NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-20-2300 August 195? 1|285

PHOTOMETRIC TESTS

OF AN

ATQ-2A AIRCRAFT SEARCHLIGHT

FQDIPPED WITH EACH OF SEVERAL REFLECTORS

*

7

W* F. Mallis

&

T, H0 Projector

Naval Aircraft Lighting Group

Photometry and Colorimetry Section

Optics and Metrology Division

Test 21R-25/53

Sponsored by

Lighting Section*, EL-5211

Department of the Navy

Washington 2?<, Do C»

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re APP roved for public release by the

unless permission is director of the National Institute of

25,d.c. Suchper Standards and Technology (NIST)

rally prepared If t On October 9, 2015

in part, is prohibited andards, Washington

port has been spe.cifi-

;port for its own use,
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Io Introduction

The A"VQ-2A aircraft searchlight employs a precision-grade* backsilvered glass parabolic reflector. From time to time* other types of

reflectors have been proposed or considered* including drop-molded

back-silvered glass (non-precision)* aluminum* and aluminum-base reflectors with special reflectance-increasing coatings® Among other

factors to be considered in determining whether such alternative types

of reflectors are satisfactory for use in the AVQ-2A searchlight* is

that of the relation between reflector quality and searchlight performance.

It is the purpose of this report to present the results of a preliminary

investigation of this relationship as determined for five reflectors* as

follows

i

#A5lli9* Precision-grade* back-silvered glass. This reflector

was selected from among the AVQ-2A reflectors on hand

at NBS as having the most nearly perfect parabolic

figure

o

#A6375>* Similar to the above except that its deviation from perfect

figure is about as large as is found in this class of

reflector.

#A32U* “Non-roll-off11 reflector* - similar to above except that it

was manufactured by an improved process* permitting maintenance

of figure farther out to the periphery of the reflector. In

“roll-off" reflectors as previously supplied (A51U9 and A6375

are of this type) the peripheral zone extending in from the

edge about 1/2 inch departs appreciably from the parabolic

figure of the remainder of the reflector and therefore does

not contribute significantly to the searchlight beam.

#Alum. 901-1* Aluminum base with a aichroic surface film designed to

produce maximum reflectance in the region of the peak of the

ICI luminosity curve.

#Alum. 901-2* Similar to 901-1.

II. Test Procedure

Candlepower distribution measurements were made on a 277°meter (909 ft.)

outdoor range® The searchlight was mounted on a goniometer at one end of

the range and the illumination measured at the other end with a luminositycorrected photocell whose output was fed into one axis of a Leeds & Northrup

automatic recorder. In order to obtain horizontal candlepower distributions*

the goniometer on which the searchlight was mounted was rotated around its

vertical axis so that the beam traversed the photocell. Tertical distributions were obtained in a similar manner by rotation of the goniometer

around its horizontal axis. Runs of $0~seeond duration were made at O.f?-

degree intervals* and the average candlepower of the last 30 seconds of each
i

*

.
o2a

run, allowing 20 seconds for stabilization* was computed* Two runs were

made at each point in the distributions* except at the peaks where at least

10 runs were made*

An AT7Q-2A searchlight with modifications as described in NBS Report

135>i|, "Instruction Book for Modification of AT7Q-2A Aircraft Searchlight"*

was used for the tests© This light burns 11 mm carbons* and normally

draws an arc current of 120 amperes* Previous experience has shown* however*

that more stable operation can be obtained at reduced current* For these

tests, therefore, the current was held at approximately 10U amperes* A

special arc regulating circuit was employed which limited the current variation to about ± U amperes© The optical figure and surface quality of each

reflector was evaluated by examining the projected shadow patterns for each

reflector as obtained on a shadow-projection device designed and built by

NBS for this purpose© The general operating principle of this device is

described in NBS Report 223U, "Methods of Focusing the AT7Q-2A Aircraft

Searchlight" (Focusing Method £3) and in the NBS Technical News Bulletin

for June 1953, pp* 93-9U*

To facilitate comparison of the reflectors* curves of spectral reflectance obtained previously (NBS Report 2358* Test 21N-63/52) for the

two aluminum base reflectors and for a back-silvered glass reflector are

presented* These curves give the total reflectance (specular plus diffuse)

for a small circular area* approximately 1 inch in diameter* at nearly normal

incidence* The luminous reflectance for each reflector was computed from

these curves and is also presented in this report*

TO* Results

Figure 1 gives the average horizontal and vertical candlepower distributions of the AVQ-2A searchlight when equipped with each of the five reflectors tested* It should be pointed out that these curves are on a comparison basis only and do not represent peak performance of the searchlight

since the arc was operated at reduced current*

Figures 2 through 6 are photographs of the projected shadow pattern

for each of the reflectors*

Figure 7 gives the spectral reflectance curves of a typical backsilvered glass reflector, the two aluminum dichroic coated reflectors and

the standard CIE (ICI) luminosity curve© The computed value of the luminous

reflectance and Ihe peak candlepower obtained with each reflector is given

in the following table*
(

.

.

.

. ..
Table I

Reflector

No.

Luminous Reflectance

(Percent)

Peak CandLepower

(Megacandles)

Alum. #901-1

Alum. #901-2

A51U9

A6375

A32U

93

93

93

90

92

67.1

61.8

65.2

59.1

65.7

IV. Discussion

Optical Figure and Distortion.

The projected shadow patterns shown in the accompanying figures are

very sensitive indicators of the optical quality of the various reflectors.

The relative freedom from distortion of glass reflector A5lii9 (See Fig. 2)

is indicated by the close conformance of the edges of the shadow with the

lines on the screen representing the outline of the mask (the center line

of each group of five lines). The rounding of the corners of the mask

shadow is caused by the "roll-off” edge. (The small dark spots on the

shadowgraph are associated with pock mark damage on the front surface of

the glass. This damage occurred during previous test work but does not

significantly reduce the efficiency of the reflector.) Reflector A6375

(Fig. 3) shows similar roll-off distortion as well as pin-cushion distortion. Reflector A32U (Fig. k) shows pin-cushion distortion but unlike

the other two reflectors, its parabolic figure is held accurately much

closer to its outside edge. This extra l/2” of non-roll-off edge amounts

to about 10$ of its entire reflective area. The additional useful area is

indicated by the trade mark and number etched near the periphery of the improved reflector. With the other two reflectors, the trade mark, located

in the same position as on the non-roll-off reflector, is cut off almost

entirely in the shadowgraphs.

The concentric fringes in the shadowgraphs for the two aluminum base

reflectors, Figs. 5 aid 6, are the result of the relatively coarse surface

finish associated with the grinding operations used in their fabrication.

This coarse finish also accounts for the rough edges of the shadow of the

mask as compared with the relatively smooth, clean edges obtained with the

glass reflectors. In addition to the coarseness of the finish shown by

the concentric fringes, moderate deviations from parabolic figure are

shown in tiie shadowgraphs by the departure of the shadow edges from the

reference lines representing the edges of the mask, and by the distortion

from circular shape of the edge of the beam. It was found also that the

aluminum base reflectors were somewhat flexible and care had to be exercised in mounting them in order to prevent excessive distortion resulting

from stresses introduced by the mounting clamps.
’

.

.

.

. i

*
Reflectance

The luminous reflectance of the dichroic film coated aluminum reflectors coupares favorably with that of back-silvered glass reflectors*

It should be noted* however* that the measurements were made at nearly

normal incidence* whereas* as used in the searchlight the bulk of the

light flux is incident at angles greater than normal* This condition

would have negligible effect on the reflectance of the glass reflectors*

but since the reflecting properties of the dichroic films of the aluminum

reflectors are appreciably affected by angle of incidence* it may be expected that the performance of the searchlight would not be in direct

proportion to the measured value of luminous reflectance at normal incidence* With the dichroic film coated reflectors the dominant hue of the

reflected light shifts toward the blue with increasing angle of incidence*

Fig* 7 shows that the centroid of the reflectance curve for reflector

Alum. #901-1 is displaced toward the blue end of the spectrum f^om the

centroid far the ICI luminosity curve while that for Alum. #901-2 lies

nearer the red end* A small shift of both curves toward the blue end*

resulting from the higher angle of incidence* could result in an increase

in liminous reflectance for the #901-2 reflector and a decrease for the

#901-1 reflector. This is in accord with comparative data given in

Table #1 and probably accounts at least in part for the lower candlepower

of #901-1*

7° Summary

It is evident from the candlepower distribution curves in Fig* 1*

that differences in quality among the reflectors tested are not accompanied

by large differences in performance in the AV3-2A searchlight* The difficulties Inherent in the test procedures* the instability of the arc lamp

even when operated at reduced current and with special precision current

control devices* and the variability of carbons result in uncertainties in

the measurements significantly close to the order of magnitude of the differences in performance found* However* the reliability of the differences

in peak candlepower is much better than that of the off-axis measurements

since much more data was obtained in the axial directions. There seems

to be a fair correlation between the quality of the reflectors and the

peak candlepowers obtained with them although the magnitudes of the differences are small* The results suggest that small scale imperfections

such as the grinding marks left on the aluminum base reflectors* although

prominently displayed in the shadowgraphs* do not materially affect performance* but that larger scale imperfections such as the distortion

represented by the pronounced pin cushioning of the shadowgraph for

reflector #A6375 do result in small but significant reductions in peak

candlepower*
. . . • • . ' « • . * . I
-5-

A complete analysis of the performance of a reflector in a searchlight

would require consideration of the entire beam. Most of the light flux is

found at angles off the axis. Unfortunately* as the zone factor by which

the candlepower must be multiplied to obtain luminous flux increases with

angular displacement from the axis so also does the uncertainty of the

candlepower measurement. It is accordingly not meaningful with the limited

data on hand to compute the luminous flux in the beam. In order to make a

complete analysis of the performance it would be necessary to obtain a considerably greater amount of data and to improve the technique of measurement

and the stability of the searchlight. One approach that may be fruitful is

to use a more stable source in place of the carbon arc. It might be difficult

to simulate the carbon arc with such a source* but it is probable that sources

might be found wherein the gain in stability would produce an improvement in

the reliability of the data that would outweigh the errors introduced by the

inexactness of the simulation.

In view bf the savings that might be effected by increasing the tolerances

for reflector quality and the indications given by the results of this test

that this might be done without appreciable reduction in performance* it is

recommended that this problem be investigated further as suggested above so

that differences in performance may be precisely analyzed and suitable methods

of specifying reflector quality be developed.
.

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Figure 7