It was a powerful (22 million candela) searchlight of 24 inches (610 mm) diameter fitted to a number of the British Royal Air Force's Coastal Command patrol bombers to help them spot surfaced German U-boats at night.
It was successfully used from June 1942 onwards to attack U-boats recharging their batteries on the surface at night. Up to then they had been relatively safe from attack at night. The aircraft would approach the submarine using its ASV (Air to Surface Vessel) 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.
Eventually time delayed flares were developed that allowed the attacking plane time to circle. The flare was fired into the air from a buoy previously dropped by the plane. The surfaced submarine could then be seen in silhouette as the plane approached.
Wing Commander Humphrey de Verd Leigh, an RAF personnel officer, came up with his own solution after chatting with returning air crew. This was to mount a searchlight under the aircraft, pointed forward and allowing the submarine to be spotted as soon as it was turned on. He then developed the Leigh light entirely off his own bat, in secret and without official sanction - even the Air Ministry were unaware of its development until shown the completed prototype. At first it was difficult to fit on aircraft due to its size. Leigh persisted in his efforts to test the idea, and garnered the support of the Commander-in-Chief of Coastal Command, Sir Frederick Bowhill. In March 1941 a Vickers Wellington DWI that conveniently already had the necessary generator on board, (it had been used for anti-magnetic mining operations using a large electromagnet) 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 nighttime 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.
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.