Library, N.W. Bldg

DEC 2 6 1963

\^ecknlcciL v2ote 198

PHOTOMETRY OF PROJECTORS AT THE

NATIONAL BUREAU OF STANDARDS

L. CHERNOFF

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards include the development and maintenance of the national standards of measurement and the provision of means and methods for making measure

ments consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to government agencies on scientific and technical problems; invention

and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications, including assistance to industry, business and con- sumers in the development and acceptance of commercial standards and simplified trade practice

recommendations. Tne work includes basic and applied research, development, engineering, instru- mentation, testing, evaluation, calibration services, and various consultation and information services. Research projects are also performed for other government agencies when the work relates

to and supplements the basic program of the Bureau or when the Bureau's unique competence is required. Ihe scope of activities is suggested by the listing of divisions and sections on the inside

of the back cover.

Publications

The results of the Bureau's research are published either in the Bureau's own series of publications or in the journals of professional and scientific societies. The Bureau itself publishes

three periodicals available from the Government Printing Office: The Journal of Research, published in four separate sections, presents complete scientific and technical papers; theTechnical

News Bulletin presents summary and preliminary reports on work in progress; and Centra! Radio

Propagation Laboratory Ionospheric Predictions provides data for determining the best frequencies

to use for radio communications throughout the world. There are also seven series of nonperiodical publications: Monographs, Applied Mathematics Series, Handbooks, Miscellaneous Publications, Technical Notes, Commercial Standards, and Simplified Practice Recommendations.

A complete listing of the Bureau's publications can be found in National Bureau of Standards

Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 (SI. 25), and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957 (SI. 50), and Miscellaneous Publication 240, July 1957 to June 1960 (includes Titles of Pcipers Published in Outside Journals 1950 to 1959) (S2.25); available from the Superintendent of Documents, Government

Printing Office, Washington, D.C, 20402.
NATIONAL BUREAU OF STANDARDS

^ecUnical "^ote 198

ISSUED DECEMBER ^7, 1963

PHOTOMETRY OF PROJECTORS AT THE

NATIONAL BUREAU OF STANDARDS

L Chernoff

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the ojjen literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 25 cents
Contents ?age

1. INTRODUCTION 1 2. EQUIPMENT 2

2.1 Ranges 2 a. 30-Ileter Range 3

b, 279--Meter Range 8

2.2 PAR Lampholder 12

2.3 Photosensors 12

a, Barrier-Layer Photocell 12

b. Vacuum Phototube 15

3. CALIBPJVTION OF THE ttIOTOMETRIC SYSTEM 22

3.1 Introduction 22

3.2 Standard Lamps 22

3.3 Attenuators 22

3.4 Calibration Procedure 23

a. External Shunt Circuit 23

b. Phototube with Electrometer Amplifier Circuit 24

c. Zero-Resistance Circuit 24

d. Special Procedures 26

4. TESTING PROCEDURE 26

5 . ACKNOWLEDCMENT 2 7

PJ^FERENCES 28

11
Photometry of Projectors at the National Bureau of Standards

L, Chernoff

A projector consists of a light source and optical system

which produces a beam of light. In the photometry; of projectors,

the intensity of the projector as a function of angle of viewing

is measured. For many years the National Bureau of Standards has

conducted photometric tests on various types of projectors, and a variety of techniques and equipment has correspondingly been developed.

Photometric testing is carried out on a photometric range

of which there are two at NBS: a short range on which the test

distance may be varied up to a maximum of 30 meters and a longer

range with a fixed test distance of 279 meters. Photometric

measurements are made by comparison with a standard lamp of known

luminous intensity in a specified direction. These comparisons

are made with photosensors which are color-corrected by filters

so that the spectral response is similar to the CIE luminous efficiency function. While precision requirements are not severe,

considerable care is required to keep the experimental errors within the desired limits,

1, INTRODUCTION

One of the most complicated problems encountered in practical photometry is the measurement of the luminous intensity of a light projector at

various angles of view, A projector consists of a light source and an optical system which produces a beam of light, A common example of a projector is the PAR-type lamp which is a sealed-reflector lamp consisting

of a filament placed at the focus of a parabolic reflector with a glass

cover. Another example is a light source placed at the focus of a Fresnel

lens.

For measuring the luminous intensity of a projector, a photometer

employing an electrical photosensor is used. The response of the photosensor is read or recorded on an electrical measuring device such as a self-balancing recording potentiometer. This response is a function of

the illuminance on the face of the photosensor; the illuminance is ex- pressed by the inverse-square law as follows:

where E^ is the illuminance at the photosensor,

I^ is the luminous intensity of the light source, and ^ is the distance between the light source and the photosensor.
The photometer is calibrated by using a standard lamp of known luminous

intensity in a specified direction oriented in that direction at a known

distance from the photosensor.

Projectors are mounted at one end of a photometric range. There

are two ranges for the photometric measurement of projectors at the

National Bureau of Standards. On the shorter range, the distance between

the projector and the photosensor can be varied to a maximum of 30 meters.

On the longer range, this distance is a fixed 279 meters.

For many years the National Bureau of Standards has made photometric

measurements of various kinds of projectors, particularly those units

used in airfield and aircraft lighting. Since these projectors have been

of many sizes, shapes, beam characteristics, and intensities, a variety

of procedures, techniques, and equipment have been correspondingly developed for their measurement,

When photometric measurements are to be made, each projector must

be considered individually, and it is not possible to put forth any one general method for testing them. The organization of this paper, therefore, is first to describe the photometric equipment most commonly used,

with emphasis on its application to particular types of projectors. Next,

the theory and practical considerations of the various calibration procedures are discussed. Finally, there is a general description of the

procedures used in these measurements.

The infoirmation and data contained herein have been obtained from

many photometric tests of projectors by the Photometry and Colorimetry

Section of the Metrology Division, NBS, and have specific reference to

the work of this section*. Some of the material has been presented in

papers listed in References but has been reviewed briefly in this paper

in the interest of completeness. I'Jhile this paper deals specifically

with photometry of projectors, the techniques and equipment mentioned

are adaptable to the photometry of other kinds of light units.

2. EQUIPMENT

2.1 Ranges

There are two ranges for the photometry of projectors at the National

Bureau of Standards. The choice of range and the actual photometric setup are determined by the intensity, size, shape, and type of the unit to

be tested.

*There are many publications which treat projection photometry in general.

Among these are:

lES Lighting Handbook, 3d ed,. Sec, 4 (Illuminating Engineering

Society, New York, N.Y., 1959).

Waldrara, J.M. , The Photometry of Projected Light, 111, Eng. 4_7,

397 (1952).

Walsh, J,W.T., Photometry, Chap. XIV (Constable and Company Ltd,,

London, England, 1953),
a, 30-Meter Range

Smaller units are usually tested on the 30-raeter range, Xi^hich is

located in the attic of the East Building of the National Bureau of

Standards, The photosensor and standard lamp for calibrating it are mounted on a movable "photometer bar", shown in figures lA and IB. By

moving the photometer bar, a maximum distance of 30 meters can be obtained between the unit under test and the photosensor. The standard lamp can be moved in and out of the calibration position by remote control.

Test units are mounted on a goniometer, a device which can rotate

the unit through known angles about a horizontal and a vertical axis.

The test unit can then be set at a given angle with respect to one axis,

and when photometric measurements are made, a traverse can be taken at

this angle by rotating the goniometer about the other axis, thus obtaining an intensity distribution.

The goniometer x^hich is located at one end of the 30-meter range is

shown in figure 2, Units mounted on it can be rotated about a horizontal

axis through two pivot points on the U-shaped inner frame. There are two

rotary tables for horizontal traverses which permit the goniometer to be^

used as either a class A or class B goniometer as described by Projector,

The inner table on which the test unit is mounted is used almost exclusively, however. This table provides rotations about a secondary axis corres- ponding to the vertical, perpendicular to the horizontal axis which in

this case is fixed. When the goniometer is operated in this manner, it

is a class A goniometer.

Horizontal traverses obtained by rotating the larger table on which

the outer frame is mounted result in rotations about a fixed vertical

axis. When the goniometer is operated in this manner, it is a class B

goniometer.

The goniometer is gear-driven in the horizontal and vertical directions and is usually turned by means of synchronous motors, I'Jhen a self-balancing recording potentiometer is used to record the output of

the photosensor, the recorder chart of the potentiometer is driven by

another synchronous motor which is powered from the same source as is the

goniometer motor. Gear ratios for the recorder and the goniometer can be

varied to make available several choices of speed of rotation and chart

speed, and hence provide a range of angular scales (degrees per division)

on the chart.

One source of error in goniometry is the backlash of the driving

gears. For horizontal traverses, the errors caused by backlash are minimized by running traverses in only one direction. In the vertical direction, backlash can result from inconstant torque and from the goniometer

cradle passing through a balance position in the course of the traverse.

To minimize these effects, with the test unit mounted on the goniometer

the pinion gear on the vertical drive is disengaged, and the inner frame
Figure lA THE PHOTOMETER BAR OF THE 30-METER RANGE- Mounted on the bar is

the equipment used in calibrating, however the shielding for stray

light between the standard lamp and the photocell has been removed,

and a white background has been substituted for the normally

black one.

h

^' —

Figure IB DIMENSIONAL DIAGRAM OF THE PHOTOMETER BAR

4
Figure 2 XHE GONIOMETER OF THE 30-METER RANGE

5
is balanced by means of the counterveights at the top of the goniometer.

After this balancing, a constant torque is applied by means of a small

weight at the end of. a cable, which passes over a pulley connected to the

vertical drive shaft. The pulley and weight are seen to the left of the

goniometer (figure 2).

In order to minimize the errors caused by stray light from spurious

reflections, the photometer bar is provided with a series of baffles.

One of these baffles is seen in figure 1. In addition, there are adjustable baffles situated along the range between the goniometer and the

photometer bar. The walls behind and around the goniometer and the background of the photometer bar are black.

The minimum test distance used in-photometry of projectors is called

the "minimum inverse-square distance". The illumination from the light

unit, measured at distances greater than this minimum, obeys the inversesquare law which is a necessary criterion for the determination of lumi- nous intensity. The photometric distance is made greater than this

minimum distance. The minimum inverse-square distance is determined by

the type and size of the light source, lens, reflector, etc., and must

be considered individually for each unit. If this distance is more than

30 meters (100 feet), the 30-meter range can not be used.

The case of the photometry of a searchlight with a finite sized

light source and emitting a collimated beam is shown in figure 3. For

this case, the angle subtended by the optic (reflector) of the searchlight at the photosensor must be less than the angle subtended at the

point on the reflector farthest from the light source by the smallest

projected dimension of the light source. If the reflector is viewed

through a telescope at the position of the photosensor, the reflector will

then appear bright over all the aperture

From these considerations, the minimum inverse-square distance, L

is given by

ad * o 6s

where

L is the minimum inverse-square distance (in feet),

_a is the distance from the point on the reflector that is farthest

from the light source to the axis of the searchlight(in inches)

d^ is the distance from the light source to point "^" (in inches),

and _s is the smallest projected dimension of the light source as viewed

from point "a." (in inches).

To illustrate, these considerations may be applied to the photometry

of two different sealed-reflector lamps of the PAR-64 type, one with a 300-watt, 6.6-ampere filament, and the other with a 120-watt, 6-volt(20-

ampere) filament. These lamps have the same overall dimensions and differ

* The number 6 appears in this equation as a result of the mixture of

length units.
H

cCOwC/J

0?J w

>

z

M

M

S

M

s

^ • w W cu

5c! rH H bOc

fn CO O r-l S CO o 3

1-1 <r H Q) <:s TJ M c s 0) S 4-J w XI H 3 W en o

en w 0) n: d H •H

1—

pc; o ^3

fe 0)

4-) ^

4JO

P^ TJ o

< <u M J^ Q H

f^

0)u

3

tiO

•>-l P^
only in the size and construction of the filaments. Both lamps have clear

covers and parabolic reflectors, and both emit collimated beams of light.

For both lamps, the dimension _a is 3.7 inches and the dimension d^ is 3,8

inches.

The filament of the 300-watt lamp is of the CC-6 type; that is, the

filament wire is wound in a helix, and the helix is again wound into a larger helix. The axis of the larger helix is perpendicular to the axis

of the reflector. The smaller helix is wound so tightly that its diameter can be considered the smallest dimension of the light source, and

its projected dimension, dimension ^, is this diameter, 0,033 inch, L^

is therefore 70 feet, which permits the lamp to be photometrically

measured on the 30-iiieter range.

The filament of the 120-watt lamp is of the C-6 type. The filament

wire is wound into a single helix. This helix is wound so loosely that

the single turns of the coil can be discerned. Therefore, the diameter

of the filament wire itself, 0.020 inch, is considered its smallest projected dimension. L is 120 feet, and on this basis this lamp should be

photometered on the 279-meter range described below. However, the separate turns of the helix cannot be discerned from the extreme regions of

the reflector. For the region of the reflector where the separate turns

of the helix can just be discerned, both a^ and d^ are about 2,2 inches,

and L computed for this region is 40 feet. At the extreme regions of

the reflector, the diameter of the helix, 0.083 inch, is the smallest projected dimension of the light source, and L is 28 feet. This lamp could

also, therefore, be photometrically measured on the 30-meter range.

The above discussion for the determination of the minimum inversesquare distance is exact only for axial measurements. For measurements

off the axis, test distances of two or three times these computed minimum

inverse-square distances are sometimes required but are often impractical.

Measurements near the axis are usually the most important in the testing

of projectors; therefore, test distances only slightly greater than the

computed minimum inverse-square distances are necessary for most practical

purposes,

b, 279-Met6r Range

Larger lights are usually tested on the 279-raeter range. The photosensor and the standard lamp are located in an enclosure on the roof of

the Industrial Building at the National Bureau of Standards, The range

has been designed with the photosensor facing south so that during the

day the photosensor views the shaded side of the building in which the

goniometer is located and the shaded sides of the intervening stray-light

baffles, A baffling tube and two additional stray-light baffles are situated on the same roof. This arrangement can be seen in figures 4A and

4B,
Figure 4A THE BAFFLING TUBE OF THE 2 79 -METER RANGE- Seen are the

baffling tube and one of the two other baffles.

Figure 4B A CLOSE-UP VIEW OF THE BAFFLING TUBE- The front of the

tube is covered with a removable protective shield.

9
The goniometer, recording potentiometer, and all electrical controls

are located in a room on the top floor of the Computer Laboratory, The

distance between the photocell and the goniometer is fixed at' 279 meters.

This goniometer was constructed from an available 60-inch coastdefense searchlight and is shown in figure 5, To obtain greater precision

in the angle settings, the old gearing, which had a great deal of play and

friction, was removed, and more precise gearing was installed. Angles

are measured through the gears because they can thereby be more conveniently read than if they were measured directly on the goniometer table.

The accuracy achieved by this procedure is sufficient for the purpose.

Tests have shown that the angles for small traverses can be set with an error of less than 0,03" in both horizontal and vertical settings.

Units mounted on this goniometer can be rotated about a fixed vertical axis and about a secondary axis perpendicular to it. Errors re- sulting from inaccurate vertical centerings of the test unit are generally negligible; e,g,, if a unit is mounted 2 feet below or above the

center of rotation of the goniometer, for a vertical rotation of 30" there

will be an error of 0,2 percent in the candlepower reading due to the

change in photometric distance, and a corresponding error in the vertical

angle readings of 0,015°, The goniometer is so sturdy that there is very

little flexure resulting from inconstant torque. It was found that under

an extremely as3mimetrical loading of a.test unit the angular error re- sulting from flexure was within 0,04**,

This goniometer is usually driven by hand although motor drives are installed. An "x-y" recorder is used to trace the goniophotometric distribution. The chart drive of the recorder, a servo-motor, is controlled

by a self-balancing potentiometer, A voltage signal from a potential

divider attached to the drive mechanism of the goniometer is connected

to the input of the y-axis amplifier, thus making the chart position a function of the goniometer angle.

The baffling tube and some of the other stray-light baffling seen in

figures 4A and AB block out all but a 6-foot diameter area of view at the

goniometer. This baffling is effective enough to permit much of the

photometry on the 279-meter range to be conducted during the day. The

illumination from the background is measured at the time of each test and

has been found to be equivalent to or less than that of a light with an intensity of about 1 kilocandle. Correction is made for background illumination in making photometric measurements. During the course of a traverse, the background illumination will usually not vary by more than

10 percent. Therefore, as a general rule, any light having a peak intensity over 100 kilocandles can be photometrically measured during daylight.

A barrier-layer photocell is usually used as the photosensor on the

279-meter range, A barrier-layer photocell is mounted in a fixed position in the enclosure at the receiving end of the range,

10
Figure 5 THE GONIOMETER OF THE 279-METER RANGE- An aircraft searchlight is mounted on the goniometer

11
As shown in figure 6 the standard lamp and photocell are so arranged

that they do not have to be moved for calibration or test measurements.

The photometric axes from the standard lamp and from the test unit are respectively 5" left and right of an axis perpendicular to the face of

the photocell. The response of the photocell used in the 279-meter range

was tested for symmetry and was found to be symmetric with respect to

these angles of incidence of the light illuminating it. Therefore, such

an arrangement introduces negligible experimental errors.

A correction for the transmittance of the 279-meter air path must be

made. The transmittance at the time of photometry is measured by means of

the transmissometer described by Douglas and is used as a correction in

the calibration of the photometer.

2.2 PAR Lampholder

A special holder for PAR-type lamps is used to facilitate the mounting of lamps of this type. Since PAR-type lamps are usually tested on

the 30-meter range, the holder for PAR-type lamps has been designed to be

mounted easily on the goniometer of this range. It is shown in figure 7.

A set of removable mounting rings makes it possible to mount any of the

several sizes of PAR lamps on the holder.

The holder contains a telescope which is used to align the holder

with a mark at the other end of the range so that the axis of the PAR

lamp reflector will coincide with the photometric axis. It is therefore

possible to remove the holder from the goniometer and to replace it at some future time, aligned as before.

2.3 Photosensors

The photosensors used in the photometric measurements are constantcurrent devices which produce currents proportional to the illuminance

on their faces. All photosensors used are color-corrected by means of

optical filters in order to make their spectral response similar to the

CIE spectral luminous efficiency function. Two different types of photosensors are in general use, the barrier-layer photocell and the vacuum

phototube. Although photomultiplier tubes are available, there has been

no occasion to use them for the photometry of projectors at the National

Bureau of Standards laboratories in the last few years,

a. Barrier-Layer Photocell

The barrier-layer photocell, a solid-state photoelectric device, is

used in most of the photometric measurements. In the selection of a photocell for photometric testing, several cells which have been colorcorrected by means of color-correcting filters are checked for linearity

and similarity of spectral response to the CIE luminous efficiecy function. In order to check the adequacy of the color-correction, the lumi- nous transmittance of several colored filters for light of a specified

12
't- / o//~

f/l

_l la ^ <m (U3

bO •H

13
Figure 7 A REAR VIEW OF THE PAR LAMP HOLDER- Seen are the telescope

for aligning^ the mounting ring^ and the back of a PAR-56

lamp which has been mounted on the holder.

14
color temperature is measured v/ith these photocells. These measurements

are compared with transmittances determined from spectrophotometric

measurements. The results of one such series of measurements, using a lamp operating at Source A (color temperature ZSSA^K) are given in table I,

The photocells with good color response are then tested for linearity; the one most nearly linear in its response is selected for use. The

response of the photocell being used at present, cell number 3 of table I, was found to be linear to better than 0,1% in the most useful range,

which is sufficiently linear for photometric testing. Here, linearity

was tested using several standard lamps of kno\.m horizontal intensity in

turn at distances from 1 to 30 meters from the photocell. The results

of the linearity measurements of this photocell are shown in figure 8,

The barrier-layer photocell is used with two different circuits.

For most photometric work with this type photocell, an external shunt is

used. The voltage drop across the shunt is amplified by means of a linear

amplifier, and the output of the amplifier is recorded on the recorder

chart of a self-balancing potentiometer. This circuit is shown in figure

9, However, when greater precision is required in the measurements, or

when the illumination of the face of the photocell is either very large

or very small, a "zero-resistance" circuit is employed, and intensity

measurements are made by using a Kohlrausch potentiometer, *

b. Vacuum Phototube

For flashing lights of short flash duration such as condenser-discharge lights, and for lights of very low intensity, a G,E, type PJ-14B

vacuum phototube is used. The phototube with its power supply is housed

in a metal box as shown in figure 10, The phototube is color-corrected

by means of a specially designed filter; the accuracy of this color cor- rection is indicated in figures llA and llB,

A d-c electrometer-amplifier is used to amplify the output of the

phototube. The electrometer has been modified so that it may also be

used as a null-detector for a self-contained potentiometer when greater

accuracy is necessary.

In the photometry of flashing lights, it is desirable to compute

the effective intensity* of the flash, which is determined from a measurement of the average intensity of the flash. In order to read the average intensity, a resistance-capacitance network is placed across the

phototube so that the photometer circuit will have a suitable time constant, A diagram of the photometer circuit employing the PJ-143 phototube, together with such a resistance-capacitance network, is shown in

figure 12, The combination of d-c amplifier and phototube is useful in

measuring steady illuminations as low as 0,00001 footcandle.

* The effective intensity of a flashing light is equal to the intensity

of a steady-burning light of the same color which will produce the same visual effect under identical conditions of observation,

15
COu

<u u

i-H •H

0)ooOuIu

(U

•1-1 u

PQo3oO

n>

M-lo

0)

COCo

(X

CO

(U CO

pes ^ <J u

V4 0) (1) o +J o

1-4 CJ >-l o oj 3 o u o

ca CO x: • U !-i H O

>-i i*^ 0) O

iH 4J ^ rH

c« •H H 1^

vOomo

CO

CU

CM

0)

00 CO <!

CT>

O CT\ o i-i un O O 0^ vO v£>

O ON CNO r^ r^ O CT» 00

CO in vo

I-H

O ON

I—I r-l vO O O C3N <t <f ON

ON 0> On vO 00 r^ o\ 00 00

O r>» <^

cs

O ON CM csi r>» O O ON

\0 >£)

O ON ON

in r^ 00

ON 00 CO

O

O <! o o <y^ ON i—i r*

ON o o\

i-H

O CM o o o

ON CM 00 ON ON 00

<! o in O O ON

O I-I r^ O ON

rH rH I-I

00 r>. iH ON ON O

00

O 00

ON ON 00

C_3

r-^ r^ 1^

iH

O ON

CM csj r* O O ON00 r^

O ON ON o

rH o r^ O CJN 00 oiH CM >*

<f CM CM O CM 00

r> Cvl in

CM iH CM >^ vo r^

Or>» iH ON

CO

O

O

O CM vt

O

O

O

CM v:t in O iH CM

m CO rH

r>- rH in

CM CO CO

O iH -vl- r-. CM <J- CO <1- <^

<JN CO

O r». f^ in

v£) in <tO CO CJN 00 ON CM O CM CO

in r^ 00

CM 00 <J- r^ MD vD

O 00 -d- CO I** in

\£> in in

CO o o

CO iH in

CM CO CO

O CM o

vO vO CnI iH iH CO > u o d o rH (U lU iH T) iH (U 3 O QJ = r (U = ^ u z ^ rH o (^ >* O pq

d

•iHu

4J

CO

CDM

4JO

(Uuoo

P^ CO u

>N CI 3

CO Clj CD (Uo

<U -H O M

cd CU 4J S w o

•H 4J S

CO 2Xi C &

CD

O

UU

4J 4Jo

^H a> o oCO .2 a

+J o

CD

U

OS MH s

CDU

bO

CD

>N u

•rloo

Xio

<udo

<u

4-1 CDd

•H

T3oooI s

CDU

bO tJ

CD •H

!>N u

•iHO

•H

4JO

Xio M Mudo

(U 4J

CDd

•H

TJ

>HoOoI>N

COdu

CO

CD

<UauuioO

J3aoupu

(U&

CO>N

(Uda

<u

4-1 0)

CO

CD

>^<U

4-1 rH

•H

MH

CU Xi

4-> MHO

(UOd

CD

4-1 4J •Ha

COd

CDU

4J

COod

•HI

16
w

u

o

H

o

ftW ' >-C 1—

^ 1—1

5 <1> pq o P OJ W ^ H 2 o U

^

UJ 1-1 T3 o W ^

< w 't; z 0) s

3 o2 _1

-J w f^ W CO ^ CO o ?

ft ^, w rt w

TM ^^

^^

o 0) >H M

H-< .—1 P^ .° <>

w i ^ -^

t-H O ^Z

CX>

0)M

dM

o o o • o o

CO u> * CM o

00 00 00 00 CO ^

Oi/Ori '33NVNIWn"l"ll Oi lN3yHnO dO OliVd

17
o UJ Q O

Ul UJ

(r \-o

a.

PiwH >Em

q: UJ z_i o§

CM

PiH

pqO

en z >

<o < £

UJ I in

1^uo

pq
yiiiiiiiinf

U3^

</) LiJ

_j c /1 1- -3 O Q- (-O

Ul

X o a. 1

1- ^ V

;;v,

O

y^ 1J>l

HUMBSCRE

TO

EMOVE

PHOTO

UBE

PLATFORM

1- tr >- - f -

CO

uo o f< 4 / \ 4

II

@

@

@

®

@

@

@

®

®

@

_l

in

CM

CM

CD

wwoHowoHo

ft

r—

I

ftu

i-t ^

19
100 -

80

60 -

>

t 40 -

20 -

1 1 1 1 1 1

VACUUM PHOTOTUBE ^

-V^^^^V""

"~^

/ \ CIE LUMINOUS

/ \ EFFICIENCY FUNCTION

^—--^1 1 1 1 1 ^"^^--^1

-

450 500 550 600

WAVELENGTH, m/i 650 700

Figure llA RELATIVE SPECTRAL RESPONSE OF UNCORRECTED PJ-14B VACUUM

PHOTOTUBE AND CIE LUMINOUS EFFICIENCY FUNCTION

70 -

1 1 1 1 1 1

if \

60 1

1

I \

\\

\\

\\

DESIGNED

50

1

\\

\\

\\ 40

1

\\

\\

\\\\

30

I

1

1

\\

\\

20 1 1 1 \

10 // //

yy

«!'*' 1 1

1 1 1 \---.l

500 550 60'0 WAVELENGTH, m/i 650 700

Figure llB SPECTRAL TRANSMITTANCE OF DESIRED AND

DESIGNED COLOR- CORRECTING FILTER

20
§io

Q.

—/^yv

a:

UJ

I- oc UJ LU

Q |_

a

LU o 4 f

CM

H t- ^ ^

H"

UJ q: CD OQ UlI ^ =) h- _ H UJ I 5 S Q ^ Z3 O <

Q- =3

H UJ

O

O

X < I > Q.

cu J3uc M

0)

C

0) •r^ s Xi U CO

(U CO ^ I—

^u M-l

15 4J >^

<u

U c

cu

*<—«fc 1— OJ

B 

flu • ttf u Xi + •H a

1—4 CO CU m +

r-^ 1 c + (U •r^

r—

c TJ

CO u

CU

CO

X CO 3

CO CO

g

•r-<

pq CU (U T3

•H (U <D H M-) >

T) •H

(ti 1 -^ E U 05 a. <u ni (U W B >^ • f-l Q H CO CU :^ § 0) CO

•* X! -H M H 4J IS 4->

i-l >

+-> H! t3 cu CO

•r-( PL. CO 4J

fM

pq -a oi CO

0) CO 1—

en bO j:: -H

•* s d

H • rH

U T3 x! 4J ff! (U c

TJ CO P^ 4J CO M-l M d (U H (U •-> pq e CO a ^ o- -u < I—

1 bO (M

ti •-J Si -H • PM aiM ^ ^ +-> f 1 00 ^1 CN \J ffi

•iH 1—

f\l (VI 1—

CU

u Di

u 3 U »i Pi

P>4

21
3. CALIBRATION OF THE PHOTOMETRIC SYSTEM

3,1 Introduction

Lamp standards of luminous intensity are used for calibrating the

photometric testing equipment; a separate calibration is made before each

test, and a record is kept of the photometer sensitivity in order to detect any irregularities. The illumination on the photosensor produced

by the standard lamp is adjusted to some typical value of the illumination

produced by the test light, usually in the range of 75% to 100% of the

peak illumination produced by the test light. This procedure minimizes

errors resulting from nonlinearity of the response of the photosensor.

The adjustment of the illumination of the standard lamp on the photosensor

is accomplished by varying the distance of the standard lamp from the

photocell and by using optical attenuators. The photometer is usually

calibrated so that it is direct reading in luminous intensity.

3,2 Standard Lamps

The standard lamps used are "working standards" whose luminous intensity in a given direction has been determined at a given voltage.

Standard lamps are available ranging in intensity from about 8 to 900

candles. When a colored light is being tested, a filter is placed between the standard lamp and the photosensor, which results in a standard

lamp-filter combination having approximately the same spectral characteristics as those of the light to be tested. This procedure minimizes

errors resulting from inadequate spectral correction of the photosensor.

In this procedure, a standard lamp of known color temperature as well as of known luminous intensity is needed,

3,3 Attenuators

Sector disks are almost always used for light attenuation, although

neutral filters are also available. A sector disk is usually placed between the standard lamp and the photosensor to calibrate the photometer

for the proper range of illumination. However, when the intensity of the

light being photometrically measured is unusually high, the sector disk

may be used to attenuate the illumination from the test light. The

range of sector disks available is from 1% to 80% transmittance.

When a sector disk is used, it is placed within a few inches of the

photocell in order to reduce error from stray light. The disk is rotated

at a few hundred revolutions per minute, which is fast enough to minimize

error from apparent flicker. When a high illumination is attenuated by

a sector disk of low transmittance, there is an error which results from

only one part of the photocell being illuminated at a time; this error

is successfully eliminated by placing a condenser of about 4 mfd, across

the output of the photocell, (In utilizing this technique, one must be

careful to obtain a capacitor which does not itself generate an emf.)

22
3.4 Calibration Procedure

« The calibration involves illuminating the photosensor with light

from a standard lamp placed at a given distance from the photosensor,

and then adjusting the sensitivity of the photometric system to some desired value.

If i^ is the photosensor current,

I^ is the intensity of the light illuminating the photosensor,

and D^ is the distance from the test unit to the photosensor,

then, since the photosensor produces a current proportional to the illuminance on its face, „ L = kl/D"^ (1)

where Ic is the sensitivity of the photosensor.

It is usually convenient to calibrate the photometer to be direct

reading, so that

I = N6 (2)

where 6_ is the reading of the potentiometer of the measuring circuit,

and where N[ is an integral power of 10 or the product of an integer,

usually 2 or 5, and an integral power of 10.

The photometer is then calibrated by using a standard lamp of known

horizontal luminous intensity. If

I is the luminous intensity of the standard lamp,

D is the distance of the standard lamp from the photosensor,

6 is the potentiometer reading and i is the photosensor

current when the photosensor is illuminated by light from the standard

lamp placed at the distance D from the photosensor, then

i = k I /D

^ (3)

s s s and since the potentiometer reading is proportional to the photosensor

current

Combining (1) , (2) , (3) , and (4) ,

i /6 = 1/6 (4)

s s

6 = I y/ND/ (5)

s s s Calibration is accomplished by the following procedure: I and D are chosen so that I /D will be approximately equal to I/D'^v/here

I^ is some typical value of the intensity of the light to be tested. A

suitable value of N is then selected. Calibration to make the photometer direct reading is completed by one of the three following procedures,

depending on the photometer circuit used.

a. External Shunt Circuit

A diagram of this circuit is shown in figure 9. In this circuit

23
fLSkI

« - ^ s s jj 2 (6)

s where S_ is the sensitivity of the photometer circuit and R is the re- sistance of the shunt.

Calibration, therefore, requires that, with the photocell illuminated

by light from the standard lamp, the external shunt resistance is set so that the potentiometer indicates the value 5 given in equation (5). The

other parameters of the calibration are usually chosen so that the shunt

resistance will be of the order of a few ohms. This order of resistance

is used as it is large enough to be set accurately, and small enough so that the voltage developed across the photocell will not cause the photocell to respond nonlinearly. The practice is to maintain the sensitivity

of the recorder at a fixed value of 5 millivolts for full-scale deflection.

The sensitivity of the preamplifier is therefore set so that this recorder

sensitivity and desired range of resistance may be used,

b. Phototube with Electrometer Amplifier Circuit

The procedure for calibration is the same as that for procedure a, (See figure 12^) The load resistor on the phototube and the controls of

the amplifier are adjusted for the optimum performance range of the am*- plifier. Also, the output of the amplifier should not exceed 5 milliamperes. Hence, other parameters are adjusted so that IL is greater than

1 ohm and is less than 5 ohms.

c, Zero-Resistance Circuit

A diagram of this circuit is shown in figure 13. In this circuit,

if the photometer is balanced so that no current flows through the galvanome te r , then

i = i a/r (7)*

a X

where

_i is the photocell current,

i^ is the current through the slidewire between and A(f igure 13)

ai^ is the resistance of the slidewire between and A (figure 13),

and r is the resistance of the resistor, r , —X ' X

Assuming the slidewire is graduated from to 100, the reading of

the indicator of the slidewire is

6 = a/a (8) o

*This equation is an approximation which depends on i_ being much greater

than i^. In practice, i is kept at about 10 milliaraperes, and the range

of i^ is from 1 to 20 microamperes. If i_ is 20 microamperes, the error

resulting from the use of this approximation will be 0.3%, For larger

values of J^, a correction in the calibration can be made.^

24
d

c: o (U («

0)u o u

4J

O eJ CO >^ o >^ 4J C« 4J o. 4-1 +j X I-l •I-l •r1 i-l CO o >. T-t > a 0) > ca •H ja u 3 •1-1 O ^1 •H 3 CO (U M-l u

•1-1 •r^ 4J cr (U OJ 4J • •H

g

w 3 •r4 (U

n o .u rt Q) CO

g

•H so CO

C ca

g

O c -«. 4J o -0 c (U to C(J ^ o C! CD a.

OS <U -H (U o a !-l •u 1-4 CO CO S

4J •u

U r-« CO C! •r-t <U CO CO >^

u nj (O OS a ra fC a 4J •H g M «\ CO •> :3. o u M u S SJ CO 42 T3 M •H i-l q; CO e u (U CO ti s <U O <U CO OJ St ^ o x: o 4J •ri -o o U •U u Gi •U O u 0) o (1) m ti o 1-1 <U u (1) o ^ 1 u S (U f-i 08 Q) o o e • B M O (U o n « > -0 o > g CO o o ^ r-l -P c o r-l C3 *% 1 03 e c: o e S «e

«s •r^ CS

O o CM •r-l j3 CO 4J 1 ^ e > tH 4J fcO 0) o ^^ 1-1 o > 3 o o <^ o r-4 o •H T3 I-l I-l T-l rH O o PQ -H ctf M »-l y-i 1 •H o CO ^ rH lO

>-•

COU CJ o St o i-H s en ao ctf

t-4 CvJ en St >< I-l U M Wi 1-1 U w a o o

Pi

Pi

Ix] HO

E-i COMW

CO

0)

}-l 3

60

•1-1

25
where a is the total resistance of the slldewlre (the resistance between

and B^ln figure 13)

Then, combining (1), (7), and (8),

r kl

5 ^

(9)

i a D

a o In the calibration of the zero-resistance circuit, i^ is usually kept

constant and r is varied,

—x

When the photocell is illuminated by light from the standard lamp,

r is adjusted to obtain a zero reading of the galvanometer when the

sXidewire is set at the value 5 of equation (5) for a given test distance, D^. With the photometer thus calibrated, the intensity of the test

light is given by equation (2)

d. Special Procedures

While photometric data are usually presented for, a test light operating under the design condition, photometry of the test light under

operating conditions other than the design condition is often desirable.

Equation (5) can be generalized, taking into account this condition as well as the transmittance of any filters or sector disks used in calibrating, so that

2

6 = Y Y FI -—r (5a)

^ ^ ^ ^ ND

2

s where y is the transmittance of the color filter at the color temperature

of the standard lamp, y is the transmittance of the sector disk, and F^ is the ratio of the output of the light under test when it is operated

under the design conditions to the output of the light when it is operated

under test conditions. This ratio may be, for example, the ratio of the

rated lumen output of the test lamp to the output of the lamp at the test

voltage. It also may be the ratio of the intensity in a given direction

at the operating voltage to the intensity in this direction at the test

voltage.

In the case of lights which are flashed in service but on which

photometric measurements are made with the light burning steadily at a selected voltage, the factor F^ is the ratio of the effective intensity

of the flash in a given direction to the steady intensity at the selected

voltage in this direction of view,

4, TESTING PROCEDURE

The photometer is calibrated as described under "Calibration Procedure" (Section 3,4) to an illuminance range determined by the intensity

of the light being tested, the test distance, and the information desired,

26
The test unit is mounted on the goniometer and is aligned. Tlie angular

settings of the goniometer are adjusted so that the origin of the goniometer settings will correspond to the desired axis. This axis usually

is chosen with respect to either the seating plane of the unit or some characteristic of the beam such as its peak.

The baffling for stray light is put into place. The eye is placed

in the position normally occupied by the photosensor. Examination can then be made to insure that the baffling is properly placed so that no obstructions exist between the light and the photosensor and so that

reflections from the walls, floor, and ceiling of the range are intercepted before they reach the photosensor. When the 279-meter range is

used, the baffling tube is periodically checked to insure that it is not

being obstructed by birds.

If a sealed-reflector lamp is being photometrically measured, the

lamp is usually operated at either rated voltage or rated current. Other

lamps, such as those used in combination with an optical system, are usually operated at or corrected to rated lumen output. Power for the

test and standard lamps is usually obtained from storage batteries, which

are periodically recharged. Voltage and current are measured on a potentiometer, and photometric measurements are not made until the lamp has

reached stability.

If the goniometer is to be motor driven, the gear ratios are chosen

so that the traverse will be slow enough to insure the accurate recording

of the characteristics of the light,

5, ACKNOWLEDGMENT

The contributions of the many members of the Photometry and Colorimetry Section of the Metrology Division, NBS, who through the years

have developed procedures and instrumentation described here is gratefully acknowledged.

27
REFERENCES

1, T, H. Projector, Versatile Goniometer for Projection Photometry,

111. Eng. 48, 192 (1953)

2, lES Lighting Handbook, 3d ed., p. 4-35 (Illiiminating Engineering

Society, New York, N.Y., 1959)

3, J, W. T, Walsh, Photometry, Chap. XIV (Constable and Company Ltd.,

London, England, 1953)

4, F. C. Breckenridge and T. H. Projector, Construction of a Goniometer

for Use in Determining the Candlepower Characteristics of Beacons,

CAA Tech, Dev. Rep. No. 39 (Feb. 1944)

5, C. A, Douglas, Visibility Measurements by Transmissometer, Electronics

20, 106 (1947)

6, Louis E, Barb row, A Photometric Procedure Using Barrier Layer Photocells, NBS J. Res. 2_5, 703 (1940)

7, T. H. Projector, M, K. Laufer, and C. A. Douglas, An Improved

"Zero-Resistance" Circuit for Photo-Cell Photometry, Rev, Sci,

Instr. 15, 107 (1944)

8, C, A, Douglas, Photometer for Measurement of Effective Intensity

of Condenser-Discharge Lights, 111. Eng, 53, 205 (1958)

9, C. A, Douglas and M. R, Freund, Potentiometer for Measurement of

Small Photoelectric Currents, NBS Rep. 6422

10. lES Lighting Handbook, 3d ed. , p. 4-5 (Illiiminating Engineering

Society, New York, N,Y., 1959)

28

MT cnTTJTiwr. mrpirp- iQRn O - 714-574


THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In genera!, each section carries out specialized research, development, and engineeringin the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

WASHINGTON, D. C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. High Voltage. Absolute Electrical Measurements.

Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology. Mass and Volume.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment. Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research. Crystal Chemistry. Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics,. Engineering Mechanics. Rheology. Combustion

Controls. Polymers. Macromolecules: Synthesis and Structure. Polymer Chemistry. Polymer Physics. Polymer Characterization. Polymer Evaluation and Testing. Applied Polymer Standards and Research. Dental Research.

Metallurgy. Engineering Metallurgy. Metal Reactions. Metal Physics. Electrolysis and Metal Deposition.

Inorganic Solids. Engineering Ceramics. Glass. Solid State Chemistry. Crystal Growth. Physical Properties.

Crystallography.

Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Metallic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics. Operations Research. Data Processing Systems. Components and Techniques. Computer Technology. Measurements Automation. Engineering Applications. Systems Analysis.

Atomic Physics. Spectroscopy. Infrared Spectroscopy. Far Ultraviolet Physics. Solid State Physics. Electron Physics. Atomic Physics. Plasma Spectroscopy.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry.' Organic Chemistry. Molecular Spectroscopy. Ele- mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.

Office of Weights and Measures. BOULDEH, COLO.

CRYOGENIC ENGINEERING LABORATORY

Cryogenic Processes. Cryogenic Properties of Solids. Cryogenic Technical Services. Properties of Cryogenic

Fluids. CENTRAL RADIO PROPAGATION LABORATORY

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research. Troposphere and Space Telecommunications. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Spectrum Utilization Research. Radio-Meteorology. Lower Atmosphere

Physics.

Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre- quency Utilization. Modulation Research. Antenna Research. Radiodetermination. Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere

Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

RADIO STANDARDS LABORATORY

Radio Standards Physics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Fre- quency and Time-Interval Standards. Radio Plasma. Microwave Physics.

Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Calibration Services. High

Frequency Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency

Calibration Services. NBS LABOilATORY ASTROPHYSICS GROUP

(Joint Institute for Laboratory Astrophysics at Univ. of Colo.).
ii?%