NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-20-2300 August 1955 1*279

PHOTOMETRIC TESTS

OF AN

A1Q-2A AIRCRAFT SEARCHLIGHT

EQUIPPED WITH A PRECISIO N-GRADE, PARABOLIC "NON-ROLL~OFFM EDGE,

BACK-SILVERED GLASS REFLECTOR

by

W. F. Mullis and T* H. Projector

Naval Aircraft lighting Grcxip

Photometry and Colorimetry Section

Optics and Metrology Division

Test 21N-21/53

Sponsored by

Lighting Section, EL-5211

Bureau of Aeronautics

Department of the Navy

Washington 25, D.C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Approved for public release by the

The publication, re| in part, is prohibited

unless permission Is director of the National Institute Of a„ da, ds, Washington

25, D.c. Such per Standards and Technology (NIST) port has been spudscal ly prepared If tl On October 9, 2015 sport for its own use,

PHOTOMETRIC TESTS

OF AN

AVQ-2A AIRCRAFT SEARCHLIGHT

EQUIPPED WITH A PRECISION-GRADE, PARABOLIC “NON-ROLL-OFF" EDGE,

BACK-SILTZERED GLASS REFLECTOR

lo Introduction, Precision back-silvered glass reflectors, as manufactured by the

Bausch and Lornb Optical Company for use in the AN/AVQ-2A aircraft searchlight, have been characterized in the past by a “roll-off“ edge. This

outer annular zone, amounting to about 1/2 inch of radial depth, departs

from a parabolic figure to such an extent that its contribution to the

beam emitted by the searchlight is negligible. Recent improvements in

the manufacturing process have permitted maintenance of the parabolic

figure much closer to the edge of the reflector, A sample of such a

“non-roll-off" reflector (#A32h) was submitted by the manufacturer to

the National Bureau of Standards in July 1953 for evaluation tests,

II, Scope of Test, This report gives the results of photometric tests conducted on an

AV3-2A aircraft searchlight equipped with a non-roll-off precision reflector (#A32i;), For comparison, results are also given on similar tests

of a searchlight equipped with conventional reflectors having roll-pff

edges. Two reflectors of the latter type were selected for the tests -

#A5>lU9 and #A6375> respectively the best and the poorest of those available,

III, 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 luminosity-corrected 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. Vertical distributions were obtained in a similar manner

by rotation of the goniometer around its horizontal axis® Runs of 50-

second duration were made at 0®5-degree intervals, and the average candlepower of the last 30 seconds of each 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 AVQ-2A searchlight with modifications as described in NBS Report

135U* “Instruction Book for Modification of the AV^-2A Aircraft Searchlight", was used for the tests. This light burns 11 mra carbons, and

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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

10li amperes# A special arc-regulating circuit was employed which limited

the current variation to about ± h 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

AVQ-2A Aircraft Searchlight" (Focusing Method #3) and in the NBS Technical

News Bulletin for June 1953* pp* 93-9Uo

IV. Results # Figure 1 gives the average horizontal and vertical candlepower distributions of the AV3-2A searchlight when equipped with each of the three

reflectors tested# Since the arc was operated at reduced current (10U

amperes) for these measurements, the candlepower obtained was appreciably

lower than that which would have been obtained at the normal operating

current of 120 amperes# However, the curves may be used for intercomparison

of the relative performance of the three reflectors#

The difference in the two types of reflectors is demonstrated by a comparison of their shadowgraphs $ Figs. 2 and 3 are those of the roll-off

type, and Fig# k that of the non-roll-off type# The effect of the non-rolloff edge of the improved reflector (A32U) is shown by the straight, clear

corners of the projected shadow of the square diaphragm and the visibility

of the Bausch & Lomb trade mark etched near its periphery as contrasted to

the rounded corners of the diaphragm shadow and the almost complete absence

of the trade marks in the shadowgraphs made with the conventional reflectors#

The trade marks are similarly located on all three reflectors# The roll-off

zone on the unimproved reflectors is about 1/2 inch in radial depth and

amounts to about 10$ of the projected area of the reflectors#

V# Discussion# The differences in the candlepower distributions of the three reflectors

are small# Accurate determination of these differences is difficult because of the inherent instability of the arc lamp, even when operated at

reduced closely controlled current, and also because of the variability of

arc carbons# Since the values of peak candlepower are based on considerably

more data, they are more reliable than values obtained elsewhere in the

candlepower distributions#

Inasmuch as the effective area of the non-roll-off reflector (#AJ2U)

is about 10$ greater than that of the other two, it was anticipated that

its peak candlepower value would be correspondingly greater# However, as
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may be seen in Fig® 1, its peak 'value is somewhat less than that obtained

with conventional reflector #Aj?l1*9* An examination of Figs® 2 and k shows

the figure of'#A5>lU9 to be exceptionally good, while that of #A32l* has

characteristic pin-cushion distortion®

It thus appears that the advantage of the additional peripheral zone

with the non-roll-off reflector is compensated for in the case of A$lh9 by

the excellent quality of its figure® The peak candlepower obtained with

the non-roll-off reflector is, however, about J% higher than that obtained

with the other conventional reflector (#A637!?)* It is probable that the

superiority of the non-roll-off reflector in this ease is partly due to

the additional contribution of the peripheral zone and partly to its

superior figure quality, as shown by the shadow-patterns in Figs® 3 and U<> It is significant that what appears to be considerable difference in

the quality of ihe three reflectors corresponds to relatively small differences in performance® The quality of superior samples of drop-molded

reflectors approaches that of #a6375, the poorer of the two conventional

precisian reflectors® It may be that the standard of quality for searchlight reflectors presently specified could be reduced to some extent

without appreciable loss of performance®

A non-roll-off reflector is estimated to provide 10% more light flux

in the beam than an otherwise equivalent conventional reflector® The

candlepower measurements made in this test were not sufficiently extensive

nor of sufficient accuracy, especially in the off-axis parts . of the beam,

where zonal contributions to total flux are very high, to verify this expectation reliably® It is felt, nevertheless, that the non-rolL-off

feature is desirable and that future procurement should call for reflectors

with this feature®
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