Leigh light

The Leigh Light (abbreviated L/L) was a British World War II era anti-submarine device used in the Battle of the Atlantic.

It was a powerful (22 million candela) carbon arc 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 used from June 1942 onwards to attack U-boats recharging their batteries on the surface at night, when they had previously been relatively safe from attack. 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 had insufficient time to dive and the bombardier had a clear view of the target. It was so successful that for a time German submarines were forced to switch to charging their batteries during the daytime, when they could at least see aircraft approaching.

Germany introduced the Metox radar warning receiver (later replaced with Naxos when Britain introduced centimetric radar) in an effort to counter the combination of ASV and Leigh Light. Metox provided the submarine crew with early warning that an aircraft using radar was approaching. Because the radar warning receiver could detect radar emissions at a greater range than the radar could detect vessels, this often gave the U-boat enough warning to dive.