The Canal Defence Light (CDL) was a British "secret weapon" of the Second World War, based upon the use of a powerful carbon-arc searchlight mounted on a tank.

A secondary use of the light would be to dazzle and disorient enemy troops, making it harder for them to return fire accurately.

[1] The idea is credited to a Greek citizen, Marcel Mitzakis, who devised the system for the de Thoren Syndicate in the 1930s; they were advised by J F C Fuller.

A flashing beam would further dazzle and disorient enemy troops by not giving their eyes a chance to adapt to either light or darkness.

It was tested during Exercise Primrose in 1943 at Tighnabruaich, Scotland; it was concluded that it was "too uncertain to be depended upon as the main feature of an invasion".

The one British squadron that had not been converted from CDLs was used in the north, 64 American CDL tanks were brought back into use with their former crews.

This included sending frogmen, using Italian underwater breathing apparatus, to plant floating mines [8] but they were discovered by US Army military police, who used Canal Defence Lights to locate and blind the swimmers.

[9] The armour of the CDLs made them more suitable for this task than conventional searchlights as, in some sectors, the East bank of the river was held by German forces who subjected the CDL tanks to considerable artillery and small-arms fire.

It landed in Normandy on 12 August 1944, seeing no action until 29 September 1944, when it was ordered to transfer all of its equipment to the 42nd and 49th Royal Tank Regiments, and was retrained to operate the American amphibious LVT-4, known by the British Army as the Buffalo Mark IV.

In their turn, the 42nd and 49th Royal Tank Regiments were largely inactive for the remainder of the war and all three units were disbanded after the end of hostilities.

Before dawn, at 06:00 on 18 November 1944, CDLs of the 357th Searchlight Battery, Royal Artillery provided hazy indirect light for the mine-clearing flail tanks supporting the infantry in Operation Clipper.

The US Tenth Army requested deployment of CDL tanks for use during the Battle of Okinawa, but fighting there was complete by the time they arrived.