

\

\

\

\

\

s

K

?in

IS

I <£

LJ

LU

J L

Figure B3. Machine drawing of filter shutter.

B3
RS.4 Vi 1 1

1

5.4 J

Part No. 101456

NOTES:

1. MAT: AL 6061 OR 5052 0.5 THK

3. REMOVE ALL SHARP EDGES AND SCRATCHES

4. UNLESS OTHERWISE SPECIFIED ALL SURFACES 1.6un

5. ALL DIM ARE BSC AT MMC

7. FINISH: BLK ANDZ TYP II PER MIL-A-8652C

8. DIM A = -1 1.5 -2 0.7

-3 STRAIGHT (NO OFFSET)

Figure B4. Machine drawing of filter retaining clip.

B4
TABLE Bl

Metric Machining Tolerances

Range/mm Linear

Tolerance/mm

Range of Short

Leg/mm

Angular

Tolerance/mrad

>0.5 to 6 ±0.1 Oto 10 ±17.4

>6 to 30 ±0.2 >10to 50 ±8.7

>30to 120 ±0.3 >50to 120 ±5.8

B5

APPENDIX C. DESIGN OF SHIPPING AND STORAGE CANISTERS AND

SPRINGS FOR OPTICAL FILTERS SRMS.

The machine drawings for the NIST SRM shipping and storage canisters are shown in

Figures CI through C4. The assembly diagram in Figure CI shows a side-on cross

section through two of the four filter storage positions. Two filter holders are shown in

the storage positions, along with a retaining spring for each one. The top view - to the

right of the side view - shows the orientation of all four-filter holders, the three filters and

the blank.

The drawing for the canister body is shown in Figure C2, and the canister lid is drawn in

Figure C3. The engraving is shown in Figure C3 for illustrative purposes, although the

filter NTRM engraving would not include the registered NIST trademarks of "SRM" and

the diamond "NIST" logo.

1 . BODY 3. CLIP 5. FILTER HOLDER

2. LID 4. FILTER HOLDER 6. FILTER HOLDER

Figure CI. Assembly diagram of shipping and storage canister.

The anti-rattle retaining clip shown in Figure C4 is used for shipping the filters, and may

be used for routine storage as well. They should be retained by the customer for return shipping the filters for recertification.

CI
Dwg No. 101497

NOTES:

1. MAT: AL 2024-T4 PER QQ-A-225/60

3. REMOVE ALL SHARP EDGES AND SCRATCHES

4. UNLESS OTHERWISE SPECIFIED ALL SURFACES J.6un

5. ALL DIM ARE BSC AT MMC

7. ALL FILLETS 0.3 MAX

8. FINISH: BLK ANDZ TYP II PER MIL-A-8625C

in

U

Figure C2. Machine drawing of canister body.

C2
The default machining tolerances for the canister components are the same as for the

filter holder components, and were given in Table B 1

.

Dwg No. 101498

L_

NOTES:

1. MAT: AL 2024-T4 PER QQ-A-225/60

3. REMOVE ALL SHARP EDGES AND SCRATCHES

4. UNLESS OTHERWISE SPECIFIED ALL SURFACES 1.6un

5. ALL FILLETS 0.3

6. FINISH: BLK ANDZ TYP II PER MIL-A-8625C

7. LETTERS AND NUMBERS SHALL BE GOTHIC CAPITALS

8. ENGRAVE AFTER ANDZ (LINE WIDTH 0.5/0.6

Figure C3. Machine drawing of canister lid.

C3
Oil? OF GRAIN

DwgNo. 101500

NOTES:

1. MAT: PH BRZ PER QQ-B-750 0.25 THK

3. REMOVE ALL SHARP EDGES AND SCRATCHES

4. UNLESS OTHERWISE SPECIFIED ALL SURFACES 1.6un

5. BEND ACROSS GRAIN

Figure C4. Machine drawing of shipping spring.

C4


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