NBS Measurement Services:

Radiance

Temperature

Calibrations

NBS

Special

Publication

250-7

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

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NBS MEASUREMENT SERVICES

RADIANCE TEMPERATURE

CALIBRATIONS

William R. Waters

James H. Walker

Albert T. Hattenburg

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987
Library of Congress Catalog Card Number: 87-619850

National Bureau of Standards Special Publication 250-7

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-7, 42 pages (Oct. 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 250-7), NBS Measurement Services: Radiance

Temperature Calibrations , by W. R. Waters, J. H. Walker, and A. T.

Hattenburg, is the seventh to be published in this new series of

special publications. It describes the measurement methods and

instrumentation used in the realization and transfer of the

International Practical Temperature Scale (IPTS-68) from 1073K to 2573K

through spectral radiance calibrations of tungsten ribbon filament

lamps (see test numbers 35010C-35060C 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

i i i
ABSTRACT: This report describes the measurement methods and

instrumentation used in the realization and transfer of the International

Practical Temperature Scale (IPTS-68) above the temperature of melting

gold. The determination of the ratios of spectral radiance of tungstenstrip lamps to a gold-point blackbody at a wavelength of 654.6 nm is

detailed. The response linearity, spectral responsivity , scattering error,

and polarization properties of the instrumentation are described. The

analysis of sources of error and estimates of uncertainty are presented.

The assigned uncertainties (three standard deviations) in radiance

temperature range from ±2°K at 2573°K to ±0.5°K at 1073°K.

KEY WORDS: calibrations; gold-point blackbody; radiance temperature;

response linearity; standards; tungsten-strip lamps.

Iv
TABLE OF CONTENTS

I. INTRODUCTION 1

II. BASIC THEORY 1

III. MEASUREMENT APPARATUS 2

1. Gold-point Blackbody 2 2. Lamp Sources 2 a. Vacuum Lamps 4

b. Gas-filled Lamps 4

3. Spectroradiometer 4

a. Fore-optics 4

b. Monochromator 4

c. Detector 5 4. Control and Data Acquisition System 5

IV. MEASUREMENT OF INSTRUMENT AND SOURCE PARAMETERS 6 1. Spectral Responsivity Function 6 2 . Linearity of Response 6 3. Polarization 7 4. Size of Source 8

V. PROCESS OF REALIZATION AND TRANSFER 8

VI. DATA ANALYSIS AND UNCERTAINTIES 9 1. Temperature Values 9

2 . Uncertainties 9 3. Quality Control 11

VII. OPTICAL PYROMETER CALIBRATIONS 11

VIII. PHOTOELECTRIC PYROMETER DEVELOPMENT 13

REFERENCES 15

APPENDIX A: Report of Calibration and Lamp Standards of

Radiance Temperature 1984

APPENDIX B: Measurement of Instrument and Source Polarization

APPENDIX C: Detailed Procedures for Routine Radiance

Temperature Calibrations

V
LIST OF TABLES AND FIGURES

TABLE I 12

FIGURE 1 3

FIGURE 2 14

vi
I. INTRODUCTION

The National Bureau of Standards Issues standards of radiance

temperature calibrated In the range 800 to 2300°C on the International

Practical Temperature Scale (IPTS-68) . The standards are tungsten stripfilament lamps with cylindrical glass envelopes. The lamps are operated on direct current and require electrical power from about 10 amperes and 1 volt at 800°C to about 41 amperes and 12 volts at 2300°C. Uncertainties

range from ±1/2 to +2°C. Visual optical pyrometers are calibrated in their

design range, with larger uncertainties. The lamp standards and pyrometer

calibrations are listed in NBS Special Publication 250 [1] as 35050C,

35060C, 35010C, 35020C, 35030C and 35040C (old numbers 7.4F, 7 . 4G , 7 . 4B

7.4C, 7.4D, 7.4E) . Photoelectric pyrometers are accepted on a special

test basis, under SP250 number 35070S (old number 7.4A).

II. BASIC THEORY (CALIBRATION DESIGN)

Temperatures above 1064. 43°C (1337. 58°K) are defined on the

International Practical Temperature Scale of 1968 [2] in terms of the

ratio of spectral radiances of two blackbody sources, one of which is

maintained at the temperature of freezing gold (gold-point blackbody)

Spectral radiance is the radiant power contained in a defined area, solid

angle, and wavelength interval,

L;^ = d^4>/dA-cos5 -dw-dA (1)

where L^;^ is the spectral radiance, $ is the radiant flux, A is the area, 6 is the angle between the surface normal and the direction of propagation,

u> is the solid angle about that direction, and A is the wavelength. The

relation between spectral radiance, wavelength and temperature is given by

Planck's Law,

L;^ = ci/TT-A^. [exp(c2/A-T) - 1] (2)

where ci and C2 are the first and second radiation constants, A is the

wavelength in vacuo, and T is the temperature. The defining equation for

the IPTS above 1337. 58°K is therefore

r = L;,(T)/La(Tau) = [exp(c2/A-TAu) " 1) ]/[exp(c2/A -T) -1] (3)

where L_i^(T) and L;^(T^,^) are the spectral radiances of the two blackbodies

at temperatures T and T^^- "^Au temperature of freezing gold defined

as 1337. 58°K, r is their ratio, and C2 is defined as 1.4388 cm-°K. In

principle, a measurement of the ratio at a discrete wavelength with a linear response instrument yields the value of T.

In practice, the radiance temperature scale is realized with an instrument of finite bandpass, and an integral form of Eqn. 2 must be

used.

1
r = jLxm-Rx-dX / jL;,(TAu)-RA-dA (4)

where R\ is the spectral responsivity of the Instrument and includes the

spectral transmittance of the wavelength- limiting element (e.g.,

interference filter or monochromator )

, the spectral transmittance of all

other optical elements, and the spectral responsivity of the detector. A

determination of R;^^ allows a calculation of T from Eqns . 2 and 4.

Determination of the ratio for a range of values provides a temperature

scale over the corresponding temperature range.

The radiance temperature scale is typically maintained and

disseminated on tungsten-strip lamps, which possess a repeatable current

vs radiance- temperature relationship not available in present variabletemperature blackbodies. Realization of the scale with these lamps and a gold-point blackbody is also practical, and is the typical procedure

followed by standards laboratories [3]. This scale is valid only for the

wavelength of realization, since the spectral distributions of the lamps

are not known functions. Tradi cionally the wavelength of realization has

been near 650 nm, a convenient region for visual optical pyrometers. At

NBS , a spectroradiometer system is presently being used and a wavelength

of 654.6 nm (wavelength of a thorium spectral line) has been chosen. This

method requires that the instrument relative spectral responsivity

function extend only over a small spectral range, or is known accurately

enough to determine the wavelength at which the integrands of Eqn. 3 have

the same ratio as the integrals

III. MEASUREMENT APPARATUS

Until 1984, radiance temperature calibrations were performed on the

NBS Photoelectric Pyrometer [4] . Component failure on this instrument led

to the transfer of the measurements to the Facility for Automated

Spectroradiometric Measurements (FASCAL) pending development and

characterization of a new photoelectric pyrometer (see section VIII)

FASCAL, which is also used for spectral radiance [5,6] and spectral

irradiance [7] calibrations, is the instrument described in this report. A

block diagram of the system is shown in Figure 1

1 . Gold-point Blackbody

The Gold-point Blackbody (GPBB) is a horizontal graphite cylinder

with a small viewing hole (diameter 1 mm) at one end, and a conical cavity

at the other. The cylinder is surrounded by 0.99999 pure gold, and the

crucible containing the gold is surrounded by heating coils and an insulated case. The construction and characterization of the GPBB have

been detailed in prior reports [4]. The estimated emissivity is 0.9999.

The duration of a melt or freeze plateau is approximately 5 minutes, and

the time delay between these observation periods is about 5 minutes.

2 . Lamp Sources

Tungsten-strip filament lamps are used both in the realization of the

radiance temperature scale and as secondary standards for scale

2
i X

L. 3
dissemination. Each filament has a small notch in one edge, about midway

along its length, to aid in determination of the filament portion to be

calibrated (target area). A small mark is placed on the rear of the lamp

envelope to permit reproducible angular positioning. The lamps are rigidly

fastened in source mounts which allow translation along, and rotation

about, three mutually perpendicular axes. These motions are required for

precise alignment and for radiometric scans in translation and rotation.

The lamps are operated on direct current, provided by stabilized (0.005%

regulation) power supplies. Current measurements are provided with the aid

of manganin shunts (0.01 or 0.001 ohms) and a 5-1/2 digit voltmeter.

a. Vacuum Lamps. Vacuum tungsten-strip lamps of the Quinn-Lee type

[8,9] are used in the scale realization. One lamp is operated at a single

current to produce a spectral radiance equal to that of the gold-point

blackbody at 654.6 nm. A second lamp is operated at a single current to

produce a spectral radiance of about eight times that of the gold-point

blackbody at 654.6 nm (about 1530°K radiance temperature). Both lamps are stable to better than 0.02% over two hundred hours when operated under

these single- level conditions.

b. Gas-filled Lamps. Gas-filled tungsten- strip lamps are calibrated

as secondary standards for dissemination of the radiance temperature

scale. Prior to calibration, the lamps are annealed at a radiance

temperature of 2350 °C for 2 hours on direct current. Each lamp is examined

for the variations in spectral radiance with angle of emission, while set

at a radiance temperature of 1700°C. The lamp orientation is chosen to

minimize variations in spectral radiance with rotation, and an alignment

arrow is then etched on the rear of the envelope. The amount and

orientation of polarization effects are also determined. Lamps selected

for calibration display a degree of polarization of 0.005 or less.

Appendix A shows a typical calibration report and accompanying descriptive

material which provide a table of radiance temperature versus current,

degree of lamp polarization, estimated uncertainties, and precautions

regarding lamp operation.

3 . Spectroradiometer

a. Fore-opticsSources are imaged with unit magnification at the

entrance slit of the monochromator by the two front- surface aluminized

mirrors shown in figure 1. The mirror surfaces are stripped and recoated

at intervals to reduce signal loss due to oxidation. The plane mirror

directs the beam to the spherical mirror (radius of curvature 1220 mm)

along a line about 1.5 degrees from the spherical mirror axis. The

spherical mirror focuses the beam onto a mirror- surfaced mask, placed

immediately in front of the entrance slit. The mask is engraved with horizontal and vertical scales with 0.1 mm divisions, and is viewed at high

magnification by either a telescope or a video camera to allow for precise

positioning of the sources. The mask determines the height of the system

field stop (source target area). The entrance slit determines the width.

The stop dimensions are 0.8 mm high by 0.6 mm wide for the scale

realization and transfer. The system aperture stop is located within the

monochromator

b. Monochromator

A prism-grating double monochromator is employed to

4
minimize spectral scattering and avoid multiple orders. The dispersion is

about 4 nm/mm at the 654.6 nm wavelength setting. The entrance aperture

(solid angle) is rectangular in shape, with a vertical angle of 0.125

radians and a horizontal angle of 0.0625 radians. The wavelength setting

is calibrated against a spectral line standard (thorium) and is repeatable

to within 0.05 nm. The entrance, intermediate, and exit slits are adjustable from 0.01 to 3.0 mm as a unit, resulting in a nearly

triangular-shaped spectral responsivity function.

c. Detector

For the radiance temperature determinations an end- on 11-

stage photomultiplier with quartz window and S-20 spectral response is

placed behind the exit slit. The detector is cooled to 258°K with a thermoelectric cooler. The anode current is amplified and converted to a 0-10 volt signal by a programmable amplifier. In order to ensure linearity

of response, the current is normally restricted to a maximum of 500

nanoamperes by selection of appropriate applied voltage for the

photomultiplier. The signal is fed to a 5 1/2 digit voltmeter, capable of

integration times ranging from one second to several minutes . To

facilitate alignment of optics or sources, a He-Ne laser is placed at the

detector position, so that its beam passes through the monochromator and

fore-optics in the reverse direction.

4 . Control and Data Acquisition System

The PASCAL system employed for the radiance temperature calibrations

permits control of the entire measurement process from a remote operator

console after initial source alignment. Component positions, instrument

settings, sequence of operations, and data collection are effected by

either stored computer programs, operator commands, or a combination of

the two

The system is directed by a microcomputer and a high-speed disk system

for program and data storage. A modular interface controller [10] provides

the link between instruments and computer. All measurement signals are multiplexed into the digital voltmeter through the interface scanner, and

the instruments are remotely programmed and controlled through interface

modules. All instrument settings and signal outputs are printed and stored

on disk for later analysis. The spectroradiometer (fore-optics,

monochromator, and detector) and a closed-circuit TV camera are mounted on

a carriage. The carriage can be positioned by remote command along a linear track, to align the spectroradiometer with one of the sources mounted at fixed stations along the track. The average move time between

stations is a few seconds, and positions are repeatable to about 0.1 mm. The TV camera presents a highly magnified image of the monochromator

entrance slit mask to displays at the spectroradiometer and at the

operator console, for initial source alignment and subsequent monitoring.

5
IV. MEASUREMENT OF INSTRUMENT AND SOURCE PARAMETERS

1 . Spectral Responslvlty Function

The relative spectral responsivity function of the spectroradiometer

is determined by the indirect method [11]. In this method, the relative

responsivity function is treated as the product of two terms, the responsivity factor and the slit-scattering function, where the responsivity factor depends only upon the wavelength of the observed flux, and the slitscattering function depends only upon the difference between the

wavelength setting of the monochromator and the wavelength of the flux.

This factorization of the spectral responsivity function is valid if the

instrument dispersion, aberrations, scattering, and diffraction are constant over the wavelength region of interest. This assumption is valid

in the central portion of the relative responsivity function, but values

for the distant wings are subject to error due primarily to changes in

scattering and dispersion.

The responsivity factor is obtained by spectrally scanning a continuous source standard of spectral radiance with narrow (0.1 mm)

slits. To determine the slit-scattering function, an integrating sphere

irradiated by a krypton or argon laser is spectrally scanned by the

spectroradiometer, with the slit widths set at the 0.6 mm width used in

the scale realization and transfer. The plot of the output signal versus

wavelength is the mirror image of the plot of the slit-scattering function

versus wavelength [11]. For a 647 nm krypton laser, the function is nearly

triangular in shape with a width at half -height of 2.2 nm. Relative to the

peak value, the measured values decrease to about lO""^ at 3 nm, 10"^ at 15 nm , and 10"^ at 70 nm from the central wavelength. At 150 nm from the

central wavelength, the value decreases to 10~^ in the short-wavelength

wing and to 10'^ in the long-wavelength wing. Scans with 488 and 514 nm

(argon), and 676 nm (krypton) yield similar results. These values were

confirmed over the central and near wings portion of the function by

measurements with the direct method, using a dye laser tuned through a series of wavelengths with the spectroradiometer set at a fixed wavelength

[10]. Since the function changes very slowly with wavelength in the

visible region, the measurement at 647 nm yields the slit-scattering

function at 654.6 nm.

2 . Linearity of Response

The degree of linearity of the spectroradiometer response is

determined with an automated beam conjoiner [13]. A beam from a constant

source is split into two branches, whose fluxes are independently

attenuated or blocked before recombination and further attenuation. The

flux contribution from both branches is equal to the sum of the fluxes

from each branch when measured separately (additivity) . The device

provides ninety-six levels of flux ranging over a factor of about five

hundred. The levels are presented in random order to avoid systematic

errors, and are interspersed with twenty-nine zero flux levels. A

microcomputer controls the attenuating filters and records the filter

positions and radiometer signals. The data is leas t- squares fitted to a polynomial response function, to determine a correction factor by which

the radiometer output signal must be multiplied to obtain a quantity

6
proportional to radiant flux.

The measured instrument response is linear to within ±0.2% for a range of photomultiplier anode currents from 1 to 500 nanoamperes . For

currents much less than 1 nanoampere, the signal is limited by noise. For

currents greater than one microampere, the linearity correction increases

rapidly, rising to 3% at 7 microamperes. The anode current is restricted

to less than 500 nanoamperes during measurements by selection of

appropriate photomultiplier tube voltage. Correction factors for the

amplifier ranges are determined from measurement of a known electrical

current and combined with the linearity correction factor.

3 . Polarization

The polarization properties of the spectroradiometer and the gasfilled lamps are measured with the aid of dichroic (linear) polarizers

positioned in motorized rotating mounts. The sheet polarizer properties

and those of the spectroradiometer are determined in an initial set of

experiments, using an illuminated integrating sphere as a source of

unpolarized radiation. The characterized polarizer and spectroradiometer

are then used to measure the polarization properties of the lamp sources.

The determination of spectroradiometer and sheet polarizer properties

consists of spectral radiance measurements of the sphere source alone,

with a pair of similar polarizers interposed in the beam and set at a number of angular positions (rotations about the optic axis) , and with

each polarizer individually placed in the beam at the same angular

positions. In order to account for departure from ideal behavior, a Mueller transmittance matrix [14] containing six parameters is assumed for

the sheet polarizers. Circular polarization is assumed to be negligible.

The spectroradiometer polarization direction is determined in a preliminary experiment and chosen as the polarization reference direction,

leaving only the degree of polarization to be determined for the

instrument. Our measurements provide about 200 equations involving ten

unknowns, whose values are then obtained by a non- linear least- squares

solution.

The sphere source is replaced by the lamp source whose properties are

to be measured using a characterized sheet polarizer and

spectroradiometer. Measurements are made with the lamp alone and with the

polarizer set at the same angular positions as before. This results in 25

equations involving the two source unknowns, whose values are obtained by

a least-squares solution. The polarization of the lamp source is specified

by the direction of maximum linear polarization 7, and the degree of

polarization

P ~ (Ljuax " Ljjjin) / (^ax ^in)

where Ljnax Win maximum and minimum readings of a polarization- indifferent radiometer when an ideal linear polarizer is

rotated in the source beam. The factor (see App . B) required to reduce the

signal ratio of Eqn. 4 to a value which would be obtained with a polarization- indifferent radiometer is

7
(1 + Bpi)-1

where = p -00327 ^.nd B is the spectroradiometer polarizance (degree of

linear polarization introduced by the spectroradiometer) . For the

spectroradiometer employed here, the measured polarizance is 0.26 at 654.6

nm. The degree of polarization of a typical lamp selected for calibration

is about 0.003. The uncertainty in the radiometer polarizance B is

estimated as ±0.0005, and the lamp polarization uncertainties are estimated as ±0.002 (uncertainties are stated at the three standard

deviations level) . A more detailed description of the measurement process

and data analysis is given in Appendix B.

4 . Size of Source

The "size of source" effect (signal contribution due to flux which

originates outside the target area and is scattered into the measured beam

by the fore-optics) is determined by observing the signal from a 0.6 by

0.8 mm segment of a uniform diffuse source, and noting the change in

signal when the surrounding area of the source is changed by placing

various masks on the diffuse source. The masks expose source areas which

closely approximate the radiant areas of the lamp and blackbody sources used in the scale realization and transfer. As a check, the effect is also

evaluated by observing changes in the near-zero signal from a "black hole"

(an absorbing cavity slightly larger than the 0.6 by 0.8 mm field stop) as the various surrounding area masks are positioned. The observed

differences are used to apply a correction to the signals observed in source comparisons. The measured effect varies from 0.04% to 0.1% at 654.6 nm depending upon the elapsed time since the last mirror recoating.

V. PROCESS OF REALIZATION AND TRANSFER

The process of realization and transfer consists of three steps, all

carried out at a wavelength of 654.6 nm. In step one, the gold-point

spectral radiance is transferred to a Quinn-Lee vacuum lamp. In step two,

a second vacuum lamp at about 1530°K radiance temperature (about eight

times the gold-point spectral radiance) is compared to the first vacuum

lamp. In step three, the second lamp is compared to the test lamp. After

resetting the test lamp current, step three is repeated for each desired

radiance temperature. If the vacuum lamps remain constant between observations, the product of the signal ratios from the three comparisons is the

signal ratio of the test lamp to the GPBB . With appropriate corrections

for size of source and polarization, the spectral radiance and thus the

radiance temperature of the test lamp can be evaluated from eqns . 4 and 2.

The polarization and the spectral radiance distributions over to, A, and A of the vacuum lamps are of no concern here, since the lamps serve only to

reproduce the spectroradiometer signal between comparisons. The only

requirement is that the vacuum lamp parameters remain constant between

comparisons with the gold-point blackbody (GPBB) and with the test lamp.

This is satisfied by the constant-current operation of the stable (0.02%

per 200 hours) vacuum lamps. Comparison with the GPBB is performed about

every 50 to 100 hours of lamp operation, and the vacuum lamps are compared

with each other weekly during experiments. The use of the first vacuum

8
lamp avoids the Inconvenience of manipulating the GPBB through its melting

and freezing point cycles and provides a continuous measure of gold-point

radiance. It also increases the number of experiments allowed between

maintenance or replacement of the gold-point furnace. The use of the

second vacuum lamp keeps all signal ratios within the linear response

region of the spectroradiometer (i.e., within the 1 to 500 nanoampere

range of anode current)

Appendix C provides a detailed description of the routine calibration process.

VI. DATA ANALYSIS AND UNCERTAINTIES

1 . Temperature Values

The signal ratio of the test lamp to the GPBB is obtained from the

product of the signal ratios measured in the first vacuum lamp comparison

with the GPBB, the second (1530°K) vacuum lamp comparison to the first,

and the test lamp comparisons to the second lamp. The ratio is then

multiplied by correction factors to account for the size-of-source disparity, polarization error, and departure from linear response where

appropriate. An estimated test lamp radiance temperature is then

calculated from the ratio of Planck functions for two blackbodies which

have the same signal ratio (one blackbody at the gold-point), using 654.6 nm as the wavelength. This temperature can then be used to determine by

iteration the exact temperature which will satisfy Eqn. 4. For the

spectral responsivity function of this spectroradiometer, this exact

temperature differs from the estimated temperature obtained from the

Planck ratio by an amount which is small (less than 0.2°K) and is a simple

function of the temperature. Therefore, to avoid the repetitive iteration

process, the temperature calculated from the Planck ratio is corrected to

the desired Eqn. 4 value by this known difference.

2 . Uncertainties

The uncertainties in the radiance temperature values assigned to the

calibrated lamps are obtained from the observed imprecision of the

measurements and the estimated systematic error in both the measured and

the provided quantities (e.g., temperature of melting gold). Uncertainties

obtained from observed imprecision and from published values for the

physical constants are based upon three standard deviations. Uncertainties

of systematic errors are estimated at the equivalent of three standard

deviations

In order to examine the contributions of the various errors to the

uncertainty in radiance temperature , an approximate equation for the

complete measurement process can be developed by using the Wien

approximation to Eqn. 2,

La (ci/tt) •A-5.exp(-C2/A-T) (6)

to express the spectral radiance of a blackbody. With this approximation,

Eqn. 3 becomes

9
r = exp(c2/A-TAu)/exp(c2/A-T) = exp [ (c2/A) • ( l/T^u " 1/T) ] (7)

Solving for T, and expressing r in terms of the measured ratios and their

correction factors, we can express the complete measurement process as

-1

T= [(1/Tau) - (A/c2)-ln[s-d-f-Mo-Mi-M2/(l+B-pi)]j (8)

where the definitions of the quantities and their estimated 3a

uncertainties are:

Mq, signal ratio GPBB vs. first vacuum lamp (0.12%)

M]^ , signal ratio first vacuum lamp vs. 1530°K lamp (0.12%)

M2, signal ratio 1530°K lamp to test lamp (0.2-0.5%)

s, size-of source correction for GPBB vs. test lamp (0.1%)

d, correction for test lamp drift during calibration (0.1%)

f, linearity-range factor correction (0.04-0.1%)

T^u, IPTS-68 temperature of freezing gold (0.4°K)

C2, second radiation constant (0.00014 cm- °K)

B, spectroradiometer polarizance (0.05%)

PI, lamp polarization component (0.2%)

A, wavelength setting at 654.6 nm (0.15 nm)

Radiance temperature uncertainties due to the factors of Eqn. 8 are obtained from the partial derivative with respect to those factors and the

estimated uncertainty in the factor (propagation of error) . Thus the

calculated uncertainty in radiance temperature at the 2300°C (2573. 15 °K)

point due to the 0.4°K uncertainty in T^^xi

AT = (2573.15/1337.58)2. (0.4)

or 1.48°K. Differences between errors calculated by Eqn. 8 and those

calculated by the exact Planck equation are negligible. Note that for the

wavelength A this process yields the error due to inserting the wrong

wavelength in the calculation, not the error due to incorrect wavelength

setting

In addition to the factors which appear explicitly in this relation,

uncertainties in the ratios Mq , M]^ , and M2 arise from errors in the

wavelength setting A, in the current measurements of the vacuum (0.2 ma)

and gas-filled (2 ma) lamps, in the alignment of lamps and in the measured

spectral responsivity function. The uncertainties in the ratios due to

wavelength setting and current are assessed by measurement of the change

in signal ratio when varying these quantities. The effect upon the signal

ratios due to the uncertainty in the measured spectral responsivity

function is determined by solving Eqn. 4 for a range of R(Ao,A) values,

using the known spectral radiance distribution of the GPBB and an approximate test lamp distribution derived from spectral scans of such

10
lamps. The radiance temperature uncertainties due to these factors are then deduced from the ratio uncertainties as before. The uncertainties in

signal ratio, wavelength setting, lamp currents and lamp alignment are considered random errors; the remaining errors are systematics.

Table I summarizes the uncertainties obtained by this process. The

calculated uncertainties in degrees are tabulated for a number of

temperatures over the calibration range. The individual values are combined in quadrature to yield the combined uncertainty for each

temperature. Issued calibration reports (see Appendix A) include a similar

table of uncertainties, with errors grouped for the convenience of the user. These uncertainties apply to the reported values at the conclusion

of the calibration.

3 . Quality Control

The calibrations resulting from this process are derived in principle

from a complete realization of the radiance temperature scale, and

therefore are not dependent upon the maintenance of in-house standards. In

practice, the realization often omits the GPBB comparison and relies upon

the value of gold-point spectral radiance which is maintained on the

stable Quinn-Lee vacuum lamp. The long-term record of this lamp's

comparison with the GPBB (each 50-100 hours of operation) is monitored to

detect any deviation from normal. The ratio of the second vacuum lamp (at

1530°K) to the gold-point lamp is monitored throughout the calibration

process in order to detect any change in either vacuum lamp.

The radiance temperature values of those lamps which have a prior

calibration (a common occurrence) are compared to the values of earlier

calibrations in order to disclose any apparent shifts in the scale.

Recalibration of a few lamps which are retained at NBS for the realization

of other radiometric or photometric scales provide further checks on scale

stability. The calibration report is carefully reviewed by a party who was not involved in producing the values.

VII. OPTICAL PYROMETER CALIBRATIONS

Visual optical pyrometers of suitable quality are calibrated on the

IPTS against the calibrated lamp standards of radiance temperature

described in this report. For calibration points above 2300°C, the

transmittance of the high range filters (absorbing glasses) of the

pyrometers are measured, and the calibration point is calculated from this

transmittance and the low range (no filter) calibration. Calibration

values are checked against a few observations on a variable temperature

blackbody in those cases where the effective wavelength of the pyrometer

is not within normal bounds. A detailed treatment of visual optical

pyrometry is given in NBS Monograph 41 [15]. Photoelectric pyrometers are occasionally accepted as a special test. Because such instruments and

their calibration requirements vary widely, the measurement procedures are determined at the time of test acceptance.

11
TABLE I

Siiminary of Estimated Uncertainties in Degrees C

Source Temperature (°C)

of

Uncertainty 800 1100 1400 1800 2300

Signal Ratio Mq (r) 0.,06 0. 10 0. 15 0. 23 0. 36

Signal Ratio Mj^ (r) 0,,06 0. 10 0. 15 0. 23 0. 36

Signal Ratio M2 (r) 0,.27 0. 27 0.,28 0.,37 0.,67

Size of Source (s) 0,,05 0. 09 0,,13 0,,20 0.,30

Lamp Drift (s) 0.,65 0. 09 0.,13 0.,20 0,,30

Linearity (s) 0..05 0,,03 0.,05 0,,13 0.,30

Temperature Freezing Gold (s) 0,.26 0.,42 0,,62 0.,96 1,,48

Second Radiation Constant (s) 0,.02 0,,0 0.,04 0.,11 0.,22

Polarization (s) 0 .04 0.,06 0,.09 0,.14 0.,21

Wavelength Setting (r) 0 .08 0.,05 0,.08 0,.20 0.,44

Vacuum Lamp Current (r) 0 .05 0..08 0,.13 0,.18 0,.29

Test Lamp Current (r) 0 .29 0,.18 0,.12 0,.09 0,.07

Spectral Responsivity (s) 0 .03 0,.01 0 .0 0,.02 0,.03

Lamp Alignment (r) 0 .13 0,.21 0 .32 0 .49 0,.76

Total estimated 3a uncertainty 0 .5 0,.6 0 .8 1 .3 2,.0 on Thermodynamic Scale

(square root of sum of squares)

Note: Random errors denoted by (r) , systematic by (s)

12
VIII. PHOTOELECTRIC PYROMETER DEVELOPMENT

The radiance temperature calibrations described in this report were previously performed by the NBS Photoelectric Pyrometer [4] . This

instrument employed refractive optics, an extended-red photomultiplier

detector, interference filters which restricted the spectral bandpass to

about 10 nm, a small internal tungsten filament lamp which served as the

reference source, and a set of composite filters (absorbing glasses) to

extend the range to high temperatures. A new version of this instrument is

now under development, and will assume the role of realizing the IPTS

above the gold-point and performing the radiance temperature calibrations.

The new pyrometer also employs refractive optics and an extended-red

photomultiplier detector, but differs from the earlier version in three

critical respects. The bandpass is restricted to about 1 nm. In lieu of

the internal pyrometer lamp, more stable external vacuum tungsten- strip

lamps are used to reproduce small multiples of the gold-point spectral

radiance. The detector is operated under measured linear conditions to

realize higher multiples of the gold-point spectral radiance, so that the

absorbing glass filters are no longer required. The measurement process is

similar to that described in this report. A schematic diagram is shown in

Figure 2

13
14
REFERENCES

[1] Uriano, G.A., Garner, E.L., Kirby, R.K. and Reed, W. P. eds

NBS Calibration Services Users Guide 1986-88. Nat. Bur. Stand.

(U.S.) Spec. Publ. 250; 1986 July. 183 p.

[2] The International Practical Temperature Scale of 1968, Metrologia

5(2) 35-44. April 1969.

[3] Supplementary Information for the IPTS-68 and the EPT-76,1983,

Committe' Consultatif de Thermometrie , Bureau Internationale Des

Poids Et Mesures Monographie ; Pavilion de Breteuil, F-92310, Sevres,

France

[4] Lee,R.D. The NBS Photoelectric Pyrometer and its Use in realizing

the International Practical Temperature Scale above 1063 °C,

Metrologia 2(4), 150-162: October 1966.

[5] Walker, J.H., Saunders, R.D. and Hattenburg, A.T. Spectral

Radiance Calibrations at NBS. NBS Spec. Publ. 250-1.

[6] Walker, J.H., Saunders, R.D. and Hattenburg, A.T. The NBS Scale of

Spectral Radiance (submitted to Metrologia)

[7] Walker, J.H., Saunders, R.D., Jackson, J.K. and McSparron, D.A.

Spectral Irradiance Calibrations at NBS. NBS Spec. Publ. 250-20.

[8] Quinn,T.J. and Lee, R.D. Vacuum Tungsten Strip Lamps with Improved

Stability as Radiance Temperature Standards: Temperature, Its

Measurement and Control in Science and Industry, (Instrument Society

of America, Pittsburgh: 1972), Vol. 4, Part 1, p. 395.

[9] Lee, R.D., Kostkowski, H.J., Quinn, T.J., Chandler, P.R., Jones,

T.N. , Tapping, J., and Kunz , H. Intercomparison of the IPTS 68

Above 1064°C by Four National Laboratories: Temperature, Its

Measurement and Control in Science and Industry, (Instrument

Society of America, Pittsburgh, 1972). Vol. 4, Part 1, p. 377.

[10] Popenoe, C.H. and Campbell, M.S. MIDAS Modular Interactive Data

Acquisition System - Description and Specification. Nat. Bur.

Stand. (U.S.) Tech. Note 790; 1973 August. 49 p.

[11] Kostkowski , H.J . The Relative Spectral Responsivity and SlitScattering Function of a Spectroradiometer , Chapter 7 of Self -Study

Manual on Optical Radiation Measurements: Part I -- Concepts, Natl.

Bur. Stand. Tech. Note 910-4, 134 pages, June 1979 pp 2-34.

[12] Saunders, R.D. and Shumaker , J.B. Apparatus Function of a Prism-Grating Double Monochromator . Applied Optics Oct. 1986.

15
[13] Saunders, R.D. and Shumaker , J . B . Automated Radiometric Linearity

Tester. Appl. Opt. 23(20): 3504-3506; 1984 October 15.

[14] Shumaker, J.B. The Distribution of Optical Radiation with Respect

to Polarization, Chapter 6 of Self-Study Manual on Optical Radiation

Measurements: Part I ... Concepts , Nat. Bur. Stand. Tech. Note 910-3,

53 pages, June 1977.

[15] Kostkowski, H.J. and Lee, R.D. Theory and Methods of Optical

Pyrometry, NBS Monograph 41, 28 p. March 1962.

16
Appendix A.

U.S. DEPARTMEhTT OF COMMERCE

NATIONAL BimOU OT BTANDAMX

Gaithersburg, MD 20B99

REPORT OF CALIBRATION

LAMP STANDARD OF RADIANCE TEMPERATURE

Requested by

(See your Purchase Order No. dated

Material

One gas-filled tungsten ribbon filament lamp has been

calibrated by NBS for this test and bears the designation

2 . Calibration

The lamp was calibrated using the equipment and procedures

described in the enclosed document, "Lamp Standards of Radiance

Temperature-1984 " . The preparation of test lamps is described in

paragraph II of this document. Note particularly paragraph III.B

which describes the orientation and operation of the test lamp.

The calibration was performed with the lamp operating on direct

current and the center contact of the lamp base at positive

potential. The lamp was aligned while operating at a radiance

temperature of 17000C. The calibration was performed at a room

temperature of 23^0. For radiance temperatures below 15000 C for

gas-filled lamps, the radiance temperature will change as the

room or ambient temperature changes (see page 14 of reference [5]

in the enclosed document).

3. Results

Table I gives the radiance temperature of the test lamp at

654.6 nm. The values in this table apply at the conclusion of

the lamp calibration. Table II gives the uncertainties of the

calibration

NBS Test No. 534/.

October , 1985 A-1
REPORT OF CALIBRATION LAMP

The polarization measurements at 654.6 nro, described in

paragraph II of the enclosed document, showed that the degree of

polarization was 0.0042. The direction of polarization of the

larger component was approximately 15 degrees clockwise from the

horizontal when viewed from the spectroradiometer

Prepared by: Approved by:

James H. Walker

Radiometric Physics Division

Center for Radiation Research

Donald A. McSparron

Radiometric Physics Division

Center for Radiation Research

NBS Test No. 534/

October , 1985

A-2
REPORT OF CALIBRATION LAMP

TABLE I

RADIANCE TEMPERATURE AT 654.6 nin

LAMP

Degrees C Direct Current

(IPTS-66> (amperefi)

800 10.909

900 11.764

1000 12.806

1100 14.054

1200 15.508

1300 17.153

1400 18.965

1500 20.922

1600 23.008

1700 25.211

1800 27.510

1900 29.913

2000 32.403

2100 34.988

2200 37.660

2300 40.437

NBS Test No. 534/

October , 1985

A-3
REPORT OF CALIBRATION LAMP

TABLE II

Summary of Eetlmated Uncertainties in Degrees C

TEMPERATURE (Degrees C>

SOURCE OF UNCERTAINTY 800 1100 1400 1800 2300

1) Calibration of Pyrometer 0.14 0.23 0.34 0.53 0.82

Lamp relative to the

International Practical

Temperature Scale (IPTS-68>

2) Temperature determination of 0.27 0.27 0.28 0.37 0.67

Test Lamp (3 x Standard

Deviation of Kean)

3) Polarization effects 0. 04 0. 06 0. 09 0. 14 0 .21

4) Current measurement 0. 29 0. 18 0. 12 0. 09 0 . 07

5) Wavelength measurement 0. 03 0. 03 0. 01 0. 06 0 .15

6> Lamp alignment 0. 13 0. 21 0. 32 0. 49 0 .76

7) Temperature scale 0. 26 0. 42 0. 62 0. 96 1 .48

Total estimated uncertainty 0. 5 0. 6 0. 8 1

.

3 2 .0

relative to S.I. units ^'^(5-^

(square root of the sum of\

the squares of the Indl- 1 vidual uncertainties /

CAUTION: Lamp current cycling has been observed to cause changes

in lamp temperature of up to 1 degree in gas lamps (see

paragraph IV of the enclosed document).

NBS Test Ko. 534/

October , 1985

A-A
LAMP STANDARDS OF RADIANCE TEMPERATURE

1984

I. Introduction Lj^IOP Description

Lamp standards of radiance temperature are calibrated by the

National Bureau of Standards for the temperature range 800 to

2300 °C. Tungsten ribbon filament lamps are calibrated in terms

of the International Practical Temperature Scale of 1968 (IPTS68) 11] by direct comparison with a standard pyrometer lamp

calibrated at 654.6 nm.

II. Preparation

Gas lamps are annealed at a radiance temperature of 2350 °C

at 654.6 nm for two hours on direct current. Vacuum lamps are

annealed at 1450 **C for about 16 hours.

Radiance temperature calibrations are performed for a rectangular target area 0.6 mm wide by 0.8 mm high, centered on

the filament at the notch. The lamp orientation chosen for

calibration minimizes the variation in lamp output while

maintaining the optical axis of the measuring instrument

approximately normal to the lamp filament. This orientation is

determined with the lamp operating at approximately 1700 ®C for a gas lamp, or 1200 °C for a vacuum lamp. An arrow is etched onto

the rear surface of the lamp envelope to allow reproducible

alignment of this orientation (see section II I. B below).

For the lamp types used in this calibration, the radiation

from the target area has been found to be slightly polarized. At

654.6 nm, the degree of polarization, P, defined as

L (max) -L (min)

P =

L (max) +L (min)

and the direction of polarization of the larger component,

L(max), are determined. The measured value of P and the

direction of polarization of L(max) are given in the report of

calibration.

III. Calibration

A. Method

The measured radiance temperature is determined by a spectral comparison of the test lamp to a standard vacuum

pyrometer lamp calibrated at 654.6 nm in terms of the

A-5
i

International Practical Temperature Scale of 1968 (IPTS-68) [11.

Measurements are made with a spectroradiometer 12] incorporated

into the NBS Facility for Automated Spectral Calibrations

(PASCAL) [3). The photomultiplier-amplif ier combination used

was determined to be linear to better than 0.1% over a photomul tiplier current range from one nanoampere to 500

nanoamperes.

B. Conditions

The values of radiance temperature measured for the test

lamp are associated with a specific target area on the filament

surface and with a specific solid angle of the radiation emitted

from the target area. These values of radiance temperature apply

ths. lamp has. h&^n aligned tsi a specified orientation j^Mi^

operating ai a designated radiance temperature and after the lamp

has reached thermal stability at each specified operating

current. The test lamp is operated on direct current with its

center contact at positive potential.

The alignment is performed with the lamp operating basedown, the filament vertical, and the optical axis of the

spectroradiometer passing through the lamp envelope and

intersecting the center of the filament at the height of the

notch. Th£. aJAsnm&nt. ^Qn& ulth th& lamp gperatins al 1700

l£x a gas lamp £X al 12SIQ. ^£ I^ll a yaiziiiim lamp.

The exact location of the target area and the rotational

alignment of the lamp are as follows.

The calibration is performed for a rectangular target area

0.6 mm wide by 0.8 mm high. The sides of the target area are

approximately parallel to the sides of the lamp filament. The

center of the target area is located at the intersection of two

orthogonal lines on the viewed filament surface. One line

bisects the filament lengthwise, and the other passes through the

point centered at the mouth of the notch.

The center ot the target area is viewed along the horizontal

optical axis of the spectroradiometer. The lamp is positioned so

that the etched arrow on the lamp envelope is to the rear, as

viewed from the spectroradiometer. A plumb line is used to make

the notch side of the filament vertical. The lamp is then

successively rotated about the horizontal and vertical

centerlines through the target area until the tip of the

arrowhead appears centered at the mouth of the notch.

lh& above d£££xil2jBd allgnmsni Is p£xiijxm£d finly ai sm£.

i£mp£xatiix£*. M all x^ihsx t£mp£xaiLUXj££ small iLexii^al and/or

horizontal translational adjustments as^ maiifi £jQ ihat tJtlfi target

ax£a vie wed la al ways centered on Jtlifi lamp lilam^nt at height

Ql ths. notcht Hfl additional rotational alignments axe performed.

The solid angle used is a rectangular pyramid whose longer

dimension is vertical. The size of the solid angle is defined by

A-6
the vertex angles at the apex of the pyramid. The vertex angles

are 0.125 radian (7.16 degrees) in the vertical plane and 0.0625

radian (3.58 degrees) in the horizontal plane.

The bandpass of the spectroradiometer is 2.2 nm at 654.6 nm.

IV. Discussion

A more detailed description of tungsten ribbon filament

lamps and their operating parameters is given in NBS Monograph

41 15].

When lamp current settings are approached from one direction

one time and from the opposite direction the next time, changes

in radiance temperature can occur. In gas lamps changes of up to

1 **C have been observed. In vacuum lamps changes of up to 0.5 °C

have been observed. Ihis calibration is performed starting a± the

lilsli££± current ££tting sud ilueu Iqm&lIhb th& £iixijeiit i£x &^£h

additional setting.

When a gas lamp is operated at its highest temperatures

(2200 and 2300 °C), the radiance temperature of the lamp will

change at a more rapid rate than if operated only at low

temperatures. If operated at its highest temperatures a typical

change is a decrease of about 1 °C at all temperatures per 10

hours burning time on a lamp.

Additional information on tungsten ribbon filament lamps can

be found in references [6] and [7].

A-7
References

"The International Practical Temperature Scale of 1968",

Metrologia, Vol. 5, p. 35-44 (1969).

Kostkowski, H.J., D.E. Erminy and A.T. Hattenburg, "HighAccuracy Spectral Radiance Calibration of Tungsten- Strip

Lamps", Advances in Geophysics, vol. 14, p. 111-127 (1970).

NBS Optical Radiation News No. 18, November 1976.

Saunders, R.D. and J.B. Shumaker, "An Automated Radiometric

Linearity Tester", Appl. Opt. 23(20): 3504-3506; 1984 Oct 15

Kostkowski, H.J. and R.D. Lee, "Theory and Methods of

Optical Pyrometry" , NBS Monograph 41 (1962).

Barber, C.R. , "Factors Affecting the Reproducibility of

Brightness of Tungsten Strip Lamps for Pyrometer

Standardization", J. Sci. Instr. 23, p. 238-242 (1946).

Jones, T.P. and J. Tapping, "The Suitability of Tungsten

Strip Lamps as Secondary Standard Sources Below 1064 °C",

Metrologia 15, p. 135-141, (1979).

AAppendix B.

Measurement of Instrument and Source Polarization

The measurement of source polarization and the determination of its

effect on the measurement of spectral radiance is carried out with a sheet

polarizer mounted in a motorized rotating mount. This mount fits a slot

milled into a bracket attached to the front of the spectroradiometer . By

this means the polarizer can be reproducibly positioned in the same location between the source and the spectroradiometer whenever required.

Before the polarizer can be used for source polarization measurements

its polarization and that of the spectroradiometer must be measured. This

is accomplished with the help of an integrating sphere and a second

polarizer similar to the first. The integrating sphere is mounted in the

radiometer source plane so that its exit port serves as a source of

unpolarized radiance. Its entrance port, located at about 90° from the

exit port, is irradiated with a suitable light source, such as a quartz -halogen lamp. Then a series of spectral radiance measurements of

this source is recorded: a) with no polarizer, b) with the first polarizer

in its slot, c) with the second polarizer added in a second milled slot in

the mounting bracket between the first polarizer and the source, and, d)

with the first polarizer removed leaving only the second polarizer. At

each wavelength of interest measurements are taken as a function of

polarizer orientatioij as the polarizers are rotated about the optic axis

The first polarizer is typically sampled every 15*-* and the second every

30°. The second polarizer is tilted in its mount by about 10° to avoid

polarizer-polarizer interreflections . All of these measurements are automated and require typically about an hour at each wavelength.

In the analysis of this data we write the first row Mueller matrix

elements of the spectroradiometer responsivity as

R = RO 1 B 0 0 (la)

where Rq is the ordinary responsivity neglecting polarization and we take

the reference direction for polarization to be in whatever direction the

radiometer polarizance lies, thus forcing the 45° component to vanish.

The reference direction was determined to be horizontal by simply

observing the effect of a hand-held polarizer of known polarization axis

upon a lamp measurement. The polarization axis of the polarizer, in turn, was confirmed by viewing sunlight reflected at glancing incidence through

the polarizer. Circular polarization is assumed to be negligible for the

type of instrument and sources employed here. The quantity B represents

the degree of linear polarization of the spectroradiometer. The Mueller

matrix of a polarizer is taken to be of the form

B-1
p = dC

dS

0

dC

X

X

dS

gSC

X

X

0

0

X

X

(2a)

where C = cos 2((() - <^o) , S = sin 2((j) - c|)q) and the parameters s, d, e, (}), g, and (bg are obtained by least-squares fitting of the measurements. The

angle q) is the orientation angle of the polarizer read from a scale on

the polarizer mount. The matrix elements marked X are not involved in

these measurements. This form of the Mueller matrix was found, by trial

and error, to fit our data well. The simpler model of an ideal dichroic

polarizer in which e = s and g = s-<\> (where, in terms of the principle

transmittances k]^ and k2 , we have s=[k]^+k2]/2 and d= [k]^ -k2 ] /2) is

adequate for our visible and infrared polarizers but fails to fit the

ultraviolet polarizer data. Finally, the Stokes vector of the unpolarized

source is given by

L = Lo (3a)

The various measurements then lead to equations of the following

forms

No polarizer: V = R • L = RqLo (4a)

Polarizer #1: V((|)) = R • P • L = RoLo(s+BdC) (5a)

Both polarizers: V((|),(t)') = R • P • P' • L (6a)

= RqLq ( s s ' +Bs ' dC+dd ' CC ' +Bd ' (|)C ' +dd ' S S ' +Bd ' gSCS ' +Bd ' eC^C ' )

Polarizer #2: V((j)') = R • P' • L = RoLo(s '+Bd' C ) (7a)

where C'= cos 2(^' - <^o' ) , S' = sin 2(<\>' - 4>o ' ) - ^ represents the

recorded radiometer signal output as a function, in general, of polarizer

orientation angles. Altogether our measurements produce about 200

equations. The values of the ten unknowns s, s', d, d' , e, (j), g, B,

and (J)q' are then obtained by a non- linear least squares solution. As far

as polarization is concerned the parameters s, d, e, (j), g, and (j)o completely characterize the first polarizer, at least within the accuracy

B-2
of the model represented by the Mueiier matrix of eq . (la), and the

parameter B characterizes the spectroradiometer . Since the factor RqLo

is common to all equations these measurements yield no information about

it.

Using the characterized polarizer and radiometer we are now able to

measure the polarization of an unknown source by recording

spectroradiometer measurements obtained from the source: a) without a polarizer, and b) with the first polarizer again mounted on the bracket as when it was characterized. As before, the measurements are made at 15°

intervals of the polarizer. We take the Stokes vector of the unknown

source to be

L = Lo PI

P2

0

(8a)

where, as always, we have neglected circular polarization. pi and p2 are given by pj^ = p -00327 s^^d P2 = p-sin27, where 7 is the direction of

polarization measured from the reference direction of the radiometer and p

is the degree of polarization,

(Lmax - Lmin)/(Lmax + Lmin) (9a)

In Eqn. (9a) Lmax and Lmin are the maximum and minimum readings of a polarization- indifferent radiometer when an ideal linear polarizer is

rotated in the source beam. Then our measurements lead to a no-polarizer

equation:

V = RoLod+Bpi) (10a)

and to 24 equations:

V(7) = RoLo[(s+BdC) + (dC+BeC^+Bf )pi + (dS+BgSC)p2] (11a)

Aside from the factor RqI^O which is common to all equations and cancels

out, the only unknowns in these 25 equations are pi and P2 which we can then obtain by a least-squares solution. These two parameters

completely characterize the state of (linear) polarization of the source. Normally, only the second described experiment of 25 measurements need be

performed to measure the polarization of a source since the polarizers and

radiometer are relatively stable and, once characterized, will only

infrequently need to be remeasured.

B-3
Equation (10a) gives the relationship between the spectral radiance of

a source, Lq , and the radiometer output signal, V, assuming linearity

of response and only linear polarization. If we apply this equation both

to the measurement of an unknown source and to the measurement of a spectral radiance standard used to calibrate the radiometer we obtain:

Lo = Lg[VA^] [(1+Bpf)/(1+Bpi)] (12a)

where Lg is the known spectral radiance of the standard, V/V^ is the

measured signal ratio, B is the radiometer polarizance, and pf and pi are the relative Stokes components measured as described above for the

standard and for the unknown source. (For a blackbody standard,

presumably, pf = 0.) This shows that the presence of polarization

introduces the extra factor

(1+Bpf)/(1+Bpi)

into the simple measurement relationship which is valid in the absence of source polarization or if the radiometer is indifferent to polarization.

The polarization measurement uncertainty by this process can be

judged by the statistics of the least-squares fitting and by the

measurement reproducibility when different polarizers are used. Both

indications suggest typical uncertainties (one standard deviation) in the

visible where the polarizers are very good and the noise level very low of

+0.0003 in the radiometer polarizance B and uncertainties of -tO.OOl in

the source relative Stokes components and P2

B-4
Appendix C

Detailed Proce

Caiibrations

dures for Routine Radiance Temperature

A. Lamp Preparation

1. Visual Inspection

The lamp that is issued as a radiance temperature standard is a specially made lamp from General Electric designated type

30A/6V/T24. Upon unpacking, each lamp is inspected to see that

a. the lamp envelope is clear.

b. the filament is within 2mm of being centered in the

envelope

c. the area through which the filament will be viewed and the

area on the opposite side of the envelope (used for

alignment) are free of striations.

d. the filament has electrical continuity with the

electrodes

2. Initial Lighting

Each lamp is positioned in a lamp mount, wired to ensure that

the center contact of the lamp will be at positive potential.

Power is applied, and the current is set at 35 amperes. If the

lamp does not fail, then the next step in the lamp preparation

procedure is performed.

THE LAMP IS ALWAYS OPERATED ON DC, WITH CENTER CONTACT

POSITIVE.

3. Annealing

An automatic pyrometer is used to set each lamp at approximately 2350°C where it is annealed for 2 hours.

4. Arrowing

In order to accurately reproduce the alignment of each lamp, an alignment arrow is etched on the rear of the lamp envelope.

a. Set the lamp temperature to approximately 1700°C with the

automatic pyrometer.

b. Position and orient the lamp so that the Helium Neon

laser beam defining the optical axis of the spectroradiometer (see A.5.b.) is normal to the envelope and

passes through the notch on the lamp filament, when the

filament is normal to the laser beam.

c. Make a small dot with a marking pen on the rear of the

lamp envelope at the point where the laser beam strikes.

C-1
. * CAUTION *

* Great care should be taken to avoid *

* any direct eye contact with the laser *

* beam. Either position a blocking *

* finger on the 45 degree plane mirror *

* to block the direct beam or translate *

* the lamp horizontally so that the lamp * * filament blocks the beam. *

Translate the lamp so that the filament image is focussed

and centered horizontally on the monochromator slit, with

the notch centered vertically on the mask opening.

Rotational mappings about the horizontal and vertical axes are now performed on the test lamp. The lamp mounts are motorized to allow automated mappings. The computer

program used to perform the mappings is "RADMAP3 . CRO"

Plot the data and locate the rotational settings which

minimize variations in spectral radiance with rotation.

Set mount at this orientation.

Align a viewing scope behind the lamp so that the scope is

normal to and views along the optical axis of the

spectroradiometer defined by the laser beam (see A.5.h.).

Block off the laser beam.

Using the scope (with a small aperture on the objective

lens to improve the depth of field) , view the position of

the dot relative to the notch in the filament and draw a picture of the relationship.

Draw an arrow on the picture with its tip centered on the

mouth of the notch and pointing into it.

Turn off the lamp and remove it from the mount.

Paint an etching wax solution on the area of lamp envelope

containing the small dot.

Allow the etching wax to dry for at least 20 minutes and

then using a sharp needle and a transparent straightedge,

scribe a small arrow in the wax at the location relative

to the dot indicated by the drawing.

Position a small wax ring around the area to be etched to

prevent acid from etching unwaxed portions of the lamp

envelope

Using a small paintbrush, put a droplet of concentrated

hydrofluoric acid on the scribed arrow.

* CAUTION * **************************************

* Do not allow the hydrofluoric acid to *

* contact the skin. Rinse thoroughly * * with cold water if skin contact occurs.* ******************************************
******************************************

p. After 3 minutes, rinse off the acid with cold water, clean

off the etching wax with xylene, and wash the entire lamp

envelope and nozzle with detergent and hot water. Then

dry thoroughly with tissues to remove any water spots. ******************************************

* CAUTION * ******************************************

* Avoid any direct breathing of the *

* xylene vapors . * ******************************************

Lamn Alignment

A precise technique for lamp alignment has been developed and

must be followed each time a lamp is aligned. A highly polished

stainless steel mask with an 0.8 mm square cut in it is

positioned just in front of the entrance slit of the

monochromator (with the cutout square centered on the slit)

The image of a source is focussed onto this mask and is

reflected back toward a low light level TV camera. The image

from the camera is viewed on a TV monitor and is used when

performing alignments. Neutral density filters can be placed

in front the objective lens of the TV camera to prevent high

level sources from saturating the vidicon tube.

a. Set the monochromator wavelength to 632.8 nm (the Helium

Neon laser wavelength)

b. Turn on and position the alignment laser located in the

detector box portion of the monochromator.

1) Push the LASER button on the detector positioning

controller circuit.

2) Position a white card in front of and centered on the

focussing mirror of the spectroradiometer

3) Manually rotate the detector positioning wheel about

1 and 1/3 turns clockwise until a maximum signal from

the laser beam appears on the white card.

4) The laser beam exiting from the entrance slit of the

monochromator and reflecting off the mirror surfaces

defines the optical axis of the spectroradiometer.

The laser beam can now be used to aid in source alignment

c. Let the alignment laser stabilize for 20 minutes.

d. With the lamp off, orient it so that the laser beam passes

through the notch in the lamp filament and the tip of the

arrowhead on the rear of the lamp envelope.

e. Set the lamp to 1700°C with the automatic pyrometer and

let it stabilize for 20 minutes.
******************************************

* CAUTION * ******************************************

* When turning a lamp on or off the *

* current should be slowly increased or *

* decreased. A period of 40 seconds to *

* 1 minute should be taken to slowly *

* increase the lamp current to its *

* operating value or to slowly decrease *

* the lamp current to turn it off. * ******************************************

Use a plumb line to make the notched side of the lamp

filament vertical.

Focus the lamp filament onto the entrance slit of the

monochromator to plus or minus 2.5 mm.

1) Replace the reflective mask in front of the

monochromator slits with a white opaque mask.

2) Remove any neutral density filters from in front of

the TV camera.

3) While viewing the image of the lamp filament on the

TV monitor, translate the lamp in a direction

parallel to the optical axis of the

spectroradiometer . Continue to perform this

translation until the filament is at its best focus on the TV monitor.

4) Replace any neutral density filters removed in step

2).

5) Replace the white opaque mask in front of the

monochromator with the reflective mask.

Position a viewing scope behind the lamp looking along the

optical axis of the spectroradiometer. Orient the scope

so that the laser beam passes through the center of the

objective lens when the lamp filament is in focus at the

center of the field of view.

1) Translate the lamp filament horizontally so that the

laser beam passes alongside it (notch side) and

exits through the rear of the lamp envelope.

2) Position a viewing scope behind the lamp and looking

toward the spectroradiometer.

3) Adjust the viewing scope so that the laser beam

passes through the center of the objective lens (a

small aperture can be placed on the objective lens to

help locate its center)

4) Translate the lamp filament to block the laser beam.

5) Adjust the scope so that the image of the lamp

filament at the notch is centered in the field

of view.

6) Repeat steps 3) to 5) until the laser beam is

centered on the objective lens while the image of the

filament notch is centered in the field of view.

Position a "blocking finger" on the 45 degree plane mirror
of the spectroradiometer . Rotate the arrow 2 degrees out

from the notch and translate the lamp so that the image of

the monochromator slit appears on the filament,

j . Translate the lamp vertically until the filament notch is

vertically centered on the image of the slit,

k. Translate the lamp horizontally to approximately center

the slit image behind the lamp filament.

1. Put a small aperture on the objective lens of the scope to

improve the depth of field, adjust the focus of the scope

to view both filament and arrow, and adjust the rotation

and the tilt of the lamp filament until the tip of the

arrow is aligned at the notch,

m. Set the dial setting to read zero for the lamp base

rotation adjustment,

n. Remove the aperture, rotate the arrow out 2 degrees, and

translate the lamp horizontally to view the slit image,

o. Translate the lamp vertically until the filament notch is

vertically centered on the slit image,

p. Rotate back to the zero setting on the rotation

adjustment

q. Translate the lamp horizontally to get a left edge and a right edge reading of the slit image on the horizontal

translation scale,

r. Calculate the reading for which the slit image will be

centered on the lamp filament then translate in the

horizontal and set that reading,

s . Recheck the focus

t. Repeat steps k. and 1. u. If the rotation or tilt was changed significantly, then

repeat steps m. to r.

6 . Polarization Measurements

Because the spectroradiometer has significant polarization

(0.26 at 654.6 nm and 0.85 at 2400 nm) , it is necessary to

measure the polarization of each test lamp so that a polarization correction can be made.

a. Use two polarizers to characterize the polarizers and the

spectroradiometer (see Appendix B) . Two motorized

polarizers are used to do an automated characterization at

654.6 nm. The computer program used to perform these

measurements is "TWOPOL-JW".

b. Use one polarizer to characterize the polarization of the

test lamp. The computer program used to perform these

measurements is "ONEPOL-JW".

B. Calibration Procedures

1 . Data Taking Procedure

The computer program used to perform the routine radiance

C-5
temperature calibration is "PLMPCALl . LIN" . The calibration begins

with the lamp set at its highest temperature of 2300°C, and

continues to successively lower temperatures down to 800°C. The

automatic pyrometer is used to set each temperature, and after

stabilization, the lamp image is recentered on the monochromator

slit by slight translational adjustments of the lamp mount. The

general data taking procedure is as follows:

a. Adjust the 1530°K standard lamp to its calibrated current.

b. With the spectroradiometer wavelength set at 654.6 nm, the

spectral radiance output from the test lamp is compared to

the spectral radiance output from the standard lamp.

1) The spectroradiometer is positioned so that it is

viewing the calibrated target area of the standard

lamp. The photomultiplier output is amplified and

;:• . converted to a voltage which is read with an integrating digital voltmeter (DVM) . The DVM reading

is recorded.

2) The spectroradiometer is positioned so that it is

viewing the target area of the test lamp. The DVM

reading is recorded.

3) The spectroradiometer is alternately positioned to

view the standard lamp and the test lamp until a set

of three readings is taken on each source.

4) The spectroradiometer is positioned in front of a shutter position and the DVM reading is recorded for

each range used in the measurements

5) For each source the mean of the three DVM readings is

calculated and the shutter reading is subtracted from

it.

6) The resulting signal is corrected for the amplifier

range on which it was taken and for the polarization

of the source.

7) The ratio of the test lamp signal to the standard

lamp signal is determined.

c. Using the mean ratio of the test lamp to the standard lamp

and the known radiance temperature of the standard lamp,

the Planck equation is used to determine the radiance

temperature of the test lamp. A small temperature

correction is applied to account for the finite spectral

bandpass

Data Reduction Procedures

1. Analysis of Spectral Radiance Data

Several computer programs are used to perform the analysis of

the spectral radiance data.

a. Run "SPLITRAD.l" to split the main data file into a
separate data fiie for each test lamp.

b. Run "REPDIF.l" to calculate the percent differences in

repeat measurements during the calibration. Measurements

are made at 1700°C at the start, near the middle, and at

the end of the calibration.)

c. Run "FLANAL.l" to calculate the following statistics

1) Number of measurements at each temperature

2) Range for repeat measurements

3) Mean value

4) Standard deviation of the mean and to store the lamp

current, mean spectral radiance, and mean radiance

temperature in a data file.

Analysis of Precision Data

The computer program "RAD' SD. RED" is used to calculate the

overall precision of the source comparisons. This result is

used in combination with the repeat measurement standard

deviation of the mean in order to derive an over- all

uncertainty in the radiance temperature determination of the

test lamp.

C-7
FORM NBS-1 14A (REV 11-84)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS/SP-250/7

2. Performing Organ. Report No 3. Publ ication Date

October 1987

4. TITLE AND SUBTITLE

"NBS Measurement Services:

Radiance Temperature Calibrations

5. AUTHOR(S)

William R. Waters, James H. Walker, Albert T. Hattenburg

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

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & 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 : 87-619850

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

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

bibliography or literature survey, mention it here)

This report describes the measurement methods and instrumentation used in the

realization and transfer of the International Practical Temperature Scale (IPTS-68)

above the temperature of melting gold. The determination of the ratios of spectral

radiance of tungsten-strip lamps to a gold-point blackbody at a wavelength of 554.6 nm is detailed. The response linearity, spectral responsivity , scattering error, and

polarization properties of the instrumentation are described. The analysis of sources of error and estimates of uncertainty are presehted. The assigned uncertainties (three standard deviations) in radiance temperature range from ±2°K at

257^ K to ±0.5° K at 1073° K.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calibrations; gold-point blackbody; radiance temperature; response linearity;

standards; tungsten-strip lamps.

13. AVAILABILITY

Unl imited Q For Official Distribution. Do Not Release to NTIS

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

20402.

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

14. NO. OF

PRINTED PAGES

42

15. Price

USCOMM-DC 85-6006


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

{KP 250-1

iSP 250-2

SP 250-3

iSP 250-4

SP 250-5

SP 250-6

SP 250-7

SP 250-8

SP 250-9

SP 250-10

SP 250-11

NBS Measurement Services:

Spectral Radiance Calibrations

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

NBS Measurement Services:

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet

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

NBS Measurement Services:

Fricke Dosimetry in High-Energy

Electron Beams

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

NBS Measurement Services:

Alpha-Particle Calibrations

J.M.R. Hutchinson

NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz,

and V.R. Weidner

NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg

NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia

NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt

NBS Measurement Services:

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

Radioactivity Calibration Capabilities

J.M. Calhoun

NBS Measurement Services:

Dosimetry for High-Dose Applications

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

SP 250-12 NBS Measurement Services:

Neutron Personnel Dosimetry

R.B. Schwartz

SP 250-13 NBS Measurement Services:

Activation Foil Irradiation

with Californiun Fission Sources

G.P. Lamaze and J. A. Grundl

SP 250-14 NBS Measurement Services:

Activation Foil Irradiation by Reactor

Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

SP 250-15 NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

SP 250-16 NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

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

SP 250-17 NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

SP 250-18 NBS Measurement Services:

Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

SP 250-19 NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

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

SP 250-20 NBS Measurement Services:

Spectral Irradiance Calibrations

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

and D.A. McSparron

SP 250-21 NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

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

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300