NBS Measurement Services:

Regular

Spectral

Transmittance

NBS

Special

Publication

250-6

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support Systemfor ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantun radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities • Accelerator Research

including the electron Linac and race track microtron; develops, • Linac Operations

designs, and builds electronic and mechanical instrumentation for • Electronic Instrumentation

radiation programs and facilities; provides national leadership • Mechanical Instrunentation

in the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrunentat ion. The I Division consists of the following Groups:

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic • Office of Radiation Measurement

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and

measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

REGULAR SPECTRAL TRANSMITTANCE

Kenneth L. Eckerle

Jack J. Hsia

Klaus D. Mielenz

Victor R. Weidner

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1987
Library of Congress Catalog Card Number: 87-619840

National Bureau of Standards Special Publication 250-6

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-6, 59 pages (July 1987)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in this document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

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

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-6), NBS Measurement Services: Regular Spectral

Transmittance, by K. L. Eckerle, J. J. Hsia, K. D. Mielenz, and V. R. Weidner, is the sixth to be published in this new series of special

publications. It describes measurement services, instrumentation,

standards, and measurement techniques for regular spectral

transmittance over the ultraviolet, visible, and near-infrared spectral

ranges, from 200 to 2500 nm (see test numbers 38060S and 38010C-38050C

in the SP 250 Users Guide). Inquiries concerning the technical content

of this document or the specifications for these services should be

directed to the authors or one of the technical contacts cited in

SP 250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

.

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research

1 1

1
Abstract

This documentation describes measurement services, instrumentation,

standards, and measurement techniques for regular spectral transmittance over

the spectral range of 200 to 2500 nm. It is divided into three parts. Part

I describes the services and standards that are available and the guiding

philosophy behind the efforts to provide a primary laboratory for spectral

transmittance measurements. Part II describes the high accuracy reference

spectrophotometer. Part III describes the high precision transfer spectrophotometer. The transfer instrument relies on master standards that are periodically calibrated using the reference spectrophotometer.

Key words: calibration; high accuracy; linearity; measurement service;

stray light; transmittance; uncertainty; wavelength.

iv
TABLE OF CONTENTS

Paqe

Abstract iv

I. Measurement Services, Philosophy and Available Standards 1 A. Measurement Philosophy 1 B. Description of Services 1 C. Standard Reference Materials and Measurement Assurance Program 3

II. Reference Spectrophotometer for Regular Spectral Transmittance. . 3

A. Definition of "True" Regular Spectral Transmittance 3

B. Instrumentation 9

1. Basic Instrument 9

2. Optical Alignment 13

3. Automation 13

a. Hardware 13

b. Software 13

4. Measurement Capabilities and Required Sample Properties . 14

a. Identification ' 14

b. Stability 14

c. Cleaning 14

d. Measured Area 15

e. Uniformity 15

f. Polarization 15

g. Temperature 16

h. Solid Angle of Collection 16

i. Angle of Incidence 16

j. Filter Size 17

k. Wavelength 17

1. Bandwidth 17

m. Transmittance Range 19

C. Instrument Calibration and Internal Quality Control 19

1. Wavelength Accuracy 19

2. Linearity of the Photometric Scale 23

3. Stability of Source and Detection System 25

4. Stray Radiant Power 25

D. Measurement Procedure 25

1. Sample Preparation 25

2. Sample Alignment 26

3. Sample Uniformity 26

4. Polarization 26

5. Data Acquisition and Reduction 26

a. Transmittance 26

b. Linearity 28

E. Uncertainties 30

1. Instrumental Bias 30

2. Random Errors 31

3. Sample-Induced Uncertainties 31

4. Total Uncertainty 33

F. Report of Test 33

G. Safety 33

V
TABLE OF CONTENTS (cont'd)

Page

III. Transfer Spectrophotometer for Regular Transmittance 35

A. Descripton of the Instrument 35

1. Wavelength Range 35

2. Bandwidth Range 35

3. Photometric Range 35

4. Temperature Control 36

5. Data Acquisition and Automation 36

B. Instrument Calibration 36

1. Wavelength 36

2. Linearity 38

3. Stray Radiation 39

4. Stability 40

C. Operational Procedures 40

1. Instrument Parameters 40

2. Measurement Sequence for Transmittance or Absorbance . . 41

3. Data Reduction 41

4. Uncertainties 41

D. Reports of Test 42

E. Safety 42

References 44

Appendix A A-1

Appendix B B-1

vi
TABLE OF CONTENTS (cont'd)

Page

LIST OF TABLES

1. Calibration Services for Spectrophotometry 2

2. Standard Reference Materials for Spectrophotometry 4

3. Range, Source, Grating, Detector and Measurement Mode for

Wavelength Ranges 18

4. Wavelengths of Atomic Emission Lines Used for Wavelength

Calibrations in the IIV and Visible Spectrum 20

a. Lines used with the .25 mm apertures 20

b. Lines used with the 1 mm apertures 20

5. Wavelengths of Atomic Emission Lines and Molecular Absorption

Bands used for Wavelength Calibration in the Near IR Spectrum . . 21

6. Typical Over-all Uncertainties for NBS Reference

Spectrophotometer 34

7. Emission Lines and Absorption Bands Used to Calibrate the Transfer

Spectrophotometer Wavelength Scale 37

Emission Lines 37

Absorption Bands of Materials 37

8. Summary of Estimated Uncertainties in Percent of Spectral

Transmittance Values for NBS Transfer Spectrophotometer [A

Representative Analysis (Three Sigma) in the Visible Region for

Tr ansmi tt ances in the Range .1-1.0] 43

vii
LIST OF FIGURES

Page

Fig. 1. Diagram Pertaining to the Definition of Regular

Transmitt ance 6

Fig. 2. Schematic of Reference Spectrophotometer as Used in the

Ultraviolet and Visible 10

Fig. 3. Schematic of Reference Spectrophotometer as Used in the

Near Infrared 12

Fig. 4. Example of Additive Wavelength Correction Curve 22

Fig. 5. Additive Correction, AT, Versus Transmitt ance T(<f)). The

Shaded Area respresents ±1 Standard Deviation 24

Fig. 6. Systematic and Random (Standard Deviation) as a Function of

Transmittance on a Log-Log Plot 32

viii
I . Measurement Services, Philosophy and Available Standards

A. Measurement Philosophy

Regular spectral transmittance measurements provide useful information

on the properties of many materials. NBS strives to provide the best

possible standards and measurement services and conducts a research program

to support the national requirements for this type of measurement. The

current NBS effort in regular spectral transmittance is based on a primary

reference laboratory with characterized instrumentation and trained

personnel. The primary laboratory is responsible for realizing the national

scales of spectral transmittance with a reference spectrophotometer

[1,2,3,4]. The reference spectrophotometer is devoted to the highest

accuracy measurements. Its systematic uncertainties are either calculable in

the sense that an upper bound can be placed on them, or are measurable with

suitable precision. The measurement precision itself is good enough to give

results with a total instrumental uncertainty less than 0.0001 transmittance

units in the visible region of the spectrum. A commercial spectrophotometer

calibrated with master standards from the primary reference laboratory is

maintained as a transfer instrument. The transfer instrument is used to

calibrate Standard Reference Materials and to provide routine and special

calibration and tests.

B. Description of Services

This document covers transmittance measurements listed in NBS Special

Publication 250-"NBS Calibration Services Users Guide 1986-88." The items

are listed in Table 1. Standard Reference Materials and a MAP service for

transmittance are described in section I.C.

Regular spectral transmittance measurements over the spectral range 200

to 2500 nm are performed either with the reference spectrophotometer or the . transfer spectrophotometer. Measurements with the reference instrument are generally made when high accuracy is needed. The transfer instrument has

been characterized in terms of the reference instrument and by other tests.

The transfer instrument allows measurements to be performed quickly at a lower cost, but with an uncertainty larger than that of the reference

instrument.

1
Table 1

Calibration Services for Spectrophotometry

Old New Instrument

Number Number

7.8A 38060S R,T Spectrophotometric measurement services

(Special Tests only) Transmittance and

Reflectance.

7.8B 38010C T Spectral transmittance standard, cobalt

blue glass, 40 wavelengths from 380 to 770 nm.

7.8C 38020C T Spectral transmittance standard, copper green

glass, 40 wavelengths from 380 to 770 nm.

7.8D 38030C T Spectral transmittance standard, carbon

ye 1 low glass, 40 wavelengths from 380 to

770 nm.

7.8E 38040C T Spectral transmittance standard, selenium

orange glass, 40 wavelengths from 380 to

770 nm.

7.81 38050C T Wavelength standard, holmium oxide glass, 240

to 650 nm at less than 2 nm bandpass. — 38080M R Transmittance Measurement Assurance Program

(MAP).

*R: Reference Transmittance Spectrophotometer (RTS)

T: Transfer Spectrophotometer

Description of Service

2
With the transfer instrument, survey spectral scans are available on a limited basis for applications where such surveys of samples are needed but

high accuracy is not required. For a reduced fee, uncorrected digital or analog data are provided without data reduction and a report of test.

C. Standard Reference Materials and Measurement Assurance Program

Transmitt ance, stray light, and wavelength standards are available as Standard Reference Materials (SRM) for spectrophotometer calibrations listed

in Table 2. They have been calibrated by the Centers for Radiation Research

and Analytical Chemistry of NBS and can be obtained from

Office of Standard Reference Materials

National Bureau of Standards

Gaithersburg, Maryland 20899.

A Transmittance Measurement Asssurance Program (MAP) service [5,6] is also

available (Measurement Service Number 38080M) for evaluating the entire

transmittance measurement process of a laboratory. This service is provided

via a set of filters called the MAP package. The MAP package consists of seven neutral density filters of nominal transmittances of 0.92, 0.70, 0.50,

0.25, 0.1, 0.01 and 0.001 measured at 548.5 nm. These are used to evaluate

the photometric linearity of spectrophotometers. A wavelength standard is

also included in the package to aid in evaluating the wavelength scale of

spectrophotometers. [7,8,9] This wavelength standard is either a didymium

glass filter or a holmium oxide solution for large and small bandpasses,

respectively. These filters are rented to the participants. The process

involves measurements by NBS followed by measurements by the participating

laboratory. Finally NBS measures the package again and the data are analyzed

by NBS to determine if there is any bias. It is strongly recommended that

the participants obtain a set of check standards for their own use to provide

continuing measurement assurance.

II . Reference Spectrophotometer for Regular Spectral Transmittance

A. Definition of "True" Regular Spectral Transmittance.

The definition of regular spectral transmittance pertinent to the NBS

calibration services described in this document may be summarized as follows

[1], [10], [11].

3
Table 2

Standard Reference Materials for Spectrophotometry

Speci al

Publication SRM Number Description

260-51 930 Glass Filters for Spectrophotometry

260-51 2030 Glass Filter for Transmi ttance Measurements

260-68 2031 Metal-on-Quartz Filters for Spectrophotometry

931b Liquid Absorbance Standards for Ultraviolet and

Visible Spectrophotometry

260-54 935 Crystalline Potassiun Dichromate for Use as an

Ultraviolet Absorbance Standard

260-32 932 Quartz Cuvette for Spectrophotometry

2032 Potassium Iodide for Use as a Stray Light Standard

2033 Potassium Iodide for Use as a Stray Light Standard

with Radiation Attenuator

260-66 2009 Didymium Glass Filters for Checking the Wavelength

2010 Scale of Spectrophotometers

260-102 2034 Holmium Oxide Solution for Use as a Wavelength

Standard in Spectrophotometry

4
Transmission is the passage (without change in wavelength) of optical

radiation through tran-sparent or translucent media. Transmission may be

regul ar (without change in direction) or diffuse (spreading out in different

directions). Transmission that is partly regular and partly diffuse is

called mixed transmission.

The (total) spectral transmittance of a given medium, for a given wavelength and a given beam geometry, is defined as the ratio of the transmitted

spectral radiant flux $^ j to the incident spectral radiant flux,

Tj(x,e') = *,yt, . (1)

Where mixed transmission occurs, the total spectral transmittance can be

divided into two parts corresponding to the two modes of transmission defined

above:

T^(x,e') = T^(x,e') + Tj(x,e') , (2)

where T^(xe') is the regular spectral transmittance of the medium and

Tj(x,6') is the diffuse spectral transmittance . Only the former will be

discussed further, denoting it from now on by T(x,0') without the subscript

'r' in equation (2). See Figure 1.

Transmittance, as defined above, is not a well-defined property of a transmitting sample. It depends, not only on the material parameters of the

sample, but also on the degree of collimation or convergence of the incident

light, its polarization, and degree of coherence. Thus it cannot serve as a meaningful quantity for accurate calibrations unless constraints are imposed

on its measurement so that the ratio in equation (1) is reduced to the

limiting form of a "true" transmittance that depends only on the properties

of the sample.

For the most commonly encountered type of a transmitting sample, a plane-parallel absorbi ng f i Iter, this true regular spectral transmittance can

be defined as follows. Referring to figure 1, consider a perfectly monochromatic, plane-polarized, plane wave of wavelength x to be incident on a perfectly plane parallel slab of infinite extent and thickness t, with all media

both isotropic and homogeneous, and the initial and final media dielectric

5
6
and the same. Let n = n(l + ik) be the complex index of refraction of the

slab medium, n' the index of refraction of the initial and final media, and

9' the angle of incidence. Under these conditions, the regular spectral

reflectance of the slab is given by:

(1 - r )e^ T(x,G') = -2 213

1 - r e

(3a;

where r is the complex Fresnel coefficient of amplitude reflectance at the

first boundary, and B is defined as

B = (2TT/X)tn cos e . (3b)

For S-polarization,

n ' co;

n ' cos 0' + n cos 9 ' s

'

^ n' cos 9' - n cos 0 _ / ,n ,^ x

and for P-polarization,

n cos 9' - n' cos 9 /v ^ =

n cos 9' + n' cos g

= ^p(® ^ ' (^^^

where 0 is complex and is defined by Snell's law

sin 9 = (n' sin 9' ) /n, (3e)

Equations (3a-e) show the above mentioned dependence of transmittance on beam

geometry (0') and polarization (r). Because of this dependence, the transmittance T(x,0') of a given sample is an instrument-dependent quantity that can differ for different angles of incidence and/or states of polarization of

the incident light. The predominant effect of angles of incidence other than

0° is a decrease of internal transmittance due to increased path lengths in

filters tilted with respect to the optical axis or illuminated by focussed

incident light of low f-number. The effect of polarization is due to the

difference of the Fresnel reflectances r for S and P polarized light and

causes further angular variations of transmittance. It also introduces a dependence of transmittance on the state of polarization of the incident

light, so that a non-polarizing filter may exhibit an apparent, instrumentdependent dichroism.

The dependence of transmittance on the coherence of the incident light

is apparent from the exponential functions in equation (3a), which contain a

7
rapidly oscillating interference term depending on (t/x) in addition to the

attenuation term that determines the absorption of the filter as a function

of the attenuation coefficient k and filter thickness t. As a consequence,

the transmittance T(x,0') can oscillate rapidly when the wavelength is

varied. Since the above equations were derived for the idealized case of

perfectly coherent light, this periodic modulation of transmittance is seldom

noticed in practical spectrophotometry. However, it is not always negligible

and may cause variations of transmittance with wavelength, bandwidth, beam

geometry, and other instrumental parameters that affect the degree of

coherence of the light.

All of these instrumental effects are eliminated when transmittance is

measured at normal incidence (0'=O) and under conditions that interference

effects are eliminated, so that equations (3a-e) are reduced to

(l-r)2T.

T = T(X,0) = '

where

r = |r\{0)|2 = kp(0)|2 =

n' - n

n' + n (4b)

is the energy reflectance of the filter surfaces for normal incidence and

either state of polarization, and

= exp(-4TTnkt/x) (4c)

is the normal internal transmittance of the filter. This averaged normalincidence transmittance t = T(x,0) is a function of the filter parameters n,

k, and t only, and may therefore be identified as the above mentioned "true"

regular spectral transmittance of the filter. As described in the following,

the NBS reference spectrophotometer for regular spectral transmittance has

been designed to measure this "true" transmittance t with high accuracy.

Samples are placed normal to the axis of a collimated beam to avoid obliquity

and polarization effects (although measurements for other than normal incidence can be made, if desired). Interference effects, if detectable, are measured and then averaged to determine the "true" transmittance. Diffraction effects are eliminated by using the same limiting aperture for the

sample and clear-space measurements so that these effects are the same for

both measurements and cancel, yielding the "true" transmittance x in spite of

the finite aperture size. The "true" transmittance as defined is measured

relative to air with density and humidity effects being negligible.

8
B. Instrumentation

The NBS reference transmittance spectrophotometer (RTS) has been

described in detail in previous publications [1,2,3,4]. A brief description

of the instrument, practical suggestions for its use, and some of its limitations will be given in this section.

1. Basic Instrument.

The major instrumental features of the RTS are a stable source, mirror

optics, circular entrance and exit apertures of the monochromator (instead of

slits), detector with averaging sphere, and linear electronics. By using

off-axis parabolic mirrors, accurate and achromatic collimation and focusing

of the light and absence of inter-reflections between sample and optics are achieved. The averaging sphere [12] is efficient and eliminates errors due

to small displacement of the light beam by the sample. A current-tofrequency converter integrates the signal over time and provides an accurate

measure of the light flux incident on the detector.

The basic instrument as it is configured for the ultraviolet and visible

is shown in Fig. 2. A 5mW He-Ne laser is used to align the optics and to

align samples for measurement. Atomic line sources are used to calibrate the

wavelength scale. A 0.4 A deuterium source with a 2 mm by 4 mm aperture is

used in the ultraviolet, and a 6V-18A (108 watt) tungsten strip lamp is used

in the visible. Both sources are used in series with a OA^ shunt resistor

for constant current regulation. The collimating mirror of the fore optics

is a 15° off-axis parabola of 177 mm focal length. The focusing mirror is a 8.5° off-axis parabola of 195 mm focal length. The state of polarization of

the beam is not altered by its passage through the monochromator. A GlanThompson prism is used to attenuate the beam as described below. A second

Gl an-Thompson prism produces a defined state of polarization.

The predisperser utilizes a quartz prism in an Ebert mount with a 1 mm

entrance slit. It has 15.2 cm focal length collimating and focusing mirrors

and a horizontal aperture ratio of f/6.

The main monochromator is a commercial Im Czerny-Turner system with an effective aperture of f/8.7. Four gratings are provided for this monochromator, each 102 mm by 102 mm. They have 1200 grooves/mm blazed at 500 nm, 600

groves/mm blazed at 200 nm, 150 grooves/mm blazed at 800 nm, and 150

9
10
grooves/mm blazed at 1250 nm respectively. The monochrornator collimatinq

and focusing mirrors are off-axis parabolas, paraboloidal to within 0.1

fringe in green light and free of zones. These mirrors can be moved along a rail and adjusted by micrometer screws.

The entrance and exit apertures of the monochrornator are circular holes

with diameters selectable as .250 mm, .500 mm, .750mm, or 1 mm. These four

apertures can be rotated alternately into position. The exit aperture thus

approximates a point source. Collimation of the monochromator output is

achieved by an 8.5° off-axis parabolic mirror with 195 mm focal length. The

degree of collimation for the 1 mm monochromator exit aperture is about 1/400

radian.

A limiting aperture defines the size and shape of the light spot at the

sample. The sample holder is a wheel accommodating up to 3 samples, or a sliding holder accommodating one filter. The latter accomodates odd sizes

and shapes of samples and can also be temperature controlled. A

pneumatically driven black cavity is used to block the beam when measuring

the dark signal. A double aperture linearity tester (described in Section

II. C. 2) can also be inserted into the beam. The averaging sphere is mounted

in front of the photomult ipl ier [12].

The photomult ipl ier is an 11-stage, end-on tube with S-20 response.

Its dynode chain is a linear potted resistor chain with a Zener diode between

cathode and the first dynode. Its high voltage power supply is regulated to

0.001%.

The anode current is measured by means of a Taylor-type [13] current-tofrequency converter. This converter, together with a high accuracy counter

used with 10-sec integration time, was found to be linear to better than

0.01% by measuring the currents produced in precision resistors by a calibrated voltage source of 0.01% accuracy. By the same method, the

current-to-frequency converter and counter were found to be precise to at least 1 part in 45,000 in the current ranges normally used.

Fig. 3 shows the instrument as configured for the near infrared. A 500

W tungsten lamp in a water cooled jacket is used as the source. The quartz

lenses are inserted in the fore optics to focus the beam at a chopper. Only

one polarizer is used to maximize throughput. The signal is amplified by a n
i 5

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12
lock-in amplifier where voltage output is converted to a current. Otherwise,

the instrument is operated in the same manner as in the ultraviolet and

visible.

2. Optical Alignment

The optical alignment of the RTS is based on pre-determi ned values of

the relative spacing of optical elements, their focal lengths, and required

angles of incidence. Using the He-Ne alignment laser, the optic axis is

established by placing masks with small central holes over the collimating

and focussing mirrors of the monochromator, and then aligning the mirrors

external to the monochromator so that the distance of the laser beam from the

table surface is constant everywhere.

3. Automation

a. Hardware.

The wavelength drive of the monochromator and the two polarizers are equipped with stepping motors. The sliding sample holder and all shutters

are operated pneumatically. The wheel is operated by a synchronous motor

with latching relays. A dedicated microcomputer with interface boards

incorporating relays, stepping motor drivers, and input-output ports is

used. An analog feedback circuit controls the light level by rotating the

polarizer nearest to the source. The light level can be adjusted to 100%,

80%, 60%, 40%, and 20% of full scale when measuring transmittance or linearity. Following manual sample insertion and alignment, all measurement

steps are performed automatically.

b. Software.

The primary computer program allows choice of sample holder, number

of samples, planes of polarization, and wavelength scans at equally spaced

intervals or arbitrary specific wavelengths. An abbreviated version of the

program allows faster data acquisition with larger statistical uncertainty.

A statistical analysis program computes the transmittance and its standard

deviation . Additional software is available for wavelength calibrations by

determining the centroid of the monochromator slit function, for linearity

measurements, stability tests, and stray light measurements.

13
4. Measurement Capabilities and Required Sample Properties

The following information is provided to acquaint users with the types of

measurements that can be performed on the RTS. Also listed are the required

properties of user-submitted samples that can be accepted for these measurements. Under each heading, standard procedures are described first and

acceptable alternatives (if any) are stated in parentheses. Where

appropriate, suggestions are made for tl^e preparation of samples prior to

submission to NBS.

a. Identification.

Each filter submitted must have permanent identification markings

which distinguish it from other similar filters. (We can inscribe identifying markings on a filter with a diamond stylus.) Generally, we will not

measure an unmarked sample.

b. Stability.

Special filters are not usually tested for stability. The stability

of each filter should be investigated over a period of time comparable to

that for which the measured values are intended to be used. If the filter is

to be a permanent standard, it should be submitted for remeasurement on the

RTS at suitable, progressively longer intervals of time until its stability

has been established.

c. Cleaning.

The filter should be cleaned in advance and shipped in a suitable

protective container. (At additional cost, a filter will be cleaned

according to the owners' specific instructions. However, we will then postpone the measurement until the filter has stabilized.) For best results, a regular cleaning routine should be established and each stability test should

involve prior cleaning following this procedure. For glass filters, the

cleaning should involve degreasing if necessary, washing with mild soap or

mild, non-f luorescing detergent, and/or isopropyl alcohol, and rinsing with

distilled water. Harsh acidic or basic cleaning agents should be avoided,

since they disturb oxide layers and other persistent surface films which can take a long time to restabilize. For exposed metal film filters, the instructions of the manufacturer should be followed. Great care should be

14
taken to avoid contact with the metal film, since scratches and other marks

can greatly degrade such a filter.

d. Measured Area.

The usual procedure for the visible spectral region is to measure an average transmittance over a central area which is smaller than the filter,

falls within a circle of 16 mm diameter, and has an area larger than

approximately 25 mm^ . The latter requirement ensures signal-to-noise ratios

adequate for high-accuracy measurements. In the ultraviolet, spot diameters

of 10 to 16 mm are normally used. In the near infrared, a spot diameter of

16 mm is normally used. Usually, we try to measure the same area as when the

filter is in use. At the ends of the wavelength ranges of the RTS larger

spot sizes may be preferable in order to reduce the influence of noise.

e. Uniformity.

Filters are not routinely checked for uniformity. (We can perform

uniformity tests at a charge for additional mounting and alignment along with

one additional measurement for each wavelength and each filter displacement

required. However, such tests are not as appropriate as one made by the user

on his own instrument under the conditions of use.) Users can check for

non-uniform transmittance of filters by observing the effects of displacing

them perpendicular to the incident beam. Because even within a given

production run some filters can be much more non-uniform than others, such

checks can save time and cost, and should be performed on each filter before

submission to NBS.

f. Polarization.

Transmittance measurements on the RTS are usually made with radiation

polarized in only one direction of the electric vector. The direction of the

electric vector with respect to the orientation of the filter will be stated

in the report. Unless a specific orientation is requested, we will choose

the orientation at our convenience. (Filters can be measured at two

orientations of the electric vector without additional mounting and

alignment, but the number of measurements will be doubled.) It is usually

desirable that a filter does not polarize the radiation passing through it.

To test whether a filter is polarization-sensitive, users should place a

15
polarizer in the spectrophotometer beam along with the filter, and measure

the transmittance of the combination with the filter rotated in 45°

increments about its normal while the polarizer remains fixed, or vice-versa.

For a uniform non-polarizing filter, the measured transmittance will be the

same for all orientations.

g. Temperature.

Measurements on the RTS are normally made at the ambient temperature

of our laboratory. The temperature has small seasonal variations, but is

constant to within tO.S'C near room temperature (usually 23''C or 25"'C) during

a given set of measurements. It will be stated in the report. (A special

temperature controlled filter holder can be used to measure certain sizes of

filters at specified temperatures over the range 15° C to 35°C for

determinations of temperature coefficients of transmittance. Such measurements are time-consuming and costly, and are only made under special

arrangements.) Users should measure filters at several expected temperatures

to determine if the temperature coefficients of the filters are acceptable.

Since filters are usually poor heat conductors, the best way to vary the

temperature of a filter is to vary the ambient temperature uniformly. This

can be done by mounting the filter in a constant temperature metal box or cylinder with minimal openings for transmittance measurements. Measurements

are made after the temperature of the filter has stabilized.

h. Solid Angle of Collection.

The solid angle of collection of the RTS is circular and subtends

approximately .0025 sr at the filter location. This solid angle can be

adjusted over the range .001 sr to .01 sr at added mounting and alignment

costs. For filters that scatter appreciably, the solid angle of collection

must be specified for meaningful interpretations of results. The scattering

of the filter can be estimated by measuring its transmittance at different

positions along the path of the incident beam. If scattering is noticeable,

a measurement of diffuse transmittance should be arranged.

i. Angle of Incidence.

Measurements on the RTS are made with collimated radiation incident

normal to the surface of the filter. (For added cost, measurements can be

made at other angles of incidence.)

16
j . Fi Iter Si ze.

Filters having the following common sizes can be measured on the RTS

either singly or in groups of two or three:

5.1 cm X 5.1 cm square

3 cm diameter circular

1.3 cm X 1.3 cm x 5.7 cm cuvette

1.1 cm X 3.05 cm rectangle.

(Other sizes of filters up to 10 cm x 10 cm can be measured singly by special

arrangement.)

k. Wavelength.

The RTS is equipped to make measurements at wavelengths from 200 to

2500 nm in three ranges as follows: 200 to 400 nm, 360 to 800 nm, and 800 to

2500 nm. Table 3 lists the equipment used for these ranges.

1. Bandwidth.

The measurement can be made at the following nominal half-height

bandwidths in nanometers:

Wavelength Range Bandwidths

(nm) (nm)

200 - 500 1.5, 1.13, 0.75, 0.38

400 - 800 1.5, 1.13, 0.75, 0.38, 0.19

800 - 2500 6.0

For the most accurate measurements, bandpasses of 1.5 nm in the 200 - 500 nm range, 0.75 and 1.50 nm in the 400 - 800 range, and 6 nm in the range 800 - 2500 result in a minimal number of measurements necessary to obtain a desired

precision. Because the RTS has circular entrance and exit apertures instead

of slits, the bandpass shape is not quite triangular, but it can be regarded

as such for practical purposes. (Data for large bandpasses can be

synthesized from narrow-band measurements at an increased cost for additional

measurements and computations. With the NBS Reference Spectrophotometer for

Diffuse Transmittance and Reflectance, regular transmittance measurements

with 5, 10, 15, and 20 nm nominal bandpass can be made at slightly higher

17
Table 3

Range, Source, Grating, Detector and Measurement Mode for Wavelength Ranges

Wavelength Source Grating Detector Niode

(nm)

200-400 Deuterium Arc 600 l/mm,200nm Photomu It ipl ier D.C.

(28w) blaze

360-800 Tungsten Strip Lamp 600 l/mm,200nm Photomu It ipl i er D.C.

(108w) blaze

or 1200 l/mm,500nm

bl aze

800-2500 Tungsten Lamp 150 1/mm, 800mm Lead Sulfide A.C.

(500w) blaze

or 150 1/mm 1250nm

blaze

18
levels of uncertainty than that for measurements made with the RTS.) Unless

there is a definite need for a qiven bandwidth, it is usually best to choose

the largest bandwidth which can provide the information about the filter.

For filters with strong spectral features, the bandwidth should be chosen as close as possible to that being used by the customer.

m. Transmittance Range.

The range of transmittances routinely calibrated on the RTS is 0.1 to

1.0 transmittance. Measurements below 0.1 transmittance can be made with the

step-down technique described in Appendix A, page A-3 . One step-down filter

and an additional set of measurements is required for each order of magnitude

of transmittance below 0.1. It is suggested that customers have their own step-down filters calibrated, since the cost will be the same if NBS filters

are used. The step-down procedure becomes complicated for spectrally

non-neutral filters and more measurements are required in this case.

C. Instrument Calibration and Internal Quality Control

Since even the best available transmittance standards may change over

time [14,15], the RTS serves as a reference for the accuracy and stability of

the NBS scale of regular spectral transmittance. For this purpose, the RTS

is monitored with respect to four parameters: wavelength accuracy, linearity

of the photometric scale, stability of source and detection system, and stray

radiant energy.

1. Wavelength Accuracy

The wavelength scale of the RTS is checked periodically to ensure

wavelength accuracy. Wavelength calibration is accomplished by means of

atomic line sources in the ultraviolet, visible, and most of the near infrared. At the longest wavelengths in the near infrared, absorption bands

are used. The atomic emission lines used to calibrate the wavelength scale

in the ultraviolet and visible spectrum are listed in Table 4. The emission

lines and absorption bands for the near infrared spectrum used are listed in

Table 5. An example of a wavelength correction curve is shown in Fig. 4.

The coefficients for wavelength corrections of transmittance measurements are stored in computer memory. The systematic uncertainty of the corrected wavelengths is 0.04 nm in the ultraviolet and visible [3] and 0.26 nm in the near infrared. [4]

19
TABLE 4

Wavelengths of Atomic Emission Lines Used for Wavelength

Calibrations in the UV and Visible Spectrum

a. Lines used with the .25 mm apertures

L. 1 CMIC 1 1 I' rt uV C 1 CI 1 U 1

1

F 1 pmp n 1" w av c 1 ci 1 ^ L 1

1

1 nm 1 vnm

;

*T J • yj 0\J Hpnc JOO • O U J 459.318 501.568

587.562

fifi7 OO Rl R / .010

;lMJ *T C. U , J.OU

Nplie

7?d R17

Hn

296.728 Cd 467.816

334.148 479.992

404.657 508.582

407.784 643.847

435.834

491.604

Rdfi n7s 7n

R7Q nfifi

b. Lines used with the 1 mm apertures

Element Wavelength

Zn 213.856

He 388.865

Hg 546.075

Rb 780.023

20
Table 5

Wavelengths of Atomic Emission Lines and Molecular Absorption Bands

Used for Wavelength Calibrations in the Near IR Spectrum

Element or Chemical (nm) Type of Spectra

Rb 780.02 Emission Line

Rb 794.76 Emission Line

Hg 1013.98 Emission Line

Hg 1128.70 Emission Line

Hg 1529.52 Emission Line

Hg 1692.1 Emission Line

Hg 1970.1 Emission Line

1,2,4-trichlorobenzene 2152.6 Absorpton Band

1 ,2,4-trichlorobenzene 2437.4 Absorpton Band

21
(uiu) N0ii3ad»03 SAiiiaav

22
These uncertainties are upper bounds and may be regarded as having a 99.7%

confidence level (3a equivalent). All upper bounds in this publication will

be equivalent to 3a uncertainties.

2. Linearity of the Photometric Scale,

All measurements with the RTS are corrected for nonlinearity of the

detector and electronics. The non-linearity is measured using a double

aperture method devised by Mielenz and Eckerle [16]. A double-aperture

linearity tester with two 5-mm beveled circular apertures separated by 10 mm

is placed in the collimated beam in the sample compartment. These apertures

can be opened and closed by pneumatically operated shutters controlled by two

solenoids located remote from the detector.

Then the following readings are taken: 1(A), the intensity with one aperture open; 1(B), the reading with the other aperture open; and I(A + R),

the reading with both apertures open. 1(A) is approximately equal to 1(B).

Then if the quantity

= 1(A) t 1(B )- ' •

is greater than 0.0001, a correction term is derived. The measurements of

1(A), 1(B), and I(A+B) are done in a time-symmetrical sequence so as to

compensate for linear drifts of the detection system(See Section D.5.b.).

The linearity test is automated by means of the data acquisition and control

system.

The non-linearity corrections for transmittance measurements in the UV

and visible, using the photomult ipl ier tube on the 3 x

10"'^ A scale, is a few

parts in 10*+ for a transmittance approximately 0.5 (see Figure 5). On the 3

X 10"8 A scale the correction is zero with an uncertainty of a few parts in

10*+. The non-linearity of the photomult i pi ier tube has been monitored since

1972 and has shown no significant changes over the years.

Experience with the PbS detector used in the near ir is still limited,

but data taken indicate that its non-linearity does not exceed the standard

deviation of approximately 1 x 10"'+ with which it can be measured. No

correction is applied in this case.

23
.2 .4 .6 .8 1.0

Fig. 5: Additive correction, aT, versus transmittance T(<^)

The shaded area represents ±1 standard deviation.

24
3. Stability of Source and Detection System

The stability required of the RTS is such that a filter with transmittance 0.5 can be measured with a standard deviation of 0.00005. Two sources of instability have been found to date. One is the photomult i pi i er power supply, and more often, the tungsten strip light source or its power

supply. The source of the instability is determined by putting a stable

betalight (beta particle activated phosphor) in front of the detection

system. If the detection system is found to be stable, the instability will

be due to the source or its power supply.

4. Stray Radiant Energy

Stray light may arise from two causes. One is room light penetrating the

instrument enclosure. It can be detected by a difference in signal with the

photomultipl ier shutter open and closed.

The other is light of other than the desired wavelength being passed by

the optical system. This stray radiant power is more difficult to detect.

Order of magnitude estimates for this stray radiant power can be obtained by

the selection of suitable sharp absorption filters, both liquid and solid,

following an ASTM Method [17]. Stray light is of the order 10"^ to

10-^[2,3,4] of full scale reading.

D. Measurement Procedure.

Since the instrument is automated and has a dedicated microcomputer, the

RTS can perform most measurements with minimal requirements for manual

operations and without errors that they can cause. A change in sample or a change in photomult ipl ier voltage are examples which require manual action.

The measurement procedures are generally straightforward. Measurements

involving unusual filter sizes, angles of incidence, or scattering

temperature control, require special attention.

1. Sample Preparation.

First, it must be determined whether a filter needs cleaning as described in Section II.B.4.C above. If it must be determined whether a filter has changed since a previous measurement, the filter should not be

cleaned but only lightly brushed with a clean camel hair brush to remove dust

particles.

25
2. Sample Alignment.

The alignment laser is used to aid in positioning the samples by means of

three alignment screws on the sample wheel or slider. With the wheel,

samples are usually aligned normal to the beam, so that the laser light is

reflected back into the exit aperture of the monochromator. Angles of

incidence up to approximately 4° from normal are also possible with the

wheel. For large angles the slider, which is mounted on a rotary table, is

more convenient. The maximum angle of incidence which may be used depends on the size and shape of the sample. A vernier on the rotary table is used to

determine the angle of rotation.

3. Sample Uniformity.

Even the best samples exhibit some non-uniformity. The wheel and slider

have vertical or horizontal translation stages which may be used manually for

uniformity checks at one or two wavelengths. For uniformity checks at a large number of positions or many wavelengths, a special automated x-y

translation device can be used.

4. Polarization

High-quality filters should not exhibit detectable polarization effects

at normal incidence. For oblique angles of incidence, the transmittance will

depend on the plane of polarization (s or p polarization) because of

Fresnel's laws. It is usually advisable to check for polarization effects at one or two wavelengths.

5. Data Acquisition and Reduction

a. Transmittance [3]

Transmittance data are taken in a manner such that linear drifts of the

system are eliminated. The method of data acquisition and reduction for

three filters is outlined below as an example. If 0 is the dark reading

(shutter closed); I^j is the jth reading of the clear-space signal; Ii^, l2j,

and l3j are the jth readings of filters 1, 2, and 3; and each of these

readings I-- is taken at time t-., the measurement sequence is: 0 Iqi 0 In

0 I21 0 I31 0 I02 0 I32 0 I22 0 I12 0 Io3 0 Ii3 0 I23 0 I33O Io4 0 I34 0 I21,

0 I^^ 0 Io5 0.

26
The dark readings are interpolated in time and subtracted from each clearspace or filter reading before any further analysis,

^01 to5

Let t^ = 2 (6)

be the time half-way between times tgi and to5 at which readings Iqi and Iqs are taken. To reduce the number of digits for computation, a new time scale

t '. . is used:

t'. .=t---t. (7)

For any fitted straight line through readings I-., for a specific value of i if m' is the normalized slope of the fitted line (m'T- is the slope) and I- is the value at t^ on this line, the predicted value for the reading will

be

r. . = (m'f. . + 1) r. (8)

This data reduction method utilizes all seventeen readings I.j to obtain a common and more accurate m' and in turn with the least square fitting and

iteration techniques to minimize the sample variance s^ of all the I^^.. For

a given m' and set of I!| values the sample variance s^ is

s2 = ,.(N.-2)-iz.(I'..- (9a)

= Ei(N.-2)-iEj.[(m't;.j+l)i;-I..]2 (9b)

The logical initial m' and I- to be selected to calculate s^ are

m' = 0,

1 r. = — r . I .

where is the maximum number of readings for a given i. The new m' is

. z.(N.-2)-iz.(i..-i;.)t'..i!

calculated from m =

^ .(^ .-2)-iz .(t

' ( I \ — * ^^^^

The new I- is

^i(m't'..+ 1)1-.

'\ ' i = 0'1'2,3 , (11)

J^ ij '

27
and a new is calculated using Eq, (9b). This process is repeated until

s2 is sufficiently small (lO"^^- The I^-'s giving this s^ are used to

calculate the transmittance for the nth filter.

T = — , n = 1,2,3

The sample standard deviation s^ of is

^n =

"^n (r)2 ^ n' ^ 0

1/2

where

E . ( r. . - I . . ) 2

N.(N.-2)

i = 0,1,2,3

(12)

(13)

is the variance of the I' for a given i

i j

b. Linearity [16]

Data for the linearity measurements can be taken at equal time

intervals. The signal currents thus obtained generally drifted in time, at a rate of typically - 3 ppm/min and exhibited random fluctuations superimposed

upon this drift. Since all quantities measured with a spectrophotometer are ratios, such as r = I/J, the drift is of no concern provided it is linear in

time and provided that a series of data,

I,J,I,...J,I, (14)

is taken at fixed time intervals. Then, the influence of the drift is

eliminated when r is expressed as the ratio, r = I/J, of the averaged values

of I and J. Under these conditions the quantity affecting the precision of

measurement is the standard deviation,

1/2

dl = J^[I^-I(vt)]2/(n - 1)] , (15a)

where

Ii, l2....In (15b)

represent n successive readings of the same signal current I, taken at the

times

t, 2t,..,nt, (15c)

28
and where I(vt) is their expectation value, obtained from a least-squares fit

of the measured data as a linear function of time.

The valves operating the two aperture shutters are actuated by the data

acquisition and control system of the spectrophotometer in a manner such that

twenty-seven readings,

0,A + B,0,A,0,B,0,A + B ,0,B, , . .A,0,A + B,0 (16a)

are effected in 20-sec intervals. Here, 0 is a dark-current reading, and

A,B, and A + B are the readings obtained with either or both apertures open.

These data are then processed as follows.

The dark currents on both sides of A,B, and A + B are averaged and

subtracted from these readings, yielding a sequence of thirteen adjusted

signal currents.

I(A + B), 1(A), 1(B), I(A + B), 1(B),. ..1(A), I(A + B). (16b)

The averages 1(A), 1(B), and I(A + B) are then used to compute

a'= I(A + B)/[I(A) + 1(B)] - 1, (16c)

as defined in Eq. 5 above.

The quantities.

Y = I(A)/I(A), I(B)/I(B), and I(A + B)/I(A + B) (16d)

are fitted to a straight line, Y(t), respective to the times t at which the

data were taken, so that 1(A) Y(t), 1(B) Y(t), and I(A + B)Y(t) represent the

expectation values of the data sequence in Eq. (16b). Hence, the standard

deviations dI(A), dI(B), and dI(A + B) are computed using Eq. (15a). Finally

the propagat ion-of -error formula below is used to calculate the estimated

standard deviation da of the measured value of o'.

The standard deviation do' of a single measurement of the nonlinearity

term o' is given by

[da-(A,B)]2 =[^-^dI(A)]' ^[TiW^^^^)]' ^[^A-^I(^^ B)]'

= [1(A) I I(B)? ^^^ ^')^tdI(A)2 + dI(B)2] + dI(A+B)2} (17)

29
where dl is the photometric error of the spectrophotometer defined by Eq.

{15a).

E. Uncertainties

1. Instrumental Bias

Because of the special design of the RTS, the combined systematic error

introduced by the spectrophotometer itself into measurements of the true

regular transmittance defined in Section II. A has been minimized. The

individual components of this error are discussed in Reference [1] and are here summarized as follows.

Beam-displacement and defocusing effects: These are undetectable due to

the parallel-beam geometry of the instrument and the use of an averaging

sphere in front of the photomult i pi ier tube.

Interref lections between monochromator exit aperture and sample: This

effect is undetectable when a clear glass plate is used as the sample, a case

in which the error is prominent in other instruments.

Obliquity errors: Path length and Fresnel reflectance effects due to

the residual non-parallelism of the beam and/or imperfect sample alignment

were calculated and also found negligible.

Polarization effects: These are accounted for by performing all

measurements in linearly polarized light.

Detector errors: All measurements are corrected for dark current and

linear drift of the photomult ipl ier signal. The non-linear component of the

drift is treated as noise and is included in the random error quoted for each

measurement. The uncertainty in the non-linearity correction (3a) applied to

all measurements is of the order of 10"^ transmittance units.

Wavelength bias: The estimated uncertainty (3o) of the wavelength

correction applied to all measurements is 0.04 nm. The corresponding

transmittance uncertainty is sample-dependent and is given by

AT = 0.04 dT/dX.

This error is smaller than 2 x 10"^ transmittance units for commonly measured

spectrally neutral filters for which the transmittance does not vary by more

than 0.005 over a 10-nm interval. When samples with pronounced spectral

30
features are measured, the error calculated from the above formula will be

included in the random error quoted for the measurement.

Stray light: The error due to stray light also depends on the

transmittance spectrum of the sample. From the measurements discussed in

Section II. C. 4 it may be estimated that it does not exceed 5 x 10"^

transmittance units (3o) in the worst case that all stray light is

transmitted by the sample.

When all these errors are considered and added in quadratu-^e, the result

is less than 10"'* in the transmittance range 0.1 to 1.0. When lower

transmittances are measured by the step-down technique described in Appendix

A, the instrumental bias decreases in accordance with the equations given in

that appendix. Hence, the over-all instrumental uncertainty shown by the

upper curve in Figure 6 has been assigned to the RTS.

2. Random Errors

Uncertainties due to light source fluctuations, electronic noise, and

shot noise are the main sources of random variations. The uncertainties due

to these fluctuations may be reduced by repeated measurements. Normally a run consisting of four measurements is satisfactory.

Measurements of a large number of filters (approximately 20) with

different transmittances made it possible to plot the dependence of the

standard deviation on transmittance. This graph is presented as the lower curve in Figure 6, and shows one standard deviation based on four

measurements, each, of transmittances between 0.0001 and 0.92.

3. Sample-Induced Uncertainties

Sample properties such as non-uniformity, thermochromism, lack of

parallelism of surfaces, and scattering must be considered separately in each

case.

Non-uniformity can be one of the major sources of error. Even the best

available filters are non-uniform. The effect on a particular application

will be determined by the spot size on the filter used and the requirements

for displacement due to alignment or use differences. Some examples of

non-uniformity are given in Appendix A and reference [6]. Some typical

31
32
values of non-uniformity are 0.1 to 0.5 percent of the transmitt ance (equivalent to a 3a systematic uncertainty).

Thermochromism becomes a problem for the measurement of colored filters.

When such filters are measured, it is necessary to monitor the temperature

carefully, or even to determine temperature coefficients for transmittance.

Thermochroism is not normally a problem for neutral filters.

Surfaces for high quality filters are quite parallel. Scattering, both

surface and volume, is negligible for quality filters. This is verified by

experiment. Not only is the scattering small but the RTS solid angle of

acceptance (approximately 0.0027 steradian) is sufficiently small to reject a diffuse component.

4. Total Uncertainty

Table 6 lists the component uncertainties and the over-all uncertainty

for high quality neutral filters. The instrumental systematic error (3a

equivalent) and the random error (la) are obtained from Figure 6. The total

instrument uncertainty is obtained by adding in quadrature (square root of

the sum of squares) the instrumental systematic error and three times the

random error. The sample induced uncertainties (3a) listed are estimated as 0.1% of the transmittance. The over all uncertainty is obtained by adding

in quadrature the instrument uncertainty and the sample induced uncertainty.

It is seen that for high transmittances, sample induced errors predominate;

and at low transmittances instrumental uncertainties become important.

F. Report of Test

A typical report (see Appendix A) contains all of the important

parameters for a given test and a description of the measurements. The

results are presented in tables with associated uncertainties.

G. Safety.

A 5mW helium-neon laser and ultraviolet-emitting low-pressure-discharge

lamps are used in the RTS laboratory. Appropriate warning signs are displayed at the entrance to the laboratory when these sources are in use. Safety glasses are worn when the ultraviolet lamps are used. A large

light-tight enclosure to the instrument must be raised to allow access when

major changes are made in the RTS. Care must be taken when raising and

33
Table 6

Typical Over-all Uncertainties for NBS Reference Spectrophotometer

Transmittance

Source of Error T = 0 .01 T = 0.1 T = 1 .0

A. Instrument al Bias (Systematic) 2.0 X 10-5 1.0 X

10-'+ 1.0 X

10-'+

B. Random Error

\o X oLanaaru ueviaLion^

1.0 X 10-5 0.6 X

10-'+ 2.7 X 10^

Total Measurement Uncertainty

(A2+B2)l/2

2.2 X 10-5 1.2 X 10-'+ 2.9 X 10^

C. Sample Induced Errors 1.0 X 10-5 1.0 X

10-'+ 1.0 X 10-3

Overall Uncertainty

(A2+b2+c2)1/2

2.4 X 10-5 1.5 X 10-'+ 1.0 X 10-3

34
lowering the enclosure to ensure that turning the lift mechanism results in

movement of the enclosure. The usual precautions are observed when using

chemicals for stray light tests. Other precautions taken are the usual ones when working with electrical laboratory equipment.

III. Transfer Spectrophotometer for Regular Transmittance

A. Description of the Instrument [18]

1. Wavelength Range

The overall wavelength range of the instrument is 185 to 3150 nm. The

monochromator uses two gratings, one covering the UV-VIS (185 to 900 nm) , and

the other covering the NIR (700 to 3150 nm). A deuterium lamp is used as the UV source below 340 nm and a tungsten-halogen lamp is used for the 340 to

3150 range. The wavelength scan rate can be selected between 0.01 and 10

nm/s, and repeated scans of selected wavelength ranges can be made

automatical ly.

2. Bandwidth Range

The spectral bandwidth can be set to any fixed value between 0.1 and 3.5 nm for the wavelength range 185 to 900 nm when using the photomult ipl ier

detector. The bandwidth, in the near infrared, may range from less than 1 nm to a maximum of 14.3 nm, depending on the available signal. The signal gain

is increased or decreased to narrow or widen the bandwidth, respectively, in

the near infrared.

3. Photometric Range

The photometric range in the transmittance mode is ordinarily 0 to 100%.

The chart recorder can be programmed to display zero to 200% (in case the

sample fluoresces). A zero suppress allows expansion of the upper portion of

the scale.

The photometric range in the transmission density mode* is 0 to 4.5. The

instrument is programmed to give a "data out of range" message on the

*The transmission density is defined to be the logarithm to the base ten of

the reciprocal transmittance.

35
parameter display monitor screen, when the density exceeds 4.5. This same message is displayed in the transmittance mode when the transmittance is less

than 0.01%. In order to measure higher densities than 4.5, special photoetched nickel screens are inserted into the reference beam. This technique

extends the density range to approximately 6. However, the density scale in

the NIR is limited to a range of 0 to 3 due to the noise of the lead sulfide

detector.

4. Temperature Control

The sample compartment is thermally insulated and equipped with water

circulating tubing for controlling the sample temperature to ±0.2"'C between

approximately 15°C and 40''C. The temperature of a sample is monitored by

means of a thermocouple probe which measures the ambient temperature inside

the sample compartment. The temperature is displayed on the parameter

monitor or recorded as a function of time during a spectral scan.

5. Data Acquisition and Automation

Spectral transmittance data can be recorded with the chart recorder, or printed in digital form by a thermal printer.

The spectrophotometer can be operated manually, or it can be controlled

by the external computer. This computer is used to store spectral data on floppy disks, and to perform data reductions, such as correcting for baseline

drift or converting absorbance to transmittance and vice-versa. There are special programs for computing. colorimetric terms, performing spectral

averaging, and plotting data.

B. Instrument Calibration

1. Wavelength

Calibration of the instrument wavelength scale is performed by comparing

the measured peaks of emission lines and absorption bands with reference data

[19] for the wavelengths of these peaks. The emission lines and absorption

bands used are listed in Table 7.

To produce the Hg, Ne, Xe, and Kr lines listed in Table 7, pen-ray lamps

are placed in the lamp compartment of the spectrophotometer. The absorption

bands are produced by 1 cm path length cuvettes placed in the sample

compartment.

36
Table 7

Emission Lines and Absorption Bands Used to Calibrate

the Transfer Spectrophotometer Wavelength Scale.

Emission Lines:

Hg: 253.65 nm, 334.15 nm, 435.83 nm, 1014 nm, 1529 nm.

D2: 486.0 nm, 656.10 nm, 1312.2 nm, 1968.3 nm, 2624.4 nm. Note: The last 3 wavelengths for D2 are the 2nd, 3rd, and 4th order

lines of the 656.1 nm line and are observed with the near infrared grating.

Ne: 703.24 nm.

Xe: 881.94 nm, 979.97 nm. Kr: 1816.73 nm.

Absorption bands of Materials;

1,2,4-trichlorobenzene (0.5 mm): 2152.6 nm. Holmium oxide (SRM 2034): 241.0 nm, 249.8 nm, 278.1 nm, 287.0 nm,

Rare-Earth Oxide (SRM 1920): 799.0 nm, 886.7 nm, 1132.4 nm, 1260.8 nm,

1535.4 nm, 1682.2 nm, 1757.8 nm, 1847.0 nm,

1932.5 nm, 1970.8 nm.

416.1

333.5 nm, 345.5 nm,

nm, 451.3 nm,

nm, 640.5 nm.

361.4 nm, 385.4 nm,

467.8 nm, 485.3 nm. 536.5

37
The D2 lines are produced by the deuterium lamp built into the spectrophotometer, and are used in automated day-to-day wavelength scale checks. In

the UV and visible the 486.0 nm and 656.1 nm lines are used in the first

grating order, and in the near IR they are used in the second, third, and

fourth orders. The zeroth order is used as a further check.

The wavelength scale error is reduced to a minimum by adjusting the

wavelength encoder mechanism. The following data are typical of the

wavelength scale error:

True Wavelength (nm) Observed Wavelength (nm) Error (nm)

2624.4 2623.8 -0.6

1968.3 1967.9 -0.4

1312.2 1312.0 -0.2

656.10 656.12 +0.02

485.00 485.02 +0.02

0.00 0.13 +0.13

In calibration reports the wavelength uncertainty is generally stated to

be ±0.1 nm in the UV-VIS and ±1.0 nm in the NIR. Since these values are upper bounds, the uncertainty is 3a in magnitude. Corrections are usually

not applied but can be made in critical cases.

2. Linearity

The linearity of the photomult ipl ier tube of the spectrophotometer was checked with a set of transmitt ance standards [5,6] following a comparison of

their measured transmittances at 770 nm with the best values recently

measured on the reference transmitt ance spectrophotometer. These

measurements indicated that no linearity correction is necessary for

measurements made with the PMT detector.

Filter No. Measured Value Reference Value Di f f erence % Difference

1-1 0.9179 0.9185 -0.0006 - 0.07

1-2 .6641 .6644 - .0003 - 0.05

1-3 .4788 .4787 + .0001 + 0.02

1-4 .3286 .3289 - .0003 - 0.09

1-5 .1669 .1674 - .0005 - 0.30

1-6 .0497 .0500 - .0003 - 0.60

1-7 .0115 .01163 - .0001 - 0.86

38
The non-linearity of the lead sulfide detector was determined by

comparing transmittance measurements at 770 nm. The following differences

were found, and are applied as non-linearity corrections for measurements

made with the lead sulfide detector.

Filter No. Measured Value Reference Value Difference % Difference

1-1 0.9214 0.9185 +0 .0029 + 0.32

1-2 .6663 .6644 + .0019 + 0.29

1-3 .4800 .4787 + .0013 + 0.27

1-4 .3300 .3289 + .0011 + 0.33

1-5 .1669 .1674 .0005 - 0.30

1-6 .0484 .0500 .0016 - 3.20

1-7 .0097 .01163 .0019 -16.34

3. Stray Radiation

The presence of stray radiation in the spectrophotometer sample beam was checked for spectral bandwidths (SBW) of 1 nm and 3.6 nm with ASTM Test

Method E387 [17] using cutoff filters and pass-band filters. The results

were as follows, indicating that the level of stray radiation in this

spectrophotometer is not significant.

Filter or Stray Radiation Levels

Wavelength (nm) Solution 1 nm SBW 3.6 nm SBW

200 KCL 3.38 x 10"^ 5.75 x lO''^ 215 NaBr 3.54 x lO'^ 1.90 x lO'^

245 Nal 4.36 x lO'^ 1.17 x 10"^

318 Acetone 5.01 x 10-^ 3.89 x 10"^

380 NaN02 2.04 x 10"^ 1.47 x 10-^

512 Red #222 2.39 x 10-^ 2.88 x 10-&

568 Red #3059 8.91 x lO'^

660 Methylene Blue 3.89 x 10"^

671 Methylene Blue 4.16 x 10"^

688 Red #3002 5.12 x 10"^

39
4. Stability

The wavelength scale error will remain constant after a warm-up time of

approximately one hour, if the temperature of the instrument remains stable.

The photometric scale drift is checked by observing changes in the baseline

(100%) calibration. A baseline drift of 0.1% in one hour is common. If

measurements require several hours, the baseline is recorded at the beginning

and ending of the run to determine the drift. The baseline is also

measured repeatedly when making multiple scans. Corrections for baseline

drift are part of the standard data reduction procedures.

C. Operational Procedures

1. Instrument Parameters

Instrument parameters (wavelength range, bandwidth, scan speed, data

interval, and choice of transmittance or absorbance scale) are set after a 30

minute warm-up period. The wavelength scale is checked at this time by

running the built-in test with the deuterium emission lines. The baseline is

set before any measurements are recorded. In setting the baseline, the

instrument scans the spectral region to be measured and stores the data in a memory that compares the sample and reference beam intensities. If these two

signals are the same, the baseline correction is zero. If they are not the

same, a record of the difference is kept in the memory. These differences

are used to correct later measurements for baseline errors. As long as subsequent measurements are made under the same instrument parameters, the

recorded baseline will be used. If the operator changes the gain, bandpass,

or wavelength range, the instrument will display a warning message on the

monitor so that a new baseline will be set for the new conditions.

The use of this baseline correction technique does not eliminate the

need for recording a 100% calibration curve at the beginning and end of a set

of measurements to determine baseline drift. A zero transmittance curve does

not have to be recorded because the instrument does not register negative

transmittances or transmittances less than 0.01% (approximately optical

density 4). In the absorbance mode, optical densities greater than 4.5 can

be recorded by special techniques.

40
2. Measurement Sequence for Transmittance or Absorbance

The measurements are recorded in the sequence: 100%, test sample, 100%.

In the absorbance mode, the sequence is the same except that the 100% becomes

the zero absorbance calibration.

3. Data Reduction

The data for the test sample are usually divided by the data for the 100%

calibration. If the 100% calibration drifts between the beginning and ending

100% measurements, the average 100% is used in adjusting the sample data. If

the drift exceeds 0.3% in the 100% calibration, the measurements are rejected

until the condition is corrected. In the case of absorbance measurements,

the zero absorbance calibration is subtracted from the sample data.

4. Uncertainties

The uncertainty (3a) for regular transmittance is generally 0.003 in the

ultraviolet-visible range and 0.008 in the near infrared range. The photometric scale uncertainties arise from factors such as non-linearity of the

photometric scale, stray radiation, random noise, stability of the

photometric scale and sample related uncertainties. The uncertainty (3a) due

to non-linearity is less than 0.001 for transmittance greater than 0.3. The error due to stray radiation is less than 0.00002. The uncertainty due to

random noise varies with a number of instrumental operational parameters.

Noise (3a) may be as small as 0.02% or as much as 2% depending on such

parameters as spectral bandwidth, wavelength range, and available reference

signal. A typical noise level is 0.0025. When random noise is relatively

high, the effects of noise on the measurements is reduced by averaging ten

measurements at each wavelength. As stated in Sec. III.B.l, the wavelength

scale uncertainty (3a) is ±0.1 nm in the ultraviolet and visible spectral

range and ±1.0 nm in the near infrared. The influence of wavelength error on the over-all uncertainty depends on the rate of change in transmittance

versus change in wavelength. A typical uncertainty (3a) in transmittance due

to wavelength error is 0.0005 for a neutral sample in the visible spectral

range. In measuring samples for which the transmittance changes rapidly

with wavelength, a correction is made for the wavelength scale error. The

stability (3a) of the photometric scale over a period of three hours is

better than 0.003. Since the average of the first and last 100% curve is

41
used, the uncertainty due to drift will not exceed 0.0015. The drift is

linear and can be minimized by recording the 100% calibration at shorter time

intervals and correcting for changes in this calibration. The estimated

typical uncertainties (3o) are summarized in Table 8 for transmitt ances in

the range 1.0 to 0.10 for the visble. The relative uncertainty increases for

lower transmitt ance. For transmittances between 0.10 and 0.001 the

uncertainty changes from approximately 0.0030 to 0.00027 for a typical case. The uncertainty for a 0.001 transmittance filter can be reduced to 0.00005 if

a linearity correction is applied. The non-linearity of the photometric

scale has not been assessed for transmittances less than 0.001.

D. Reports of Test

The "Report of Test" (see Appendix B) is an important documentation of

services performed. The report includes such information as: (1) The purpose

of the test; (2) a description of the test samples; (3) a description of the

instrumentation, measurement parameters, and procedures; (4) details of the

data reduction and estimates of the uncertainties; (5) comments and

references; and (6) tables of data and calculated results.

E. Safety

The same safety precautions are followed with the transfer instrument as those described above for the reference instrument.

42
Table 8

Summary of Estimated Uncertainties in Percent of Spectral

Transmittance Values for NBS Transfer Spectrophotometer [A

Representative Analysis (Three Sigma) in the Visible Region for

Transmi ttances in the Range .1-1.0]

Wave length

Stray radiation

Li nearity

Random noise

Stability (Drift)

5.0 X 10-^

2.0 X 10-5

6.0 X lO-"* 2.5 X 10-3

1.5 X 10-3

Total Uncertainty (3o)

(Square root of sum of squares)

3.0 X 10-3

43
REFERENCES

1. Mielenz, K. D., Eckerle, K. L., Madden, R. P., and Reader, J., "New

Reference Spectrophotometer", Appl. Opt. j2 (7), 1630 (July 1973).

2. Mielenz, K. D., and Eckerle, K. L., "Design, Construction, and Testing

of a New High Accuracy Spectrophotometer", NBS Technical Note 729 (June

1972).

3. Eckerle, K. L., "Modification of an NBS Reference Spectrophotometer",

NBS Technical Note 913 (July 1976).

4. Eckerle, K. L., Weidner, V. R., Hsia, J. J., and Chao, Z. W., "Extension

of a Reference Spectrophotometer into the Near Infrared", NBS Technical

Note 1175 (April 1983).

5. Eckerle, K. L., Weidner, V. R., Hsia, J. J., and Kafadar, K., "Measurement Assurance Program Transmittance Standards for Spectrophotometric

Linearity Testing: Preparation and Calibration", J. of Res. of the

NBS, 88, 1 (Jan. -Feb. 1983) .

6. Eckerle, K. L., Hsia, J. J., and Weidner, V. R., "Transmittance MAP

Service", NBS Special Publication 692, (March 1985).

7. Venable, W. H., and Eckerle, K. L., "Didymium Glass Filters for Calibrating the Wavelength Scale of Spectrophotometers - SRM 2009, 2010, 2013,

and 2014", NBS Special Publication 260-66, (Oct. 1979).

8. Eckerle, K. L., Chang, S., and Hsia, J. J., "Calibration in 1976 and

1983 of Didymium Glass Filters Issued as NBS Standard Reference

Materials", Color Res. and Appl., W_ (32) (Spring 1985).

9. Weidner, Victor R., Mavrodineanu, Radu, Mielenz, Klaus D., Velapoldi,

Ranee A., Eckerle, Kenneth L., and Adams, Bradley: Spectral

Transmittance Characteristics of Holmium Oxide in Perchloric Acid

Solution, J. Res. Nat. Bur. Stand. 90, 115 (March-April 1985).

10. Mielenz, K. D., "Physical Parameters in High-Accuracy Spectrophotometry", NBS Special Publication 378, (May 1973)..

44
References (cont'd)

11. CIE Publication No. 17 (E-1.1) 1970, International Lighting Vocabulary , 3rd Edition, Bureau Central de la CIE, Paris (1970).

12. Eckerle, K. L., Venable, W. H. Jr., and Weidner, V. R., "Averaging

Sphere for Ultraviolet, Visible, and Near Infrared Wavelengths: A

Highly Effective Design", Appl. Opt. lb_ (3) (March 1976).

13. Taylor, D. J., Rev. Sci. Inst., 40. 559 (1969).

14. Burke, R. W. and Mavrodineanu, R., "Accuracy in Analytical

Spectrophotometry", NBS Special Publication 260-81, (April 1983).

15. Mavrodineanu, R. and Baldwin, J. R., "Glass Filters as A Standard

Reference Material for Spectrophotometry - Selection, Preparation,

Certification, Use, SRM 930", NBS Special Publication 260-51 (Nov.

1975)

.

16. Mielenz, K. D., and Eckerle, K. L., "Spectrophotometer Linearity Testing

Using the Double Aperture Method", Appl. Opt. jl (10) (Oct. 1972).

17. Estimating Stray Radiant Energy, 1970 Annual Book of ASTM Standards,

Part 30, p. 1154 (1970).

18. Varian-Cary Model 2390 UV-VIS-NIR Spectrophotometer.

rd

19. CRC Handbook of Chemistry and Physics, 63 Edition, CRC Press, Inc.

(1982-1983)

.

45

APPENDIX A

U.S. Department of Commerce

NAT[ONAL BUREAU OF STANDARDS

GAITHERSBURG. MARYLAND 20899

REPORT OF TEST

for

Spectral Transmi ttance

of

Four Neutral Filters

Submitted by

Laboratory A

(Authorization for this test was given in Purchase Order XYZ dated

January 13, 1976.)

1. Purpose.

The purpose of this test is to determine the spectral transmittance

of four glass filters at the wavelength 694.3 nm.

2. Material.

The four neutral filters, which were supplied by Mr. Jones, are each approximately 51 mm square and had been cleaned and engraved by

him. Each filter was engraved with the letters "NBS" in one corner

along with the designation 1.00, 2.00, 3.0 and 4.0E in another corner. The engraved numbers denote the approximate optical density of a particular filter.

3. Measurements.

Measurements of spectral transmittance of the filters were made

using the reference spectrophotometer. A detailed description of the

spectrophotometer has been published as NBS Technical Note 729, and a discussion of the systematic and random errors is included in the paper

"New Reference Spectrophotometer" published in the July, 1973 issue of

Applied Optics. Several developments have occurred since those publications appeared, including the installation of new source optics

(mirrors), acquisition of a 600 groove/mm grating, and the use of an averaging sphere of a new type. The instrument has also been automated

and is controlled by a minicomputer. The latter development led to

acquisition of data at variable time intervals and to an iterative

procedure for eliminating the drift of the system.

The approximate spectral bandpass was 1.5 nm, and the temperature

was 25.0 ± 0.5°C. The filters were illuminated with collimated, polarized radiation normal to the specimen surface. Measurements were made

NBS Test No.: 534/ABC

Date: January 26, 1976

A-1
APPENDIX A

REPORT OF TEST Page 2

with the electric vector of the incident radiation horizontal (0°) and

vertical (90°). The solid angle subtended at the sample by the detection system was approximately 0.0027 steradians. The illuminated area

on the samples (centered unless otherwise specified) was a 16 mm diameter circle. The radiation was incident on the side of the filter on which the letters "NBS" were engraved, and these letters were in the

upper left corner of the filter when viewed from the direction of

incidence.

For specimens with transmittances between 0.1 and 1.0, measurements

are usually made relative to air. However, when the transmittance of

the test filter is less than 0.10, increased accuracy can be obtained by

using a step-down technique. The following page contains a description

and an error analysis of this type of measurement. Therefore, the

filter designated "1.00" was measured relative to air, filter "2.00"

relative to filter "1.00", filter "3.0" relative to filter "2.00", and

filter "4.0E" relative to filter "3.0". Since the filter designated

"1.00" was measured relative to air, the final transmittance for all of

the filters will be relative to air.

Transmittance measurements of the test filters were also made after

shifting the filter in the illuminating beam by 4 mm in two directions

normal to the beam. The resulting data indicate the magnitude of errors

that would be caused by misalignment and nonuniformi ty of the filters.

The Tables contain the transmittance, instrumental systematic

error, and standard deviation based on four measurements. The errors

due to properties of the filters such as non-uniformity, and those due

to variations in the filter environment, such as temperature changes,

can cause further uncertainty in a particular measurment. To obtain

the total uncertainty to the 99.7% confidence level, the instrumental

uncertainty, three times the standard deviation, and other filter dependent uncertainties should be added in quadrature.

Table 1 contains transmittance data for the filters at 694.30 nm along with the estimated systematic errors and standard errors based on four measurements. The uniformity of the filters is indicated in Table

2.

Prepared by: Approved by:

NBS Test No.: 534/ABC

Date: January 26, 1976
APPENDIX A

Error analysis for measurements of transmittance using the reference

The transmittance T of a filter relative to air can be calculated

from the expression

where T„ is the transmittance of a reference filter, and Tn is the transmittance of the filter being tested relative to the transmittance of the

reference filter.

An error analysis shows that the error of measurement by this

procedure is smaller than the error that results when the test filter is

measured directly relative to air.

The inaccuracy in the value of T due to systematic errors will be

called AT<; and is given by

where AT^ is the inaccuracy of T^ and ATg is the inaccuracy of Tg.

The standard error in T due to random fluctuation will be called

ATp and is given by

where AT^ is the standard error for the transmittance measurement of

filter A (T„) and ATg is the standard error for the transmittance mea- surement of filter B relative to filter A (Tn).

spectrophotometer and the "step-down" technique.

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A-5

APPENDIX B

U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG. MARYLAND 20899

REPORT OF TEST

for

Spectral Transmi ttance

of

One Tiffen Neutral Density Filter

Submitted by:

Laboratory B

(Reference Purchase order No. ABC dated January 6, 1986.)

1. Purpose

The purpose of this test is to measure the spectral transmi ttance

of one Tiffen neutral density filter over the spectral range 380 to

770 nm.

2. Materials

The Tiffen neutral density filter is identified by the designation #511.

3. Measurements of Spectral Transmi ttance

Measurements of spectral transmittance of the Tiffen neutral density filter were made by means of a Varian-Cary Model 2390 recording

spectrophotometer over the spectral range 380 to 770 nm with a spectral

band pass of 1-nm.

The filter was placed in a sample beam at the focal plane where

an image of the exit slit of the monochromator forms. The image is

approximately 8 mm by 2 mm at the center of the filter. The sample

beam converges at the focal plane with a semi -angle of approximately

1 degree.

4. Data Reduction and Results

The spectral data of the test filters and zero density calibration were recorded on an optical density (-log T) scale at 10-nm

intervals.

NBS Test No.: 534/XYZ-86

Date: January 16, 1986

FOIIM NBS—2SS (EV. e.as)

B-1
APPENDIX B

REPORT OF TEST Page 2

The data for the test filter were corrected for small variations

in the zero calibration and converted to equivalent values of spectral

transmi ttance.

The results of the data reduction are listed in table 1.

5. Uncertainties

The spectral transmi ttances listed in the table are believed to be

uncertain by no more than ±0.3 percent (three sigma). These uncertainties

are based on knowledge of the linearity of the Gary Model 2390 photometric scale as determined through extensive measurements of a set of

master neutral density filters that are periodically calibrated on the

NBS Reference Spectrophotometer for high accuracy transmi ttance measurements .

Prepared by: Approved by:

NBS Test No.: 534/XYZ-86

Date: January 16, 1986
APPENDIX B

TABLE 1

TIFFEN N.D. FILTER 60% #511

TRANSMITTANCE

WAVELENGTH (NM) {%)

380 59.95

390 61 .82

400 62.70

410 62.59

' 420 62.42

430 62.33

440 62.22

450 62.37

460 62.56

470 62.57

480 62.57

490 62.56

500 62.60

510 62.60

520 62.56

530 62.52

540 62.47

550 62.42

560 62.33

570 62.22

580 62.04

590 61.97

600 61 .80

610 61.66

620 61.48

630 61.31

640 61 .12

650 60.95

660 60.72

670 60.51

680 60.33

690 60.12

700 59.88

710 59.68

720 59.44

730 59.21

740 58.97

750 58.71

760 58.49

770 58.25

NBS TEST No.: 534/XYZ-86

Date: January 16, 1986

roilM NBS-l 14A (REV. 11 -84)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions) | NBS/SP-250/6

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No 3. Publication Date

July 1987

4. TITLE AND SUBTITLE

NBS Measurement Services: Regular Spectral Transmittance

5. AUTHOR(S)

Kenneth L. Eckerle, Jack J. Hsia, Klaus D. Mielenz, and Victor R. Weidner

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction sj

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report 8e Period Covered

Final

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

Same as item #6 above.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619840

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bi bliography or literature survey, mention it here)

This documentation describes measurement services, instrumentation,

standards, and measurement techniques for regular spectral transmittance over

the spectral range of 200 to 2500 nm. It is divided into three parts. Part

I describes the services and standards that are available and the guiding

philosophy behind the efforts to provide a primary laboratory for spectral

transmittance measurements. Part II describes the high accuracy reference

spectrophotometer. Part III describes the high precision transfer spectrophotometer. The transfer instrument relies on master standards that are periodically calibrated using the reference spectrophotometer.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calibration; high accuracy; linearity; measurement service; stray light;

transmittance; uncertainty; wavelength

13. AVAILABILITY

[x] Unlimited

Fo'' Official Distribution. Do Not Release to NTIS

[x] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, DC

20402.

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

14. NO. OF

PRINTED PAGES

59

15. Price

f'U.S. Government Printing Office : 1987 - 181-100/62525 USCOMM DC 83-«006

I

PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

J.Z. Klose, J.M, Bridges, and W.R. Ott

SP 250-4 NBS Measurement Services:

Fricke Dosimetry in High-Energy

Electron Beams SP 250-15

C.G. Soares, E.L. Bright and M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-15

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4Tr" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services:

Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with Californium Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor

Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services:

Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachyther apy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H, Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

J.S. Pruitt, C.G. Soares, and M. Ehrlich
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300