Canal Defence Light

Offensive armoured combat searchlight
The searchlight turret included a station for an operator, who had the task of changing the light's carbon electrodes when they burned out. 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 disorient enemy troops by not giving their eyes a chance to adapt to either light or darkness. For secrecy the construction was dispersed. Conversion of the M3 to take the CDL was by the American Locomotive Company as "Shop Tractor M10", turrets were produced by Pressed Steel Car Company as "coast defence turrets", and the arc lamps were sourced through the Corps of Engineers. The final assembly of the CDL tank was at Rock Island Arsenal. By the end of 1944, Alco had produced 497 tanks. American crews were trained at Fort Knox and in the California Arizona manoeuvre area. The six battalions of tanks then moved to the UK to join the British CDL tanks in Wales.

The British and American CDL units deployed to the continent did not cross over to France until August, the British as part of the 79th Armoured Division. The British 35th Tank Brigade and US 9th Armored Group were retained in the UK. The system was highly secret as surprise was considered essential to its use. This hampered its employment, as commanders were often unfamiliar with, or did not know of it, and did not consider it when drawing up plans for attack.

The Allies used the CDLs to protect the Ludendorff Bridge after it was captured intact during the Battle of Remagen. The Germans used virtually every weapon at their disposal to try to destroy the bridge. This included sending frogmen, using Italian underwater breathing apparatus, to plant floating mines but they were discovered by US Army military police, who used Canal Defence Lights to locate and blind the swimmers. The armour of the CDLs 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. The use of the system resembled its name, which had been intended to be spurious. Later, the battle moved eastwards and the CDLs were used to illuminate the bridges for the benefit of engineers carrying out maintenance. Conventional searchlights would have been more suitable, but none were available. The CDLs were eventually replaced by captured German searchlights.

The only surviving CDL equipped Matilda tank is in the collection of the Royal Armoured Corps at The Tank Museum, Bovington, Dorset, in Britain. One CDL equipped M3 Grant is displayed at Cavalry Tank Museum, Ahmednagar in India.