NBS Measurement Services:

Photometric

Calibrations

NBS

Special

Publication

250-15

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major coniponent of the National Measurement Laboratory in the National Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

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

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

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

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

Atomic and Plasma Radiation Division

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

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

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

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

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

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

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

tions; conducts theoretical and experimental research with electron,

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

measurement techiques. The Division consists cf the following Groups:

Radiometric Physics Division

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

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

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

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

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities

Including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instruiientation for

radiation programs and facilities; provides national leadership

In the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrumentation. The

Division consists of the following Groups:

• Accelerator Research • Linac Operations

• Electronic Instrunentation

• Mechanical Instrunentation

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic •

data for applications of ionizing radiation; develops new methods of •

chemical and physical dosimetry; conducts theoretical and experimental •

research on the fundamental physical and chemical Interactions of

ionizing radiation with matter; provides essential standards and •

measurement support services to the National Measurement Support • System for Ionizing Radiation; and develops and operates radiation • sources needed to provide primary radiation standards, fields, and • well-characterized beams of radiation for research on radiation •

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and Improves the quantitative

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

PHOTOMETRIC CALIBRATIONS

Robert L. Booker

Donald A. McSparron

Center for Radiation Researcii

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987
Library of Congress Catalog Card Number: 87-619878

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
PREFACE

The calibration and related measurement services of the National Bureau

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

measuring instruments in achieving the highest possible levels of

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

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

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

Users Guide .

The Users Guide is being supplemented by a number of special

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

calibration services. These documents provide a description of the:

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

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

assessment of measurement uncertainty including random and systematic

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

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

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

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

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

of information more readily available to the user.

This document ( SP 250-15), 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

i i i
ABSTRACT

The National Bureau of Standards supplies calibrated standards of

luminous 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.

iv
TABLE OF CONTENTS

Page

Abstract iv

1. Introduction 1 1.1 Photometry, Physical Photometry, and Radiometry 7. 1.2 Photometric Scale 3

1.3 Photometric Quantities 7 1.4 Safety 9

1.5 Acknowledgements 9

2. Luminous Intensity Calibrations 10

2.1 Test Lamp 10

2.1.1 Description 10

2.1.2 Preparation 11

2.1.3 Orientation 11

2.1.4 Operation 11

2.2 Photometer 12

2.2.1 Detector and Amplifier ". 12

2.2.2 Optical Bench 12

2.3 Calibration Procedure 13

2.3.1 Photometer Calibration 13

2.4 Measurement Schedule 13

2.5 Data Reduction 14

2.6 Uncertainty 14

2.6.1 Precautions 16

3. Luminous Flux Calibrations 20

3.1 Test Lamps 21

3.2 Sphere Photometers 23

3.3 Standards 24

3.4 Calibration Procedures - Working Standards 25

3.5 Calibration Procedures - Test Lamps 27

3.6 Uncertainties - Large Lamps 28

3.7 Uncertainties - Miniature Lamps 30

3.8 Care and Use 32

I

V
TABLE OF CONTENTS (continued)

Page

4. Color Temperature Calibrations 36

4.1 The NBS Color Temperature Scale 36

4.2 Test Lamps 39

4.2.1 Preparation 39

4.2.2 Orientation 39

4.2.3 Operation 39

4.3 Color Temperature Apparatus 40

4.4 Calibration Procedure 41

4.5 Uncertainty 42

4.6 Di scussion 42

References 45

Appendix A - Detailed Procedures for a Luminous Intensity Calibration

1. Introduction A-1

2. Lamps A-1

2.1 Test Lamp A-1

2.2 Working Standards A-1

3. Measurement Preparation A-1

3.1 Detector Alignment A-1

3.2 Lamp Alignment A-1

3.3 Shunt Box A-2

3.4 Volt Box A-2

3.5 Lamp Connections A-2

4. Run Cards A-2

4.1 Working Standards A-2

4.2 Test Lamp A-2

vi
TABLE OF CONTENTS (continued)

5. Measurements A-3

5.1 Working Standards A-3

5.2 Test Lamp A-3

5.3 Remaining Working Standards A-3

5.4 Remaining Measurement Runs A-3

6. Data Reduction A-3

6.1 Calibration Factor A-3

6.2 Quality Control Factor A-3

6.3 Test Lamp Luminous Intensity Value A-4

6.4 Final Value A-4

Appendix B - Detailed Procedures for a Routine Luminous Flux Calibration

1. Introduction B-1

2. Lamps B-1

2.1 Test Lamp B-1

2.2 Working Standards B-1

3. Run Card Preparation B-1

3.1 Working Standards B-1

3.2 Test Lamp B-1

4. Measurement Preparation B-2

4.1 Shunt Box B-2

4.2 Volt Box B-2

4.3 Detector B-2

4.4 Lamp Connections B-2

4.5 Final Adjustments B-2

5. Measurements B-3

5.1 Data Taking B-3

5.2 Test Lamp B-3

vi i
TABLE OF CONTENTS (continued)

Page

5.3 Remaining Working Standards B-3

5.4 Remaining Measurement B-3

6. Data Reduction B-3

6.1 Calibration Factor B-3

6.2 Quality Control Factor B-4

6.3 Test Lamp Luminous Flux Value B-4

6.4 Final Value B-4

Appendix C - Detailed Procedures for a Routine Color Temperature

Cal ibrat ion

1. Introduction C-1

2. Lamps C-1

2.1 Test Lamp C-1

2.2 Working Standards C-1

3. Preparation of Run Cards C-1

3.1 Working Standard C-1

3.2 Test Lamp C-1

4. Measurement Preparation C-1

4.1 Photometer C-1

4.2 Allignment C-2

4.3 Photometer Sensitivity Adjustments C-2

5. Measurements C-2

5.1 Working Standards C-2

5.2 Test Lamp C-2

6. Data Reduction C-2

6.1 Test Lamp C-2

V i i i
TABLE OF CONTENTS (continued)

Appendix D - Reports of Calibration

Page

Report of Calibration for Luminous Intensity of Three 500-watt

Inside-Frosted Lamps D-1

Report of Calibration for Luminous Flux of one 200-watt

General-Purpose Lamp D-2

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

ix
LIST OF FIGURES

Page

Figure 1 Spectral Luminous Efficiency Function V(x) Defining

the Standard Photopic Observer 4

Figure 2 Photometric Chain Showing Derivation of Photometric

Scales From the Gold Point Blackbody 6

Figure 3 Concept of Luminous Intensity 8

LIST OF TABLES

Table 1 Important Quantities, Units, and Symbols Used in Radiometry

and Photometry 5

Table 2 Uncertainty Estimates (%, 3o) For the Spectral Irradiance

and Photometric Scales at the Time of the Photometric Scale

Realization 15

Table 3 Uncertainties {3a) Associated With the NBS Scale of

Luminous Intensity 17

Table 4 Operating Characteristics of Miniature Lamp Standards

of Luminous Flux 22

Table 5 NBS Working Standards of Luminous Flux 25

Table 6 Uncertainties (3o) Associated with Large Lamps Calibrated

for Luminous Flux 31

Table 7 Uncertainties (3a) Associated with Miniature Lamps Calibrated

for Luminous Flux 33

Table 8 Color Temperature Uncertainties (3o) 43

X
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 3o 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 k

specified color temperature. Operation at 2856 K (CIE 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

permanently 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.

1
will be replaced with a modern high-quality rail and carriage system in a slightly different configuration. The V( X)-corrected selenium barrier layer

photocells will be replaced with V(X )-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 the branch of science known as physical

photometry.

According to the CIE conventions, a generalized radiometric quantity,

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

G 5 A " V equation of the form:

Q = K / Q ,V(x)dX ( 1)

2
where V(x) 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. 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 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

convent ions.

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.

3
400 500 600 700

Wavelength A (nm)

Figure 1. Spectral Luminous Efficiency Function V(A)

Defining the Standard Pho topic Observer.

4
Table 1 Important Quantities, Units and Synbols used in Radiometry and Photometry

Radiometric Quantity Unit Photometric Quantity Unit

radiant flux,

radiant power {(^^)

watt (W) lumi nous

flux ((|)^)

lumen (Im)

radiant intensity (1^) watt per luminous

steradian (W sr~M intensity (I^)

candela (cd)

radiance (L ) e watt per square

meter and per

steradian (W m"^ sr"M

luminance (L ) candel a per

square meter

(cd m-2)

irradiance (E^) watt per square

meter (W m"^

illuminance (E

v)

lumen per

square meter

( Im m~2 )

, lux (1 x)

radi ant

exitance (M ) e

watt per square

meter (W m"^

1 umi nous exitance (M )

lumen per

square meter

(Im m-2)

distribution

temperature

kelvin (K)

Note: Should there be no possibility of confusion, the subscripts may be

omitted from the symbols for the quantities. Also, when the spectral concentration 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 X; example, spectral radiance, L .

5
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

SPECTRAL IF^RADIANCE

LUMINOUS INTENSITY GEOMETRICALLY TOTAL

LUMINOUS FLUX

COLOR

TEMPERATURE

6
1.3 Photometric Quantities

The photometric quantities we are concerned with here are luminous

intensity and luminous flux. Luminous intensity,

\= d <t>/da3 (2)

is the luminous flux per unit solid angle in the direction in question. For

purposes of measurement, luminous intensity is defined by

and similarly for luminous flux,

*v = ^

/ *e.X dX (4)

where I and <^ are the spectral concentrations for radiant intensity and

radiant flux.

An equivalent definition of luminous intensity makes use of the quantity

illuminance (see Table 1) which in the context of Figure 3, is simply

defined as the quotient of the luminous flux d(}) 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,

d(t) E = -TT- (5)

V dA ^

Since du = we can substitute this and equation (5) back into equation

(2) and obtain

(6)

In equations (3) and (4), V(x) is the spectral luminous efficiency function

for the CIE standard photometric observer and is the coupling constant

previously discussed in section 1.1. Although \l{x) is a psychophysical

7
dA

< r

^

Figure 3. Concept of Luminous Intensity

8
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 detectorbased scale of radiometry, in fact, is one of the goals of the NBS photodetector 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.

9
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 tt 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 PASCAL, 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 1^ ^ 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 illuminance

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-13B filaments. The 100 watt lamps

10
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

n
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

12
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 58) 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 SO 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
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 group (NBS 5612

group) is shown in Figure 2. The uncertainties (3a) 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(x) 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 time 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 intensity reference lamp group. In the buildup of the uncertainty terms shown in

Table 2, the factors in the physical models [6, 12] enter mult ipl icat ively

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
Table 2. Uncertainty Estimates (%, 3a) for the Spectral Irradiance and

Photometric Scales at the Time of the Photometric Scale

Real ization

400

nm 450

nm 500nm 555nm 600nm 654.6

nm 700nm

v(x)

Weighted

I. NBS SPECTRAL

RADIANCE SCALE

a. Absolute error

(with respect

to SI Units)

0.92 0.82 0.73 0.66 0.61 0.56 0.54 0.65

b. NBS long term

reproduc i bi 1 i ty

0.46 0.40 0.35 0.32 0.29 0.28 0.29 0.32

II. NBS SPECTRAL

IRRADIANCE SCALE

a. Systematic

errors

0.21 0.19 0.18 0.16 0.16 0.15 0.14 0.16

b. Random errors

(3 0 orerisinn)

0.07 0.07 0.07 0.07 0.06 0.06 0.07 0.07

III. TRANSFER CALIBRATION

OF A TEST LAMP

—

a. Systematic

errors

0.26 0.24 0.22 0.20 0.19 0.18 0.17 0.20

b. Random errors

(3 a precision)

0.20 0.20 0.20 0.15 0.15 0.15 0.20 0.15

IV. UNCERTAINTY OF

REPORTED VALUES

a. With respect to

SI units

1.00 0.90 0.81 0.72 0.68 0.62 0.62 0.71

b. NBS long term

reproducibi 1 ity

0.60 0.55 0.49 0.45 0.42 0.40 0.42 0.44

IVa is a quadrature combination of items la, II and III.

IVb is a quadrature combination of items lb, II and III.

15
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 i ntercomparison 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 i ntercompari son 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
Table 3

Uncertainties (3a) Associated with the NBS Scale of Luminous Intensity

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

Group)

a. With respect to SI units 0 .7%

b. NBS lonq term reproducibility 0,At

II. Transfer Calibration of the NBS Working Group

a. Random uncertainty of the mean of the eight lamps 0,.2%

(3a)

III. Substitution Calibration of a Test Lamp

a. Random uncertainty (3a) 0..5%

b. Nonrandom shifts 0..5%

IV. Total Uncertainty of a Reported Value

a. With respect to SI Units ( a quadrature 1..0%

combination of items I ,11, and III.)

a b. NBS long term reproducibility (a quadrature 0. 8%

combination of items I^^, II, and III.)

17
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 liqhted.

2. In order to orolong 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 edqes 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.

18
7. The variation of luminous intensity of a gas-filled tungsten lamp is

approximately related to the variation in its current by the equation

S . di

I = 6.25 f- (7)

where 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%.

19
3. LUMINOUS FLUX CALIBRATIONS

Radiant flux [c})^] 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 4ti steradians. A practical method

is 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

characterist ics.

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

1 amp

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
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 S26 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

working 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
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

Flux

Lamp Standards of

Lamp

Type

Current

(set)

Voltage Approximate

Color Temp.

Nominal

Luminous Flux

( amperes) (volts) ( kel vi ns) ( 1 umens) (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
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 dianeter

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. Baffles, 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
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 recal ibration 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 5. NBS Working Standards of Luminous Flux.

Lamp Group Nominal

Designation Wattage

Bulb Filament Approx. Lumens

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 = (f^/^ir) ) , 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
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 ]% or less.

Similar measurements are made in the 0.76-meter sphere using a 28-volt,

40-watt, part i al ly-si Ivered-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 hor i zontal -bar photometer by an inverse-square method. The test photocell is mounted on the bar at a known

26
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 assi/nption 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
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- Larqe 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

3a). 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 \% 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
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
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 a test lamp, in this case the type 1183 miniature lamp working

group, against the 100-watt working group was estimated to 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
Table 6. Uncertainties (3a) Associated with Large Lamps Calibrated

for Luminous Flux

. 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%

Uncertainty of NBS Working Standards relative to primary

reference group:

a. Geometric differences 1.0%

b. Spectral differences 0.4%

c. Random variations (3a) 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 (3a): 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
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
Table 7 Uncertainties (3a) Associated with

for Luminous

Miniature Lamps Calibrated

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 (3a)

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
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 wal 1

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]:

where cj) 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 luminous

flux is to be no greater than 0.2% due to the electric current uncertainty.

34
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

1 inearity

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
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
with three meltinq 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 (T^) and the applied voltage

(V), of the form

T^ = A + B (V)i/2 (9)

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
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

I = A + BT^ + CT^2 (10)

relating the color temperature T^ 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 spectroradiometrical 1

y

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
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

frost ing.

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
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
photomult ipl ier. 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 D.

41
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 spectral quality (relative spectral distribution) of the optical radiation considered. 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 +red, -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 1 imit-of-error calculation and thus is approximately a 3a 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
Table 8. Color Temperature Uncertainties (3a)

Color Temp Systematic Uncertainty

K K

2000 7.0

2400 9.0

2600 10.0

2856 12.5

3000 14.0

Transfer

Precision (3a)

K

8.0

8.0

4.5

3.5

3.5

Total

Uncert ai nty

(Quadrature)

K

10.5

12.0

11.0

13.0

14.5

43
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 lamos, 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:

dl = (B + 2CJ^) dJ^ (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 10-3, and C = 1.4 X 10"^.

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
REFERENCES

1. V.I. Burns and D.A. McSparron, Photometric Calibration Procedures, NBS

Technical Note 594-3, November 1972.

2. G.A. Uriano, et al. Editors, NBS Calibration Services Users Guide

1986-1988, NBS Special Publication 250, July, 1986.

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

4. CIE Publication #18.2, The Basis of Physical Photometry, 1983. (See note

below)

See also: F.E. Nicodemus, H.J. Kostkowski , and A.T. Hattenburg, Self-Study

Manual on Optical Radiation Measurements, Chapter 1, NBS Technical Note

910-1, March 1976.

5. W.R. Waters, J.H. Walker, and A.T. Hattenburq, NBS Measurement Services:

Radiance Temperature Calibrations, NBS Special Publication 250-7.

6. J.H. Walker, R.D. Saunders, and A.T. Hattenburq, NBS Measurement Services:

Spectral Radiance Calibrations, NBS Special Publication 250-1, January,

1987.

7. 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

Or. K.D. Mielenz, Secretary

USNC-CIE

National Bureau of Standards

Room B306, Metrology Building 220

Gaithersburg, MD 20899

Telephone Number (301) 975-2316

45
8. See, for instance: C.G. Hughes III, Silicon Photodiode Absolute Spectral

Response Self-Calibration Using A Filtered Tungsten Source, Appl. 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).

9. 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, J7, 133 (1984).

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

10. E. Zalewski, et al. Photometric Instrumention and Research (1970 to 1971)

NBS Technical Note 594-2, pp. 29, 32, 33, September 1972.

11. V.K. Zworykin and E.G. Ramberg, Photoelectricity, Chapter 11, John

Wiley & Sons, Inc., New York, N.Y. (1949).

12. See, for instance: G. Wyzecki and W.S. Stiles, Color Science: Concepts

and Methods, Ouantitive Data and Formulae, 2nd edition, John Wiley and

Sons, New York, N.Y., 1982

13. 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.

14. J.W.T, Walsh, Photometry, Dover Publications, Inc., 180 Varick Street, New

York, N.Y., 1965.

15. A. Kingslake, Applied Optics and Optical Engineering, Academic Press, New

York, N.Y., 1965.

46
16. A.R. Schaefer and K. Mohan, A New Gonioradiometer for Total Flux

Measurements, J. Ilium. Enqr. Soc. 3_, 349, 1974.

17. 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.

18. P. Giacomo, Metrologi a j^, p. 55-61 (1980), News from the BIPM. This

article reports the actions of the 16th Conference Generale des Poids et

Measures

19. lES Lighting Handbook, Reference Volume (1981), J.E. Kaufman, Editor,

Section 4, Measurement of Light and Other Radiant Energy.

20. F.E. Nicodemus, Self-Study Manual on Optical Radiation Measurements,

Chapter 4, NBS Technical Note 910-2, Feb. 1978.

21. E.H. Eberhardt, Source - Detector Spectral Matching Factors, Appl. Opt.

7_, 2037, 1968.

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

1970.

23. 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.

24. 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.

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

44, January 1950.

26. 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
27. D.A. McSparron, et al, Spectroradiometry and Conventional Photometry, An

Interlaboratory Comparison, pp. 5-12, NBS Technical Note, November 1970.

28. 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
APPENDIX A

DETAILED PROCEDURES FOR A

LUMINOUS INTENSITY CALIBRATION
TABLE OF CONTENTS - APPENDIX A

Page

1. Introduction A-1

2. Lamps A-1

2.1 Test Lamp A-1

2.2 Working Standards A-1

3. Measurement Preparation A-1

3.1 Detector Alignment A-1

3.2 Lamp Alignment A-1

3.3 Shunt Box A-,2

3.4 Volt Box A-2

3.5 Lamp Connections A-2

4. Run Cards A-2

4.1 Working Standards A-2

4.2 Test Lamp A-2

5. Measurements A-3

5.1 Working Standards A-3

5.2 Test Lamp A-3

5.3 Remaining Working Standards A-3

5.4 Remaining Measurement Runs A-3

6. Data Reduction A-3

6.1 Calibration Factor A-3

6.2 Quality Control Factor A-3

6.3 Test Lamp Luminous Intensity Value A-4

6.4 Final Value A-4
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 test 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 center line extending between the center of the detector

I

A-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 1st 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
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
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
APPENDIX B DETAILED PROCEDURES FOR A

ROUTINE LUMINOUS FLUX CALIBRATION

I
TABLE OF CONTENTS - APPENDIX R

Paqe

1. Introduction B-1

2. Lamps B-1

2.1 Test Lamp B-1

2.2 Working Standards B-1

3. Run Card Preparation B-1

3.1 Working Standards B-1

3.2 Test Lamp B-1

4. Measurement Preparation B-2

4.1 Shunt Box B-2

4.2 Volt Box B-2

4.3 Detector B-2

4.4 Lamp Connections B-2

4.5 Final Adjustments B-2

5. Measurements B-3

5.1 Data Taking B-3

5.2 Test Lamp B-3

5.3 Remaining Working Standards B-3

5.4 Remaining Measurement B-3

6. Data Reduction B-3

6.1 Calibration Factor B-3

6.2 Quality Control Factor B-4

6.3 Test Lamp Luminous Flux Value B-4

6.4 Final Value B-4
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 sinqle 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 1st 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

B-l
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-W 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

ampl if ier

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 1st 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.

B-2
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 1st

measurement run.

5.4 Remaining Measurements

Take out the 1st 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 1st run.

6. Data Reduction

6.1 Calibration Factors

Divide the assigned flux value for each working standard by the measured

detector output from the 1st run. This yields a K-factor in lumens/volt for

B-3
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%.

; • ~s . 6.3 Test Lamp Luminous Flux Value

Multiply the detector output from the test lamp by the average K-factor

from the 1st 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
APPENDIX C

DETAILED PROCEDURES FOR

A ROUTINE COLOR TEMPERATURE

CALIBRATION
TABLE OF CONTENTS - APPENDIX C

Page

1. Introduction C-1

2. Lamps C-1

2.1 Test Lamp C-1

2.2 Working Standards C-1

3. Preparation of Run Cards C-1

3.1 Working Standard C-1

3.2 Test Lamp C-1

4. Measurement Preparation C-1

4.1 Photometer C-1

4.2 Alignment C-2

4.3 Photometer Sensitivity Adjustments C-2

5. Measurements C-2

5.1 Working Standards C-2

5.2 Test Lamp C-2

6. Data Reduction C-2

6.1 Test Lamp C-2
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

cal ibrat ions.

C-1
4.2 Aliqnment

Install 1st 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 photomultipl ier 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.

C-2
APPENDIX D REPORTS OF CALIBRATION
TABLE OF CONTENTS - APPENDIX D

Page

Report of Calibration for Luminous Intensity of Three 500-watt

Inside-Frosted Lamps D-1

Report of Calibration for Luminous Flux of one 200-watt

General-Purpose Lamp D-2

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

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.
(Rcv. u-m)

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF BTANOAROS

Gaithersburg, MD 20899

REPORT OF CALIBRATION

for

Luminous Intensity

of

Three 500-Watt Inside-Frosted Lamps

Supplied to:

(See your Purchase Order No. 053^88, dated April 16, 1985)

1 . Material

Three 500-watt lamps were callbarated for luminous Intensity.

The lamps have C-13B filaments In T-20 Inside-frosted bulbs with

medium blpost 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 insidefrosted lamp standards of luminous Intensity. Note especially

paragraphs I-C and I-O which describe the orientation and operation of the test lamps. The photometric distance used for this

callbaration was 3 meters.

3. Results

Lamp No. Direct Current

(amperes)

Voltage Luminous Intensity

(volts) (candelas)

NBS 10i<99

NBS 10500

NBS 10501

3.900

3.900

3. 900

1 08 . 87

109.3^

1 08 . 72

676

720

703

Prepared by:

Ronald L. Wilkinson

Radiometric Physics Division

Center for Radiation Research

Approved by

Donald A. McSparron

Radiometric Physics Division

Center for Radiation Research

NBS Test No. : 53^/

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 1 of the present

document for the updated uncertainty values.

D-1
(NCV. U-U)

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. Cal ibration

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.

Date: March

534/

13, 1986 Page 1 of 2

D-2
REPORT OF CALIBRATION

Lamp No,

Set

Direct Current

(amperes)

Voltage

(volts)

Luminous Flux

(lumens)

NBS10279 1.645 119.81 3485

Prepared by:

Ronald L. Wilkinson

Radiometric Physics Division

Center for Radiation Research

Approved by:

Donald A. McSparron

Radiometric Physics Division

Center for Radiation Research

NBS Test No.: 534/

Date: March 13, 1986 Page 2 of 2

D-3
(MXV.

U.S. DEPARTMENT OF COMMERCE

NATIONAL. BUREAU OT BTANOAIIDS

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 mi nature 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
REPORT OF CALIBRATION

Lamp No. Direct Current VoUage Luminous Flux

(Amperes -Set) (volts) (lumens)

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:

Donald A. McSparron

Radiometric Physics Division

Center for Radiation Research

Approved by:

Klaus D. Mielenz, Chief

Radiometric Physics Division

Center for Radiation Research

NBS Test No.: 534/

Date: March 10, 1986 Page 2 of 2

D-5
roMu Has->U>

(ncv. t»-M)

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 in a clear T-20 bulb with a medium bipost

base. The lamp designation listed below is etched on the bulb.

2. Cal i bration

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

Lamp No.

NBS10496

Direct Current

(amperes)

2.460

3.228

4.152

4.463

Voltage

(volts)

43.21

71.21

112.76

128.46

Color Temperature

(IPTS 1968)

(kelvins)

2000

2400

2856

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
REPORT OF CALIBRATION

The coefficients from the least squares fit to the equation

I = A + BT + CT^

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 10'^

C = 1.4305 X 10"^

Prepared by:

Ronald L. Wilkinson

Radiometric Physics Division

Center for Radiation Research

Approved by:

Donald A. McSparron

Radiometric Physics Division

Center for Radiation Research

NBS Test No. : 534/

Date: December 24, 1985 Page 2 of 2
NBS.n4A (REV, z-ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/SP-250/15

2. Performing Organ, Report No. 3. Publication Date

October 1987

4. TITLE AND SUBTITLE

NBS Measurement Services;

Photometric Calbi rations

5. AUTHOR(S)

Robert L. Booker. and Donald A. McSparron

6. PERFORMING ORGANIZATION (if joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Galthersburg. MP 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

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

SAME AS ITME //6 ABOVE

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619878

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

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

bibliography or literature survey, mention it here)

The National Bureau of Standards supplies calibrated standards of

luminous 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. Details of the uncertainty information

currently available, including estimates and procedures for determining

uncertainties of the reported values, are also presented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration procedures; color temperature; luminous flux; luminous intensity;

photometry

13. AVAILABILITY

I Unlimited

I I For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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

14. NO. OF

PRINTED PAGES

83

15. Price

li-U.S. Government Printing Office : 5 987 - 201 -5 97/7 2481 USCOMM-DC 6043-P80
PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

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

NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

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

NBS Measurement Services:

Fricke-Dosimetry in High Energy

Electron Beams SP 250-15

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

NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

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

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

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

SP 250-8 NBS Measurement Services:

Spectral Reflectance

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

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "Att" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

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

NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with Californiun Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor

Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

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

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

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

NBS Measurement Services:

Spectral Irradiance Calibrations

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

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

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

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300