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 high-powered laser is spectrally scanned by the spectroradiometer, with
the slit widths set at the 0.6 mm width used in the seale 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. For a
647 nm krypton laser, the function is nearly triangular in shape with a
width at half-height of 2.5 nm. Relative to the peak value, the measured
values decrease to about 10°? at 3 nm, 107 at 15 nm, and 10°” at 70 nm
from the central wavelength. At 150 nm from the central wavelength, the

8 in the short-wavelength wing and to 10°? in the

value decreases to 10°
long-wavelength wing. Scans with 488 nm, 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 [11].

The measurement at 647 nm yields the slit-scattering function at 654.6
nm, where the spectral distribution mismatch of the two blackbodies
requires am accurate determination of the relative responsivity function.
However, the measurements in the visible cannot be applied with confidence
to the short-wavelength region, since the dispersion varies by about a
factor of 24. For this region, the central portion and near wings of the
slit-scattering function are determined by scans of a spectral line
discharge source, and values in the distant long-wavelength wing are
deduced from a measurement of the integrated spectrally-scattered
radiation. With the wavelength set at a selected value in the 200 to 250 nm
region, the signal from a calibrated lamp (radiance temperature 2475 K at
654.6 nm) is recorded. A glass filter which blocks all radiation in the
vicinity of the wavelength setting and passes about 90% of the radiation at
longer wavelengths is inserted into the beam. The ratio of signals with and
without filter is taken as the fractional contribution of spectrally
scattered radiation to the signal. A second (identical) filter is added to
insure that only scattered light is being observed in the filtered beam.

Results with filters of different cut-off wavelengths (Corning filters CS

14


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REPORT OF CALIBRATION LAMP Q93
Department of The Navy

LAN? NO. 093 TAMEDA = 654.6. 01
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NBS Test No. 534/G-44928-2
September 23, 1985


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REPORT OF CALIBRATION LAMP Q93
Department of The Navy

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NBS Test No. 534/G-44928-2
September 23, 1985


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References

“The International Practical Temperature Scale of 1968",
Metrologia, Vol. 5, p. 35-44 (1969).

Kostkowski, H.J., D.E. Eriminy, 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.

Nicodemus, F.E., H.J. Kostkowski, and A.T. Hattenburg, Self-Study
Manual on Optical Radiation Measuremente: Part I -- Concepts,
Chapters 1 to 3, Nat. Bur. Stand. (U.S.), Tech Note 910-1, p. 52
(March 1976).

Blevin, W.R., “Corrections in Optical Pyrometry and Photometry for
the Refractive Index of Air", Metrologia, Vol. 8, p. 146-147
(1972).

Coleman, c.c., W.R. Bozman, and W.F. Meggers, Table of
Wavenumbers, Vols. 1 and 2, Nat. Bur. Stand. (U.S.), Monograph =
(May 1960).

Saunders, R.D. and J.B. Shumaker, “An Automated Radiometric
Linearity Tester", (submitted to Applied Optics).


of a high density graphite tube, which is resistively heated in an, argon
atmosphere. Electric current is supplied to the graphite tube through
water-cooled electrical connections at each end. The tube is surrounded by
a double-walled graphite radiation shield, with carbon black fill between
the walls. This assembly is surrounded by a water-cooled metal housing,
with an observation port which can be sealed during evacuation of the
atmosphere within the housing prior to flushing with argon. A window is
provided at the top of the housing for visual pyrometer observation of the
temperatures along the tube interior. A second window at the rear of the
housing allows radiation from the rear wall of the graphite tube to fall on
a silicon photodiode, whose signal provides automatic control of the
saturable-reactor power supply for the tube. A germanium photodiode, whose
response extends further into the infrared region, replaces the silicon
cell for operation at low temperatures. The blackbody mounting provides
adjustment in two angular and three translational degrees of freedom,
allowing for precise positioning and radiometric scanning over the target

area and the beam solid angle.

The graphite tube is about 200 mm long, with an inner diameter of abouc
11 mm. The outer surface is tapered to improve temperature uniformity along
its length. The wall is about 4 mm thick at mid-length. A 2 mm diameter
hole in the wall at this point allows for observation of the emitted flux.
The tube is partitioned into small cylindrical sections by a series of thin
graphite disks separated by thin graphite cylinders located at intervals
along the bore. Holes in the graphite disks permit measurement of the
temperatures in the middle and upper sections with a visual pyrometer. The
holes vary in diameter from 6 mm for the uppermost disk to 0.75 mm for the
disk below the central section. The central cylindrical section, which
provides the observed flux, is 9 mm high and has a 10 mm diameter. The wall
is threaded to reduce the partial reflectivity. Figure 3 shows a cross-

sectional view of the central section.

The blackbody emissivity has been assessed by measurements of its
solid angle subtended by the cavity opening, partial reflectivity of the
graphite material [6], temperature gradients, and absorption by gases [2].

The solid angle subtended at the rear wall of the cavity by the inner edge


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:

L, = LGLV/V"] (1+Bp3)/(1+Bp, )] (12a)
where Ls is the known spectral radiance of the standard, V/V" is the
measured signal ratio, B is the radiometer polarizance, and Py and p,
are the relative Stokes components measured as described above for the
standard and for the unknown source. (For a blackbody standard,
presumably, Py = 0.) hie shows that the presence of polarization

introduces the extra factor
Ss.
(1+Bp,)/(1+Bp, )

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 rela-

tive Stokes components Py and Py:


The various measurements then lead to equations of the following

forms.

No polarizer: V-R-+-L=R UL) (4a)
Polarizer #1: V(¢) -R- P-L = RL (s+BdC) Ga)
Both polarizers: V(¢,¢’')=R-+P-P'- L (6a)

- RL, (ss! +Bs! dCtdd’ CG’ 4+Bd! £6! +dd’ SS!-+Ba’ gSCS‘+Bd' eC")
Polarizer #2: V(¢') =R- P'- L= RyL, (s'+Bd'C") (7a)

where C'= cos 2(¢' - $9)» S' = sin 2(¢' - $y)» 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’, 4d, d’, e, f, g, B, by
and $' are then obtained by a non-linear least squares solution. As far

as polarization is concerned the parameters s, d, e, f£, g, and $0
completely characterize the first polarizer, at least within the accuracy

of the model represented by the Mueller matrix of eq. (10), and the

parameter B characterizes the spectroradiometer. Since the factor RL
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


Turn on the lamp and let it stabilize for 20 minutes.

JHE AAAI IAAI
* CAUTION *
FEI AE TTA
* 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.
EE et cet terre reer rere

Use a plumb line to make the notch 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 mono—
chromator 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 par-—
allel 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 along side it 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.


i.
i.

o.
p-

Throw 208V breaker on the wall behind the spectrorad—

iometer to ON position (Up).

Depress yellow button on C.A.T. control unit until

meter reads about 30 and check that the top of the

blackbody housing is cool and that the cooling water

is flowing. (If not flowing, depress red button and

return meter reading to zero.)

Let blackbody run at this level for 5-10 minutes.

Check for wanted temperature on the "T(BB) vs. O/P

Current Read" plot and depress the yellow and red

buttons until the O/P Current Read meter at the

blackbody displays the wanted setting.

Allow about 10 minutes for the blackbody to stabilize

at this setting.

Set the null detector to the 10 millivolt range and

adjust the potentiometer setting until the monitor

detector signal is nulled to zero.

Push the "AUTO" button to go into the automatic contra?

mode.

Turn the null detector scale sensitivity switch

counterclockwise until oscillation occurs and then

back off one scale setting.

Allow 5-10 minutes for the blackbody to stabilize.

When ready to view the blackbody with the spectrorad—

iometer

1) Adjust the Argon flow valve on the green line so
that its flow gauge reads 8 CFH.

2) Adjust the Argon flow valve on the copper line so
that its flow gauge reads 4 CFH.

3) Open the blackbody window (turn counterclockwise).

Standard Lamp and Test Lamp Turn On and Alignment

Perform as described in A.5.

Adjust Blackbody Level

The blackbody level is adjusted so that its output

approximately matches the output of the test lamps at

654.6 nm.

The adjusting procedure is the same as described

in C.1i. j. toa.

Blackbody Alignment

Once the blackbody has stabilized at its operating

temperature it is aligned as follows:


Appendix C. 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 tits 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 polari-
zer similar to the first. The integrating sphere is mounted in the radio-
meter 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 orientation 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= n,|2 8 0 o| (la)

where R) is the ordinary responsivity neglecting polarization and we take


PROCESS OF REALIZATION AND TRANSFER

The process of realization and transfer consists of the temperature
determination of the variable temperature blackbody (VTBB) at a reference
wavelength, a spectral radiance comparison of the VIBB and the gas-filled
test lamps at chosen calibration wavelengths, and a redetermination of the

VIBB temperature.

The temperature determination of the VIBB is composed of three steps.
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 VIBB. 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 blackbodies, from which the spectral radiance and thus
the temperature of the VIBB can be evaluated from eqns. 5 and 2. The
polarization and the spectral radiance distributions over w, A, and. 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 lamp parameters remain constant between comparisons
with the gold-point blackbody (GPBB) and with the VIBB. 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, the vacuum lamps are compared with each other
weekly during experiments, and the 1530 K lamp is compared to the VIBB
before and after each set of comparisons of the VIBB to the gas-filled
lamps. The use of the first vacuum 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).

In principle, the three comparisons comprising the VIBB temperature

18


LAMP STANDARDS GF SPECTRAL RADIANCE
1984

I. Introduction and Lamp Description

Standards of spectral radiance are issued by the National Bureau
of Standards to cover the spectral range 225 nm to 2400 nm. Gas-filled
tungsten filament lamps (General Electric type 30/T24/13 with quartz
windows and mogul bipost bases) are calibrated by direct comparison
with a high quality blackbody. The blackbody temperature is determined
in terms of the International Practical Temperature Scale of 1968
CIPTS-68) C13. for each of the 34 wavelengths for which spectral

radiance values are reported. The calibration includes a detailed
characterization of the radiation field, and the degree of
polarization). Such characterization measurements are performed at

approximately 655 nm.

II. Preparation and Screening

At the beginning of testing, an identification number is painted
on the rear surface (the side opposite the quartz window) of each lamp
envelope. All test lamps are annealed at a brightness temperature of
about 2620 K at 655 nm for 2 to 3 hours on direct current (the longer
filament support post at positive potential). @ calibration current is
then chosen to produce a brightness temperature of about 2480 K. Drift
rates are determined by monitoring the spectral radiance at 654.4 am
and 350 nm for a 20 hour period. Lamps exhibiting a drift rate of less
than 0.04% per hour at 654.6 nm are selected for calibration. The
measured drift rates for individual lamps are given in the Report of
Calibration.

Spectral radiance calibrations are performed for a rectangular
target area 0.6 mmwide by 0.8 mm high which is centered on the
filament at the height of the notch. Angular orientation of the target
area is) determined from goniometric scans about three mutually
perpendicular axes centered on the target area. The lamp orientation
chosen for the calibration is one which minimizes the variation in lamp
output while maintaining the optical axis of the spectroradiometer
approximately normal to the filament and passing approximately through
the center of the quartz window. An arrow is etched on the rear surface
of the lamp envelope to allow reproducible alignment of this orientation
(see section III.B below). Detailed mappings of the lamp output are
performed and the results given in the Report of Calibration (see
section IV below).

For lamps of the type used for this calibration, the radiation
from the target area as viewed through the quartz window has been found
to be slightly polarized. At 655 nm, the degree of polarization, P,
defined as


TABLE OF CONTENTS

INTRODUCTION...... eee eee teen eee e eee e ett eee eee eeeee teeeeeel
SCALE DERIVATION AND TRANSFER: A GENERAL DESCRIPTION............-0- eee eee 2

MEASUREMENT APPARATUS... 2.2... cece cece eee eee eee teen ener e tenn nnee
1. Gold Point Blackbody...
2. Variable Temperature Blackbody
3. Lamp Sources

a. Vacuum Lamps....
b. Gas-filled Lamps
4, Spectroradiometer..
a. Fore-optics..
b. Monochromator...
c. Detectors....... see
5. Control and Data Acquisition System.

MEASUREMENT OF INSTRUMENT AND SOURCE PARAMETERS.
Spectral Responsivity Function
Linearity of Response...
Polarization...
Size of Source

PROCESS OF REALIZATION AND TRANSFER.
Scale Stability

DATA ANALYSIS AND UNCERTAINTIES.
Uncertainty Estimation

CONCLUSION... 0... ccc cece eee eee ea ate e nent ee es we 23

REFERENCES .

APPENDIX A: Report of Calibration and Lamp Standards
of Spectral Radiance 1984

APPENDIX B: Fascal Description in Optical Radiation News
APPENDIX C: Measurement of Instrument and Source Polarization

APPENDIX D: Detailed Procedures for Routine Spectral
Radiance Calibrations


REPORT OF CALIBRATION LAMP @93
Department of The Navy
TABLE I

Measured Values of Spectral Radiance and Blackbody Temperature
for 893 at 40.370 amperes

Wavelength in air Spectral Radiance Blackbody Temperature
(nm) (WW ocm7> sr-*) K CIPTS-68)
225 7: 150E+00 2654.2
230 1.073E+01 2653.3
240 2.286E+01 2651.68
280 4.542Es01 2650.2
260 8. S13E+01 8
270 1.499E+02 2646.3
280 2.518E+02 2643.6
290 4.061E+02 2641.4
300 6.268E+02 2638.2
325 1.607E+03 2629.4
350 3.490E+03 2619.9
375 6.633E+03 2609.5
400 1.136E+04 2598.5
aso 2.626E+04 2575.4
475 3.649E+04 2563.6
500 4.834E+04 2551.6
5s0 7.570E+04 2526.9
375 9.049E+04 2514.4
600 1.054E+05 2501.6
654.6 1.373E+05 2473.4
675 1.486E+05 2463.1
700 1.614E+05 2450.2
750 1.836E+05 2423.6
600 2.003E+05 2396.2
900 2.185E+05 2340.6

1050 2.147E+05 2234.4
1200 1.919E405 2169.3
1300 1.719E+05 2114.0
1550 1.241E+05 1981.0
1700 1.002E+05 1906.2
2000 6.507E+04 1770.0
2100 5.641E+04 1727.9
2300 @.204E+04 1642.9
2400 3.709E+04 1610.7

NBS Test No. 534/G6-44928-2
September 23, 1985


0-56 and CS 0-52) both indicate an integrated scattered light contribution
of less than 0.2% at 225 nm. The slit scattering function calculated from
this result and the known source distributions and responsivity factor is
less than 10° at wavelengths greater than 200 nm from the central

wavelength, in good agreement with the values measured in the visible.

Linearity of Response

The degree of linearity of the spectroradiometer response is determined
with an automated beam conjoiner [12]. 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
contributions 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 micro-computer 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 proportional to radiant
flux.

The response function of the spectroradiometer is dependent upon the
detector-amplifier employed. With the photomultiplier tube in place
(spectral range 200 to 850 nm), the instrument response at all wavelengths
is linear to within 0.2% for a range of 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 correction
increases rapidly, rising to 3% at 7 microamperes. The anode current is
restricted to less than 500 nanoamperes during measurements by selection of
appropiate tube voltage. Correction factors for the amplifier ranges are
determined from measurement of a known electrical current and combined with
the .linearity correction factor. Linearity tests of two PbS detectors
resulted in a correction factor which is a linear function of the signal
over the range 1 to 280 millivolts. The correction varies from 0.1% at 3 mv

to about 9% at 300 mv. To avoid reliance on large corrections, the sources

15


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 of 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 spectral radiance, an approximate equation for the complete
measurement process can be developed by using the Wien approximation to
Eqn. 2,

Ly = (e,/x)-27? -exp(-¢5/4-T) (10)

The signal ratio of the VIBB to the GPBB at the reference wavelength can
then be expressed as

Sy/Sa. = exPl(Co/A,)C/Ta - VT] > qi)
and the spectral radiance of the test lamp at a calibration wavelength as
-5
Ly = (S;/Sy) “Ly = (S,/Sy) + (ey /m)-A ~ -expl-c4/(A-TY)] (12)

where ly is the spectral radiance of the VIBB. Solving Eqn. 11 for Ty,
substituting this expression into Eqn. 12, inserting the various correction

factors heretofore omitted, and rearranging, we have

APr
L = Jeq:ep'¥,/048-p))| [e,/[x-9° -expley/O-T,,)]] [s_-£,-M,/¢] r (13)

where the definitions of the quantities and their estimated uncertainties

are:

21


indifferent radiometer when an ideal linear polarizer is rotated in the
source beam. The correction factor required to reduce the signal ratio of
Eqn. 8 to a value which would be obtained with a polarization-indifferent
radiometer is

(+ Bp.)

where p, = p-cos2y and B is the spectroradiometer polarizance (degree of
linear polarization introduced by the spectroradiometer). For the
spectroradiometer employed here, the polarizance varies from zero at 225 nm
to 0.26 at 655 nm and 0.85 at 2400nm. 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 .0005 in the visible and .001
in the extremes of the wavelength range. The lamp polarization
uncertainties are estimated as 0.002 in the visible and 0.003 at cxtreme

wavelengths. A more detailed description of the measurement process and

data analysis is given in Appendix C.

Size of Source

The "size of source" effect (signal contribution due to flux which
originates outside the target area ard 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
effect is measured at wavelengths of 654.6 and 350 nm, and values for other
wavelengths are estimated from the assumption of an inverse wavelength
dependence. The correction varies from 0.04% to 0.1% at 654.6 nm depending

upon the elapsed time since the last mirror recoating.

17


REPORT OF CALIBRATION LAMP @93
Department of The Navy

TABLE I!

Summary of Estimated Uncertainties in Percent of Spectral Radiance Values

SOURCE OF UNCERTAINTY 225 250 950 654.6 900 1550 2400
i) Blackbody quality 0.29 0.24 0.13 0.00 0.04 0.09 0.11
ty Calibration of Pyrometer 0.70 0.63 0.45 0.24 0.17 0.10: 0.07

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

3) Temperature determination 1.05 0.90 0.36 0.27 0.36 0.30 0.24
of blackbody and transfer
of blackbody spectral
tadiance to lamp (30)

4) Polarization effects 0.00 0.00 0.01 0.04 0.06 0.09 0.15
35> Current measurement 0.14 0.13 0.09 0.05 0.04 0.02 0.01
6) Wavelength measurement O.11 0.10 0.08 0.01 0.01 0.01 0.01
7) Temperature scale 1.43 1.28 0.93 0.49 0.40 0.20 0.13
Total estimated uncertainty 1.9 1.7 1.1 0.6 0.6 0.4 0.3

relative to 5.1. units

square root of the sum of
the squares of the indi-

vidual uncertainties

NBS Test No. 534/G6-44928-2
September 23, 1985


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. Detectors. Two interchangeable detectors are used to cover the wave-
length range of the spectroradiometer. For the 200 to 850 nm range, an end-
on 1l-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. The current is normally
restricted to a maximum of 500 nanoamperes, by selection of detector
applied voltage, to ensure linearity of response. A lead sulfide detector,
cooled to 240 K by a thermoelectric cooler, is used for the 800 to 2600 nm
range. The detector and the exit slit are placed at the foci of an
ellipsoidal mirror, which images the slit upon the detector with a
demagnification of 7. The detector output is amplified and converted to a
0-1 volt signal by a phase-sensitive lock-in voltmeter, which is keyed to a
78 hertz sector disk placed just before the plane mirror. The signal
from either detector-amplifier combination 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.

5. Control and Data Acquisition System.

The spectral radiance calibrations are carried out on the NBS
Facility for Automated Spectroradiometric Calibrations (FASCAL). After
initial alignment, the FASCAL system permits control of the entire meas-
urement process from a remote operator console. Component positions,
instrument settings, sequence of operations, and data collection are
effected by either stored computer programs, operator commands, or a combi-

nation of the two.

The system is directed by a microcomputer equipped with a CRT terminal
and keyboard, and a high-speed disk system for program and data storage. A

modular interface controller [9] provides the link between instruments and

12


position, and performs the calculations to determine the

ratios of the test lamp readings to the reference lamp

readings.)

a. Test lamps are set at a current to give them a radiance
temperature of about 2470 K at 654.6 nm (This current
is usually between 38 and 43 amperes.)

b. The test lamps are allowed to stabilize at this level
for about one hour.

c. The radiometric output of the test lamps is then
measured and compared to the radiometric output from a
highly stable reference lamp at 654.6 nm and also at
either 400 or 350 nm.

d. The reference lamp is turned off and the test lamps
are burned overnight (20 to 24 hours).

e. The reference lamp is turned back on and the radio-—
metric output from the test lamps is measured again
and compared to the output from the reference lamp at
the same two wavelengths.

*. The two ratios of each test lamp to the reference lamp
are used to determine the percent change in the output
of each test lamp and a drift rate is calculated.

Mapping Measurements

Translational and rotational mappings about the
aligned position are performed on each test lamp at
654.6 nm. The lamp mounts have been motorized so that
automated mappings can be performed. The computer program
used to perform the mapping measurements is "“RADMAPS.CRO".
The mappings performed are

a. Translation along the horizontal axis
b. Translation along the vertical axis
c. Rotation around the horizontal axis
d. Rotation around the vertical axis

e. Rotation around the optical axis

Mapping is not performed for focussing because spectral
radiance is not critical with respect to focus.

Polarization Measurements

Because the spectroradiometer has significant polar-—
ization (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 3). Two motorized


radiance variations over w and A generally will not be weighted equally by
the instrument responsivity, and the variation of spectral radiance over
wavelength for a non-blackbody source will not be a known function. The
errors due to the variations over w and A may ve minimized by making w and
A small, and treating Ly as a weighted average Qy) av. over these
parameters. If the two sources are operated so that their relative spectral
distributions are identical over the instrument bandpass, i.e. so that
(ly) = KD, (6)

av.

and (Ly av. = KD, (7)

where Dy is the relative spectral distribution and is the same for each

source, then [4]

n

2 = Ka Jay ROQ ADA Ky hy ay, (8)

1 Ry -Tay ROAD q Cy av.

na

and the ratio of weighted-average spectral radiances is equal to the signal
ratio. Any polarization of the non-blackbody source requires a correction

‘to the observed signal

These procedures are used in the realization and transfer of the NBS
scale of spectral radiance. Calibration of lamp standards at wavelengths as
low as 225 nm requires a blackbody temperature of about 2600 K, nearly
twice the temperature of the gold point blackbody. The resultant spectral
distribution mismatch occurring in the comparison of the two blackbodies
requires an accurate determination of the spectral responsivity function
that is used in eqn. 5. The transfer of spectral radiance from the
blackbody to another source, using Eqn. 8, requires small values of w and
A, selection of a source with minimal variation in these parameters and
minimal polarization, and close matching of the relative spectral
distributions of the two sources. The transfers at the shortest wavelengths
also require a low value for the long-wavelength wing of the spectral
responsivity function to avoid large spectral scattering contributions from

; 4a
the visible region, where the source spectral radiances are about 10° times


U.S. DEPARTMENT OF COMMERCE
(NATIONAL BUREAU OF STANDARDS

hens neem
Gaithersburg, MD 20899

REPORT OF CALIBRATION

LAMP STANDARD OF SPECTRAL RADIANCE
Requested by

Department of The Navy
Commanding Officer
Naval Rework Facility
North Island, San Diego, CA 92135

(See your Letter dated 10/16/84 signed by J. B. Mackinnon.)
1. Material

One gas-filled General Electric type 30/T24/13 tungsten ribbon
filament lamp has been supplied by NBS for this test and bears
the designation Q93.

2. Calibration

The lamp was calibrated using the equipment and procedures
described in the enclosed document, “Lamp Standards of Spectral
Radi ance-1984". The selection and preparation of test lamps are
described in paragraph II of this document. Note particulary
Paragraph _I1I.B which describes the orientation and operation of
the test lamp. The calibration was performed with the lamp
operating at a direct current of 40.370 amperes with the longer
filament support post at positive potential. The environmental
temperature was 26°C and the relative humidity was 55%.

3. Results

Table I gives the measured spectral radiances of the test lamp.
Table II gives the uncertainties of the calibration. Figures 1
through 5S show the results of the lamp mappings described in
Paragraph IV of the enclosed document.

The stability measurements at 654.6 nm and 350 nm described in
paragraph II of the enclosed document showed that the spectral
radiance of the test lamp changed at a rate of -—0.029% per hour
and -0.055% per hour, respectively. The relative change in
spectral radiance at other wavelengths is expected to be
approximately inversely proportional to the wavelength. It is
therefore recommended that the shortest wavelengths be
transferred to other lamps first.

NBS Test No. 534/6-44928-2
September 23, 1985


d. Run “FLANAL.1" to calculate the following statistics
1) Number of measurements at each wavelength
2) Range for repeat measurements
3) Mean value
4) Standard deviation of the mean
and to store the wavelength, mean spectral radiance,
and mean blackbody temperature in a data file.

e. Run “DELRAD.1" to calculate and store the percent
difference at each wavelength between the spectral
radiance of the test lamp and the spectral radiance
of a reference lamp measured several years ago.

f. Run "PLOT" to plot the percent difference as a function
of wavelength on the HP plotter.

g- The plot should form a smooth curve to about plus or
minus 0.5% if the test data is good. Here is an

example:
Q77 % DIFFERENCE PLOT
ow
SS 18.8
° 8.8
= .
S 6.8
& 48
Es 2.6 "855 go 00 oo
a 6.644 T T 7, T 1
268 «6648 «61888 «1528 «1968 9488
ae

WAVELENGTH (nom)

Analysis of Precision Data

The computer program ."RAD‘SD.RED" is used to calculate
the the overall precision the source comparisons. This re-
sult is used in combination with the repeat measurement
standard deviation of the mean in order to derive an over-—
all uncertainty in the temperature determination of the
blackbody and the transfer of the blackbody spectral rad-
iance to a test lamp.


TABLE I

SPECTRAL RADIANCE UNCERTAINTY (%)

Source Wavelength (nm)
of

Error 225 250 350 655 900 1550 2400
Thu (s) 1.43 1.29 0.92 0.49 0.36 0.21 0.13
M (r) 0.60 0.52 0.37 0.23 0.20 0.11 0.12
My (r) 1.27, 0.61 0.46 0.19 0.22 0.17 0.16
Ss) (s) 0.29 0.26 0.19 0.10 0.07 0.04 0.03
8) (s) 0.10 0.10 0.10 0.10 0.10 0.10 0.10
f, (s) 0.29 0.26 0.19 0.10 0.07 0.04 0.03
d (s) 0.10 0.10 0.10 0.10 0.10 0.10 0.10
& (s) 0.19 0.16 0.09 0.00 0.03 0.06 0.07
fy (s) 0.15 0.13 0.10 0.05 0.04 0.02 0.01
Py (s) 0.01 0.01 0.03 0.05 0.06 0.17 0.25
AL setting (r) 0.03 0.04 0.03 0.01 0.02 0.01 0.01
d setting (r) 0.24 0.22 0.16 0.08 0.06 0.03 0.01
Spectral Resp. (s) 0.09 0.08 0.06 0.03 0.02 0.02 0.01
Lamp currents:

Quinn-Lee (r) 0.12 0.11 0.08 0.04 0.03 0.02 0.01

1530 K (r) 0.06 06 0.04 0.02 0.02 0.01 01

Test (r) 0.11 -10 0.07 0.04 0.03 0.02 0.01
B, cy (Ss) wee eee eee neee (less than 0.001%) ---------------
Total 30 Uncertainty (%) 2.09 1.61 1.16 0.62 0.51 0.38 0.39

(square root of

sum of squares)

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

24


INTRODUCTION

Spectral radiance, denoted L, is defined as the radiant flux at a given
point, direction, and wavelength per unit of projected area, solid angle,

and wavelength interval. Mathematically,

= a f(an-cosé-do-a) eo)
where @ is the radiant flux, A is the surface area at that point, @ is the
angle between the normal to the surface and the direction of propagation, w
is the solid angle about the direction subtended at A, and A is the
wavelength. Spectral radiance is the basic quantity for specifying the
distribution of radiant flux relative to position, direction, and
wavelength, and all other classical radiometric quantities may be derived
from it. The National Bureau of Standards (NBS) issues lamp standards of
spectral radiance calibrated at 34 wavelengths over the spectral range 225
to 2400 nm. These lamp standards are designated in NBS Special Publication
250 as items 39010c, 39020c, and 39030c (former designations 7.5b, 7.5c and
7.5d respectively).

The NBS scale of spectral radiance is based upon the radiant flux emit-
ted by a blackbody of known temperature. In 1960, Stair [1] developed tung-
sten strip lamp standards of spectral radiance in the 250 to 2600 nm spec-
tral range by comparison to blackbody sources, whose temperature was
measured with visual optical pyrometers. Estimated accuracies ranged from
8% at 250 nm to 3% at 2600 nm. In 1965, Kostkowski [2] developed more
accurate lamp standards in the 210 to 850 nm range, using a spectroradio-
meter which doubled as a photoelectric pyrometer. The pyrometer was
calibrated in accordance with the definition of the International Practical
Temperature Scale [3]. A variable temperature blackbody, operable at
temperatures up to about 2700 K, was used as the comparison source, and all
radiometric matches were made between sources of nearly equal spectral
radiance (null measurement). The present NBS spectral radiance scale
derives from these procedures and instrumentation, but has undergone many
refinements: determination of the spectroradiometer response linearity

allows accurate comparison of unequal sources, the temperature of the


TABLE I.

FIGURE 1.

FIGURE 2.

FIGURE 3.

LIST OF TABLES AND FIGURES

Spectral Radiance Uncertainty (%)

Spectral Radiance Apparatus

Variable Temperature Blackbody Schematic

Central Section of Variable Temperature Blackbody


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. Additional details on the FASCAL system are given in Appendix B.

The spectroradiometer (fore-optics, monochromator, and detectors), a
closed-circuit TV camera, and a photoelectric pyrometer 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. The pyrometer is used
for the initial setting of the variable temperature blackbody to its
approximate temperature.

MEASUREMENT OF_INSTRUMI D_S! CEP. [ETERS

Spectral Responsivity Function

The relative spectral responsivity function of the spectroradiometer is
determined by the indirect method [10]. 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.

13


where R(A,,A) is the spectral responsivity function of the
spectroradiometer and depends on the instrument wavelength setting ro and
on the wavelength A of the measured radiation, Ly is the spectral radiance
of the source, cos#-dw is an element of projected solid angle, and dA is an
element of source target area. In general, both R(A),4) and L, vary with w,
A, 4, and state of polarization. Each element of flux is weighted by the
corresponding value of the responsivity. Thus the indicated spectral
radiance is an instrument-weighted average which may vary from instrument
to instrument, and RQ, A) must be determined over the spectral range of

the spectroradiometer.

When the sources being compared are both blackbodies, the spectral
radiance is unpolarized and is uniform over w and A. Thus the responsivity

weighting over these parameters is the same for both sources, so that

S= Ja Ly: [ahs R(g,A) -0088-do- aa] + ad (A)

and the ratio of signals is given by

sf, - Jukao%,°%//fateo 4% ©)

where R,Qo)) is the responsivity averaged over w and A (the bracketed

integral in Eqn. 4). When source 1 is the gold point blackbody, is

Ma
given by eqn 2, and the value of the integral in the denominator is
determined. By estimating a temperature for the second blackbody, and

inserting the corresponding expression for L, from eqn. 2 in the

2,2
integral in the numerator, the temperature which satisfies eqn. 5 can be

found by iteration.

If the sources being compared are not both blackbodies, the spectral


determination can be performed at any wavelength within the signal-to-noise
limitations of the instrument. At present, they are done at a wavelength
setting Xo of 654.6 nm (location of a convenient thorium line), where a
long history of vacuum lamp behavior has been accumulated, and where

comparisons with the IPTS temperature scale are facilitated.

The spectral radiance comparisons of the VIBB and the test lamps are
typically carried out on a group of lamps, each operated at its chosen
calibration current throughout the experiment. For the 800 to 2400 nm
spectral range the VIBB is set at various temperatures to yield spectral
radiances within 10% of those of the lamps, in order to avoid reliance upon
large linearity corrections. In the ultra-violet and visible spectral
region, the VIBB is operated at a temperature of about 2580 K. At this
temperature, the VIBB spectral radiance is about half that of the lamps at
225 nm and about twice that of the lamps at 800 nm, well within the linear
range of the photomultiplier detector. Under these conditions, the spectral
distribution similarity in the two sources over the central instrument
bandpass permits the use of Eqn. 8 to within 0.05%. Spectral scattering
from the distant wings of the slit-scattering function requires a
correction of about 0.1% for the comparisons below 250 nm. Spectral
radiance comparisons are made at 654.6 and 250 nm at the start and finish
of the full range of comparisons, to detect and correct for any effects of

lamp drift. Detailed procedures of the process are listed in Appendix D.

Seale Stability

The calibrations resulting from this process are derived in principle
from a complete realization of the spectral radiance 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 spectral radiance values of each group of calibrated lamps are

compared wavelength by wavelength to the values of earlier calibrations in

19


NBS MEASUREMENT SERVICES:
SPECTRAL RADIANCE CALIBRATIONS

James H. Walker
Robert D. Saunders
Albert T. Hattenburg

Center for Radiation Research
National Measurement Laboratory
National Bureau of Standards
Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
Issued January 1987


ot,

tous

Spticel Radiation (
Tews

NO, 18

Prepared by the staffs of the Optical Radiation and the Radiometric Physics Sections
of NBS toreport on items of interest in optical radiation measurements. Inquiries may
tbe directed to Robert L. Booker, B308 Metrology Bidg., NBS, Washington, D.C. 20234
(301) 921-3864.

November 1976

FASCAL

The Facility for Automated Spectroradiometric Calibrations (FASCAL), the most recent
and ambitious product of the Optical Radiation Section's automation efforts, has reached a
stage of development whereby routine spectral irradiance calibrations have been automati-
cally performed on this equipment since June.

Since there have been no previously published descriptions of this facility, its main
technical features will be described here in some detail along with a description of current
usage.

a | Light-tight Servopositioned (=r
Compartment t On Instrument Irts
Elliptical tt Cer Carriage Diaphragn
Motor _| PDS Detector] <—>
—_]
it TV Diagonal | |
wn) / |
Ped
a Double 2
sates rentcrontor ef
) 2
Detector Entrance =
--- Platform Slit —_e
Integrating Sphere: Le Diagonal 6" Spheric:
Chopper ‘Mirror Hirror
Radiance Black-
Source fo}
Stations —f body
4 1 Irradiance
U Source Station
Figure 1. OPTICAL ARRANGEMENT OF FASCAL SYSTEM

Top View


snjoinddy juewesnspay aouDIppDY |Dujoeds *; FyNoI4

AGOBNOVIE
AGOBYOWTE
son BenUvesGNaL ANIOd G09 |
Pav ISVA IN
are save ana

O

YONSIN

HOUUIN
Tvoldalids
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~ ONILYHD-WSId

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uoghd ea Sad NOUMIN

WOUdITIS



greater than in the short wavelength region. Both comparisons require a
high degree of response linearity in the spectroradiometer for the valid

determination of those signal ratios which depart significantly from unity.
MEASUREMENT APPARATUS

A block diagram of the measurement apparatus is shown in figure 1. The

principal components of the system are:
1. Gold Point Blackbody
2. Variable Temperature Blackbody

3. Lamp sources
a. Vacuum Lamps

b. Gas-filled Lamps

4. Spectroradiometer
a. Fore-optics
b. Monochromator

c. Detectors

5. Control and data acquisition system

1. Gold Point Blackbody.

The gold point blackbody was developed for the purpose of realizing
the International Practical Temperature Scale above the gold point at NBS,
and its construction and characterization have been detailed in reports of
this work [5]. The estimated emissivity is 0.99999. 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. Variable Temperature Blackbody

A schematic cross-section of the variable-temperature blackbody is

shown in figure 2. The blackbody cavity is located in the central portion


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.

i. Position a “blocking finger" on the 45 degree plane
mirror on the spectroradiometer. Rotate the arrow
2 degrees out from the notch and translate the lamp
so that the monochromator slit image can be viewed.

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

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, 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 l.

u. If the rotation or tilt was changed significantly, then
repeat steps m. tor.

B. Preliminary Measurements

1.

Stability Measurements

Stability measurements are made in order to determine
an approximate drift rate on each lamp. The measurements
are performed at 654.6 nm and at either 400 or 350 nm.

The computer program used to perform the stability meas—
urements is "RCSTAB2.LIN". (This program reads lamp
currents, sets wavelength, positions the spectroradiometer,
takes the detector readings for the lamps and a shutter


B. Conditions

The values of spectral radiance 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 spectral radiance apply when the lamp has reached
thermal stability at a specified operating current (about a one-hour
warm-up is sufficientTM) and has then been aligned to a specified
orientation. The test lamp is operated on direct current with the
longer filament support post at positive potential.

The lamp is oriented base down with the filament vertical and the
optical axis of the spectroradiometer passing through the quartz window
and intersecting the center of the filament at the height of the notch.
The exact location of the target area and rotational alignment of the
lamp are as follows.

The calibration is performed for a rectangular target area 0.46 mm
wide by 0.8 mm high. The height dimension of the target area is
approximately parallel to the sides of the lamp filament. The centroid
of the target area is located at the intersection of two lines, both of
which lie on the viewed filament surface. The lines are perpendicular
to each other, one line bisecting the filament along its length and the
other passing through the point centered at the mouth of the V-notch.

The centroid of the target area on the filament is viewed along

the horizontal axis of the spectroradiometer. The lamp is positioned
so that the long filament support is to the left and the etched arrow
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 axis through the centroid of
the target area and fiormal to the line of sight, and about the vertical
axis through the centroid of the target area, until the tip of the
arrowhead appears centered at the mouth of the filament’s V-notch.

The solid angle used is a rectangular pyramid whose longer
base dimension is vertical. The size of the solid angle is defined by
the vertex angles at the apex of the pyramid. The vertex angles are
0.125 radians (7.16 degrees) in the vertical plane and 0.0625 radians
(3.58 degrees) in the horizontal plane.

The bandpass of the spectroradiometer varies from 0.6 nm at 225 nm,
to 2.2 nm at 655 nm, to 1.6 nm at 2400 nm.

“For lamps that have not been burned for more’ than three weeks, a
warm-up time of at least two hours is recommended.


7]

[8]

[9]

[10]

{11]

[12]

[13]

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. (to be submitted for pub.)

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

26


The FASCAL facility incorporates all of the usual elements required for precision
temperature-based radiometry. They are configured in a linear arrangement, dominated by a
heavy truss bridge structure 7.3 m long supporting a pair of ground 14-inch diameter stain-
less steel raile. These rails were carefully aligned by a laser beam to within +.03 mm
tolerance in straightness and parallelism. Riding the rails on linear ball bushings is an
instrument carriage in the form of a rectangular box 240 cm long, 68 cm wide and 50 cm in
height. All optical elements, dispersers and detectors are mounted within the carriage and
can be positioned along the track to intercept the radiation from various radiation sources
placed at stationary points alongside the rail structure. There are twenty-five available
source mounting stations spaced along the length of the structure.

The instrument carriage is driven through roller chains by a digital servo-positioning
system and may be automatically positioned with a resolution of 0.1 mm and a repeatability
of 0.01 mm. Positioning speed is sufficiently rapid that an average move between two sta-
tions takes only about two seconds. The actual carriage position is tracked by an incre-
mental rotary encoder which has a digital visual display. The digital position information
is also entered into a minicomputer. There is a unique central position, designated the
“home” position, to which the carriage may be driven at any desired time to verify the func-
tioning of the incremental encoder and to reference it to an absolute scale.

The instrument carriage is divided into two compartments. One is light-tight and
houses detectors, and the other contains optics and associated electronics. The latter
compartment is almost entirely enclosed to minimize dust collection and scattered light.
The optical arrangement of the instrument carriage is shown schematically in Figure 1.

The dispersing element is a prism-grating double monochromator, mounted so that the exit
slit is within the light-tight detector compartment. Monochromatic radiation from the exit
slit can be directed to either of two detectors on a detector platform that can be posi-
tioned remotely. Radfation in the visible and ultraviolet is detected by a thermoelectri-
cally cooled photomultiplier having an extended S-20 response, while infrared radiation is
detected by a thermoelectrically cooled PbS detector placed at the focus of an elliptical
mirror. The wavelength of the monochromator may be remotely set by a high-speed stepping
motor with a resolution of .025 nm through a range of 200 nm to 2500 nm. An absolute
encoder having .025 nm resolution permits the actual wavelength setting to be visually
displayed and entered into the computer. A He-Ne laser, also mounted on the detector
platform, can be automatically positioned so that its beam passes through the monochromator
and optics in the reverse direction to facilitate alignment of optics and sources.

The fore-optics are arranged to enable measurements to be made on either radiance or
irradiance sources. Radiance sources are focused on the entrance slit by a 6-inch diameter
spherical mirror with the optical path being turned approximately 90° by a flat diagonal
mirror. The spherical mirror is rotated a few degrees for irradiance measurements. This
allows the exit aperture of a small integrating sphere to be imaged on the entrance slit
of the monochromator, Irradiance sources are placed 50 cm from the entrance aperture of
the integrating sphere, and extensive light baffling:of the irradiance sources and sphere
minimizes the effects of scattered light. A motor-driven chopper is positioned in the
optical path when the PbS detector is used.

The monochromator entrance slit is almost completely inaccessible to direct viewing
due to the enclosures, baffling and closely packed optical elements and associated equip-
ment. To provide a view of this slit, a closed-circuit television (CCTV) camera is mounted
within the carriage and focused on the monochromator entrance slit through a small flat
mirror mounted near the 6-inch spherical mirror, This has been invaluable in the initial
alignment of radiance sources and in monitoring the positioning of the source image on the
slit (or slit mask used to limit the slit height) during the course of experiment: The
image of the entrance slit is magnified some twenty-five times such that the notch on
tungsten ctrip lamps imaged onto the elit is readily observable on the monitor screens.

The instrument carriage also houses ancillary equipment such as power supplies for the
thermoelectric coolers, a remotely programmable high-voltage power supply for the photo
multiplier, and an automatic photoelectric pyrometer. The latter ie focused on the radi-

ance sources, including a variable-temperature, graphite-tube blackbody source that occupies
the "home" position.

B- 2


the calibration method, the lamp positioning, orientation, target area and
solid angle, and dependence upon lamp current. A typical report with
attachment is shown in Appendix A. In addition to the uncertainties which
apply at the conclusion of the calibration, additional uncertainties accrue
as the lamp is operated as a standard source. The reported spectral
radiance values may be adjusted with operating time by use of the reported
drift rate, but this rate cannot be assumed to remain constant over a long
time period. Therefore, it is recommended practice to promptly transfer the
calibration values to other lamps, use such lamps as working standards, and

monitor their changes by occasional comparison to the original lamp.

REFERENCES

[1] Stair, R., Johnston,R.G., and Halbach,E.W. Standard of Spectral
Radiance for the Region of 0.25 to 2.6 Microns. J. Res. Nat.
Bur. Stand. (U.S.). 64A(4):291-296. 1960 July-August

[2] Kostkowski,H.J., Erminy,D.E., and Hattenburg,A.T. High Accuracy
Spectral Radiance Calibration of Tungsten-Strip Lamps. Advances in

Geophysics, Vol. 14. New York, NY: Academic Press Inc. 111-127 (1970)

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

[4] Kostkowski,H.J. and Nicodemus,F.E. An Introduction to the Measurement
Equation, Chapter 5 of Self-Study Manual on Optical Radiation Measure-
ments: Part 1 -- Concepts, Nat. Bur. Stand. (U.S.) Tech. Note 910-2,
118 pages February 1978 pp. 58-92

[5] 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

{6] De Vos, J. C. Evaluation of the Quality of a Blackbody. Physica 20:
669-689; 1954

25


n. Read over "End of Day Check List" (located on K-6 Pot)
and perform the necessary operations.

1)

13)
14)
15)
16)

Turn off the blackbody.

a) Throw main breaker on back wall to off.

b) Turn off cooling water.

c) Close blackbody window and turn off Argon flow
at the blackbody and at Argon tank.

d) Turn off power switch on blackbody control
panel and turn pot to zero.

e) Turn off null detector microvoltmeter.

#) Turn off and unplug blackbody alarm.

Turn off auto pyrometer and cover objective with

a lens cover.

Turn off power supplies for positioning spectro—

radiometer and for setting wavelength.

Turn off TV monitors (2) and TV camera.

Turn off lamp power supplies and record off time.

Shutter photomultiplier tube.

Turn off potentiometer null detector.

Turn off stepping motor power supplies.

Cover 45° plane mirror with dust cover.

Cover V25 with dust cover.

Put back panel on spectroradiometer box.

Address MIDAS using CRT keyboard:

a) Open all switches (18,J8).

b) Push Clear button on MIDAS control board.

c) Turn CRT power off.

Remove floppy disks from disk drives.

Turn printer off.

Turn computer terminal power off.

Turn all water off. .

D. 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
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 654.46 nm at the start,
near the middle, and at the end of the calibration.
Measurements are made at 250 nm at the start and
near the end of the calibration. Calculating the
the differences for these repeat measurements
allows for drift rates to be calculated and for
drift corrections to be made if necessary.

c. Run “ORDERFL.1" to order the data by wavelength.

D- 12


variable temperature blackbody is determined by nearly direct comparison to
the gold point blackbody, the wavelength range has been extended to 2400
mm, accuracy estimates have been more rigorously evaluated, and the
instrument has been incorporated into an automated calibration facility,
utilizing a computer for operational control, data acquisition, and data
reduction. This report is a detailed description of the present scale

realization and its associated uncertainties.

SCALE DERIVATION AND TRANSFER: A GENERAL DESCRIPTION

Planck's law defines the spectral radiance Ly of an isothermal cavity,
or blackbody, as

Ly = ey Jeena? (Lexpleg /n-A-T)] -1) (W/em® sr) (2)

where c, = 3.7418-10°! (w-em?) and cy = 1.4388 (cm-K) are the first and
second radiation constants, n is the refractive index of air, (cm) is the
wavelength in air and Tis the thermodynamic temperature (K). For a
blackbody at the temperature of equilibrium between melting and freezing
gold (gold point), the temperature is defined as 1337.58 K on the
International Practical Temperature Scale (the best estimate of the
thermodynamic temperature) [3]. In principle, such a blackbody establishes
a spectral radiance scale, but the energy is too weak at wavelengths below
about 500 nanometers for practical use. Measurement of the ratio of
spectral radiance of the gold point blackbody to that of another blackbody
at known wavelength determines the temperature of the second blackbody, and
therefore its spectral radiance at any wavelength. These values can be
transferred to other sources by direct comparison at selected wavelengths,

thereby establishing a practical scale of spectral radiance.
The ratio of spectral radiance of two sources is deduced from the

corresponding ratio of signals from a lincar spectroradiometer sect at a

selected wavelength. For each source, the resulting signal is [4]

S= fan in [80929-10080 -do-aa-an (3)


are typically operated at near equality in the PbS spectral region.

Polarizati

The polarization properties of the spectroradiometer and the gas-
filled lamps are measured with the aid of sheet (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
separately in the beam at the same angular positions. A Mueller
transmittance matrix [13] containing six parameters is assumed for the
sheet polarizers, to account for departure from ideal behavior. 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, which 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, resulting in 25 equations involving the
two source unknowns, which 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= Cnax . Lain) / Cnax + Lin) {9}

min’

where Lax and Lain are the maximum and minimum readings of a polarization-

16


m. 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 Sparkleen and
hot water. Then dry thoroughly with Kimwipes to re-
move any waterspots.

28H HE

* CAUTION *
FHI EAE ITT AE
* Avoid any direct breathing of the *
* xylene vapors. *

HAA HINA EHTS IAA IIIA ANIA ATI
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 spectro—
radiometer. 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.


Q-

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

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 polariza—
tion of the source.

7) The ratio of the blackbody signal to the standard
lamp signal is determined.

Set the spectroradiomter to the test wavelength and

compare the spectral radiance output from each test lamp

to the spectral radiance output from the blackbody. (The

procedure is similar to the steps listed in section b.

except the signals from several test lamps are compared

to the signal from the blackbody.)

Set the spectroradiomter to 654.6 nm and repeat step b-

Using the mean ratio of the blackbody to the standard

lamp and the known radiance temperature of the standard

lamp, the Planck equation is used to determine the
temperature of the blackbody. A small temperature cor—
rection is applied to account for the finite spectral
bandpass.

The Planck equation is used to determine the spectral

radiance of the blackbody at the test wavelength and the

spectral radiance of the test lamps is determined.

For each test wavelength the spectral radiance and the

blackbody temperature of each test source is saved to

to one data file and the precision of the source com—

parisons is saved to a second data file.

Data Taking Sequence

Adjust ‘blackbody level to approximately match the out-—

put of the test lamps at 654.6 nm.

Measure the test lamps at 654.46 nm using the photo-

multiplier tube.

Adjust blackbody level to approximately match the out-—

put of the test lamps at 250 na.

Measure the test lamps at 250 nm using the photomulti-

Plier tube.

Measure the test lamps from 2400 to 900 nm using PbS

detector.

1) Set at the test wavelength, position the PbS detec—
tor and adjust the blackbody level to approximately
match (within 5%) the output of the test lamps.
This is necessary because the responsivity of the
PbS detector has not been accurately determined and
it is being used at a signal level where saturation
is beginning to occur.


MAEMO mM MEE

REPORT OF CALIBRATION LAMP Q93
Department of The Navy

LAM? NO. LAMBDA = 654.6 4M
LAMP CURRENT 40.376 AMPERES 0.61 0.0 WM TARGET AREL
Po oe — ont: — ane. —s
abe
we :
: * :
a . e a :
: e I
2 * :
: t I
456 ‘
Ade +
“sé ‘
denneee: wenene! -
3 a4 7 a1 - ‘ 1 2 3 4 $s
DEGREES FROM SPECIFIED SETTING( INCREASING ©, ELEVATES LAMP MOZZLE)

NBS Test No. 534/G-44928-2
September 23, 1985


MAZE ROP RN Meme

REPORT OF CALIBRATION LAMP Q93
Department of The Navy

LAMP WO. G99 LAMBDA = 654.6 MM
LAMP CURRENT 40.376 AMPERES 0.62 0.8 MM TARGET AREA
wontons: one: a tenes: eee tecweenene +
ade
as ‘ ‘
: ‘ :
te . . ® . +
- . -
. ) -
Ass
e
AS +
+ es
3 4 1 3 -l ‘ 1 2 3 4 $
DEGREES. FROM SPECIFIED SETTING (INCREASING %& ROTATES LAMP CLOCKWISE WHEN VIEWED FROM ABOVE)

NBS Test No. 534/G-44928-2
September 23, 1985


L= Ly py (8a)

where, as always, we have neglected circular polarization. Py and Py are
given by Py = p:cos2y and Py = 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. 17 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 = ROL (14Bp,) (10a)

and to 24 equations:
Vy) = RyL|[(s+Bdc) + (ac+Bec”4BE)p, + (dS+BgSC)p,]. (11a)

Aside from the factor RL, which is common to all equations and cancels
out, the only unknowns in these 25 equations are Py and Py 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.

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

a source, Ly; and the radiometer output signal, V, assuming linearity


M, signal ratio of the VIBB-test lamp comparison (0.16 to 1.27%)

M. signal ratio of the GPBB-VIBB comparison (0.2%)

8), size-of-source correction for the VIBB-test lamp comparison (0.1%)
ep? effective emissivity of the VIBB (0.1%)

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

B, spectroradiometer polarizance (0.05 to 0.1%)

Py> lamp polarization component (0.2 to 0.3%)

Si size-of-source correction for the GPBB-VIBB comparison (0.1%)

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

Tay ’

c
1?
Cos second radiation constant (0.000135 cm-K)

IPTS-68 temperature of melting gold (0.4 K)
first radiation constant (0.0006-10°!* w-cm’)

A, wavelength of the VIBB-test lamp comparison
ae wavelength of the GPBB-VTBB comparison

Spectral radiance uncertainties due to the factors of Eqn. 13 are
obtained from the partial derivative with respect to those factors and the
estimated uncertainty in the factor. Thus the percent uncertainty in
spectral radiance at 300 nm due to the 0.4 K uncertainty in Thy is

100-AL,/L, = 100-fey/A-Tp,7}-aT = 100-[1.4388/3-107>- (1337.58)2]- (0.4)

Au
or 1.07%, while the percent uncertainty at 900 nm due to the uncertainty

in the VIBB emissivity is
100-AL,/L, = 100-[1-(A,/4)]-Aeg = 100-[1-(654.6/900)] - (0.001)

or 0.03%. Differences between errors calculated by Eqn. 13 and those
calculated by the exact Planck relation are negligible. Note that for the
wavelengths A and La this process yields the error due to inserting the
wrong wavelength in the spectral radiance calculation, not the error due to

an incorrect wavelength setting.

In addition to the factors which appear explicitly in this relation,
uncertainties in the ratios My and ML arise from errors in the wavelength
settings . (0.1 nm) and » (0.05 nm), in the current measurements of the

22


c.

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

AKI HH EHH HEHEHE EEE
* CAUTION *
HHH TE TET TETE ME HEHEHE HEHE HEHEHE HEH HEHEHE ETE EEE EEE
Great care should be taken to avoid
any direct eye contact with the laser
beam. Either position a blocking
#inger on the 45 degree plane mirror
to block the direct beam or translate
the lamp horizontally so that the lamp

filament blocks the beam.
FAH IE

xe RK RK
keK RK RK

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.S.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 hydrofluoric
acid on the scribed arrow.

JEST TIE
* CAUTION *
HIE IETHER INIT IAI III
* Do not allow the hydrofluoric acid to *
* contact the skin. Rinse thoroughly *
* with cold water if skin contact occurs.#
EHH HH HEHE EEE EEE HEE EERE EEE EERE


k.

2) Return to 654.6 nm, position the photomultiplier
tube and determine the blackbody temperature by
comparing its output to the output of the standard
lamp.

3) Set at the test wavelength, position the PbS detec—
tor and compare the output from the test lamps to
the output from the blackbody.

4) Repeat step 2).

5) Set at the next test wavelength and repeat steps
1) to 4).

Adjust blackbody level to approximately match the out-—

put of the test lamps at 654.6 nm

Measure the test lamps fram 900 to 300 nm using the

photomultiplier tube.

Adjust blackbody level to approximately match the out-

put of the test lamps at 250 nm.

Measure test lamps from 300 to 225 nm using the photo-

multiplier tube.

Adjust blackbody level to approximately match the out-—

put of the test lamps at 4654.6 nm.

Measure test lamps at 654.6 nm using the photomulti-

multiplier tube.

Standard Lamp and Test Lamp Turn Off

Turn lamps down slowly as described in CAUTION after

section A.5.d.

Blackbody Turn Off Procedure

a.
b.

f.
q-
h.
i.
i.
k.

1.

Turn null detector to the 100 millivolt scale.
Depress red button on the C.A.T. control unit and hold
down until the meter reads zero and then continue to
hold down for 4 more seconds.

Throw 208V breaker on the wall behind the spectrorad-
iometer to OFF position (Down).

Turn the potentiometer setting to read zero.

Leave the cooling water and Argon flow on for at least
10 more minutes to give the blackbody time to cool
down.

Turn off cooling water.

Turn off and unplug blackbody alarm.

Close blackbody window (turn clockwise).

Turn off the Argon flow on the green line.

Reconnect the vacuum and turn it on.

Turn off the vacuum line and allow the Argon to flow
until the vacuum pressure gauge reads 1 atmosphere
and then turn off the Argon flow valve on the copper
line. Turn off valve at Argon supply tank.

Turn null detector microvoltmeter OFF.

Turn blackbody power switch OFF.

D- 11


ABSTRACT: This report describes the measurement methods and
instrumentation used in the realization and transfer of the NBS scale of
spectral radiance. The application of the basic measurement equation to
both blackbody and tungsten strip lamp sources is discussed. The
polarizance, spectral responsivity function, linearity of response and
“size-of-source effect" of the spectroradiometer are described. The
analysis of sources of error and estimates of uncertainty are presented.

The assigned uncertainties in spectral radiance range from about 1.75% at
225 nm to 0.25% at 2400 nn.

KEY WORDS: calibrations; blackbody; response linearity; slit-scattering

function; spectral radiance; standards.


TABLE 1

SAMPLE OF DATA AND UNCERTAINTIES

Typical Sp

Spectra] Radiance

(Wom sr)
Blackbody Temperature
(K IPTS-68)

SOURCE OF UNCERTAINTY

1) Blackbody quality

2) Calibration of Automatic
Optical Pyrometer relative
sto the International Prac-
tical Temperature Scale
(IPTS-68)

3) Temperature determination
of blackbody and transfer
of blackbody spectral
radiance to lamp

4) Polarization effects

5) Current measurement

6) Wavelength measurement

7) Thermodynamic Temperature
Scale relative to IPTS-68

225

6.679

2.09

1.43

350

3.427x10°

1.34

0.48
0.01
0.09
0.02

0.93

Radiance Values and Estimated Uncertainties (Percent)

Wavelength (nm)

654.6 800 1550

1.369x10 1.999x105

2472.7 2395.5 1979.7

0.03% 0.09%

0.62

0.36

0.04
0.05
0.61

0.49 0.20

1.237x10°

3.756x1C

1616.1

0.11%

0.25

0.21
0.13
0.01
0.01

0.13

Total estimated uncer-
tainty relative to S.I.

the squares of the indi-
vidual uncertainties

(ise root of the sum a)

2.8%

0.9% 0.8% 0.5%

15

0.4%


IV. Mappin

Translational and rotational mapping about the aligned position
are performed on each test lamp at 654.6 nm. The variation in spectral
radiance is measured as the filament is translated horizontally and
vertically along lines which intersect at the centroid of the target

afea. The horizontal translation is done along a line, x, parallel to
the width of the filament and the vertical translation is done along a
line, Vs Parallel to the length of the filament. The variation in

spectral radiance of the target area is mapped as a function of the
angle of rotation about three mutually perpendicular axes which
intersect at the centroid of the target area. Rotation about the
horizontal axis in the filament plane is denoted 6,. Ratation about the
vertical axis in the filament plane is denotated e,. Rotation about the
optical axis is denoted 6..

v. Discussion

Table I gives typical values of spectral radiance for lamps of
this type and typical values of the associated uncertainties. For each
calibration run, the uncertainties are redetermined and these results
are given in the Report of Calibration.

The change in spectral radiance for a change in lamp current has
been measured at 654.6 nm for a typical lamp of the type issued for
calibration. A one milliampere change corresponded to approximately a
0.013% change in spectral radiance. To calculate the approximate
percent change per milliampere at other wavelengths, the following
formula can be used:

aly 654.6
--- = ~--- x 0.015] % per milliampere.
ai »
The transmittance of the quartz window may be non-uniform. Fora

solid angle that is different from the one described above, the user
must determine, by measurement, the appropriate corrections.



Lb

So

FIGURE 3. Central Section of Variable
Temperature Blackbody


DETAILED PROCEDURES FOR ROUTINE SPECTRAL RADIANCE CALIBRATIONS

€@. Lamp Preparation

1.

Visual Inspection

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

type 30A/T24/13. Upon unpacking, each lamp is inspected to
see that

a. the quartz nozzle has a clear window

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

ec. the area of the lamp envelope opposite the window
does not have striations which distort the view of
the filament

d. the filament has electrical continuity with the
electrodes.

Initial Lighting

Each lamp is positioned in a lamp mount, 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.

Annealing

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

In order to accurately reproduce the alignment of each

lamp, an alignment arrow is etched on the rear of the lamp
envelope.
a. Turn the lamp on and set it at approximately 1700°C.
b. 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 viewing window
and passes through the notch on the lamp filament.


polarizers are used to do an automated characterization
at 654.4 nm. The computer program used to perform these
measurements is "“TWOPOL.JW".

Use one polarizer to characterize the polarization of
the test lamp. The computer program used to perform
these measurements is "ONEPOL.JW".

C. Calibration Procedures

1.

Blackbody Turn On Procedure

a.
b.
c.
d.

e.

Turn on cooling water and check for flow.

Turn blackbody power switch ON.

Turn null detector microvoltmeter ON.

Zero null detector on 100 microvolt scale and check

batteries.

Blackbody Flushing Procedure

1) Turn on vacuum line and wait 30 seconds.

2) Turn on Argon flow using valve on the copper line.
3) Turn off vacuum line, let Argon flow until pressure
gauge on vacuum line reads 1 atmosphere and turn

off Argon flow.

4) Wait 2-3 minutes to allow Argon to diffuse within
the blackbody housing.

5) Turn on vacuum line and let pump for 30-60 seconds.

6) Repeat steps 2) to 5) three more times.

7) Turn on Argon flow, turn off vacuum line and when
the pressure gauge reads 1 atmosphere, unplug the
vacuum line and check for Argon exhaust flow.

@) Adjust the Argon flow valve on the copper line so
that the flow gauge reads 8 CFH.

Plug in blackbody alarm and turn switch ON. This alarm

will output a warning signal if the Argon flow is too

low or if blackbody housing starts heating up.

4A IAAI ATT TAS
* CAUTION *
JAH E HEHEHE ETE IEE
* If the blackbody alarm sounds:

* 1) Check the Argon flow.

* 2) If Argon flow is normal then turn
* down blackbody by pushing the red
* button and throw breaker on back
* wall to OFF position.
*
*
*

eee RR

3) If Argon flow is low then connect*

up a new tank of Argon and adjust #*

the flows. *
JIA AAA IAA IAAT


radiance temperature of 2620 K (at 655 nm) for 2 to 3 hours. Each lamp is
examined for the variations in spectral radiance with time, with position
within the central area, and with angle of emission. The amount and
orientation of polarization are also determined. Lamps selected for
calibration display a degree of polarization of 0.005 or less anda drift
rate of less than 0.04% per hour at 654.6 nm. The drift rate is measured at
two wavelengths over a 20-hour period by comparison with a stable vacuum
lamp, and included in the calibration report. Typical values in percent
change per hour are 0.025 + 0.004. Appendix A shows a typical calibration
report and accompanying descriptive material which provide a table of L,
values versus wavelength, and variations in spectral radiance with

position, angle, polarization, time, and lamp current.

4, 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 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
minimize spectral scattering and avoid multiple orders. The wavelength
range is 200 to 2600 nm. The dispersion varies with wavelength from about 1
to 4 nm/mm. 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 spectral line

standards Gy, thorium, neon, helium) and is repeatable to within 0.05 nm.

ll


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 250-1), NBS Measurement Services: Spectral Radiance
Calibrations, by J. H. Walker, R. D. Saunders, and A. T. Hattenburg, is
the first to be published in this new series of special publications.
It covers the calibration of the spectral radiance of tungsten ribbon
filament lamps over the wavelength range of 225 to 2400 nm (see test
numbers 39010C-39030C 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 author 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


Ct ee er oe

REPORT OF CALIBRATION LAMP Q93
Department of The Navy

LAMP WO. O93
LAMP CURRENT 40.370 AMPERES

LAMBDA = 654.6 MH
0.61 0.6 MM TARGET ARED

a ataenee. o.

as

Ce a

‘

wu

*
.

Oa]

AS

Ce 2

SHH eden rem ererereremrererenes

le

Y DISTANCE FROM CENTROID IN MM (POSITIVE VALUES ABOVE NOTCH)
FIG. 2

NBS Test No. 534/G-44928-2
September 23, 1985


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.

DATA ANALYSIS AND UNCERTAINTIES

The signal ratio of the VIBB to the GPBB is obtained from the product
of the signal ratios measured in the first vacuum lamp comparison with the
GPBB, the second vacuum lamp comparison to the first, and the average of
the VTIBB comparisons to the second lamp. The ratio is then multiplied _by
correction factors to account for the size-of-source disparity, the
estimated departure from unity emissivity, and the departure from linear
response where appropriate (total correction 0.1 to 0.6%). An estimated
VIBB temperature is then calculated from the ratio of Planck functions for
the two blackbodies. This temperature can then be used to determine by
iteration the exact temperature which satisfies Eqn. 5. For the spectral
responsivity function of this spectroradiometer, this exact temperature
differs from the ratio of Planck functions approximation by an amount which
is small (less than 0.2 K) and is a simple function of the temperature.
Therefore, to avoid the repetitive integration process, the temperature
calculated from the Planck function ratios is corrected to the desired Eqn.

5 value by this known difference.

For each calibration wavelength, the measured signal ratio of the test
lamp-VIBB comparison is corrected for the size-of-source, non-unity
emiesivity, and polarization factors (total correction less than 0.5%). The
spectral radiance of the test lamp is then obtained by using the measured
VIBB temperature in Eqn. 2 to determine the spectral radiance of the VIBB
at the calibration wavelength, and inserting this spectral radiance and the
corrected signal ratio into Eqn. 8. The spectral radiance values at each
wavelength are corrected for any lamp drifts determined by the initial and

final measurements at 654.6 and 250 nm.

Uncertainty estimation
The uncertainties in the spectral radiance values assigned to the

calibrated lamps are obtained from the observed imprecision of the

20


Library of Congress Catalog Card Number: 87-619800

National Bureau of Standards Special Publication 250-1
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-1, 68 pages (Jan. 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



Cables from all equipment housed in the instrument carriage are bundled and passed
through the bottom of the carriage, where they ride in a smooth trough between the rails
as the carriage passes back and forth. All control and measurement cables from the car-
riage and source supplies lead to the operator's console.

Measuring instruments and equipment to communicate with the operator and the computer
are housed in the console. The operator interface consists of analog and digital displays,
a CCTV monitor displaying the monochromator entrance slit and a CRT terminal and keyboard.
Instrumentation includes a 5% digit DVM, a lock-in amplifier, a picoammeter, and remote
controls for eight source power supplies. A MIDAS system!*? provides the interface between
instruments and computer. All measurement signals are multiplexed into the DVM through the
MIDAS scanner, and all instruments may be remotely programmed and controlled through MIDAS
modules. In addition, the current supplied to the lamp sources can be automatically mon-
itored and controlled.

The MIDAS controller communicates serially at a rate of 1200 baud with a minicomputer
having 32 K of core memory. The computer is equipped with a high-speed disk system for
program and data file storage. It is operated in a time-shared mode so that it can service
other experiments, in addition to FASCAL, simultaneously. Programming is done exclusively,
in BASIC interactive language.

FASCAL is currently being used for routine spectral irradiance calibrations. These
calibrations are carried out on groups of twelve 1000 W tungsten-halogen lamps preselected
for stability, spectral composition, and directional uniformity. Three lamps from the group
and one of our in-house standards (calibrated as described in the soon-to-be-published NBS
Tech Note 594-13) are mounted in four of the outer row of lamp stations shown in Figure 1.
Lamp alignment is accomplished with "reverse optics" using the laser in the detector com-
partment and a special alignment jig that is interchangeable with the integrating sphere.
This jig includes a diagonal mirror that directs the laser beam to the lamp station for
positioning the lamp in the plane perpendicular to the optical axis, and a 50 cm arm termi-
nating in a dial gauge for setting the lamp-to-detector distance. Using the integrating
sphere input to the monochromator, these four lamps are then intercompared automatically by
FASCAL at our twenty-six customary wavelengths. Then another three lamps and a different
in-house standard are measured in the same way. This process is performed sixteen times,
with permutations of lamps and stations, until each of the twelve lamps has been compared
against each in-house standard and each of the sixteen lamps has been measured in each of
the four lamp stations. The redundancy of these measurements permits us to sense any
systematic errors. For example, any dependence upon lamp station, or discrepancy among our
four in-house standards would be thoroughly investigated.

We are now in the process of adding routine spectral radiance calibrations to FASCAL's
repertoire. This is a more complicated problem requiring the rest of the features of
FASCAL and we hope to report on these calibrations in the near future.

Optical Radiation News, No. 8 (March 1975).

2¢. H. Popenve and M. S. Campbell, MIDAS, Modular Interactive Data Acquisition System --
Description and Specification, NBS Tech Note 790 (August 1973).

ANEW TECHNIQUE FOR TRANSLUCENCY EVALUATION

The translucent phenomenon that causes flux loss in spectrophotometric measurements
involving a finite-size sample port is well recognized. A technique utilizing an inte-
grated point spread function and laser and conventional light sources has been developed
by J. J. Hsia of the spectrophotometry group in the Radiometric Physics Section. The
details of the method, mathematical derivation, experimental ect ups, and reoulte are
described in NBS Technical Note 594-12, "The Translucent Blurring Effect - Method of


REPORT OF CALIBRATION LAMP O93
Department of The Navy

The polarization measurements at 655 nm, described in paragraph
Ir of the enclosed document, showed that the degree of
polarization was 0.0020. The direction of polarization of the
larger component was approximately 6 degrees counterclockwise
from the horizontal when viewed from the spectroradiometer.

Prepared by: Approved by:

Gt 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/G6-44928-2
September 23, 1985


L(max) + L(min)

and the direction of polarization of the larger component, L(max), are
determined. L(max) and L(min) are the maximum and minimum readings of a
polarization-indifferent radiometer when an ideal linear polarizer
is rotated in the source beam. Only lamps with P less than or equal to
-005 are calibrated. The measured value of P and the direction of
polarization of L(max) are given in the Report of Calibration.

III. Calibration
A. Method

Measurements are made with a spectroradiometer [2] incorporated
into the NBS Facility for Automated Spectral Calibrations (FASCAL) [31].
In the infrared, the spectroradiometer uses a lead sulfide detector and
the measured spectral radiance* is determined by a spectral comparison
of the test lamp to a high-quality variable temperature blackbody of
approximately equal spectral radiance. In the visible and ultraviolet
the spectroradiometer uses a photomultiplier tube. The
photmultiplier—amplifier combination was determined to be linear [£71 ta
better than 0.1% over a photmultiplier current range from i X 10-7

amperes to 5S xX 10-7 amperes. Therefore, in the visible and
ultraviolet, it is not necessary to closely match the blackbody to the
test lamp. The blackbody temperature is determined by using a

Pyrometer lamp calibrated at 654.6 nmin terms of the International
Practical Temperature Scale (IPTS-68) [1]. Within the present state of
thermometry, this scale is the best approximation to the Thermodynamic
Temperature Scale. The reported spectral radiance (in air**) and
blackbody temperature are related by the Planck equation [51]

L = (C,/7n2aATM) / Cexp(C2/ndT) — 1]

where Ca is 3.741832 x 10-22 W cm? sr-*, nis the index of refraction
of air (461, is the wavelength (in air) in cm, C2 is 1.4388 cm K, and
T is the IPTS-68 blackbody temperature in K. -

“The spectral radiance values in this report are average values over
the target area and solid angle set by the spectroradiometer. Detailed
descriptions of these parameters are given later in this documentation.

“If the spectral radiance in a medium other than air is desired, it can
be obtained from the Planck equation and the L n> invariance
relationship (41.


vacuum (0.2 ma) and gas-filled (1 ma) lamps, and in the measured spectral
responsivity function. The uncertainties in the ratios due to wavelength
setting and current are assessed at a number of wavelengths by measurement
of the change in signal ratio when varying these quantities. The effect
upon the signal ratios due to the uncertainties in the measured spectral
xesponsivity function is determined by solving Eqn. 5 for a range of
RQg)A) values, using the known spectral radiance distributions of the
blackbodies and an approximate test lamp distribution derived from the
calibration values. The spectral radiance uncertainties due to these
factors are then deduced from the ratio uncertainties as before. The signal
ratio, lamp current and wavelength setting errors are considered random;

the remaining errors are systematic.

Table I lists the uncertainties obtained by this process. The
calculated uncertainties in percent of spectral radiance are tabulated for
a number of wavelengths over the calibration range. The individual values
are combined in quadrature to yield the combined uncertainty for each
wavelength. These uncertainties apply to the reported values at the

conclusion of the calibration.
CONCLUSION

(he realization of the NBS scale of spectral radiance and its transfer
to calibrated lamp standards has been described in this report. This
gpectral radiance scale also serves as the basis for the NBS scales of
spectral irradiance, luminous intensity, luminous flux, and color
temperature. It has therefore been the subject of continuing investigations
to decrease and assess uncertainties and to increase its efficiency. Recent
improvements in the process are the determination of the system response
linearity, the polarization characterization of instrument and sources, and
accurate measurement of the spectroradiometer spectral responsivity
function. The thermodynamic temperature of melting gold remains as the

chief source of uncertainty in the scale realization.

The calibrated lamp standards of spectral radiance are provided to the

user with a report of calibration and an attachment which briefly describes

23


PYROMETER

?

CONTROL
DETECTOR

REMOVABLE
WINDOW

]
|
|

MUAY INT TT

4
~~ GRADIENT OBSERVATION
an WINDOW
mn AT.
- Al) |
BH |.
1 aAo| ft] CARBON-BLACK
as i a
iA | Bl
came
A |G
ia
Bi ay.
———_—+ ig |. .
—t4 2a
' a 7
2

a

GRAPHITE TUBE

FIGURE 2. Variable Temperature Blackbody Schematic


e.
.

The spectroradiometer detector signal is converted to
an analog voltage which is displayed on a strip-chart
recorder.

The full scale signal is biased to about 5% of its
original value and then is amplified by a factor of 10
so that one division on the strip-chart represents
about 0.1% in radiance.

The blackbody is translated horizontally (in a
direction perpendicular to the optical axis of the
spectroradiometer) until the 1% drop offs are found on
each side of center. These horizontal scale position
readings are noted and the blackbody is translated until
the scale reading is half way between the two readings.
@ similar procedure is done using the vertical trans—
lation of the blackbody.

Step c. is repeated one more time.

The blackbody is rotated around the vertical axis
Passing approximately through the center of the black-—
body rod until the 1% drop offs are found on each side
of center. The rotational scale position readings are
noted and the blackbody is rotated until the scale
reading is half way between the two readings.

A similar procedure is done using rotation around the
horizontal axis passing through the viewing opening of
the blackbody rod.

Steps c. through e. are repeated one more time.

Data Taking Procedure

The computer program used to perform the routine

spectral radiance calibration is "RCBB2A.LIN". The general
data taking procedure is as follows.

a.

b.

Adjust the 1530 K standard lamp to its calibrated

current.

With the spectroradiometer wavelength set at 654.4 nm,

the spectral radiance output from the blackbody 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 inte-—
grating digital voltmeter (DVM). The DVM reading
is recorded.

2) The spectroradiometer is positioned so that it is
viewing the opening in the blackbody rod. The DVM
reading is recorded.

3) The spectroradiometer is alternately positioned
to view the standard lamp and the blackbody until
a set of three readings is taken on each source.

D- 9


Evaluation and Estimation", which is now available. Inquiries concerning this technique
and its possible use in error evaluation and estimation should be directed to:

Dr. Jack J. Hsia

Room A317, Metrology Building
National Bureau of Standards
Washington, D.C. 20234

Phone: (301) 921-2791.

Copies of Tech Note 594-12 are now being distributed to CORM members. Copies may also
be obtained from the Superintendent of Documents, U. S. Government Printing Office, Wash-
ington, D.C. 20402. Order by SD Cat. No. C13.46:594-12, Price 75¢. Contact the editor of
this newsletter if difficulty is encountered.


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

s ag ds 0

Pe dC eC +£ gSC 0 (2a)
ds x x x
0 x x x

where C = cos 2(¢ - $9)» S = sin 2(¢ - $9) and the parameters s, d, e, f,
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-f (where, in terms of the principle

1

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

(3a)

ooor


of the observation hole is about 0.03 sr. The measured partial reflectivity
of the graphite is 0.02 srt, The measured temperature gradient over the
length of the viewing cavity is less than 1K. Experimental investigations
of possible absorption of radiation by gases has disclosed only weak
absorption lines at 589 and 589.6 nm (Na) and at 766.5 nm (K). The
resulting estimate of emissivity is 0.999.

3. Lamp Sources.

Tungsten-strip filament lamps are used both in the realization of the
spectral radiance scale and as secondary standards for scale 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
at. constant current, provided by stabilized (0.005% regulation) DC power
supplies. Manganin shunts (0.01 or 0.001 ohms) and a 5-1/2 digit voltmeter
provide the current measurement. Lamps are allowed to stabilize at the set

current for about 1 to 1-1/2 hours before measurements are begun.

a. Vacuum Lamps Vacuum tungsten-strip lamps of two types are used in
the scale realization to facilitate the temperature measurement of the
blackbody. A Quinn-Lee lamp [7,8] is operated at a single current to
produce a spectral radiance equal to that of the gold-point blackbody at
654.6 nm. A commercial vacuum lamp with plane windows [8] 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 spectral radiance scale.
The lamps have a quartz window to transmit near-ultraviolet radiation. They
are operated at a single current, chosen to produce a radiance temperature

of about 2470 K at 654.6 nm. Prior to calibration, lamps are annealed at a

10