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The Matilda CDL (Canal Defence Light) was a version of the Matilda Infantry Tank Mk II that carried a powerful searchlight instead of its main gun and that was designed to win control of the night-time battlefield.
The Canal Defence Light was developed in an attempt to win control over the night-time battlefield by using bright rapidly flickering lights to blind any opponents. The idea was originally developed by Commander de Thoren of the Royal Navy during the First World War, but wasn't developed. De Thoren continued to work on the idea after the war, working with a naturalised Greek engineer Marcel Mitzakis. They won the support of the Duke of Westminster, and got J F C Fuller to act as their tactical advisor. They sold the idea to the War Office in 1937, but little progress was made. They were even allowed to try and sell it to the French.
Some work must have been carried out, as in September 1939 an improved light armoured turret was ordered.
The idea was demonstrated at a British Army trial at Lulworth in June 1940. At this stage, with the war situation fairly desperate, the Army was willing to try anything, and the project was given high priority.
The theory was that rapidly flickering bridge lights would cause the target's pupils to rapidly dilate and contrast, effectively rendering them blind. If two light-equipped tanks were used against the same target then they would also create a triangle of dark ground between their beams, which could be used by the attacking infantry. The flickering lights would brightly illuminate the defenders, making their vulnerable.
The new device was given the code name Canal Defence Light, to give the idea that it was being produced to defend British canals against a possible German invasion. The light was to be carried in an armoured turret, effectively an armoured lighthouse. These were armed with a Besa machine-gun and a dummy 2-pounder gun (in an attempt to make them look like normal tanks, at least from a distance). The turret was split in two. On the left was the light operator, whose main job was to replace the carbon elements in the arc light, using asbestos gloves. The light itself, along with a mirror, was carried in the right hand side of the turret. The new turret could be quickly swapped with a standard Matilda turret, and a crane attachment was produced for the Matilda CDL to make this easier. The flickering was produced using a shitter on a narrow slit in the front of the turret.
The Matilda CDL only had space for a crew of two (driver and light operator).
A total of 300 light turrets were ordered, enough to equip one brigade in the Middle East and another in Britain. The 11st RTR converted to the Matilda CDL in 1942, and then in June was sent to Egypt.
By January 1943 two regiments were equipped with the Matilda CDL, but with production of the Matilda ending it wasn't a long term prospect.
The big problem with the CDL was the intense secrecy that surrounded it. As a result the people planning attacks were rarely aware of its existence, or of its abilities, and it wasn't included in any attack plans until the Rhine crossing of 1945. The two British units that were equipped with it (1st and 35th Army Tank Brigades) were posted to the Lake District, and rather faded from view. There was also a tendency to want to wait for the exact right moment to use the new weapon, in the believe that it would probably be most effective in its first few uses.
The vehicles sent to the Middle East were less secret, but the aim was to use the new weapon in Europe first, so they never got their chance.
The CDL was finally used in its intended role during the crossing of the Rhine in 1945, but by this point the Matilda had largely been replaced with the Grant tank.