873 Movement Light Squadron, R.E.

Horn Lane, Acton, 1961 - 1993


Light and generator specifications

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Carbon-Arc lights

The principal is quite simple. A flame, or arc, is produced by electricity jumping between the tips of two carbon rods acting as electrodes, one positively charged, the other negative. The gap between the tips needs to be set to produce a steady and intense arc and as the rods burn away they have to be moved to keep the gap constant.

The light produced is intensified by a concave reflector or mirror, just as with a torch, and searchlights are designated by the diameter of the reflector/ mirror, in metric except in the USA where inches are used.

Most lights are adjustable to provide a narrow beam or a wide flood of light.


90cm was the size of the first searchlight used by the British Army, and that size remained the most popular with the last version entering service in RE and RA units in the late 1930s and used in WW2 for both ground lighting and in 'ack-ack' roles illuminating enemy aircraft.

They were in service with 873 ML Bty RA, 1947 -1960 and 873 ML Sqn RE from 1960 to 1977 for battlefield lighting and until 1985 for tattoo lighting.

The specification included motorised racks that moved the rods as they burnt away, automatically maintaining the optimum gap. However the initial gap had to be set manually, and sometimes it was necessary to over-ride the automatic adjustment, skills all operators of carbon-arcs have needed to master. One rod had lasted only 20 minutes, the other an hour.

The lights had an undercarriage with four small crawler tracks, one pair of which swivelled, and on firm ground two men could tow and manouvre a light. However, they were normally used on the rear of a 3 or 4 ton truck with the rear part of the body uncovered. The truck also towed a trailer-mounted generator.

a short-arc Xenon bulb
photo from Wikipeadia

Xenon Bulb searchlights

photo from Francis website

Francis xenons
AN/TVS xenon on show with reeded glass filter opened. Trailer in original US spec, later modified to meet UK requirements
(Not normally towed like this)
(courtesy Larry Hayward)

AN/TVS-3, 30-inch (American)

In service at 873 1977-1990

During 1978 the unit re-equipped with ex-American army trailer-mounted lights.  These AN/TVS-3 30-inch Xenon Searchlights were rated at 800 Million Candle Power, more than double the power of the Carbon Arcs, yet light enough to be towable by a light truck. The US army evaluated nine of these in March 1969 and deployed them soon after.
Horizontal spread beam of 1.75˚, and an adustable beam which could be varied from 1.75 in focused mode to 9 defocused.

Layout of generator & controls in rear of Land Rover body
(photo from user handbook,
courtesy of Larry Hayward)

Francis xenons
Francis Xenon at factory, 1991
(courtesy Francis Searchlight Co)

Francis FX710, 63cm (British)

In service at 873 1990-93 and elsewhere to at least 2010.

These are a military version of the Moonraker Xenon light, the largest in their Extravaganza range.
Features a nickel parabolic mirror which can focus from "laser-like beam to floodlight", with instant strike and restrike, with no warm-up time.
Stove enameled marine paint.

The army version is much heavier and much higher than the standard model.

Francis xenons
Francis Xenon Specification sheet
(courtesy Francis Searchlight Co)

Table of general specifications

30 inch
ex-US Xenon
Francis Xenon
civvy model(1)
Francis Xenon
army version
Model/type   AN/TVS-3 Moonraker FX710
rating     7kW 7kW
mirror     nickel parabolic nickel parabolic
reflector/ mirror diam 90cm, 35½" 30 ins ? 630mm, 24¾" 630mm, 24¾"
barrel diam     710mm, 28" 710mm, 28"
barrel length     980mm, 39" 980mm, 39"
overall height   1980mm, 6½ft incl trailer 1500mm, 5ft 1833mm, 6ft (estimated)
overall width     700mm, 28" 1088mm, 42¾"
overall length     850mm, 34" 982mm, 38⅝"
weight   680kg, 1500lbs incl 2-whl trl(3) 500kg, 1100lbs not known
pan     180˚ 320˚
tilt up     90˚ 90˚
tilt down   90˚ 30˚ 30˚
focus divergance   1.7 - 9 1.5-10˚ 2-10˚
mounting rear of 3 or 4 ton truck (Bedford RL) integral trailer towed by L/R.   rear of 4-ton truck
(Bedford MK, and AWD MJ)
cooling top-mounted fan air and water 2 side-mounted filtered fans 2 side-mounted filtered fans
colour filters
red and blue hung at front   single or dichromic  
power generator diesel engined Lister, type JP4.
110V dc, 24kVA
trailer mounted.
3-phase 15kW

25kVa engine-driven
alternator in L/R body(5)
208-230V or 380-415V
Hunting Engr
208-230V or 380-415V originally on truck; later on trailer
light output   800 million candlepower   1,000 million candlepower
range     10 lux at 10 km 10 lux at 10 km
ambient temp     -20C(2) to +50C  

Sources & notes
(1) Francis Searchlights Ltd
(2) can be modified to work at -40C
(3) weight is for the combined unit before the trailer was "modified" to comply with UK requirements - eg addition of mudguards, no.plate
(4) US army spec. UK not known
(5) UK spec
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back to top of page page created Jan 2013 last edited/amended 27 January 2022 please e-mail me