Canal Defence Light The Canal Defence Light ( CDL ) was a British " secret weapon " of the Second World War . It was 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 disorient enemy troops , making it harder for them to return fire accurately . The inaccurate name " Canal Defence Light " was used to conceal the device ' s true purpose . For the same reason , in US service they were designated the " T10 Shop Tractor " . Description . The idea is credited to a Greek citizen , A V M Mitzakis , who devised the system in 1937 . The British War Office showed no interest until 1940 , when 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 by , its comparative smallness reduced the chance of damage to the optical system by the entry of bullets . The beam diverged at 19° horizontally and 1 . 9° vertically , forming a pool of light around at a range of . The turret could rotate 360° and the light beam elevated or depressed 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 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 disorient 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 and better armoured tank , yet also faster and so better able to keep up with tanks such as the Sherman . It was armed with a hull – mounted , 75 mm gun , which was unaffected by the searchlight turret . A dummy gun - barrel fitted to the searchlight turret made it resemble a conventional , unmodified M3 tank . The project was shrouded in secrecy . It was tested during Exercise Primrose in 1943 at Tighnabruaich , Scotland with the result that it was determined to be " too uncertain to be depended upon as the main feature of an invasion " . Units that were equipped with CDL . The 11th Royal Tank Regiment was raised in January 1941 and designated for the CDL role in May 1941 . The unit trained at Lowther Castle near Penrith , and were based at Brougham Hall , Cumberland , spent 1942 and 1943 in the Middle East without seeing action , and returned to the UK in April 1944 . They landed in Normandy on 12 August 1944 , and saw no action until 29 September 1944 when they were ordered to transfer all their equipment to the 42nd and 49th Royal Tank Regiments , and were 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 , when all three units were disbanded after the end of hostilities . Battalions of the American 9th Tank Group trained using the Grant variant of the CDL tank at Camp Bouse in the Arizona desert . Before deployment in the European Theatre of Operations . They continued training in 1944 on the Preseli Hills in Pembrokeshire , West Wales . At 06 : 00 , before dawn , on 18 November 1944 , giant searchlights ( " canal defence lights " ) of the 357th Searchlight Battery , Royal Artillery provided hazy indirect light for the mine - clearing flail tanks supporting the infantry in Operation Clipper . Deployment and Combat . The system was highly secret as surprise was considered essential to its use . This hampered its employment as commanders were often unfamiliar with it and did not consider it when drawing up plans for attack . CDLs were never used for their intended purpose ; however , they saw use by US forces in protecting bridges after the crossing of the Rhine in March 1945 . The Germans attempted to attack the bridges at night using swimmers and floating mines , dropped into the river upstream . The armour of the CDL ' s 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 . Curiously , the actual use of the system resembled its name , which was intended to be spurious . Later , the battle moved East and the CDLs were used to illuminate the bridges for the benefit of engineers carrying out maintenance . Conventional searchlights were more suited for this but none were available . The CDLs were eventually replaced by captured German searchlights . Surviving examples . A CDL equipped Matilda tank still exists as part of the collection of the Royal Armoured Corps Tank Museum at Bovington , Dorset , in Britain . It is the only one of its type known to exist . One CDL equipped M3 Grant is displayed at Armoured Corps Museum , Ahmednagar , in India .