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14


separate data file for each test lamp.

b. Run "REPDIF.1" to calculate the percent differences in
repeat measurements during the calibration. Measurements
are made at 1/00°C at the start, near the middle, and at
the end of the calibration.)

c. Run "FLANAL.1" 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.


5.

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.

FEI III IRI III IR II IRI IRI RII III ER

* CAUTION *
* Avoid any direct breathing of the *
* xylene vapors. *

Lamp 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.

c-3


(1 + Bpy) 74

where pj = p-cos2y and 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 observa-
tions, the product of the signal ratios from the three comparisons is the
signal ratio of the test lamp Lo Che 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 w, A, and X
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

g&


REPORT OF CALIBRATION LAMP

The polarization measurements at 654.6 nm, 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:

am
James H. Walker Donald A. McSparron
Radiometric Physics Division Radiometric Physics Division
Center for Radiation Research Center for Radiation Research

NBS Test No. 534/
October » 1985


TABLE OF CONTENTS

I. INTRODUCTION. 0.0... eee eee eee cence eee eee e eee teen teens 1
Il. BASIC THEORY... 0... . cece cece eee eee eee ee teen eee nett eens 1
III. MEASUREMENT APPARATUS +2
1. Gold-point Blackbody. 2

2. Lamp Sources. . 2

a. Vacuum Lamps..... A

b. Gas-filled Lamps. 4

3. Spectroradiometer. 4

a. Fore-optics... 4

b. Monochromator 24

5

25

Iv. MEASUREMENT OF INSTRUMENT AND SOURCE PARAMETERS. . .-6
1. Spectral Responsivity Function 6

2. Linearity of Response a)

3. Polarization 27

4. Size of Source .8

Vv. PROCESS OF REALIZATION AND TRANSFER........ 0.00 e eee cee eee ne ene 8
VI. DATA ANALYSIS AND UNCERTAINTIES a)
1. Temperature Values... a)

2. Uncertainties 9

3. Quality Control 1

VII. OPTICAL PYROMETER CALIBRATIONS......... 0s cece cece eee reece eee 11
VIII. PHOTOELECTRIC PYROMETER DEVELOPMENT...........- 0: eect eee re ences 13
REFERENCES .. 2... cee ee eee eee teen ee eee tenet eee 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


Appendix C.

A.

Detailed Procedures for Routine Radiance Temperature

Calibrations

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.

b.

c.

the lamp envelope is clear.

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

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.

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 approx-
imately 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.

Position and orient the lamp so that the Helium Neon
laser beam defining the optical axis of the spectro-
radiometer (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.

Make a small dot with a marking pen on the rear of the
lamp envelope at the point where the laser beam strikes.


{1]

[2]

(3)

[4]

[5]

[6]

(7]

(8]

(9]

[10]

{11]

(12]

REFERENCES,

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.

The International Practical Temperature Scale of 1968, Metrologia
5(2) 35-44. April 1969.

Supplementary Information for the IPTS-68 and the EPT-76,1983,
Committe’ Consultatif de Thermometrie, Bureau Internationale Des

Poids Et Mesures Monographie; Pavillon de Breteuil, F-92310, Sevres,
France.

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.

Walker, J.H., Saunders, R.D. and Hattenburg, A.T. Spectral
Radiance Calibrations at NBS. NBS Spec. Publ. 250-1.

Walker, J.H., Saunders, R.D. and Hattenburg, A.T. The NBS Scale of
Spectral Radiance (submitted to Metrologia).

Walker, J.H., Saunders, R.D., Jackson, J.K. and McSparron, D.A.
Spectral Irradiance Calibrations at NBS. NBS Spec. Publ. 250-20.

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.

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.

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.

Kostkowski,H.J. The Relative Spectral Responsivity and Slit-
Scattering 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.

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

15


LAMP STANDARDS OF RADIANCE TEMPERATURE
1984

I. Introduction and Lamp 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 (IPTS-
68) [1] 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 III.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


won WRSO63 Appendix A.

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

LAMP STANDARD OF RADIANCE TEMPERATURE

Requested by

(See your Purchase Order No. ‘dated

aterial

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 17009C. The calibration was performed at a room
temperature of 23°C. For radiance temperatures below 1500°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


» ot

*

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.

ee
+e eH HH

RIK

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.*

C-2


TABLE I

Summary of Estimated Uncertainties in Degrees C

Source Temperature (°C)

of

Uncertainty 800 1100 1400 1800 2300
Signal Ratio Mg (r) 0.06 0.10 0.15 0.23 0.36
Signal Ratio My (r) 0.06 0.10 0.15 0.23 0.36
Signal Ratio Mg (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) 0.05 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 30 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


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-optics. Sources 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 hori-
zontal 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


of the model represented by the Mueller matrix of eq. (la), and the
parameter B characterizes the spectroradiometer. Since the factor Rolo

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 (8a)

where, as always, we have neglected circular polarization. py, and pg are
given by py = p-cos2y and pj = p:sin2y, where y is the direction of
polarization measured from the reference direction of the radiometer and p
is the degree of polarization,

p = (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 = RoLo(1+Bp1) (10a)

and to 24 equations:
V(y) = RoLo[(stBdG) + (dC+BeC2+B£)py + (dS+BgSC)p2]. (lla)

Aside from the factor Rolo which is common to all equations and cancels
out, the only unknowns in these 25 equations are p, and pg 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


{1]

[2]

[3]

[4]

[5]

[6]

{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, “High-
Accuracy 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).

A-8


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 strip-
filament 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,

Ly = 438/dA-cosd+dw-da qd)

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,
w is the solid angle about that direction, and \ is the wavelength. The
relation between spectral radiance, wavelength and temperature is given by
Planck's Law,

Ly = e4/m-d9-[exp(eg/A:T) - 1] (2)

where cy and cg are the first and second radiation constants, \ is the
wavelength in vacuo, and T is the temperature. The defining equation for
the IPTS above 1337.58°K is therefore

x = L\(T)/L\(Tau) = [exp(c2/A-Tau) - 1)]/[exp(e2/a-T) -1] (3)

where L)(T) and L)(Tay) are the spectral radiances of the two blackbodies
at temperatures T and Tay, Tay is the temperature of freezing gold defined
as 1337.58°K, x is their ratio, and cg 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.


TABLE 1.

FIGURE 1

FIGURE 2

LIST OF TABLES AND FIGURES

+12

vi


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


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REPORT OF CALIBRATION

TABLE I

RADIANCE TEMPERATURE AT 654.6 nm

Degrees C
CIPTS-68)

600
900
1000
1100

1200
1300
1400
1500

1600
1700
1800
1900

2000
2100
2200
2300

NBS Test No. 534/

October

1985

LAMP

Direct Current
Camperes)

10.3903
11.764
12.806
14.054

15.508
17.153
18.965
20.922

23.008
25.211
27.510
29.913

32.403
34.988
37.660
40.437

LAMP


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


IV. MEASUREMENT OF INSTRUMENT AND SOURCE PARAMETERS

1. Spectral Responsivity 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 respons-
ivity factor and the slit-scattering function, where the responsivity fac-
tor depends only upon the wavelength of the observed flux and the slit-
scattering 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 107° at 3 nm, 10-4 at 15
nm, and 10°’ at 70 nm from the central wavelength. At 150 nm from the
central wavelength, the value decreases to 10°8 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 least-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


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 FASCAL 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.


P= dc ec2+p = gsc 0 (2a)
ds x x x
0 x x x

where C = cos 2(p - $9), S = sin 2( - $0) and the parameters s, d, e, $,
g, and $9 are obtained by least-squares fitting of the measurements. The
angle \ 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 kj, and kg, we have s=[kj+k9]/2 and d=[k,-ky]/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

1
L =1ly9 | 0 (3a)
0

(0)

The various measurements then lead to equations of the following
forms.

No polarizer: V=R- L= Rolo (4a)
Polarizer #1: V(>) =R - P - L = Rolo(s+BdC) (Sa)
Both polarizers: V(p,f’) =R- P+ P!- L (6a)

= RoLo(ss’+Bs'dG+dd’ CC’+Bd’$C' +dd’ 88’ +Bd’ gSCS’+Bd’ eC2C’ )
Polarizer #2: V(p') =R- P! - L = Rolo(s’+Bd'C’) (7a)

where C'= cos 2($’ - $9’), S' = sin 2(p' - $9’), and V_ 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, $, g, B, $0

and bo! are then obtained by a non-linear least squares solution. As far
as polarization is concerned the parameters ss, d, e, >, g, and $9
completely characterize the first polarizer, at least within the accuracy

B-2


Appendix 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 orientatioyg 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 B00 (la)

where Rg 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


x = fLy(T)-Ry-dd / fly (Tay) “Ry dd (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 variable-
Lemperature 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. Traditionally 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


{13]

[14]

(15)

Saunders, R.D. and Shumaker,J.B. Automated Radiometric Linearity
Tester. Appl. Opt. 23(20): 3504-3506; 1984 October 15.

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.

Kostkowski, H.J. and Lee, R.D. Theory and Methods of Optical
Pyrometry, NBS Monograph 41, 28 p. March 1962.

16


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.

nm. 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.

oO. Translate the lamp vertically until the filament notch is
vertically centered on the slit image.

Pp. 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 8B). 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 "PLMPCALI.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.

ce. 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.
C. 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.1" to split the main data file into a

C-6


International Practical Temperature Scale of 1968 (IPTS-68) [1].
Measurements are made with a spectroradiometer [2] incorporated
into the NBS Facility for Automated Spectral Calibrations
(FASCAL) [3]. The photomultiplier-amplifier combination used
was determined to be linear to better than 0.1% over a
photomultiplier 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
when the lamp has been aligned to a specified orientation while
operating at 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 base-
down, 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. The alignment is done with the lamp operating at 1700 *C
for a gas lamp or at 1200 °C for a vacuum 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.

The above described alignment is performed only at one
temperature. At all other temperatures small vertical and/or
horizontal translational adjustments are made so that the target
area viewed is always centered on the lamp filament at the height
of the notch No additional rotational alignments are performed.

The solid angle used is a rectangular pyramid whose longer
dimension is vertical. The size of the solid angle is detined by

A-6


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 tungsten-
strip 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.


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 calibra-
tion 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,

Ly = (c4/m)+A75-exp(-c9/A°T) (6)

to express the spectral radiance of a blackbody. With this approximation,
Eqn. 3 becomes


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


PREFACE

The calibration and related measurement services of the Natiqnal 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
instrument ation 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

iii


He

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 mn.

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

*
*

+e He
eee eH HO

c-4


1)

2)

3)

4)

S>

6)

7)

REPORT OF CALIBRATION

SOURCE OF UNCERTAINTY

Calibration of Pyrometer
Lamp relative to the
International Practical
Temperature Scale (IPTS-68)

Temperature determination of
Test Lamp (3 x Standard

TABLE II

LAMP

Summary of Estimated Uncertsinties in Degrees C

Deviation of Mean)

Polarization effects

Current measurement

Wavelength measurenent

Lamp alignment

Teaperature acale

Total estinated uncertainty
relative to S.I. units Geo)

(

eaquare root of the sum of
the aquares of the indi-

vidual uncertainties

CAUTION:

NBS Test No.

October

)

800

0.14

0.27

0.04
0.29
0.03
0.13

0.26

TEMPERATURE (Degrees C)

1100

0.18

0.03

0.21

0.42

1400

0.12

0.01

0.32

0.62

1800

0.03

0.06

0.49

0.96

0.07

0.15

0.76

1.48

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).

534/
1985

An“


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


Equation (10a) gives the relationship between the spectral radiance of
a source, Lo, 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 = L§LV/VS] [ (1+Bp$)/(1+Bp1) J (12a)

where L§ is the known spectral radiance of the standard, V/VS is the
measured signal ratio, B is the radiometer polarizance, and pf and pj
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+Bp1)

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 +0.001 in
the source relative Stokes components pj, and pa.


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 (5).

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. This calibration is performed starting at the
highest current setting and then lowering the current for each

daiti 1

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].

A7


proportional to radiant flux.

The measured instrument response is linear to within +0.2% for a
vange of plholomulliplier 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 gas-
filled 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 y, and the degree of
polarization

P = (Lmax - Lmin) / (Lmax + bmin) (5)

where LIpax and Lpjn are the 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


_x = exp(c2/dTay)/exp(eg/d-T) = exp[(c2/A)*(1/Tau - 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

T= [otaw - (O/eg) -In[s-8-£ MoM -M/C148-01))] (8)

where the definitions of the quantities and their estimated 30
uncertainties are:

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

My, signal ratio first vacuum lamp vs. 1530°K lamp (0.12%)
Mj, 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%)

Tau, IPTS-68 temperature of freezing gold (0.4°K)

eg, second radiation constant (0.00014 cm-°R)

B, spectroradiometer polarizance (0.05%)

pi, lamp polarization component (0.2%)

, 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 (25/3.15°K)
point due to the 0.4°K uncertainty in Tay is

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 Mo, Mj, and Mj arise from errors in the
wavelength cetting 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(Ag,) values,
using the known spectral radiance distribution of the GPBB and an
approximate test lamp distribution derived from spectral scans of such

10