873 Movement Light Squadron, R.E.
Horn Lane, Acton, 1961 - 1993
Light and generator specifications
Jan 2013. This page is an early draft with much more to be added.
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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
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
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 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 Xenon Specification sheet
(courtesy Francis Searchlight Co)
Table of general specifications
| 90cm carbon-arc |
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" (estimated) |
||
| overall length | 850mm, 34" | 982mm, 38⅝" (estimated) |
||
| 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 optional |
red and blue hung at front | single or dichromic | ||
| power generator | diesel engined
Lister, type JP4. 110V dc, 24kVA trailer mounted. |
3-phase 15kW 120/208V(4) 25kVa engine-driven alternator in L/R body(5) |
3-phase 208-230V or 380-415V |
Hunting Engr 3-phase 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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