Leigh light
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The Leigh Light (abbreviated L/L) was a British World War II era anti submarine device used in the Second Battle of the Atlantic. It was a powerful 22 million candela searchlight of 24 inches (610 mm) fitted to a number of Britain's Coastal Command patrol bombers to help them spot surfaced German U boat submarines at night.
It was successfully used from June 1942 onwards to attack U boats recharging batteries on the surface at night. The aircraft would approach the submarine using its radar and only switch on the searchlight beam during the final approach. The U boat would not have sufficient time to dive and the bomb aimer would have a clear view of the target. It was so successful that German submarines were forced to switch to daytime battery charging when they could at least see aircraft approaching. After its introduction allied shipping losses from U boats dropped from 600,000 to 200,000 tons per month.
Development
Early air-to-surface radar sets, namely the ASV Mk. II, had a fairly long minimum detection range. As the aircraft approached the target, it would disappear off the radar at a range that was too great to allow it to be seen by eye at night. At first aircraft solved this problem by dropping flares to light up the area, but since the flare only lit up the area directly under the aircraft, a string would have to be dropped until the submarine was spotted. Once it was spotted the aircraft would have to circle back to attack, the entire process giving the submarine a fair amount of time to dive out of danger.
Squadron Leader Humphry de Verde Leigh suggested that a better solution was to mount a searchlight under the aircraft, pointed forward and allowing the submarine to be spotted as soon as it was turned on. At first it was difficult to fit to aircraft due to its size. Leigh persisted in his efforts to test the idea, and garnered the support of the C-in-C of Coastal Command, Sir Frederick Bowhill. In March 1941 a Vickers Wellington that had been used for anti-mining operations was modified with a retractable "dustbin" holding the lamp, and proved the concept sound.
At this point the Air Ministry decided that the idea was worthwhile, but that they should instead use the Turbinlite, a less effective system which had been originally developed as an aid for nightime bomber interception. After trials they too eventually decided to use Leigh's system, but it was not until mid-1942 that aircraft started being modified to carry it.
Operation
By June 1942, aircraft equipped with ASV radar and the Leigh light were operating over the Bay of Biscay intercepting U-boats moving to and from their home ports on the coast of France. In the five months prior not one submarine had been sunk, and six aircraft had been lost. The Leigh light turned the tables, and by August the U-boats preferred to take their chances in daytime when they at least had some warning and could fight back.
External link
Technical details of the Leigh Light (http://uboat.net/allies/technical/leigh_light.htm)