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. It was intended to be used during night-time attacks, when the light would allow enemy positions to be targeted. A secondary use of the light would be to dazzle and disorientate enemy troops, making it harder for them to return fire accurately. The name Canal Defence Light was used to conceal the device's true purpose. For the same reason, in US service they were designated T10 Shop Tractor. Description 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. The device was demonstrated to the British War Office in 1937. Although three examples were ordered for tests, the trials did not begin until 1940, when the War Office took over and ordered 300 lights for fitting to tanks. A prototype was constructed using a Matilda II tank. The tank's normal turret was replaced with a cylindrical one containing both a 13 million candlepower (12.8 million candela) searchlight and a machine gun. The searchlight turret included a station for an operator, who had the task of changing the light's carbon electrodes when they burned out. The light emerged from a vertical slit that was just wide by tall, a small size which reduced the chance of battle damage to the optical system. The beam diverged at 19° horizontally and 1.9° vertically, forming a pool of light of around at a distance of . The turret could rotate 360° and the light beam could be elevated or lowered by 10° from the horizontal. Blue and amber filters allowed the light to be coloured as well as white. A shutter could flash the beam on and off, up to twice a second. It was found that the blue light caused the CDL tank to appear to be at a greater distance, and blue and amber light beams from two CDL tanks could combine to illuminate a target with white. A flashing beam would further dazzle and disorientate enemy troops by not giving their eyes a chance to adapt to either light or darkness. The Matilda tank was later replaced by the US M3 Grant, which was superior in several ways. It was a larger, roomier