Canal Defence Light

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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 to dazzle and confuse enemy troops. A demonstration had shown that the use of a vehicle mounted searchlight both disoriented the units facing it and masked activities behind the searchlight.

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 the searchlight (the light emitting through a vertical slit) 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, better able to keep up with tanks such as the Sherman and it had a hull mounted gun which was unaffected by the replacement of its normal turret with the searchlight turret.

The light could be varied in two ways to further enhance any effect.

Addition of blue or amber filter would make the light source seem further away or closer respectively.
the operation of a shutter would create a flickering effect.

The project was shrouded in secrecy. It was tested during Exercise Primrose in 1943 at Kilbride Bay with the result that it was determined to be "too uncertain to be depended upon as the main feature of an invasion".

A CDL equipped Matilda tank still exists as part of the collection of the Bovington Tank Museum in England.