The British War Office became convinced of the idea of high-powered searchlights to blind enemy combatants and illuminate battlefields as early as 1937. Design studies and practical trials were then ordered resulting in production of a special tank turret sporting a powerful searchlight for 1939. World War found Europe for a second time with the German invasion of Poland on September 1st and development work on what would become the "Canal Defence Light" system continued with greater resolve.
The Matilda II infantry tank of the British Army was selected as the first recipient of the concept. The standard turret - complete with its main armament fit - was wholly removed and, in its place, the CDL turret installed. Other internal modifications were forced by the installation of this turret but the general form of the Matilda tank remained. The searchlight component lay behind a shutter that was manually operated. With a fast "open-close" action, the bright light was intended to disorient the enemy at range as allied tanks closed in on their position for the kill.
From this work spawned an order for three hundred such turrets to be affixed to existing British Army tank hulls. The initial large-scale offering was the Matilda II which produced the "Matilda CDL" designation. The vehicles were too late for service in the North African Campaign so attention turned to using the newfound "weapon" in the planned Normandy Invasion landings of Northern France. By this time, the Matilda was an aging, ill-equipped combat vehicle so the turrets were set upon American M3 Grant Medium Tanks then in British Army service - thus the "Grant CDL" designation was born.
Even in this guise the CDL concept was never used as intended despite the crucial nature of the Normandy landings in retaking Europe. During the major river crossings of early-1945, the CDLs were in place solely to illuminate nighttime operations but never forced upon the awaiting enemy as designed. They also served in other post-invasion actions as searchlight vehicles and little more.
For a time, the Americans believed in the CDL concept and ordered 355 searchlight-equipped turrets all their own. These were installed on outgoing M3 Lee Medium Tanks to produce the "T10 Shop Tractor" designation (the name intended to hide the vehicle's true battlefield role). Like the British design, the T10 was not used as intended during the last push towards Berlin (and Tokyo for that matter) and the concept eventually fell to history for all interested parties.
The "T52" was a short-lived 1944 American attempt to convert M4 Sherman tanks to the CDL form. This did not proceed as regular searchlight-equipped Shermans could easily handle the role of the specialized CDL at a lower cost with little conversion work to be had.