2. LUMINOUS INTENSITY CALIBRATIONS

The luminous intensity of a source is the luminous flux emitted per solid
angle. The unit of luminous intensity is the candela which is equal to one
lumen per steradian. A source having a luminous intensity of 1 candela in all
directions radiates luminous flux equal to 4 m lumens.

Inside-frosted lamp standards of luminous intensity are calibrated at the
National Bureau of Standards by a substitution method on a calibrated
horizontal bench photometer. The photometer is calibrated at the time of the
measurements in terms of the unit of luminous intensity (candela) at
approximately the color temperature of CIE standard illuminant A (2856 K-IPTS
68) as maintained at NBS. The luminous intensity scale is derived from the
NBS spectral irradiance scale and is maintained in a group of nine 500-W
inside-frosted lamps designated as the NBS 5612 groups.

The luminous intensity of these lamps was determined by measuring the
spectral irradiance of each lamp on FASCAL, an automated high-accuracy
spectroradiometer that serves as the transfer instrument, and then calculating
the luminous intensity according to equation (3), with the spectral
intensities I, ad evaluated according to a radiometric form of equation (6).
The measurements were performed at a distance of 1.5 meters. A complete set
of measurements on the lamps in the 5612 group was performed in 1978 and again
in 1984 with some measurement checks in between. Using the i] luminance
substitution method described in section 2.3.1, the luminous intensity scale
was further transferred from the nine lamps in the NBS 5612 group to eight
lamps which comprise the NBS 7796 group. The lamps in the NBS 7796 group are
the working standards used in routine calibrations.

2.1 Test Lamp
2.1.1 Description

Gas-filled, inside-frosted lamp standards of luminous intensity are
calibrated by NBS in 100-, 500-, and 1000-watt sizes. The corresponding
luminous intensities are approximately 90, 700, and 1400 candelas. The 100-
and 500-watt lamps have T-20 bulbs; the 1000-watt lamps have T-24 bulbs.
They all have medium-bipost bases and C-133 filaments. The 100 watt lamps

10


Table 6. Uncertainties (30) Associated with Large Lamps Calibrated
for Luminous Flux

1. Uncertainty of primary reference group:

a. Uncertainty of distribution measurements 0.3%
b. Uncertainty in spectral irradiance scale and in
transfer to luminous flux group 0.7%
Total Uncertainty of NBS primary Reference Group 0.8%

2. Uncertainty of NBS Working Standards relative to primary
reference group:

a. Geometric differences 1.0%
b. Spectral differences 0.4%
c. Random variations (30) 0.2%

Total uncertainty of working standards relative to NBS primary
reference group (a, b, c combined in quadrature) 1.1%

3. Uncertainty for lamps differing in size or color temperature from
those of working standards:
a. Geometrical differences 1.0%
b. Spectral differences 0.4%

4. Random variation in transfer from working standard to test
lamp (30): 0.4%

Total Uncertainty of Test Lamp

1. Lamps the same size and color temperature as the working standards:
a. Relative to SI - combine 1, 2, and 4 in quadrature 1.4%
b. Relative to NBS - combine 2 and 4 in quadrature 1.2%

2. Lamps of sizes or color temperatures different from that of the
working standards:
a. Relative to SI - combine 1, 2, 3, and 4 in quadrature 1.8%
b. Relative to NBS - combine 2, 3, and 4 in quadrature 1.6%

31


distance from a stable, clear-bulb, monoplane straight-wire filament lamp.
The filament is approximately 2 centimeters square. (The use of a planar
filament lamp avoids any invalidation of isotropic assumption implicit in the
inverse square law that can occur with, for instance, a helical - coil
filament lamp when different portions of the rear of the coiled filament are
obscured by the front portions as the source-to-detector distance is varied
[20]). The photocell is moved along the bar and its response noted at various
distances from the lamp. The deviation of the response from that expected by
applying the inverse-square law is attributed to the nonlinearity of the
detection system. Illuminance levels ranging approximately from 3 lux to 800
lux are produced at distances ranging from 0.75 to 4.17 meters. Maximum

deviations from linearity for the three detectors are typically a few percent
or less.

The measured luminous flux values for all lamps in a group, averaged over
the three measurements, constitute the assigned value of the luminous flux

attached to that group.

3.5 Calibration Procedures - Test Lamps

Test Lamps for issuance to the public are calibrated in groups of ten to
twenty. Three measurements, one on each of three days, are made on each lamp
in the group. Each lamp is placed in the sphere and allowed to warm up at its
designated current or voltage for ten minutes when using the 2-meter sphere
and five minutes when using the 0.76 meter sphere. Then measurements of
voltage, current, and luminous flux are made. The lamps are measured in the
following order: two standards, one-half of the test lamps, two standards,
the other half of the test lamps, and the remaining two standards. The three
photocells described earlier are used, one on each of the three days. Test
lamps in the 2-meter sphere are compared to the standards which are closest in
luminous flux. Corrections for the nonlinearity of the photocell and
difference in absorption are applied. When miniature lamps are calibrated in
the 0.76-meter sphere, no corrections for lamp absorption or system
non-linearity are necessary since the miniature working standards and test
lamps are of the same type and have nearly the same light output.

27


4. COLOR TEMPERATURE CALIBRATIONS

Incandescent lamp color temperature standards are calibrated at NBS
using a red-blue ratio substitution method. The basic instrumentation
consists of a 20-cm diameter integrating sphere and a commercially-available
telephotometer. The color temperature instrument is calibrated at the time of
the measurement in terms of the color temperature scale maintained by NBS.

4.1 The NBS Color Temperature Scale

NBS has established and maintains a scale of color temperature. Careful
distinctions should be made between the concept of color temperature and the
related concepts of radiance temperature, correlated color temperature, and
distribution temperature. Radiance temperature is defined as the "temperature
of the full radiator for which the radiance at the specified wavelength has
the same spectral concentration as for the radiator considered" [22]. Color
temperature is defined as the "temperature of the full radiator which emits
radiation of the same chromaticity as the radiation considered" [22]. That
is, the test source and the full radiator (i.e. blackbody) have the same
appearance to a human observer. Note particularly that the corresponding
relative spectral power distributions are not necessarily identical or even
similar. If the test source and the corresponding blackbody have identical
relative spectral power distributions in the spectral region of interest, the
correct designation is distribution temperature [22]. If one wishes to assign
a "temperature" to describe a source whose appearance, and hence whose
relative spectral power distribution, is markedly different from a blackbody,
such as a fluorescent lamp, the correct designation is correlated color
temperature [22]. Mathematical procedures are available for determining the
correlated color temperature of any source [23]. In applying the values of
color temperature reported by NBS, the user should bear these distinctions in
mind. Particular care should be exercised in utilizing the widespread

practice of treating color temperature values as synonymous with distribution
temperatures.

The NBS scale of color temperature was first established in 1934 [24] by
visual comparison of monoplane, coiled tungsten filament, incandescent lamps

36


Luminous Efficiency

as
oO

ad
CJ

S
a

S
bh

id
ie)

400 500 600 700
Wavelength A (nm)

Figure 1. Spectral Luminous Efficiency Function V(X)

Defining the Standard Photopic Observer.



4.2 Alignment

Install Ist working standard and note carriage position on bench. Turn
on lamp at reduced current. Adjust mounting height of lamp so that the
filament center is at the same height as the center of the sphere opening.
Bring lamp up to assigned current and set timer for 7 minutes.

4.3 Photometer Sensitivity Adjustments

Select a combination of ND-filter and aperture setting that does not
saturate the photomultiplier tube. A typical setting is ND-2 with a 1°
aperture setting.

5. Measurements
5.1 Working Standards

Measure and record signal levels through both the R and B filters. Power
down the lamp. Repeat procedures for other two working standards. Compute
the R/B ratio for each working standard and then average these 3 values. This
average R/B ratio then becomes the target for the test lamp.

5.2 Test Lamp

Mount the test lamp and warm-up for 7 minutes at the estimated
calibration voltage. Find two currents where R/B is approximately 5% above
and below the target R/B ratio. Also record the voltages. Repeat the above
procedures for 2 more runs with the working standards and the test lamp.

6. Data Reduction
6.1 Test Lamp

For each run, do a linear interpolation between the high and low currents
and find the current corresponding to the target R/B ratio. Final voltage is
determined in the same manner. Average the currents from each of the three
runs. This is reported as the calibration current for 2856 K and will be the
set current for the subsequent luminous intensity calibration (if any). The
average voltage is also reported.


10.

11.

12.

13.

14.

15.

See, for instance: C.G. Hughes III, Silicon Photodiode Absolute Spectral
Response Self-Calibration Using A Filtered Tungsten Source, App]. Opt.
21, 2129 (1982).

R.L. Booker and J. Geist, Induced Junction (Inversion Layer) Photodiode
Self-Calibration, Appl. Opt. 23, 1940 (1984).

J. Geist, E.F. Zalewski, and A.R. Schaefer, Spectral Response
Self-Calibration and Interpolation of Silicon Photodiodes, Appl. Opt. 19,
3795 (1980) . >
See, for instance: E.F. Zalewski and W.K. Gladden, Absolute Spectral
Irradiance Measurements Based on the Predicted Quantum Efficiency of a
Silicon Photodiode, Optica Pura y Aplicada, 17, 133 (1984).

L.P. Boivin, Calibration of Incandescent Lamps for Spectral Irradiance by
means of Absolute Radiometers, Appl. Opt. 19, 2771 (1980).

E. Zalewski, et al, Photometric Instrumention and Research (1970 to 1971)
NBS Technical Note 594-2, pp. 29, 32, 33, September 1972.

V.K. Zworykin and E.G. Ramberg, Photoelectricity, Chapter 1], John
Wiley & Sons, Inc., New York, N.Y. (1949).

See, for instance: G. Wyzecki and W.S. Stiles, Color Science: Concepts
and Methods, Quantitive Data and Formulae, 2nd edition, John Wiley and
Sons, New York, N.Y., 1982

J. Bonhoure, Rapport sur la Cinquieme Comparaison des etalons nationaux
d'intersite et de flux lumineux (1969), p. 86, 7th session of the
Consultative Committee on Photometry (CCP), 1971

J.W.T. Walsh, Photometry, Dover Publications, Inc., 180 Varick Street, New
York, N.Y., 1965.

A. Kingslake, Applied Optics and Optical Engineering, Academic Press, New
York, N.Y., 1965.

46


1. The lamps should be turned on and off slowly (15-30 seconds). The
lamp should not be moved while lighted.

2. In order to prolong the useful life of the lamps, it is recommended
that they be used sparingly, and great care should be taken so that at no time
will the current appreciably exceed the value stated in the report. For
general use, working standards should be prepared by calibrating them relative
to the lamps supplied by NBS. When a laboratory procures standard lamps from
NBS it is well to obtain at least three such lamps in order to be able to
detect changes that may occur.

3. Measurements should be made only after the lamp has stabilized
(approximately 10 minutes after turn on).

4. Stray light should be excluded from the measuring instruments. This
includes light from the standard which may be reflected from the background or
from instruments being used.

5. Differentiation of equation (10) yields:
dI = (B+ 2cT¢) Ty (11)

Typical values of the constants A, B, and C in eq. (11) for the airway beacon
lamps of the type 500T20/13 are A = -.45, B= 1.0 X 1073, and C = 1.4 X 1077.
Evaluating eq. (11) in the range 2000 K to 3000 K, shows that the sensitivity
of the current setting ranges from 0.0016 to 0.0018 amps/K. Since these lamps
typically draw about 4 amps at 2856 K, a current setting accuracy of at least
0.1% is required if the color temperature of the lamp is not to be affected by
more than 2 K.

44


TABLE OF CONTENTS - APPENDIX A

Page
Introduction. ©. 6 we ee eee oe Al
Lamps... ee ee A-1
2.1 Test Lamp. .... Se A-1
2.2 Working Standards... 1... ee ee A-1
Measurement Preparation... . 2... 2... eee se eee A-1
3.1 Detector Alignment. . 2... ee ee ee ee eee ee A-1
3.2 Lamp Alignment. 2... 1 eee eee ee ee ee ee ee Aol
3.3. Shunt Box... A-2
3.4 Volt Box. 2 2 2. co ee ee ee ee ee An?
3.5 Lamp Connections. ..........084 eee ee ee eee AR?
Run Cards 2 6 ee ee A-2
4.1 Working Standards»... ee ee ee A-2
4.2 Test Lamp... 2... ee A-2
Measurements. © 6 6 6 ee ee ee ee A-3
5.1 Working Standards . 2. 2. ee ee A-3
5.2 Test Lamp... ee ee ee ee ee AB
5.3 Remaining Working Standards... .... 2... ee ee eee A-3
5.4 Remaining Measurement Runs... 2... ee ee ee ee A-3
Data Reduction, . 2... ee A-3
6.1 Calibration Factor, 2... eee ee « A-3
6.2 Quality Control Factor... 2... . ee ee A-3
6.3 Test Lamp Luminous Intensity Value. ......... we ee ARG
6.4 Final Value... ee ee ee oe AWG


should equal the sum of the negative differences (allowing for rounding).
Neglecting signage, sum the differences. Divide the sum of the differences by
the number of working standards to get the average difference. Divide the
average difference by the average of the assigned values for the working
standards, and multiply by 100 to convert to percent. The resulting so-called
"delta" is used as an internal quality control check and should be less than
0.5%.

6.3 Test Lamp Luminous Intensity Value

Multiply the average K-factor in (cd/v) by (4.17/2.9)2 to correct for
distance. Multiply the distance-corrected average K-factor by the detector
output for the test lamp, thereby producing the luminous intensity of the test

lamp in candelas.

6.4 Final Value

Repeat steps 6.1 through 6.3 for the 2nd and 3rd runs. Average the three
luminous intensity values to determine the final reported calibration value.
Also report the corresponding average current and voltage of the test lamp.

A-4


4.5 Uncertainty

The NBS color temperature scale is based on the NBS spectral irradiance
scale. The uncertainties associated with the spectral irradiance scale have
been presented in Table 2. Color temperature is a parameter related to spec-
tral quality (relative spectral distribution) of the optical radiation consid-
ered. The uncertainties associated with the NBS working lamp standards of
color temperature have been estimated by the following procedure. Wavelengths
of 650 nm and 450 nm were choosen as representative of red and blue components
of the lamp spectrum. The red-to-blue ratio (R/B ratio) was computed for
blackbodies at the temperatures of interest. The R/B ratio was altered by the
percentage uncertainties given in Table 2 (uncertainty with respect to SI).
Computations were performed for the tred, -blue case and the -red, +blue case.
The resultant shift in blackbody temperature in K is taken as the uncertainty
associated with the NBS working standards. This computation has the nature of
a limit-of-error calculation and thus is approximately a 30 error. These

results are listed in the systematic uncertainty column in Table 8.

The present NBS color temperature comparator is an interim apparatus, as
discussed in section 1, and has not been extensively characterized. Further,
the data base acquired on this instrument is extremely limited. The transfer
precision of the instrument has been estimated from the data acquired on five
lamps measured over the range 2000 - 3000 K. Each lamp was measured three
times at four color temperatures in this range. Pooled estimates of the
standard deviation of a single measurement at each color temperature were
computed. The uncertainty asigned to the transfer calibration of a test lamp
is taken as three times the standard deviation of the mean of three
measurements. Table 8 summarizes these uncertainties

4.6 Discussion

Lamps supplied to other laboratories as standards of color temperature
represent the color temperature scale as maintained by NBS. They are
expensive laboratory equipment and deserve the utmost care in handling and
use. In addition to some obvious precautions, such as keeping the lamps clean
and avoiding mechanical shock to the filament, the following are a few

precautions, sometimes overlooked, which should be used with these standards.

42


2.5 Data Reduction

Individual photometer calibration factors (lux per volt output of the
operational amplifier) are computed for each working standard. After checking
these factors to ascertain that no significant drift in the sensitivity of the
photometer has taken place during the calibration of the test lamps, an
average photometer calibration factor is computed. This average factor is
used to compute the illuminance produced by the test lamps. Test lamp
intensities are then computed by multiplying the measured illuminance by the
square of the distance between the lamps and the detector. The entire
measurement procedure is repeated at least three times, each with a different
selenium barrier-layer photocell. The test lamp intensities reported are the
averages of these determinations. A typical luminous intensity calibration
report is shown in Appendix D.

2.6 Uncertainty

The lineage of the prime luminous intensity reference lamp qroup (NBS 5612
group) is shown in Figure 2. The uncertainties (30) associated with this
chain through the calibration of a test lamp for spectral irradiance have been
documented elsewhere [7]. Table 2 is an expanded version of the uncertainty
table in reference [7] specifically applicable to the spectral irradiance
calibration of the luminous intensity reference group. The last column in
Table 2, headed V(A) weighted, is a photometric weighting of the radiometric
uncertainties according to equation (3). Note that the actual experiment
utilized the alternative definition of luminous intensity, equation (6). Note
also that, since these measurements were performed immediately following a
spectral irradiance scale realization, the Lime drift model error normally
associated with the use of the four working standard spectral irradiance lamps
is not applicable and therefore not included in Table 2. The values listed
under IV in Table 2 are taken as the uncertainty of the prime luminous inten-
sity reference lamp group. In the buildup of the uncertainty terms shown in
Table 2, the factors in the physical models [6, 12] enter multiplicatively
with one exception. This exception is exponential in form and has a value of
approximately 1.00 in the range of interest. Hence, the summing of relative
percents in quadrature is justified.

14


due to the differences in spectral distribution since the lamps were operating
at approximately the same color temperature. Differences in the spectral
distribution are therefore small. For the type 51 vacuum lamps an allowance
of 2% is taken as a "best guess" estimate for the uncertainty due to spectral
differences. In 1974, measurements were made both with photopically corrected
selenium cells and with photopically corrected silicon cells. A difference of
2% was observed depending on the type of cell used. The present measurements
are limited to photopically corrected selenium cells, but it is interesting to
note that the observed shift in the assignment to the NBS type 51 working
group was also approximately 2%.

For random variations, including the uncertainty in the linearity
corrections, a statistical analysis of the measured luminous flux values was
performed. Using the data from the 1981/82 calibration of the NBS working
lamp groups, the deviations from mean assignment were computed for each lamp
(99 degrees of freedom). Then the standard deviations were pooled to give an
overall estimate of the precision of the measurements for the working groups.
This analysis resulted in an estimate of 0.3% for the random variations in
deriving the assignments for the NBS working miniature groups.

The uncertainty due to random variations in the transfer calibration of a
test lamp was obtained by pooling the observed variation in the measurements
over all test lamps (80 degrees of freedom). The pooled estimate of the
standard deviation was 0.15%. An allowance of 0.3% (three times the standard
deviation of the mean of the three measurements made on each test lamp) is
made for the uncertainty due to this source.

The uncertainty due to all suspected errors associated with a reported
value for a lamp standard of luminous flux is obtained by combining in
quadrature the uncertainties in the measurement chain that generated the
value. Table 7 summarizes these uncertainties

3.8 Care and Use

Lamps supplied to other laboratories as standards of luminous flux
represent the unit of luminous flux (lumen) as maintained at the National

32


Detailed Procedures for a Routine Color Temperature Calibration

1. Introduction

The example chosen here is the color temperature calibration at 2856 K of
an inside-frosted luminous intensity standard, which is a commonly requested
calibration. A color temperature standard, per se, would be a clear lamp.

2. Lamps
2.1 Test Lamp

The test lamp is a 500-W, inside-frosted, medium bipost lamp. The lamp
has been seasoned at rated voltage for 4% of its rated life and has an
identifying NBS number etched on the bulb.

2.2 Working Standards

The working standards are the three lamps of the NBS 9171 group. They
are identical to the test lamp.

3. Preparation of Run Cards
3.1 Working Standards

A single run card is prepared for each color temperature listing all the
working standards on that card. All 3 runs will be recorded on the single
card. List the color temperature, assigned current sets for each lamp, and
note the fixed shunt value.

3.2 Test Lamp

Describe lamps, note color temperature, identifying NBS number, and test
number if appropriate.

4. Measurement Preparation
4.1 Photometer

The telephotometer is warmed-up overnight prior to color temperature
calibrations.

C-1


TABLE OF CONTENTS

Abstract. ©. 1 ee ee ee

1. Introduction... ....... bo ee ee
1.1 Photometry, Physical Photometry, and Radiometry
1.2 Photometric Scale... . 1... eee eee
1.3. Photometric Quantities. .... we
1.4 Safety... .. 2... eee ee ee
1.5 Acknowledgements... ... swe eee

2. Luminous Intensity Calibrations .

2.1 Test Lamp... . ee

2.1.1 Description. 2... . ee ee ee

2.1.2 Preparation. ©... ee

2.1.3 Orientation. 2. ee

2.1.4 Operation. 2. ee ee

2.2  Photometer. 2... ee ee ee

2.2.1 Detector and Amplifier... ... 0.2.2... -.2.0200000

2.2.2 Optical Bench... 1... 2... So ee ee

2.3. Calibration Procedure... .. ee ee ee

2.3.1 Photometer Calibration... . 2... ee

2.4 Measurement Schedule... .....-...04.%

2.5 Data Reduction... .. 2. ee eee

2.6 Uncertainty... ........2.000.

2.6.1 Precautions... . . ee
3. Luminous Flux Calibrations... .... ee

3.1 Test Lamps. . Se

3.2 Sphere Photometers. . 2... 2... . ee ee ee eee

3.3. Standards... 2... Se ee

3.4 Calibration Procedures - Working Standards... ......

3.5 Calibration Procedures - Test Lamps... .

3.6 Uncertainties - Large Lamps... ..... ee ee

3.7 Uncertainties - Miniature Lamps... ..

3.8 Care and Usen sw. ee

ye

o©onw

10
10
10
11
11
11
12
12
12
13
13
13
14
14
16

20
al
23
24
25
27
28
30
32


TABLE OF CONTENTS - APPENDIX D

Page
Report of Calibration for Luminous Intensity of Three 500-watt
Inside-Frosted Lamps. . 6 1 6. ee ee ee D-1
Report of Calibration for Luminous Flux of one 200-watt
General-Purpose Lamp. . 2... 1. ee ee 0-2
Report of Calibration for Luminous Flux of Ten Miniature Lamps .. . 0-4
Report of Calibration for Color Temperature of one 500-watt Lamp . . 0-6

Note:

The “enclosed descriptions" referred to in these Reports of Calibration
are earlier descriptions of the Calibration procedures and have been

superceded by the present document.


APPENDIX D
REPORTS OF CALIBRATION


vonm MBS-43
(REV. 48)

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

for
Luminous Flux
of
One 200-Watt General-Purpose Lamp

Supplied to:

(See your purchase order number LY~3842 dated September 25, 1985.)
1. Material

One 200-watt general-purpose incandescent lamp was calibrated for
luminous flux. The lamp designation listed below is etched on the
bulb.

2. Calibration

The lamp was calibrated in a two-meter integrating sphere in accordance
with the enclosed. description "Calibration of Incandescent Lamps for
Luminous Flux". The values reported below are based on the NBS scale
of spectral irradiance in agreement with the international redefinition
of the photometric units in 1979. Values reported on this scale are
0.25% less than those reported on the previous NBS flux scale. This
shift is less than the uncertainty associated with the reported lumi-
nous flux values. The uncertainty is discussed in Section V and sum-
marized in the final table of the enclosed description.

NBS Test No.: 534/
Date: March 13, 1986 Page 1 of 2

D-2


Table 5. NBS Working Standards of Luminous Flux.

Lamp Group Nominal Bulb Filament Approx. Lumens
Designation Wattage Color (Group

Temp. Mean)

NBS 162 500 PS 35 clear gas-filled C-7 2800K 6426
NBS 6807 200 PS 30 frosted gas-filled C-9 2900K 3252
NBS 3158 100 A-21 frosted gas-filled C-9 2900K =: 1637

In the 1981-82 calibrations, flux assignments for the miniature lamp
working standards groups were derived from the primary flux standards (NBS 526
group) in several steps. In the first step, the six lamps in the NBS 3158
group were compared directly against the primary flux group in the 2-meter
sphere. This group was then compared in the 2-meter sphere with a group of
ten type 1183 lamps operated at the two levels listed in Table 4. The type
1183 group, operated at 32 mean spherical candlepower (MSCP = o,/4n) ) 5 was
then used in the 0.76-meter sphere to calibrate the group of type 1133 lamps.
In turn, the type 1133 lamps were used in the 0.76-meter sphere as standards
to calibrate the other working groups listed in Table 4.

3.4 Calibration Procedures - Working Standards

Prior to a calibration run, each group of working standards and test lamps
is divided into subgroups so that measurements can be spaced throughout the
day in a balanced distribution and thus average out any linear drifts in the
measurement process. Each comparison of working standard and test lamps is
performed on three different days using different photodetectors. Electrical
measurements are made with an integrating digital voltmeter and/or a
potentiometer to an accuracy of 0.02% or better. Power is applied slowly to
the lamps, taking at least 15 seconds to reach the desired current. The lamps

25


operate at 32 volts and the 1000-watt lamps operate at 120 volts; both types
are designed to have approximately 1000 hours life. The 500-watt lamps are
designed to have 500 hours life at 120 volts.

2.1.2 Preparation

The lamps are seasoned by operating them at design voltage (dc) for
approximately 5% of their rated life and an identifying number is etched on
each bulb.

2.1.3. Orientation

Test lamps are calibrated while operating base down on a horizontal bench
photometer with the identifying number turned away from the detector. Lamp
orientation is accomplished by aligning the lamp socket so that the lamp
posts are held vertically, and the plane formed by the axes of the posts is
perpendicular to the optic axis of the photometer. A special multi-pin jig
and silhouette procedure has been developed for this alignment. The bottom of
the lamp bulb is 10.16 cm below, and the lamp pins are equidistant from, the
optic axis. Source-to-receiver distances are measured from the plane formed
by the axes of the biposts to the sensitive surface of the detector.

The use of inside frosted instead of clear lamps greatly reduces the
variation in luminous intensity that can result from small changes in the
pitch orientation of the lamps [10]. In lamps that are issued to the public
the luminous intensity of each lamp varies by less than 0.2% for (1) a+ 1.5°
rotation about a horizontal axis intersecting the optic axis in the plane
formed by the axes of the posts (pitch) and (2) a +1.5° rotation about a
vertical axis contained in the post plane and intersecting the optic axis
(yaw).

2.1.4 Operation

All measurements are performed with the lamps operated on dc power.
Electrical measurements are made with a digital multimeter to an accuracy of
0.02%. After positioning and alignment, the lamps are slowly (15-30 seconds)
brought up to the designated electrical operating point and allowed to

i


Table 3

Uncertainties (30) Associated with the NBS Scale of Luminous Intensity

Uncertainty of the Calibration of the Reference Lamp Group (NBS 5612

Il.

Ill.

IV.

Group)

a. With respect to SI units 0.7%

b. NBS long term reproducibility 0.4%

Transfer Calibration of the NBS Working Group

a. Random uncertainty of the mean of the eight lamps 0.2%

(30)

Substitution Calibration of a Test Lamp

a. Random uncertainty (30) 0.5%

b. Nonrandom shifts 0.5%

Total Uncertainty of a Reported Value

a. With respect to SI Units ( a quadrature 1.0%
combination of items I> II, and III.)

b. NBS long term reproducibility (a quadrature 0.8%

combination of items I II, and III.)

b?

7


from light reflected off the socket. Light emitted by the lamp in the
direction away from the receiver is absorbed with a light trap placed 40 cm
behind the lamp.

2.3 Calibration Procedure
2.3.1 Photometer Calibration

The photometer is calibrated, at the time of the measurements, in terms of
the illuminance produced at the detector by each of a group of 500-watt lamps
of the same type and construction as the test lamps. These working standards
are periodically compared with a group of 9 similar lamps (NBS 5612 group)
which represents the unit of luminous intensity at approximately the color
temperature of CIE standard illuminant A (2856 K-IPTS 68) as maintained at
NBS.

An illuminance substitution method is used for the measurements.
Source-to-detector distances (photometric distances) are chosen to produce
approximately equal illuminances at the detector and thus avoid any need for
linearity corrections. 500-watt lamps are calibrated at a photometric
distance of 3 meters for both standards and test lamps. This produces
approximately 80 lux at the photocell. When calibrating 100-watt lamps the
photometer is calibrated with the 500-watt working standard at a photometric
distance of 3 meters and again with a distance of 4.17 meters (approximately
40 lux). The test lamps are then measured at two distances, one distance to
produce approximately 80 lux and another distance to produce approximately 40
lux. The 1000-watt lamps are measured at 4.17 meters with the photometer
calibrated with the 500-watt working standards at a distance of 2.9 meters
Detailed procedures for luminous intensity calibrations are given in Appendix
A.

2.4 Measurement Schedule

Test lamps are measured in groups of approximately 18. Normally 8 working
standards are used to calibrate the photometer. Measurements of the working
standards are interspersed before, after, and within the group of test lamps
to check for drifts in the detector sensitivity.

13


are allowed to warm up at least 5 minutes in the closed sphere. All lamps are
set at their assigned currents, and the operating voltage is read at the time

their light output is recorded. Detailed procedures for a routine luminous
flux calibration are given in Appendix B.

At the conclusion of one day's measurements, a photometer calibration
factor is calculated for each working standard lamp in the run by dividing the
value of the flux assigned to the lamp by the corresponding signal from the
photocell. The average calibration factor for all standards in that day's run
is then calculated. The uncorrected luminous flux for each test lamp is
computed by multiplying this average calibration factor by the signal from the
photocell when illuminated by the test lamp. These uncorrected flux values
are converted into measured flux values by correcting for the differences
between the working standards and test lamps' self-absorption in the sphere

photometers [19], and for the nonlinearity of the particular photocell being
used.

In order to determine the correction due to lamp absorption in the
2-meter sphere, a 200-watt lamp is placed at the bottom of the sphere. Light
from the 200-watt lamp is baffled from the sphere window and from unlit lamps
placed in turn at the center of the sphere. Readings are taken of the
detector output when each of the unlit working standards is in the sphere and
again when the unlit test lamps are in the center of the sphere. The
correction is the ratio of the average reading taken with each group of
working standards to the average reading of the test lamps being measured.
Corrections are typically 1% or less.

Similar measurements are made in the 0.76-meter sphere using a 28-volt,
40-watt, partially-silvered-bulb lamp near the bottom of the sphere. The
corrections for absorption in the 0.76-meter sphere relative to the type 1133
lamps are typically a few tenths of a percent.

For each of the three selenium photocells used in these calibrations, the
deviation from linearity of the response of the detection system is
determined. The determination is made on a horizontal-bar photometer by an
inverse-square method. The test photocell is mounted on the bar at a known

26


Point Source

Figure 3.

Concept of Luminous Intensity


Pons NES463
(any. 3-48)
U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

for
Luminous Intensity
of
Three 500-Watt Inside-Frosted Lamps
Supplied to:

(See your Purchase Order No. G53488, dated April 16, 1985)
1. Material

Three 500-watt lamps were calibarated for luminous intensity.
The lamps have C-13B filaments in T-20 inside-frosted bulbs with
medium bipost bases. The lamp designations listed below are
etched on the bulbs.

2. Calibration

The lamps were calibrated for luminous intensity in accordance
with the enclosed description of the calibration of inside-
frosted lamp standards of luminous intensity. Note especially
Paragraphs I-C and I-0 which describe the orientation and opera-
tion of the test lamps. The photometric distance used for this
calibaration was 3 meters,

3. Results

Lamp No. Direct Current Voltage Luminous Intensity
(amperes) (volts) Ceandelas)

NBS 10499 3.900 108.87 676

NBS 10500 3.900 109.34 720

NBS 10501 3.900 108.72 703
Prepared by: Approved by:
Vbretdrd Y)thver Bemetel Q 741 Sheen,
Ronald L. Wilkinson Donald A. MceSparron
Radiometric Physics Division Radiometric Physics Division
Center for Radiation Research Center for Radiation Research
NBS Test No.: 534/
Date: January 7, 1986 Page 1 of 1

*The enclosed description referred to above includes an uncertainty estimate
that has been updated in the present document. See Table 2 of the present
document for the updated uncertainty values

D-1


and the light center. Set the zero position. Cover the shadowgraph grid with
the black cloth. Position the baffles as described in section 2.2.2 of the
text. Remove the jig and put in the Ist working standard.

3.3 Shunt Box

The electrical measurement circuitry includes a shunt box in series with
the lamp. Shunt values must be selected that are greater than the operating
current of the working standards or test lamp. Since the 1000-W lamp draws
approximately 7.7A, use the 15 A shunt.

3.4 Volt Box

The electrical measurement circuitry includes a volt box in parallel with
the lamp. Put the volt box setting on 150 volts.

3.5 Lamp Connections

Make electrical connections to the lamp. Turn on the power supplies with
the coarse control turned all the way down. Slowly increase the power until
the current is about 1/2 the set value. Let the power supplies and
measurement circuitry warm up in this fashion for a short period. During the
warm-up period, prepare the run cards.

4. Run Cards
4.1 Working Standards

Three separate run cards are prepared, one corresponding to each selenium
cell detector. List the working standards, operating current, shunt value,
operational amplifier setting, the source to detector distance, and the bar
marking for the detector and source carriages. Reverse the sequence of the

working standards on the second run card.

4.2 Test Lamp

Prepare a lamp card for each test lamp. List test number, (if
applicable), lamp number, lamp description and current value.

A-2


Page
LIST OF FIGURES

Figure 1 Spectral Luminous Efficiency Function V(A) Defining

Figure 2

Figure 3

Table

Table

Table

Table

Table
Table

Table

Table

on

the Standard Photopic Observer... 1... 5... ee eee ae 4
Photometric Chain Showing Derivation of Photometric

Scales From the Gold Point Blackbody........... . 6
Concept of Luminous Intensity. ...........20- . 8

LIST OF TABLES

Important Quantities, Units, and Symbols Used in Radiometry

and Photometry. .......220% rr ra 5
Uncertainty Estimates (%, 30) For the Spectral Irradiance

and Photometric Scales at the Time of the Photometric Scale
Realization. 2... ee ee ee 15
Uncertainties (30) Associated With the NBS Scale of

Luminous Intensity. ......... ee 17
Operating Characteristics of Miniature Lamp Standards

of Luminous Flux. . 6 6 eee ee ee ee ee ery 22
NBS Working Standards of Luminous Flux... ....+---. » 25
Uncertainties (30) Associated with Large Lamps Calibrated

for Luminous Flux... 2. ee eee eee Pe ee ee 31
Uncertainties (30) Associated with Miniature Lamps Calibrated

for Luminous Flux... 1 ee ee ee ee ee 33
Color Temperature Uncertainties (30), .........505008 43


3.2 Sphere Photometers

There are two integrating spheres used for the luminous flux measurements;
one with a diameter of 2 meters and one with a diameter of 0.76 meters. The
2-meter sphere is coated with powdered barium sulfate, prepared by mixing 95%
ethyl alcohol with reagent grade barium sulfate, and then spraying the
resulting suspension onto the sphere wall. The alcohol evaporates leaving a
pure barium sulfate coating. About 30 coats are necessary to produce the
required thickness of 1 to 2 millimeters. A 7.6-centimeter diameter
observation port is located in the sphere wall. The 0.76-meter diameter
sphere is coated with a commercial sphere paint made of a mixture of magnesium
carbonate, latex, and mineral spirits. A 4.3-centimeter diameter observation
port is located in the sphere wall.

The observation ports in both spheres are fitted with white diffusing
plastic filters (Plexiglas color W2447 Rohm and Haas white) mounted flush with
the inside sphere walls. 8affles, coated with the commercial sphere paint,
are placed between the lamps and the observation ports. In both spheres, lamps
are located at the nominal center of the sphere. When measuring lamps up to
500 watts in the 2 meter sphere, the baffle used is a 20 cm disk, located 0.55
meters from the observation port. For special measurements of higher wattage
lamps, a baffle 45 cm in diameter is used. In the 0.76 meter sphere, a
rectangular baffle is used, 6 cm by 7.6 cm, placed 0.24 meters from the
observation port.

Because of the selective spectral reflectances of the sphere coatings and
selective spectral transmittances of the windows, pale blue filters are
inserted adjacent to the windows in both spheres. Each pale blue filter was
selected from a set of Corning 5900 filters of graded color temperature
altering power. The net spectral effect of the sphere reflectance, window
transmittance and the blue filter is that the relative spectral distribution
of the flux arriving at the detector is approximately the same as that leaving
the lamp. The proper filter is determined just before a series of
calibrations is started. Typical corrections are 350 K for the 2 meter sphere
and 500 K for the 0.76 meter sphere

23


5. Measurements

5.1 Data-Taking

When the timer goes off, recheck the current set. Rotate the input
selector switch so the DVM reads the op amp. Set the DVM controls to take an
average of 10-30 readings of the op amp output. Record this data on the run
card. Rotate the switch and record the lamp voltage. Slowly power down the
lamp. Turn off the power supplies. Open the sphere. Remove the lamp and
replace with the next lamp in the 6807 group. Close the sphere. Turn on the
power supplies. Slowly power up the lamp and set the timer. In this manner,
run 1/2 of the working standards.

5.2 Test Lamp

Put the test lamp in the sphere. Slowly increase the power. When the
electrical operating point is reached, set the timer. When the timer goes
off, use the integrating DVM to take an average of 10-30 readings and record
the current, voltage, and op amp reading. Power down the lamp, turn off the
power supplies, open the sphere, and remove the test lamp.

5.3 Remaining Working Standards

Run the three remaining working standards. This completes the Ist

measurement run.

5.4 Remaining Measurements

Take out the Ist detector cell and insert the 2nd cell. Perform the 2nd
measurement run. This procedure will be the same as the first run, except the
sequence of the working standards is reversed. A 3rd run is performed with
the remaining detector cell, in the same sequence as the Ist run.

6. Data Reduction
6.1 Calibration Factors

Divide the assigned flux value for each working standard by the measured
detector output from the Ist run. This yields a K-factor in lumens/volt for

B-3


APPENDIX C
DETAILED PROCEDURES FOR
A ROUTINE COLOR TEMPERATURE
CALIBRATION


TABLE OF CONTENTS - APPENDIX C

Introduction. .....

Lamps . . .
2.1
2.2

Test Lamp

Working Standards

Preparation of Run Cards.

3.1
3.2

Working Standard.
Test Lamp . .

Measurement Preparation .

4.1 Photometer. . .
4.2 Alignment

4.3

Measurements... ..
5.1 Working Standards
5.2 Test Lamp....

Data Reduction. ....
6.1

Test Lamp . .

Photometer Sensitivity Adjustments

Page

. C-1

C-1

. C-1
. C-1

. C-l

C-1

. C-1

- C-1

. C-1
. C-2
. C-2

. C-2

C-2
C-2

+ C-2


3. LUMINOUS FLUX CALIBRATIONS

Radiant flux [o,] is radiant energy per unit time. Luminous flux is
radiant flux weighted with the photopic luminous efficiency function V(A),
which is defined in a way that limits our concern to the visible spectrum.
When the geometrically total flux (in lumens) of a lamp is known, then the
luminous efficacy of the lamp in lumens/watt is easily determined. Lamps are
rated in this manner throughout the lighting industry and light fixtures are
evaluated according to the fraction of a lamp's flux output that they direct
to a task or a surface.

The fundamental method of determining the total luminous flux emitted by
a source is to measure the luminous intensity in a large number of directions
distributed in space about the source, and then integrate the measured
luminous intensity over the solid angle of 4m steradians. A practical method
jis to measure the illuminance from the source at points on the surface of an
imaginary sphere. Large source-to-detector path lengths are not necessary
when deriving the lumen (total flux) from illuminance, rather than intensity
measurements, and it is not necessary that the source be at the center of the
sphere [16]. Since the detector can be brought significantly closer to the
source, there will be a greater sampling of the surface surrounding the source
for the same number of measurements taken. National standardizing
laboratories use a goniophotometer, consisting of a photometer which can be
rotated about the source, to measure directional light distribution
characteristics.

A theoretically equivalent procedure is to mount the light source on a
goniometer which allows the source to be rotated about its horizontal and
vertical axes. The illuminance is measured by a single fixed detector. This
method is generally not used by standardizing laboratories because it
introduces uncertainties due to changing the orientation of the operating
lamp.

Once a scale of total luminous flux has been established, a more
convenient way to transfer calibrate other lamps is by means of a substitution

20


ronu MES443
(nev. as)
U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

for

Color Temperature
of

One 500-Watt Lamp

Supplied to:

(See your purchase order number 1511 dated July 23, 1985.)
1. Material

One 500-watt airway beacon lamp was calibrated for color temperature.
The lamp has a C-13B filament ina clear T-20 bulb with a medium bipost
base. The lamp designation listed below is etched on the bulb.

2. Calibration

The lamp was calibrated in accordance with the enclosed description* of
the calibration of incandescent lamp standards of color temperature.

The currents through the lamp required to produce color temperatures of
2000 K, 2400 K, 2856 K, and 3000-K were determined. The coefficients of
the color temperature equation result from a least squares fit of the
measured data points to a second order polynomial.

3. Results
coer Temperature

Lamp No. Direct Current Voltage IPTS 1968)

(amperes) volts kelvins
NBS10496 2.460 43.21 2000

3.228 71.21 2400

4.152 112.76 2856

4.463 128.46 3000
NBS Test No.: 534/
Date: December 24, 1985 Page 1 of 2

*The enclosed description referred to above includes an uncertainty estimate
that has been updated in the present document. See Table 8 of the present
document for the updated uncertainty values.

D-6


TABLE OF CONTENTS (continued)

5. Measurements. 2... ee ee ee ee
5.1 Working Standards . 2. 2 1 wee ee ee ee ee woe
5.2 Test Lamp... we ee
5.3. Remaining Working Standards... 1 ee ee ee ee ee ee
5.4 Remaining Measurement Runs. 2... 1 1 ee eee ee ee ee

6. Data Reduction, ©... ee ee ee
6.1 Calibration Factor... 1 1. ee ee ee ee :

6.2 Quality Control Factor... ... 2. ee ee ee eee
6.3 Test Lamp Luminous Intensity Value. ..........00.
6.4 Final Value... . - . ee eee ee ee

Appendix B - Detailed Procedures for a Routine Luminous Flux Calibration

1. Introduction. ©... ee

2. Lamps... we ee ee ee see eee
2.1 Test Lamp. ..... ry
2.2 Working Standards... 1. ee ee ee

3. Run Card Preparation. . 2... 1 ee ee ee ee we
3.1 Working Standards... 2. ee ee ee :
3.2 Test Lamp... ww ee ee

4, Measurement Preparation... 1 1. ee ee
4.1 Shunt Box . 2. ee
4.2 Volt Box... .. ee. Ss
4.3 Detector. ........ ce
4.4 Lamp Connections. .. 6. 2 eee ee ee ee ee see
4.5 Final Adjustments . 6. 6. ee ee

5. Measurements. 2. 6 6 eee ee ee
5.1 Data Taking... .... ee. So
5.2 Test Lamp. .... cy

vii

A-3
A-3
A-3
A-3

A-3
A-3
A-3
A-4
A-4

B-1

B-1

B-1

B-1

B-1

B-1

B-2
B-2


TABLE OF CONTENTS (cont inued)

Page
4. Color Temperature Calibrations... .........02.00- .. 36
4.1 The NBS Color Temperature Scale. .............. 36
4.2 Test Lamps. ..... a
4.2.1 Preparation... ... rr cn eee ee ee 89
4.2.2 Orientation. . 2... eee ee ee nS)
4.2.3 Operation. 2... ee 39
4.3. Color Temperature Apparatus... ....... rr 0)
4.4 Calibration Procedure... . 1... ee ee ee eee wee Al
4.5 Uncertainty. 2... ... ee ee ee ~. 42
4.6 Discussion... . 1... ee ee ee eee 4
References. ©. 6 6 ee ee oe 45

1. Introduction, ©... ee eee A-1
2. Lamps... . 2... eee i eee Al
2.1 Test Lamp... 2 2. eee ee eee ee ee Al
2.2 Working Standards... 2... ee ~ . Al
3. Measurement Preparation. ...... ry A-1
3.1 Detector Alignment... ....... Ce ee we ee A-1
3.2 Lamp Alignment. 2. 2 2 2. ee ee ee A-1
3.3. Shunt Box... 2... Se A-2
3.4 Volt Box, . ee . A-2
3.5 Lamp Connections. .........2000- So ee A-2
4, Run Cards. 2... eee eee rrr soe ee ee AM?
4.1 Working Standards ........ Se ee wee ee A?
4.2 Test Lamp... ... eee eee rr A-2

vi


function, its numerical weighting is fixed by international agreement and
hence photometric measurements are physical measurements in the same sense as
the measurement of analogous radiometric quantities. Table 1, produced from
reference [4], shows the relationship between analogous radiometric and
photometric quantities and units.

In principle, based on equation (1), it is also feasible to establish a
radiometric/photometric scale that is based on the physics of silicon
photodiodes instead of the physics of a gold-point blackbody. A detector-
based scale of radiometry, in fact, is one of the goals of the NBS photo-
detector program. Procedures have been developed whereby "self-calibrating"
silicon photodiodes are used to establish a detector-based scale [8] which
compares quite well [9] with established source-based scales.

1.4 Safety

A 5mw helium-neon laser is occasionaly used for alignment of the
Photometric bench. An appropriate warning sign is displayed at the laboratory
entrance. High current laboratory power supplies are used to power the higher
wattage lamps. Appropriate precautions are used when working with these and
other electrical laboratory equipment.

1.5 Acknowledgements

We express our appreciation to R. D. Saunders for his discussions
assisting in the preparation of the uncertainty analyses and to R. L.
Wilkinson for his assistance in the preparation of the detailed calibration
procedures in Appendices A, B, and C.


TABLE OF CONTENTS (continued)

Page
Appendix D - Reports of Calibration
Report of Calibration for Luminous Intensity of Three 500-watt
Inside-Frosted Lamps. ....... Se D-1
Report of Calibration for Luminous Flux of one 200-watt
General-Purpose Lamp. .. 2... 2... eee eee eee ee ee ew DH?

Report of Calibration for Luminous Flux of Ten Miniature Lamps. .... D-4

Report of Calibration for Color Temperature of one 500-watt Lamp. ... D-6


Table 2.

Uncertainty Est imates
Photometric Scales at
Realization.

(%, 3c) for the Spectral Irradiance and
the Time of the Photometric Scale

400 450 500 555 600 654.6 700 v(a)
nm nm nm am am nm nm Weighted
1. NBS SPECTRAL
RADIANCE SCALE
a. Absolute error 0.92 0.82 0.73 0.66 0.61 0.56 0.54 0.65
(with respect
to SI Units)
b. NBS long term 0.46 0.40 0.35 0.32 0.29 0.28 +29 0.32
reproducibility
II. NBS SPECTRAL
IRRADIANCE SCALE
a. Systematic 0.21 0.19 0.18 0.16 0.16 0.15 0.14 0.16
errors
b. Random errors 0.07 0.07 0.07 0.07 0.06 0.06 0.07 0.07
(3 o precision)
III. TRANSFER CALIBRATION
OF A TEST LAMP
a. Systematic 0.26 0.24 0.22 0.20 0.19 0.18 17 (0.20
errors
b. Random errors 0.20 0.20 0.20 0.15 0.15 0.15 20 0.15
(3 0 precision)
IV. UNCERTAINTY OF
REPORTED VALUES
a. With respect to 1.00 0.90 0.81 0.72 0.68 0.62 62 0.71
SI units
b. NBS long term 0.60 0.55 0.49 0.45 0.42 0.40 42 0.44
reproducibility
IVa is a quadrature combination of items Ia, II and III.
IVb is a quadrature combination of items Ib, II and III.


NBS MEASUREMENT SERVICES:
PHOTOMETRIC CALIBRATIONS

Robert L. Booker

Donald A. McSparron

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

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987


These calibrations have since been validated by checks against the NBS
spectral irradiance scale. Lamp NBS 9341 was calibrated from spectral

irradiance measurements.

In addition, lamps NBS 9098, 9099, 9166, 9246, 10155, and 10423 are used
in the range 2600-3200 K. These lamps were calibrated from spectral
irradiance data obtained with a high accuracy spectroradiometer. Using the
procedure described by Kelly [23], chromaticity coordinates were computed for
these lamps, converted to u, v space, and correlated color temperatures
determined at a number of currents

Once spectral irradiance data are used to establish the color temperature
calibration at fixed points, it is convenient and straightforward to
interpolate and extrapolate the calibrations to other color temperatures.
Empirical equations of the form

T=At BT, + CT 4 (10)

relating the color temperature To and current I through the lamp, were used to
establish the scale in increments of 1 K throughout each lamp's range.

In addition to the clear lamps just described, which collectively
comprise the color temperature scale over the range 2000-3200 K, three
inside-frosted lamps, NBS 9171, 9172 and 9173, are used at 2856 K (CIE Source
A) to calibrate inside-frosted test lamps at that color temperature
Initial calibration of these lamps was performed spectroradiometrically
against the 500-watt, clear-bulb working standards of color temperature. The
spectroradiometer used for this calibration [27] determined the ratios of the
spectral radiant powers of the frosted and clear lamps at every 10 nm from 380
nm to 760 nm with a spectral bandpass of 5 nm. The data reductions assumed
that the relative spectral power distribution of the clear-bulb standards was
the same as the relative power distribution of a Planckian radiator. The
relative spectral power distribution of the inside-frosted lamps thus
determined was used to compute their chromaticity coordinates. The procedure
described by Kelly [23] was then used to compute the correlated color
temperature of the inside-frosted lamps.

38


will be replaced with a modern high-quality rail and carriage system in a
slightly different configuration. The V(A)-corrected selenium barrier layer
photocells will be replaced with V(A)-corrected silicon photodiodes.

1.1 Photometry, Physical Photometry, and Radiometry

An understanding of the relationship among these three branches of science
is fundamental to the application and use of the photometric standards
provided by NBS. This topic is treated in a concise, but comprehensive manner
in CIE Publication 18.2 [4]. The following discussion is intended to provide
the reader with a brief overview of these relationships. The reader is urged
to consult the literature and obtain a more comprehensive understanding.

Photometry deals with the measurement of optical radiation evaluated
according to the visual effect it produces. Visual effects result from a
processing of light stimuli by the human visual system - the eye, the optic
nerve, and the more central visual parts of the brain. It is a very complex
phenomenon and obviously of a subjective nature. Quantification of these
visual efects depends on the choice of observer, the state of adaptation of
the eye, the angular size and retinal location of the visual field, the
intensity of the incident radiation, its wavelength, and other factors. In
order to provide a convenient, precise, and reproducible basis of measurement
for science and commerce, the CIE has adopted a number of conventions defining
a standard observer. Use of these conventions allows one to relate a
characterization of optical radiation in purely physical terms (radiometry) to
the visual effects it produces, though only approximately. Objective
measurements made incorporating these conventions and using physical detectors
of optical radiation constitute Lhe branch of science known as physical
photometry.

According to the CIE conventions, a generalized radiometric quantity,

a, » is related to its analogous physical photometry quantity, Q., by an

equation of the form:

Ve

% = Ki, J Qo VOar (1)


1 INTRODUCTION

Photometric calibration facilities at NBS were most recently described in
1972 [1]. The present document updates the 1972 Technical Note, particularly
in the area of uncertainty estimates. Throughout this document uncertainties
are estimated at the 30 level. The specific calibrations described in this
document are luminous intensity [candela], luminous flux [lumen] and color
temperature [Kelvin]. These quantities are defined later in sections 1.3 and
4.1

Brief descriptions and prices for the individual standards and
calibrations are listed in NBS Special Publication 250 [2] and its appendices
(published semi~annually). Luminous intensity standards are listed in the
SP 250 Appendix as items 37010C through 37070C (formerly items 7.7B through
7.7H), luminous flux standards as items 37080C through 37130C (formerly items
7.71 through 7.7M and item 7.72Q) and the color temperature standards as items
37140C through 37160C (formerly items 7.7N through 7.7P). Lamps are often
calibrated for luminous intensity or luminous flux while operating at a
specified color temperature. Operation at 2856 K (cle* Source A) [3] is most
often requested.

The material presented in this technical note is a description of
photometric calibration facilities and procedures as they existed in July,
1986. Both the photometric bench and the color temperature apparatus will
undergo modernization in the near future. In both cases, the measurement
principles now in use are still valid and will remain the same. Changes to
the color temperature apparatus will involve replacing the presently used
Pritchard telephotometer with red- and blue- filtered silicon photodiodes
permanent ly mounted to viewing ports on the sphere. The photometric bench

*the CIE (Commission Internationale de L'Eclairage; International Commission
on Illumination) is a voluntary standards organization devoted to
international collaboration and exchange of information on illuminating
engineering, radiometry, photometry, and colorimetry.


Detailed Procedures for a Luminous Intensity Calibration

1. Introduction

This procedure describes the luminous intensity calibration of a single
1000-W, inside-frosted lamp. A single lamp is used to illustrate the
procedure; in actual practice a group of test lamps would be calibrated.

2. Lamps
2.1 Test Lamp

The Lest lamp is a 1000-W, inside-frosted, medium bipost lamp with a T-24
bulb. The lamp has been seasoned at rated voltage for approximately 5% of its
rated life and has an identifying NBS number etched on the bulb.

2.2 Working Standards

The working standards are the eight 500-W lamps in the NBS 7796 group.
They are also inside-frosted, medium bipost lamps, but have a T-20 bulb.

3. Measurement Preparation

3.1 Detector Alignment

Mount the first selenium cell on the detector carriage. Bring the
carriage up to the shadowgraph. Remove the aperture mask from the detector.
Turn on the shadowgraph light source and align the detector to the zero
Position on the shadowgraph grid. Note the carriage position on the bar.

Replace the aperture mask and move the detector carriage back 2.9 m.

3.2 Lamp Alignment

Put the lamp mount back on the bar. Mount the alignment jig in the lamp
mount. Using the alignment jig and the shadowgraph, adjust the height using
the cross hairs corresponding to a 4" light center height. Then using the
pitch and rotation adjustments, orient the alignment jig so that it is
perpendicular to the centerline extending between the center of the detector


Bureau of Standards with the best precision now available. They are expensive
laboratory equipment and deserve the utmost care in handling and use.

In addition to some obvious precautions, such as keeping the lamps clean
and avoiding mechanical shock to the filament, the following are a few
precautions which should be followed. Precautions 1 through 7 apply to both
large lamps and miniature lamps.

1. The electrical power to the lamps at turn on or turn off should be
increased or decreased gradually, taking at least 15 seconds to reach the
specified electrical operating points.

2. In order to prolong the useful life of the lamps as standards, it is
recommended that they be used sparingly and great care should be taken so that
at no time will the current through the lamp appreciably exceed the value
stated in the report. For general use, working standards should be prepared
by calibrating them relative to the lamp standards supplied by NBS. When a
laboratory procures standard lamps from NBS, it is well to obtain at least
three such lamps in order to be able to detect any changes that may occur.

3. A baffle should be used between the lamp and the observation window.
The baffle should be as small as possible but large enough to prevent any
light from the lamp reaching the window without undergoing at least one
reflection. The baffle surface should have the same reflectance properties as
the sphere wall.

4. The variation of luminous flux of a tungsten filament lamp is
approximately related to the variation in its current by the equation [14,15]:

a
ole

= di
=ko (8)

where > is the luminous flux, i is the lamp current and K is 6.25 for
gas-filled lamps and 6.05 for vacuum lamps. Thus a current measurement
accuracy of at least 0.032% is required if the uncertainty in the Tuminous
flux is to be no greater than 0.2% due to the electric current uncertainty.

34


25-watt inside-frosted, vacuum, tungsten lamps and 60-, 100-, 200-, and
500-watt inside-frosted, gas filled, tungsten lamps are calibrated for
issuance as luminous flux standards. The corresponding values of luminous
flux are approximately 270, 870, 1600, 3300, and 10,000 lumens. The 25- and
60-watt lamps have A-19 bulbs, the 100-watt an A-21, the 200-watt a PS-30, and
the 500 watt has a PS-35 bulb. All of the lamps have medium screw bases,
except the 500-watt which has a mogul screw base.

Miniature lamps are also calibrated for issuance to the public. Miniature
lamps currently issued to the public are part of a stock resulting from a
1981-82 calibration of 6 types of miniature lamps ranging in total flux output
from 6 to 400 lumens. Electrical leads covered with white insulation are
soldered to the base of each lamp. The lamp types and their approximate
operating characteristics are listed in Table 4. Lamps submitted by users for
calibration must be of the types listed in Table 4.

Table 4. Operating Characteristics of Miniature Lamp Standards of
Luminous Flux

Lamp Current Voltage Approximate Nominal
Type (set) Color Temp. Luminous Flux
(amperes) (volts) (kelvins) (lumens) (MSCP )

1183 5.80 5.0 2950 400 32

1183 5.55~ 4.7 2856 300 24

1133 3.50 5.3 2856 220 18
87 1.80 6.1 2856 130 10
81 1.00 6.1 2856 66 5.2
63 0.63 7.4 2750 44 3.5
51 0.197 6.3 2400 6 0.5

22


4.2 Test Lamps

Clear airway beacon lamps are issued by NBS as color temperature
standards in the range 2000-3000 K. These nominally 500 watt, 120 volt lamps
have clear T-20 bulbs, C-13B filaments, medium bipost bases, with a rated life
of 500 hours at 120 volts.

At present, inside frosted lamps are issued as color temperature
standards at a single color temperature only, 2856 K. The physical
characteristics of these lamps are the same as the clear lamps except for the
frosting.

4.2.1 Preparation

Initially, the lamps are seasoned by operating them at 120 volts dc for
20 hours (4% of rated life). An identifying number is then etched on each
bulb.

4.2.2 Orientation

Test lamps are calibrated while operating base down with the etched
identifying number turned away from the comparator. The clear lamps are
aligned so that their planar filaments are centered on a perpendicular to the
optical axis of the entrance aperture of the collecting sphere of the color
temperature comparator.

Orientation of the inside frosted lamps is accomplished by aligning the
lamp socket so that the lamp posts are held vertically with the plane formed
by the axes of the posts perpendicular to the optical axis of the comparator.
The light center of the lamp is on, and the lamp pins are equidistant from
this optical axis.

4.2.3 Operation

All measurements are made with the lamps operating on dc power.
Electrical measurements are made with a digital voltmeter to an accuracy of

39


method using an integrating sphere. According to the theory of the
integrating sphere [17], the reflected flux reaching any point on the sphere
wall is directly proportional to the total flux of a source within the sphere
Since it is only the reflected flux that is proportional, it is necessary to
baffle the source to prevent direct light from reaching the detector. The
baffling is a perturbation to the sphere theory. In routine substitution
measurements, baffles and lamp mounting hardware are left in place in the
sphere and their effects approximately null out. However, the lamps
themselves (standards and test lamps) and their sockets may differ in
self-absorption, and corrections for these effects may be necessary.

In June, 1980, the group of opal-bulb lamps (NBS 526 Group) that serves as
the primary NBS reference standard of luminous flux was recalibrated in accord
with the 1979 international redefinition of photometric units [18], using the
goniometric distribution of the lamps. This calibration was then transferred,
using the substitution method, to three working groups, consisting of lamps
ranging from 100 to 500 watts. The 1980 substitution calibration of the
working group lamps is described here, and the luminous flux scale realization
is summarized briefly. Lamps calibrated for issuance to the public are
measured against the working group in a manner similar to the calibration
of the working groups against the NBS primary reference standards.

For miniature lamps, luminous flux scales are derived from the 100-watt
work ing group (NBS 3158 group) by substitution methods in integrating sphere
photometers. The present scale, realized in this manner, was first issued in
1974 and was re-determined in 1981/82.

3.1 Test Lamps

Two general types of incandescent lamps, large lamps and miniature lamps,
are calibrated for issuance to the public. Lamps are seasoned at rated
voltage for 4% of their rated life before calibration and each bears an
identifying number etched on the neck of the bulb. Lamps are usually supplied
by NBS but may be submitted by the user. The so-called large lamps are
general-purpose, inside-frosted lamps ranging from 25 to 500 watts. The
clear-bulbed, miniature lamps range from about 1.25 to 29 watts.

21


lamp, test number from the NBS test folder, and corresponding run numbers from
the run cards for the working standards.

4. Measurement Preparation
4.1 Shunt Box

The measurement electrical circuitry includes a shunt box in series with
the lamp. Select a shunt value greater than the operating current of the
working standards or test lamp. For the 200-N luminous flux lamps, the 3.0 A
shunt is the proper one. (approximately twice the lamp current).

4.2 Volt Box

The measurement circuitry includes a volt box in parallel with the lamp.
Put the volt box setting on 150 v.

4.3 Detector

Mount one of the cells in the tube extending from the sphere window. Do
not lock the detector in place. Connect the detector to the operational
amplifier.

4.4 Lamp Connections

Open sphere and mount the appropriate base/lamp holder in the sphere
(medium-screw for the 200-W lamps). Put in Ist lamp of the 6807 group and
close the sphere. Complete connections to power supply. Turn on the power
supplies with the coarse control] turned all the way down. Slowly increase the
power until the set current is reached. Set the timer for 7 minutes.

4.5 Final adjustments

Determine final position of detector cell in the tube (distance from
diffusing window). The cell must not be saturated and should operate at
80-90% of full scale. Leave the cell in this position for the remainder of
the run. Determine the final range on the operational amplifier.


TABLE OF CONTENTS (cont inued)

Page

5.3. Remaining Working Standards... 2... 2. ee ee eee ee B-3

5.4 Remaining Measurement ...... So ee we ee BB

6. Data Reduction, 2... ee B-3

6.1 Calibration Factor... ...... So ee see ee BAB

6.2 Quality Control Factor. .............04. we. BAA

6.3 Test Lamp Luminous Flux Value. . 2. 2 eee ee ee 2. BG

6.4 Final Value... . ee see ee BA
Appendix C - Detailed Procedures for a Routine Color Temperature

Calibration

1. Introduction, ©. 6 6 eee ee wee CHl

2. Lamps..... Sc C-1

2.1 Test Lamp... . ee C-1

2.2 Working Standards . 2 1 2 we ee ee ee ~. C-l

3. Preparation of Run Cards... 1... 1. eee eee se ee C-1

3.1 Working Standard... ... ar C-1

3.2 Test Lamp... . ee C-1

4. Measurement Preparation... 1... eee ee ee ee ee ee C-1

4.1 Photometer. 2. 2... 2 eee ry C-1

4.2 Allignment. ...... Ce C-2

4.3. Photometer Sensitivity Adjustments. .........2.0. ~ . C-2

5. Measurements. ..... ear - . C-2

5.1 Working Standards .. 2... 1... ee eee see ee ee C2

5.2 Test Lamp. . 2... ee ee ee rn Oy 4

6. Data Reduction. . 2 6. ee C-2

6.1 Test Lamp 2. 1. ee eee ee bo ee ee C-2

viii


stabilize for at least 10 minutes before measurements are made. Normally, the
lamp current is set and measurements are made of the luminous intensity and of
the potential drop across the pins of the bipost base

2.2 Photometer
2.2.1 Detector and Amplifier

The photometer detector is a selenium barrier-layer photocell [11]
equipped with a filter which modifies its spectral response to approximately
match the CIE luminous efficiency function [3,12]. Measurements are usually
made at a photocell illumination level of approximately 80 lux. The entire
photosensitive surface is directly illuminated by the entire test or standard
lamp, and no auxiliary optics are used. The photocell is fully illuminated
during the 10 minute stabilization period of the lamps.

The detector photocurrent is measured with a unity gain operational
amplifier (current to voltage converter) arranged in a typical closed loop
configuration. Thus the photocell is operating into a near "zero-resistance"
circuit (no voltage across the terminals of the photocell). A 5 1/2 digit
voltmeter is used to measure the output of the operational amplifier.

2.2.2 Optical Bench

All measurements are made on a 4.5-meter optical bench. The optical bench
is housed in an enclosure 61 cm wide and 117 cm high. The enclosure is
covered with black "suedine" cloth on all four sides. The optic axis is 38 cm
above the bench and is located approximately 69 cm from the bottom, 48 cm from
the top, and 30.5 cm from the sides of the enclosure. Source-to-receiver
distances of 1.3 to 4.2 meters are typically used. For the 100- and 500-watt
lamp sizes, a baffle with an aperture 7.6 cm wide and 17.8 cm high is placed
25 cm from the plane of the biposts and centered with respect to the
lamp bulb. For the 1000-watt test lamps, this baffle aperture is 10 cm wide
and 23 cm high. Additional baffles are placed between the source and the
detector to screen the detector from any directly reflected light. A small
baffle is also attached directly to the lamp support to shield the detector


APPENDIX A
DETAILED PROCEDURES FOR A
LUMINOUS INTENSITY CALIBRATION


REPORT OF CALIBRATION

The coefficients from the least squares fit to the equation
T= A+ et. +¢1
c c

in which I is the current. through the lamp in amperes, and T, is the
color temperature in kelvins, are:

A = -0.6819

B = 1.2851 x 1079

C = 1.4305 x 1077

Prepared by: Approved by:

Sonate When)

Conall a: 6 Taney

Ronald L. Wilkinson
Radiometric Physics Division
Center for Radiation Research

NBS Test No.: 534/
Date: December 24, 1985

D-7

Donald A. McSparron
Radiometric Physics Division
Center for Radiation Research

Page 2 of 2


TABLE OF CONTENTS - APPENDIX B

Page
Introduction, ©... ee ee ee +. Bel
Lamps... 1 ee ee ee ee ee wee eee eee ee ee ee Bel
2.1 Test Lamp... .... ee. eee eee eee ee ee ee Bel
2.2. Working Standards... 1... 2. eee eee wee eee ee Beal
Run Card Preparation. . 2... 2 ee ee ee ee ee ~ ee. Bel
3.1 Working Standards .. 2... 2... ee eee eee ee eee Bl
3.2 Test Lamp... . . . ee ee ee eee ee es Bl
Measurement Preparation... .....-2-.208. woe ee B-2
4.1 Shunt Box . 6 6 ee ee ee ee B-2
4.2 Volt Box... 6... ee sees B-2
4.3 Detector. ©... . ee ee ee eee ee ee BH?
4.4 Lamp Connections. ©. 1 .. ee eee . B-2
4.5 Final Adjustments . 2... 2 2. ee ee ee ee eee ee we BH?
Measurements. . .... a B-3
5.1 DataTaking. 2... ee ee ee eee BHD
5.2 Test Lamp. ....... 2.4 Se ee ee B-3
5.3 Remaining Working Standards ........-.4. eee eee BD
5.4 Remaining Measurement . 2... 6 1 ee ee ee ee ee B-3
Data Reduction. ....... er B-3
6.1 Calibration Factor. ..... es oe. BD
6.2 Quality Control Factor. ..... 2... eee wee ee ee BH
6.3 Test Lamp Luminous Flux Value... .....-- oe eee + BAA
6.4 Final Value... ee ee B-4


of the light produced by a lamp operating in the sphere, compared to the
relative spectral distribution of the light produced by the same lamp
operating outside of the sphere. The relative spectral response for a "true"
photopically corrected detector is used in the calculation, and contrasted
with the "actual" response of the detectors [21]. The calculations were made
for conditions where the test lamps operated at color temperatures of 2700 K
to 3000 K, compared to a flux standard operated at a color temperature of
2856 K. Under NBS working conditions, the combined effect of all these
factors would yield an uncertainty due to spectral effects that would not
exceed 0.4%.

For random variations, the standard deviation computed from 138
measurements (six measurements on each of the five or six lamps in each of the
three groups of working standards plus the opal reference standards) is 0.14%.
The uncertainty assigned for the random variations is three times the standard
deviation of the mean of the six measurements on each lamp or 0.2%. The three
uncertainties (geometric, spectral, and random) combined in quadrature give an
uncertainty of 1.1% for the NBS working standards relative to the primary
reference group.

The uncertainty of the transfer from the NBS working standard to test
lamps supplied to other laboratories depends on the size and operating color
temperature of the test lamps relative to the standard group used to calibrate
them. For test lamps of sizes and color temperatures differing from those of
the working standards, the uncertainties of the transfer from NBS working
standards to test lamps are the same as for the transfer from the primary
group; 1.0% for geometric differences, and 0.4% for spectral differences. In
addition there is a random variation in the transfer to a test lamp that
applies to all lamps. This is computed as three times the standard deviation
of the mean of the three measurements made of each of the test lamps and is

0.4%.

The uncertainty due to all suspected errors associated with a reported
value for a lamp standard of luminous flux, is given by combining in
quadrature the uncertainties of the primary reference group, the uncertainties
of the NBS working standards relative to the primary reference group, the

29


REPORT OF CALIBRATION

Set
Lamp No. Direct Current Voltage Luminous Flux
(amperes) (volts) (Tumens)
NBS10279 1.645 119.81 3485

Prepared by:

Breath) LW Mower

Ronald L. Wilkinson
Radiometric Physics Division
Center for Radiation Research

NBS Test No.: 534/
Date: March 13, 1986

0-3

Approved by:

Ponte A. PWNS hewn ery

Donald A. McSparron
Radiometric Physics Division
Center for Radiation Research

Page 2 of 2


uncertainty of the transfer to the test lamp when that lamp is not similar to
the working standards, and the random error typical of the lamps calibrated.
Therefore the combined total uncertainty of the assigned value for a lamp
ranges from 1.4% to 1.8% depending on similarity to the working standards.

The uncertainties for large lamp luminous flux calibrations are
summarized in Table 6.

3.7 Uncertainties - Miniature Lamps

The estimated uncertainties in the calibration of the NBS primary
reference group (the NBS 526 group), the transfer of the scale to the 100-watt
working group (the NBS 3158 group), which served as the starting point for the
calibration of the miniature lamps, and the calibration of a test lamp against
this working group have just been described. The uncertainty of the
calibration of atest lamp, in this case the type 1183 miniature lamp working
group, against the 100-watt working group was esLimated Lo be 1.8% relative to
SI units, and 1.6% relative to the NBS primary group. Additional
contributions to the uncertainty associated with the calibration of a
miniature test lamp result from: the transfer from the 2-meter sphere to the
0.76-meter sphere; absorption and linearity corrections; geometrical and
spectral differences between the standards and the test lamps; and random

errors in the operation of the lamps and the measurement of their luminous
flux:

An estimate of the possible error encountered in transferring calibrations
from the 2-meter sphere to the 0.76-meter sphere was taken from the
measurements made on the type 1183 and type 1133 lamp groups. These groups
were measured in both spheres and the relative assignments shifted by 0.7%.
This observed difference is believed to be due to differing geometrical
distributions of the luminous flux from the lamps and/or uncertainties in the

absorption corrections. An allowance of 0.7% is taken as the uncertainty due
to these factors.

No detailed analysis of the uncertainties due to spectral differences has
been made. For the gas-filled lamps, no allowance is made for the uncertainty

30


REFERENCES

1.

V.I. Burns and D.A. McSparron, Photometric Calibration Procedures, NBS
Technical Note 594-3, November 1972.

G.A. Uriano, et al, Editors, NBS Calibration Services Users Guide
1986-1988, NBS Special Publication 250, July, 1986.

CIE Publication #15, Colorimetry, 1971. (See note below)

CIE Publication #18.2, The Basis of Physical Photometry, 1983. (See not
below)
See also: F.E. Nicodemus, H.J. Kostkowski, and A.T. Hattenburg, Self- tudy

Manual on Optical Radiation Measurements, Chapter 1, NBS Technical N ec
910-1, March 1976.

W.R. Waters, J.H. Walker, and A.T. Hattenburg, NBS Measurement Services:
Radiance Temperature Calibrations, NBS Special Publication 250-7.

J.H. Walker, R.D. Saunders, and A.T. Hattenburg, NBS Measurement Services:
Spectral Radiance Calibrations, NBS Special Publication 250-1, January,
1987.

J.H. Walker and D.A. McSparron, NBS Measurement Services: Spectral
Irradiance Calibrations, NBS Special Publication 250-20.

NOTE: In the U.S., CIE documents can be purchased by contacting the office of

Dr. K.D. Mielenz, Secretary
USNC-CIE
National Bureau of Standards
Room B306, Metrology Building 220
Gaithersburg, MD 20899
Telephone Number (301) 975-2316

45


represent the unit of luminous intensity (candela) as maintained at the
National Bureau of Standards. They are expensive laboratory equipment and
deserve the utmost care in handling and use.

In addition to some obvious precautions, such as keeping the lamps clean
and avoiding mechanical shock to the filament, the following are a few
precautions, sometimes overlooked, which should be used with these standards.

1. The lamps should be turned on and off slowly (15-30 seconds). The
lamp should not be moved while lighted.

2. In order to prolong the useful life of the lamps, it is recommended
that they be used sparingly and great care taken so that at no time will the
current appreciably exceed the value stated in the report. For general use,
working standards should be prepared by calibrating them relative to the lamps
supplied by NBS. When a laboratory procures standard lamps from NBS it is
well to obtain at least three such lamps in order to be able to detect any
changes that may occur.

3. The lamps should be carefully aligned in accordance with the procedures
described above. Photometric measurements should be made only after the lamp
has stabilized (approximately 10 minutes after turn on).

4. Stray light must be excluded. One source of stray light that is
sometimes overlooked is the standard lamp itself. The background of the lamp
on the side away from the detector should not reflect light back along the
optic axis. NBS uses a light trap made of two pieces of black glass set at
approximately 60° to each other and set behind the lamp. Black cloth
positioned 45 to 60 cm behind the lamp is convenient and usually adequate.

5. Baffle aperture edaes should be very thin or beveled so as not to
reflect light to the detector. It is of little use to coat baffle edges with
black paint or black cloth, since most flat surfaces reflect light reaching
them at large angles of incidence regardless of whether or not they are

blackened.

6. It is preferable not to confine the lamp to a small space, especially
with poor ventilation. Excessive noise in the measurement often results.


27.

28.

D.A. McSparron, et al, Spectroradiometry and Conventional Photometry, An
Inter laboratory Comparison, pp. 5-12, NBS Technical Note, November 1970.

See for instance: J.S. Preston, Phys. Soc., Proc. 47, 1935, p. 1012.

H.G. W. Harding, Phys. Soc., Proc. 63, 1950, p. 685. J. Sci. Inst., 29,
1952, p. 145. W.J. Brown, J. Sci. Inst., 31, 1954, p. 469.

48


Detailed Procedures for a Routine Luminous Flux Calibration

1. Introduction

In order to clarify the detailed procedures for a luminous flux
calibration, the example which follows is limited to the calibration of a
single 200-W lamp. In actual practice, it is likely that more than one lamp
would be calibrated.

2. Lamps
2.1 Test Lamp

The test lamp is a 200-W general purpose, inside-frosted, incandescent
lamp having a medium-screw base and a PS-30 bulb. The lamp has been seasoned
at rated voltage for 4% of its rated life and has an identifying NBS number
etched on the neck of the bulb. The customer will usually request the
measurement at a specific current, voltage, or color temperature.

2.2 Working Standards

The working standards are the six 200-W lamps in the NBS 6807 group.
They are identical to the test lamp, which makes unnecessary any measurements
to correct for differences in bulb absorption.

3. Run Card Preparation
3.1 Working Standards

Prepare three run cards, one corresponding to each of the three selenium
cells. Label each card as to run number and cell number. On each card, list

the calibration current and corresponding luminous flux for each of the
working standards. The sequence of the six working standards is the same for
the Ist and 3rd cards but is reversed for the 2nd card.

3.2 Test Lamp

Prepare a lamp card for the test lamp. Information on this card should
include the lamp designation (NBS number), size of the lamp, type base of the


ABSTRACT

The National Bureau of Standards supplies calibrated standards of
juminous intensity, luminous flux, and color temperature on a routine basis
The procedures, equipment, and techniques used to perform these calibrations
as of July 1986 are described. Detailed estimates and procedures for
determining uncertainties of the reported values are also presented.

Key words: calibration procedures; color temperature; luminous flux; luminous
intensity; photometry.


with three melting point blackbodies: platinum at 2045 K, rhodium at 2236 K,
and iridium at 2720 K. The nine lamps thus calibrated (three at each
temperature) were then used to calibrate three working standard lamps across
the color temperature range. An empirical approach was adopted for
interpolating between the three fixed temperature points. Taking advantage of
the cavity effect due to filament coiling, pyrometric determinations of the
radiance temperature were made at a position on the inside of a single turn of
the coiled filament. It was observed that the differences between the
radiance temperatures so determined and the color temperatures of the lamp as
a whole were a smoothly varying function of the voltage applied to the lamp.
Pyrometric determinations of the radiance temperatures of three different coil
turns of the test lamps yielded consistent color temperature values for the
interpolated region. Further, from this data it was empirically determined
that an equation relating the color temperature (To) and the applied voltage
(V), of the form

T. = A+B (V)1/2 (9)

c
was adequate to express all of the data within the estimated uncertainties.
The 1934 color temperature scale was also extrapolated up to 3200 K and down
to 1500 K. Sources with known color temperatures in the extrapolated regions
were produced by modifying incandescent lamp outputs (assumed to be sources
whose distribution temperatures equalled their color temperatures) with blue
and amber filters whose spectral transmittances had been measured.
Incandescent lamps and associated equations of the form of eq. (7) have been
used since 1934 to maintain the NBS color temperature scale. The scale has
been adjusted twice, in 1949 [25] and in 1970 [26], to take account of changes
in the International Practical Temperature Scale.

The present scale of color temperature has been transferred to a number
of clear bulb incandescent working standard lamps that are used in the actual
calibration of color temperature standards for issuance to the public. Lamps
BS 9021, BS 9022, and NBS 9341 are used in the range 2000-2600 K; and lamps
NBS 1923, 1924, and 1925 are used in the range 2300-2900 K. With the
exception of lamp NBS 9341, these working standards were calibrated by visual
comparison with the lamps used to originally establish the scale in 1934.

37


5. Some detectors are not linear in their response. Detectors of a
single. type made by a single manufacture sometimes differ significantly in
this respect. If lamps of different flux outputs are to be compared, the
response of the detector should be studied to determine its departure from
linearity.

6. Most sphere coatings are somewhat spectrally selective in reflectance.
Diffusing windows commonly used in spheres also have spectrally selective
transmittances. Corrections should be made when comparing lamps operating at
different color temperatures. In addition to this, a detector whose spectral
response agrees with the CIE luminous efficiency function should be used.

7. Lamps of different physical size and blackening absorb different
amounts of flux reflected to them by the sphere wall. The amount of flux
absorbed also varies with the size of the sphere being used and with the
reflectance of its coating. Therefore, when lamps of different physical size,
bulb material and/or blackening are being compared, corrections for the lamp
absorption should be made. The absorption correction should be determined in

the sphere in which the comparison is made.

8. The coated wire leads on the miniature lamps should be left attached
to the lamp. If they are removed, the voltage measured across the lamp will
not be the reported value.

35


each working standard lamp; check for drift. Average the six K-factors.
Multiply the measured detector output for each working standard by the average
K-factor. This yields the measured lumen output of each lamp for this run.

6.2 Quality Control Factor

Determine the difference between the assigned output and the measured
output for each of the working standards. The sum of the positive differences
should equal the sum of the negative differences (allowing for rounding).
Neglecting signage, sum the differences. Divide the sum of the differences by
the number of working standards to get the average difference. Divide the
average difference by the average of the assigned values for the working
standards, and multiply by 100 to convert to percent. The resulting so-called
“delta" is used as an internal quality control check and should be less than

0.5%.

6.3 Test Lamp Luminous Flux Value

Multiply the detector output from the test lamp by the average K-factor
from the Ist run producing a lumen output value for the test lamp.

6.4 Final Value

Repeat Steps 6.1 through 6.3 for the 2nd and 3rd runs. Average the three
computed lumen output values. This is the final reported calibration value
Also report the average current and voltage of the lamp.

B-4


Table 1
Important Quantities, Units and Symbols used in Radiometry and Photometry

Radiometric Quantity Unit Photometric Quantity Unit
radiant flux, watt (W) luminous lumen (1m)
radiant power (,) Flux (¢,)

radiant intensity (1,) watt per luminous candela (cd)

steradian (W sr7!) intensity (1))

radiance (L,) watt per square Tuminance (Ly) candela per
meter and per square meter
steradian (W m2 sr7}) (cd m2)

irradiance (E,) watt per square illuminance (Ey) lumen per
meter (W m7?) square meter

(Im m72), lux (1x)

radiant watt per square Juminous Jumen per

exitance (M,) meter (W m-2) exitance (M.) square meter
(im m-2)

distribution kelvin (K)

temperature

Note: Should there be no possibility of confusion, the subscripts may be
omitted from the symbols for the quantities. Also, when the spectral concen-
tration of a radiometric quantity, such as radiance, is considered, it is
designated by the same term preceded by the adjective spectral, and by the

same symbol with the subscript 4; example, spectral radiance, uy.


16.

17.

18.

19.

20.

al.

22.

23.

24,

26.

A.R. Schaefer and K. Mohan, A New Gonioradiometer for Total Flux
Measurements, J. Illum. Engr. Soc. 3, 349, 1974.

See, for instance: R. Ulbricht, ETZ 21, 595 (1900).
E. Rosa and A. Taylor, Sci. Papers NBS p. 447, 281 (1922).
D. Goebel, Generalized Integrating Sphere Theory, Appl. Opt. 6. 125, 1967.

P. Giacomo, Metrologia 16, p. 55-61 (1980), News from the BIPM. This
article reports the actions of the 16th Conference Generale des Poids et
Measures.

IES Lighting Handbook, Reference Volume (1981), J.E. Kaufman, Editor,
Section 4, Measurement of Light and Other Radiant Energy.

F.E. Nicodemus, Self-Study Manual on Optical Radiation Measurements,
Chapter 4, NBS Technical Note 910-2, Feb. 1978.

E.H. Eberhardt, Source - Detector Spectral Matching Factors, Appl. Opt.
7, 2037, 1968.

CIE Publication #17, International Lighting Vocabulary, 3rd Edition,
1970.

K.L. Kelly, Lines of Constant Correlated Color Temperature Based on
MacAdam's (U,V) Uniform Chromaticity Transformation of the CIE Diagram, J.
Opt. Soc. Am., 53, No. 8, pp. 999-1002, August 1963.

H.T. Wensel, D.B. Judd, and W.F. Roeser, Establishment of a Scale of Color
Temperature, Bur. of Stand. J. Res., 12, pp. 527-536, May 1934.

. D.B.dudd, the 1949 Scale of Color Temperature, J. Res. Nat. Bur. Stand.,

44, January 1950.
Color Temperature, Luminous Efficacy and the International Practical

Temperature Scale of 1968, Nat. Bur. Stand. Technical News Bulletin, pp.
206-207, 54, No. 9, September 1970.

47


5. Measurements

5.1 Working Standards

Slowly bring the first working standard up to the calibration set current
and set the timer for 7 minutes. When timer goes off check current setting,
and then record current, voltage, and detector output. Slowly power down the

lamp, remove, and repeat for next three working standard lamps.

5.2 Test Lamp

Increase the source-detector distance to 4.17 meters and re-position the
baffles. Mount the 1000-W test lamp, warm-up for 7 minutes, and record

current, voltage, and detector output.

5.3 Remaining Working Standards

Decrease the source-detector distance to 2.9 m and run the 4 remaining

working standards.

5.4 Remaining Measurement Runs

Measurements runs must be repeated with the other two selenium cells.

Each detector must be aligned as described in section 3.1 of this appendix.

6. Data Reduction
6.1 Calibration Factor

Divide the assigned luminous intensity value for each working standard by
the corresponding measured detector output from the first run. This yields a
K-factor in candelas/volt for each working standard. Average the eight
K-factors. Multiply the average K-factor by the measured detector output for
each working standard. This yields the measured luminous intensity of each
working standard for this run in candelas.

6.2 Quality Control Factor

Determine the difference between the assigned output and the measured
output for each of the working standards. The sum of the positive differences

A-3


1.3. Photometric Quantities

The photometric quantities we are concerned with here are luminous
intensity and luminous flux. Luminous intensity,

Iyed 4, /dw (2)

is the luminous flux per unit solid angle in the direction in question. For
purposes of measurement, luminous intensity is defined by

I= Ky f I, y V(A) da (3)

and similarly for luminous flux,

ty = KS G4 WA) od (4)

where I, and o, 4 are the spectral concentrations for radiant intensity and
> >

radiant flux.

An equivalent definition of luminous intensity makes use of the quantity
illuminance EY (see-Table 1) which in the context of Figure 3, is simply
defined as the quotient of the luminous flux do over the elementary area dA
illuminated by the source. In this definition, we assume that the area dA is
perpendicular to the direction (axis of cone) of illumination; thus,

Ey a (8)

Since dw = a we can substitute this and equation (5) back into equation
(2) and obtain

I
fy = (6)

In equations (3) and (4), V(A) is the spectral luminous efficiency function
for the CIE standard photometric observer and Ky is the coupling constant
previously discussed in section 1.1. Although V(A) is a psychophysical


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 (desiqnated 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 250-15), NBS Measurement Services: Photometric
Calibrations, by R. L. Booker and D. A. McSparron, is the fifteenth to
be published in this new series of special publications. It covers the
calibration of standards of luminous intensity, luminous flux, and
color temperature (see test numbers 37010C-37160C in the SP 250 Users
Guide). Inquiries concerning the technical content of this document or
the specifications for these services should be directed to the authors
or one of the technical contacts cited in SP 250

The Center for Radiation Research (CRR) is in the process of publishing
21 documents in this SP 250 series, covering all of the calibration
services offered by CRR. A complete listing of these documents can be
found inside the back cover

NBS would welcome suggestions on how publications such as these might
be made more useful. Suggestions are also welcome concerning the need
for new calibration services, special tests, and measurement assurance
programs.

George A. Uriano Chris E. Kuyatt
Director Director
Measurement Services Center for Radiation Research

iii


Lamps are calibrated base up in the 2-meter sphere and base down in the
0.76-meter sphere. All lamps are operated on dc electrical power with the
center contact at positive (+) potential. Typical calibration reports for
both large lamps and miniature lamps are shown in Appendix D.

3.6 Uncertainties- Large Lamps

The uncertainty of the primary reference group (the 300-watt opal lamps)
is made up of two components: the uncertainty of the measurement of each
lamp's goniometric distribution, and the uncertainty of the radiometric
measurement of its spectral irradiance. A thorough analysis of the errors of
both processes has been done. Based on repeated measurements, the uncertainty
due to goniometric distribution is 0.3 % (range of measurements, equivalent to
30). The uncertainty of the spectral irradiance transfer, including the
uncertainty in the radiometric scale, is 0.7% as shown in Table 2. Comments
on this uncertainty estimate were presented in section 2.6.

The uncertainty of transfer from the primary reference flux group to the
NBS working standards may be divided into three parts, those uncertainties due
to: (1) geometric effects, i.e., the presence of objects, such as the lamp,
its support, and the baffle, in the sphere and their geometrical arrangement ;
(2) spectral effects, i.e., differences in the spectral distribution of the
lamps being compared or a difference between the spectral response of the
detector being used in the CIE spectral luminous efficiency function; and (3)
random variations of the measurements. While a detailed analysis of the
geometric effects has not been made, differences as large as 1% have been
observed when using the sphere geometry described above compared to a very
different geometry requiring more objects in the sphere during the
measurements. An allowance of 1% has therefore been made for geometric

sources of error.

The uncertainties due to spectral effects have been determined from
calculations based on measurements of the relative spectral characteristics of
the integrating sphere, of the photopically corrected photodetector, and of
the standard and test lamps. The spectral throughput of the integrating
sphere was determined from measurements of the relative spectral distribution

28


ad
(aay. 8)

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

Gaithersburg, MD 20899

REPORT OF CALIBRATION

for
Luminous Flux
of
Ten Miniature Lamps

Submitted by:

(See your Purchase Order Number 1344-122-2535 dated January 29, 1986.)
1. Material

Ten minature incandescent lamps were calibrated for luminous flux.
The lamp designations listed below are scribed on the lamp bases.

2. Calibration

The lamps were calibrated in a 0.76-meter integrating sphere in accordance
with the enclosed description "Calibration of Miniature Lamps for Luminous

Flux - 1982". The values reported below are based on the NBS scale of
spectral irradiance in agreement with the international redefinition of
the photometric units in 1979. Values reported on this scale are 0.25%
less than those reported on the previous NBS flux scale. This shift is
less than the uncertainty associated with the reported luminous flux
values. The uncertainty is discussed in Section VII and summarized in
Table 2 of the enclosed description.

NBS Test No.: 534/
Date: March 10, 1986 Page 1 of 2

D-4


Two additional measurements enter into the uncertainty calculation for an
individual test lamp: the substitution calibration of the working standards
group, NBS 7796 group, and the substitution calibration of the test lamp
against the working group. Each of these experiments involves a direct com-
parison of essentially identical lamps. The only systematic error that could
affect the results would come from the spectral response of the detector. In
principle, if the sources are identical in spectral distribution, no error
occurs, no matter what the properties of the detector are. In practice, the
detector is approximately matched to the V(A) curve and systematic errors are
assumed to be neqlible. Each of these experiments is also subject to the
random variations of the measurements. A study was conducted on 124
measurements of 36, 500-watt lamps and 80 measurements on 20, 100-watt lamps.
This study yielded a pooled estimate of the standard deviation of a single
measurement of 0.3%. Since it is customary to make three measurements of a
test lamp, an allowance of + 0.5% is made for the random variation of the
measurements on a test lamp (three times the standard deviation of the mean of
three measurements). Similarly for the eight lamps of the working standards
group, the appropriate random uncertainty is the precision associated with the
group mean, 0.2%. Examination of this data base also revealed occasional
non-random changes in light output. Usually these changes were of the order
of 0.2-0.3%, though a few were as large as 1%. Presumably these changes were
due to shorting in the filament coil structure. In order to allow for
possible discrete changes in lamp output, an additional uncertainty of 0.5%
(1/2 the largest observed change) is assigned. Table 3 sumarizes these
various uncertainties for an individual test lamp.

In 1968-70, an international intercomparison of the candela at the color
temperature of CIE illuminant A showed a range of 1.7% among the eight
participating national standardizing laboratories including NBS [13].
Preliminary results from an international intercomparison in 1984-86 show a
similar range. These results are consistent with the + 1.0% uncertainty

estimated for the NBS calibrations as shown in Table 3.

2.6.1 Precautions

Lamps supplied to other laboratories as standards of luminous intensity

16


Library of Congress Catalog Card Number: 87-619878

National Bureau of Standards Special Publication 250-15
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-15, 83 pages (Oct. 1987)
CODEN: XNBSAV

Commercial products—materials and instruments —are identified in this document for the sole pur-
pose of adequately describing experimental or test procedures. In no event does such identification
imply recommendation or endorsement by the National Bureau of Standards of a particular product;

nor does it imply that a named material or instrument is necessarily the best available for the purpose
it serves.

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 1987

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


0.02%. After positioning and alignment, the lamp is slowly (15-30 seconds)
brought up to approximately the required electrical operating point and
allowed to stabilize for at least 10 minutes before measurements are made.
Measurements are then made of the current through the lamp and the potential

drop across the pins of the bipost base necessary to produce a color match to
a comparison source.

4.3 Color Temperature Apparatus

Numerous devices based on the red/blue ratio principle have been described
in the literature [28]. The present apparatus has not been optimized, and
will be modernized in the near future (see Introduction). It has, however,
been shown to possess adequate sensitivity and precision. The standard
deviation computed on the basis of repeated measurements of the same color

temperature on the same test lamp ranges from about 3.5 K at 3000 K to about
8.0 K at 2000 K,

The present interim apparatus consists of a 20-cm diameter integrating
sphere and a Pritchard model 1980 telephotometer. The integrating sphere is
coated with a commercial sphere paint made of a mixture of magnesium
carbonate, latex, and mineral spirits. A single light source is typically
located 70 cm from the 7.5 cm diameter sphere opening. Both the sphere and
light source are located in a rectangular light-tight enclosure. A baffle is
located between the source and sphere, 25 cm from the sphere. There is a 2.5
cm aperture opening in the baffle that allows for viewing of the entire lamp.

Measurements are made based on the red-to-blue ratio principle [28]. The
lamp is powered by three, 60-volt power supplies connected in series. The
telephotometer is used with its own internal broad-band red and blue filters
and is equipped with a close-up lens focused at the plane of the 3-cm diameter
viewing port of the sphere. A 3-cm diameter black tube is attached to the
sphere and extends 7 cm to the lens of the telephotometer.

Radiation collected in the sphere is measured separately through the red
and blue filters and the ratio of these signals is determined. A sufficiently
dense neutral density filter must be used to prevent saturation of the

40


Selenium, barrier-layer photocells [11] (Weston model 856 YYLSV) are used
with the spheres. Each is equipped with a filter to correct its spectral
response to approximately match the CIE luminous efficiency function. The
photocells are mounted in the tubes fitted to the windows of the spheres. The
tubes are coated on the inside with a non-glossy black paint. The current
produced by the photocells is converted into an amplified voltage signal by a
unity-gain operational amplifier used with closed loop feedback. Thus the
photocells operate into a zero-resistance circuit [11], with negligible
voltage across the terminals. A 5 1/2 digit voltmeter is used to measure the
output of the operational amplifier.

3.3 Standards

A group of five 300-watt opal-bulb lamps (the NBS 536 group) operating at
a color temperature of approximately 2720 K serves as the primary NBS
reference standard of luminous flux. For the June 1980 work, the
international redefinition of the photometric units agreed to in 1979 [18]
was implemented by performing a complete recalibration of the primary NBS
reference group in terms of the NBS scale of spectral irradiance [7]. The
relative goniometric distribution of the light produced by each of the five
lamps of the NBS 526 group was measured at five wavelengths within the visible
spectrum. These distribution measurements yielded the ratio of the
geometrically total spectral flux emitted by the lamp to the flux emitted in a
single "reference" direction. The flux emitted in the reference direction was
measured in terms of the NBS scale of spectral irradiance. The total luminous
flux emitted by each lamp was then computed from equation (4). The mean
assignment of the primary NBS reference group was shifted 0.25% by this
measurement from the assignment of the previously maintained scale, which had
been based on the platinum point blackbody definition of the candela.

The NBS working standards are periodically recalibrated in the 2-meter
sphere photometer by comparing them in six runs, using the three photocells
mentioned above, with the 300 watt opal-bulb lamps. The NBS working standards
are listed in Table 5. Each of the three groups of standards consists of six
lamps.

24


Table 7
Uncertainties (30) Associated with Miniature Lamps Calibrated
for Luminous Flux

1. Uncertainty of NBS 100-watt working standards
a. Relative to SI 1.8%
b. Relative to NBS 1.6%

2. Uncertainty in transfer of scales from measurements in the 2-meter
sphere to measurements in the 0.7-meter sphere 0.7%

3. Random errors in the transfer to the NBS miniature lamps working
standards (30)

a. Type 1183 lamps vs. NBS 100 W lamps 0.3%
b. Type 1133 lamps vs. type 1183 lamps 0.3%
c. Type 87, 81, 63, and 51 lamps vs. type 1133 lamps 0.3%

4. Random errors in the substitution calibration of test lamps 0.3%

5. Uncertainty due to spectral differences (type 51 lamps only) 2.0%

Total Uncertainty of Test Lamps

Lamp Quadrature Combination Relative Relative
Type of Items to SI to NBS
1183 1, 2, 3a, & 4 2.0% 1.8%
1133 1, 2, 3a, 3b, & 4 2.0% 1.8%
87, 81

& 63 1, 2, 3a, 3b, 3c, & 4 2.0% 1.8%
51 1, 2, 3a, 3b, 3c, 4&5 2.8% 2.7%

33


7. The variation of luminous intensity of a gas-filled tungsten lamp is
approximately related to the variation in its current by the equation

dl .
— = 6.25 db (7)

I oO
v
where ly is the luminous intensity and i is the lamp current [14,15]. Thus a

current measurement accuracy of at least 0.032% is required if it is not to
affect the luminous intensity by more than a factor of 0.2%.


where V(A) is the photopic (light adapted) spectral response function for the
"standard observer" of the CIE [4]. This function is shown by the curve in
Figure 1. Kn is a coupling constant and fixing its value defines the physical
photometry units in terms of the radiometric units. In 1977 the International
Committee for Weights and Measures (CIPM) adopted a Kn value of 683
lumens/watt for monochromatic radiation at a wavelength of 555 nm. Note that
implicit in the use of an integral in equation 1, is a convention of
arithmetic additivity for photometric quantities, although such a law is

obeyed imperfectly by actual observers.

Table 1 lists the quantities, units and symbols used in radiometry and
photometry [4]. NBS supplies standards utilizing the conventions of physical
photometry and based upon a radiometric standards chain. For the remainder of
this document, the modifier physical will be dropped and the term photometry
should be understood to mean physical photometry and its associated
conventions.

1.2 Photometric Scale

The NBS photometric scales are derived from the NBS scale of spectral
irradiance. Figure 2 shows the major steps in the derivation of this spectral
irradiance scale from a blackbody operated at the temperature of freezing
gold. Briefly, the gold point blackbody is used to realize the International
Practical Temperature Scale (IPTS-68), which in turn is used to measure the
temperature of a variable temperature blackbody (VTBB). The VTBB provides us
with a scale of spectral radiance and this in turn provides the basis for the
NBS scale of spectral irradiance. All of the steps in this chain have been
documented in a series of calibration documents being published as supplements
to SP 250 [5, 6, 7]. All of the measurements necessary for implementing the
chain shown in Figure 2 are made on the NBS Facility for Automatic
Spectroradiometric Calibrations (FASCAL). The NBS scale of spectral
irradiance is maintained in a group of four 1000-watt quartz-halogen lamp
working standards. Primary calibration of the photometric working standards
is performed by making spectral irradiance measurements on FASCAL against
these four quartz halogen lamps.


APPENDIX B
DETAILED PROCEDURES FOR A
ROUTINE LUMINOUS FLUX CALIBRATION


REPORT OF CALIBRATION

Lamp No. Direct Current Voltage Luminous Flux
(Amperes-Set) (volts) (Tumens)
NBS 9876 5.800 4.868 405.1
NBS 9878 5.800 4.946 435.5
NBS 9883 5.800 4.999 431.9
NBS 9815 1.000 6.279 69.77
NBS 9816 1.000 6.217 67.90
NBS 9817 1.000 6.317 70.45
NBS 9546 0.197 6.158 6.23
NBS 9548 0.197 6.488 7.43
NBS 9565 0.197 6.035 5.78
NBS 9566 0.197 6.882 6.87

Prepared by:

Kermetel G. MI idgparron

Donald A. McSparron
Radiometric Physics Division
Center for Radiation Research

NBS Test No.: 534/
Date: March 10, 1986

D-5

Approved by:
NB NA
Klaus D. Mielenz, Chief

Radiometric Physics Division
Center for Radiation Research

Page 2 of 2


photomultiplier. Once the particular internal neutral density filter is
selected, it remains in position for both the red and blue measurements. The
sphere-telephotometer apparatus has been used for color temperature
calibrations in the range 2000 K to 3000 K. The use of accessory color
compensating filters at the extreme ends of this range is not necessary.

Equality of two red-to-blue ratios is interpreted to mean equality in
color temperature. Since the measurement involves the inference of a visual
property of the test source (chromaticty match to the standards) from the
measurement of a different physical property, severe requirements are placed
on the test and standard sources. Specifically, their relative spectral power
distributions must be identical throughout the entire visible region of the
spectrum, Only test lamps substantially identical to the working color

temperature standard lamps are calibrated by this method.

4.4 Calibration Procedure

A simple substitution procedure is used to calibrate lamps for color
temperature on the sphere-telephotometer apparatus. At the beginning of each
calibration run, the proper internal neutral density filter is selected and
the telephotometer is calibrated in turn against each lamp of the relevant
working group.

Each lamp of the working standard group is placed in the measuring
position and allowed to operate for 10 minutes at the current required for the
desired color temperature. Separate measurements with the red and blue
filters in place are recorded and the red-to-blue ratio determined. After
each lamp of the working group is measured, the test lamp is placed in the
measurement position and, after warm-up, red and blue measurements are made
while adjusting the current of the test lamp to bracket the red-to-blue ratio
average of the working standard lamps. A linear interpolation between the
high and low currents determines the test lamp current corresponding to the
target R/B ratio. The entire procedure is repeated two more times. The three
current values thus determined for the test lamp are averaged to find the
final reported calibration value. Detailed procedures for a routine color
temperature calibration are given in Appendix C. A typical calibration report
is shown in Appendix 0.

41


Table 8. Color Temperature Uncertainties (30)

Total

Transfer Uncertainty

Color Temp Systematic Uncertainty Precision (30) (Quadrature)

K K K K

2000 7.0 8.0 10.5
2400 9.0 8.0 12.0
2600 10.0 4.5 11.0
2856 12.5 3.5 13.0
3000 14.0 3.5 14.5

43


Figure 2. Photometric Chain Showing Derivation of Photometric Scales From
the Gold Point Blackbody

GOLD POINT
BLACKBODY
IPTS-68

VARIABLE TEMPERATURE
BLACKBODY (VTBB)

SPECTRAL RADIANCE

Ly

SPECTRAL IRRADIANCE

Ey
LUMINOUS INTENSITY GEOMETRICALLY TOTAL COLOR
LUMINOUS FLUX TEMPERATURE
ly ey