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.
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The searchlight was mounted in an armoured turret fitted to a tank. Initially the Matilda tank was used, replacing its normal turret with a cylindrical one containing both a searchlight and a machine gun. This was later replaced by the US M3 Grant which was superior in several ways. It was a larger, roomier tank, yet was also faster and so better able to keep up with tanks such as the Sherman. It was armed with a hull–mounted gun, which was unaffected by the replacement of its normal turret with the searchlight turret.
The searchlight turret included a station for an operator. The light emerged from a vertical slit that was just 2 inches (5.1 cm) by 24 inches (61 cm), 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 34 by 340 yards (31 × 310 m) at a range of 1,000 yards (910 m). The turret could rotate 360° and the light beam elevated or depressed by 10° from the horizontal.[1]
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.[1]
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".
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.
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.[1] 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.[1]
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. There are no Grant examples.