NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS

0201-20-2300 April, 1956

EVALUATION FIELD TESTS

OF

SEARCHLIGHTS AND REFLECTIVE MATERIALS

IN * SEARCH AND RESCUE OPERATIONS

by

J. W. Lane, Sr*

T. H. Projector

W. A. Hall

L. R. Noffsinger

Naval Aircraft Lighting Group

Photometry and Colorimetry Section

National Bureau of Standards

Washington 25, D. C.

Test No. 21N-41/55

Sponsored by

Lighting Section EL-5211

Bureau of Aeronautics

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, i

unless permission

25, D.C. Such p<

cal ly prepared If

Approved for public release by the

director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

f in part, is prohibited

Standards, Washington

eport has been specifireport for its own use,

REPORT

46o6

1. INTRODUCTION

The only two visual aids presently available to an

aviator forced down at sea during the hours of darkness

are a small, single-celled, flashlight, and a flare. The

flare, of course, is expended after only one use, and the

flashlight is visible for only a few hundred yards, even

when its battery is fresh and the atmosphere is clear.

Then too, both of these aids require that the man be alive

and able to operate them. To be effective at all times a

visual aid must function independently of any cooperation

from the downed airman. There are a number of possible

approaches to the solution of these problems, such as improved pyrotechnics or a better light for the pilot. Improvements such as these would be helpful, but would still

fall far short of providing effective visual aid in the

variety of situations in which search and rescue operations

must be carried out. A more complete solution to the problem has been proposed, involving the use of a light-weight

searchlight on the rescue helicopter together with effective

reflectorization of the pilot’s clothing, life raft, and

life vest. This report describes an investigation into

this proposal in which several searchlights and reflectorized targets were tested at a site where the search and

rescue operations could be simulated under controlled conditions .

2. PRELIMINARY INVESTIGATIONS

Preliminary investigations were made from the Potomac

River Bridge at Ludlow, Maryland,, The searchlights were

mounted on the roof of a truck which furnished power to

operate the lights. The truck was then positioned on the

bridge in such a way that the lights could be directed over

the water from a point approximately 150 feet above the

water surface. This site was found unsatisfactory because

of excessive bridge traffic and because of difficulty in

controlling the location of targets in the river. The test

site was then transferred to the Chesapeake Bay Annex of the

Naval Research Laboratory at Chesapeake Beach, Maryland.

This location proved to be much more satisfactory, and was

used for all subsequent tests.

Many different types and sizes of light sources are

available for use in an airborne helicopter searchlight.

For the purposes of this test, the choice was narrowed to

three representative types: a 3000-watt incandescent lamp,
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a 3’250-watt carbon arc lamp, and a 1000-watt mercury

capillary arc lamp. All three were mounted in AN/AVQ-2A

searchlights from which the carbon arc mechanism had been

removed. For comparison, an AN/AVQ-2A searchlight with

its regular 8400-watt carbon arc lamp was included in the

test.

It became apparent early in the investigations that

treatment of the target surfaces with retroref lective

materials resulted in a considerable increase in detectability. Retroreflectors have the property of returning a

substantial portion of the light incident upon them in a

narrow beam aimed toward the source of light. The reflected light is narrowly confined in this manner even

when the orientation of the retroref lector itself varies

within wide limits. Thus with an observer located close

to a searchlight, as is the case in the searching helicopter,

retroreflectors appear much brighter in the searchlight beam

than do painted surfaces. Two commercially available types

were chosen for this test. The first, “Stimsonite 1*, is of

the trihedron mosaic plaque type. Both red and white reflectors were tested. The second, **Scotchlite 11

, is made

of glass beads, imbedded in plastic _ sheets . Red, white,

and yellow samples were tested. The gain in brightness of

a target covered with “Scotchlite1* compared with the same

target covered with a good paint, is of the order of 500

times. The gain when the target is covered with ^Stimsonite 8® is of the order of 5000 times. These comparisons presuppose

that the areas covered are equal in size and that the colors

are the same.

3. TEST SITE AND FACILITIES

The Chesapeake Bay Annex of the Naval Research Laboratory is located just south of Chesapeake Beach, Maryland.

Fig. 1 is a general view of the area. The equipment was

located on the edge of a 100-foot high cliff, overlooking

the Bay. The target area was a radial sector 40° wide and

2500 yards in radius, extending outward from the shore with

the searchlights at its apex. The searchlights were mounted

in cradles on top of the power truck which contains generating and auxiliary equipment for operating the searchlights.

The lights were placed on the edge of the truck so that

their beams could be directed out over the cliff. The hand

control units were mounted on a board which was clamped to

a stand about 25 feet to the rear of the truck. A plotting

chart consisting of a map of the area and a sighting device

was used to ascertain angles of azimuth. An optical range
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finder was used to measure the distance to the targets

in the test area. Placement and anchoring of the targets

in the area were carried out from a Navy launch with which

radio communication was maintained from the observation

site. The arrangement of the test equipment is shown in

Figs. 2 and 3.

4. SEARCHLIGHTS

The incandescent lamp, shown mounted in a searchlight

in Fig. 4, is a 28-volt, 3-kw searchlight lamp with a mogul

bipost base, a C -6 filament and a T-24 bulb. It has a

rated life of 10 hours. The filament coil is about 1=1/2

inches long and 1/4 inch in diameter. The lamp is 11-1/2

inches long. It was mounted with the axis of its filament

coil horizontal, and perpendicular to the axis of the re=

flector. The lamp produces a beam 11.3° by 3*6® with a

peak intensity of about 6 , 000,000 candles.

The L-9 arc mechanism, shown in Fig. 5, is a standard

motion picture projection lamp adapted for use in an early

experimental airborne searchlight. It is rated at 45

amperes at 26.5 volts and operates continuously for 20 min=

utes on one trim of carbons. The lamp mechanism is 24=1/2

inches long. The beam produced has a peak incensity of

45,000,000 candles, and a beam width of 3°

The BH=6 air-cooled mercury arc lamp, shown mounted

in a searchlight in Fig. 6 , is approximately 3-1/4 inches

in length and is constructed of quartz tubing with tungsten

electrodes sealed into each end, extending through mercury

pools into the lamp. The light source is approximately 1.5

by 25.0 millimeters and has an output of 60,000 lumens. The

lamp requires 17 amperes at 115 volts, 60 cycles. The aircooling system for this lamp requires a volume equivalent

to about 7 cubic feet of free air per minute at room temperature. This flow must be maintained at a% pressure of 20

p.s.i. at the nozzles of the mount. A 3/4 horsepower compressor is required to supply the necessary air. With the

lamp mounted perpendicular to the axis of the reflector

(Fig. 6 ) the searchlight beam has a width of 8,1° and a

vertical spread of 2.3 c

o The peak intensity is about 22,000,000

candles.

The AN/AVQ-2A searchlight is the standard airborne

searchlight now in use by the Navy for anti-submarine warfare. The source is a high intensity carbon arc. The power
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requirements are 120 amperes at 70 volts, dc, and 8.5

amperes at 28 volts, dc. With the 19-inch reflector this

searchlight produces a 3-75° beam with a peak intensity of

about 85,000,000 candles. The mirror is back-silvered

glass, having a focal length of 6 inches. A remote hand

control mounted in the cockpit of the aircraft directs the

light beam in azimuth 26*5° right or left and in elevation

from 5° up to 45° down. Table I summarizes the characteristics of all four searchlights. Fig. 7 gives the candlepower distributions of the searchlights.

5. TARGETS

The ^Scotchlite** retroref lective sheeting used to cover

the targets in the test is a ^wide-angle , flat=topttE design

manufactured by the Minnesota Mining and Manufacturing

Company. It consists of a plastic sheet in which minute

glass beads are imbedded. The beads in turn are covered

over with a plastic film so that the upper surface is smooth.

This material can be wet by rain or spray without appreciable

impairment of the retroref lective properties. It will act as

a retroref lector when illuminated by a beam of light incident

on the surface at an angle as great as 80 to 85 degrees,

although with a very much reduced efficiency. The sheeting

used in this test was about .005 inch thick and can be applied

readily to irregular surfaces.

The u Stimsonite u tetrahedron mosaic plaques used in the

test were manufactured by the American Gas Accumulator Company. Two sizes of plaques were used. One was 1-13/16 inches

in diameter and about 5/16 inch in diameter (1.6 square inches)

The other was 1-5/16 inches in diameter and about 1/16 inch

thick, with an effective surface about 1-3/16 inches in

diameter (1.1 square inches). The plaques are made of a

transparent plastic with the tetrahedron mosaic molded into

the rear surface. The rear mosaic surfaces were protected

by a plastic backing in the large plaques and by a thin metal

sheet in the smaller plaques.

The mosaic plaques have a narrower angle of divergence

(angular width of reflected beam) than the glass bead sheeting and a correspondingly higher efficiency. Since the

divergence angle required in the search and rescue operation

is very small, well within the effective range of ^Stimsonite**,

a given area of “Stimsonite 11 will appear much brighter than

the same area of tt Scotchlitel% except at high angles of in-
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cidence on the surface, when the wide-angle feature of

*Scotchlite* yields higher brightnesses. Since the

target surfaces are very irregular, it is difficult to

predict overall performance accurately. The estimates

given earlier in this report (500 times the brightness

of good paint for B Scotchlite w and 5000 times for

tt Stimsonite”) were based on a consideration of the known

properties of these retroreflectors together with a very

approximate estimate of the weighting of the angles of

incidence on the various parts of the targets.

The targets used were as follows:

a. Four two-man life rafts: one each was

covered with red, yellow, and white n Scotchlite‘* and

one was left plain. A “Scotchlite* covered raft is shown

in Fig. 8.

b. Four ttMae West 14 life vests: one was left

plain and the others were covered with red, yellow, or

white “Scotchlite 6* as shown in Fig. 9.

c. Seventeen helmets: These were mounted on

metal plates which were strapped to innertubes. The

method of mounting is shown in Fig. 10. Thirteen of the

helmets were covered with red or white "Scotchlite".

“Stimsonite1* buttons were affixed to the remaining four.

Fig. 13 shows the manner in which the retroref lective

materials were placed on helmets and their bright appearance when illuminated by a light source placed close to

the line of sight.

The targets were anchored in a 40° radial sector at

ranges from 750 to 2500 yards. The arrangement of the

various targets and reflective materials was changed from

time to time to determine the visibiZity of the reflectors

under different. conditions and at different ranges. A

typical target layout is shown in Fig. 11.

To demonstrate the proposed use of searchlights

and retroreflectorized targets, ax* incandescent searchlight was mounted and flown on an H04S helicopter. This

searchlight was similar to the incandescent unit previously

described. The light was mounted on a Mark 50 bomb rack

on the port side of the helicopter, just forward of the

rear landing gear strut.
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In order to provide general illumination as the

helicopter approaches the scene of the rescue three

V50~watt floodlights were mounted on the helicopter

aimed forward and downward to illuminate the surface

of the water just ahead of the aircraft. A fourth

floodlight was mounted on the starboard rear wheel

strut, illuminating the surface below the aircraft to

facilitate the rescue. Once the target has been

located, these four lights enable the pilot to fly

contact on the target while the rescue is being performed. Fig. 12 shows the helicopter installation of

the searchlight and floodlights.

6. RESULTS

There was considerable variation in the ranges

at which the targets were detectable, depending on the

amount of haze present in the atmosphere. On clear

nights all of the retroref lectorized targets were vis°

ible with any of the lights at ranges up to 2500 yards.

On hazy nights, however, there were marked differences

in detectability of different targets with various

lights. Except on moderately hazy nights all of the

retroref lectorized targets were visible using the L-9

searchlight. On these same nights the incandescent

searchlight provided adequate illumination to approximately 1250 yards, beyond which the targets were too

dim for positive recognition.

The H6 mercury capillary lamp illuminated the white

and yellow targets better than did the incandescent light,

but because of the spectral distribution of the light

emitted by the mercury source the red targets did not

appear as bright as with the incandescent source which

has only one-third the intensity.

The AVQ-2A searchlight, while providing the highest

peak candle power, sometimes failed to illuminate the

targets as well as did a light of less candle power because of the large amounts of back-scattered illumination.

Upon completion of the test wr»^k a demonstration

sponsored by the Bureau of Aeronautics, was conducted

for interested military personnel and civilians. The

demonstration set-up can be seen in Figs. 1,2, and 3.

The chart on the side of the small building in the photographs shows a proposed flight pattern for search. The

demonstration included an examination of the incandescent

searchlight and floodlight installation on the H04S helicopter, a static demonstration of the four searchlights

mounted on the truck and a simulated helicopter rescue,

showing a proposed technique for search, hovering and the

actual pickup.

7. DISCUSSION

The arrangement used in this test to simulate a helicopter searching for a downed aviator in open sea differs

from the actual situation in several important respects.

While fixing the location of the searchlights and the

targets in a carefully measured test area provides accurate information in many respects, it eliminates the uncertainty as to the location of the targets. This difference might well affect the decision as to preferred equipment, particularly as to the searchlight selected. In this

test, the L-9 carbon arc searchlight with a narrow beam

performed very well. In an actual search operation the

narrow beam might increase the chance of overlooking a target that is in range of the light. For this reason, and

because it is the simplest and lightest of the lights tried

will operate for long periods without attention, and will

operate from a 28-volt dc power system, the incandescent

searchlight is considered preferable for helicopter installation.

Treating targets with retroref lective material is

highly advantageous. The white retroref lectors were more

efficient than red or yellow. In the test situation, the

water surface was relatively calm. In actual rescue operations, whitecaps might be present and might easily be confused with a white target as the searchlight sweeps the

area looking for survivors. To avoid this possible confusion, the use of red retroreflective materials is recommended.

Both types of retroref lectors were found very useful.

"Stimsonite" is much more efficient than ^Scotchlite*4 for

the same area, but the ease of application of ^Scotchlite”

may be decisively advantageous. It may also be that larger

areas could be covered practically with w Scotchlite tt than

with “Stimsonite® and this might reduce the advantage in

efficiency of the “Stimsonite*.
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The assistance of Mr. J. A» Bartelt of BuAer,

Lt. Comdr. W. Frankie, USN, Mr. Paul Hannen of Patuxent

NATC, of the personnel of the Chesapeake Bay Annex of

The Naval Research Labroatory, and Of Mt. T. 0. Twist,

Mr. S. W. Wilson, Mt. W. F. Mullis, Mr. L. C. Bluebond,

and Mr. G. A. Dines, of the Photometry and Colorimetry

Section of the National Bureau of; Standards in the performance of these tests was greatly appreciated.

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TABLE I

SEARCHLIGHT UNIT

POWER

REQUIREMENTS

PEAK

CANDLEP0WER BEAM WIDTH

TOTAL

WEIGHT***

AN/AVQ-2A 120 Amps at

70 Volts, dc

13.45 Amps

at 28 Volts,

1.3 Amps at

115 Volts,

400 Cycles

85, 000,000

dc

3.75° 145 lb.

AN/AVQ-2A with 3°

KW incandescent

lamp

100 Amps at

28 Volts, dc

1.3 Amps at

115 Volts,

400 Cycles

6,000,000 11.3°H

X

3.6®V 120 lb.

AN/AVQ-2A with

B-Ho mercury

capillary lamp

1000 Watts

at 115 Volts

.

400 Cycles*

6.2 Amps at

28 Volts, dc

1-KW**

22,000,000 8. 1°H

X

2.3°V

240 lb.

AN/AVQ-2A with

L-9 carbon arc

mechanism

50 Amps at

28 Volts, dc

1.3 Amps at

115 Volts,

400 Cycles

45 , 000,000 3® 150 lb.

* A 60-cycle power supply was used for this test,

** Approximately this amount of power is required for an air

compressor to provide the necessary cooling for this lamp. *** Estimated for helicopter installation including control

unit, amplifier, and necessary lamp auxiliaries, exclusive

of power generating equipment.