Diffused lighting camouflage was a form of active camouflage using counter-illumination to enable a ship or plane to match its background, the night sky. You could light up a ship or plane to make it harder to see:
An equivalent strategy, known to zoologists as counter-illumination, is used by many marine organisms, notably cephalopods including the midwater squid, Abralia veranyi. The underside is covered with small photophores, organs that produce light. The squid varies the intensity of the light according to the brightness of the sea surface far above, providing effective camouflage by lighting out the animal’s shadow.
In 1940, Edmund Godfrey Burr, a Canadian professor at McGill University, serendipitously stumbled on the principle of counter-illumination, or as he called it “diffused-lighting camouflage”. Burr had been tasked by Canada’s National Research Council (NRC) to evaluate night observation instruments. With these, he found that aircraft flying without navigation lights remained readily visible as silhouettes against the night sky, which was never completely black.
Burr wondered if he could camouflage planes by somehow reducing this difference in brightness. One night in December 1940, Burr saw a plane coming in to land over snow suddenly vanish: light reflected from the snow had illuminated the underside of the plane just enough to cancel out the difference in brightness, camouflaging the plane perfectly.
Burr informed the NRC, who told the RCN. They realized that the technique could help to hide ships from German submarines in the Battle of the Atlantic. Before the introduction of centimetre radar, submarines with their small profile could see convoy ships before they were themselves seen. Diffused lighting camouflage might, the RCN believed, redress the balance.
[…]
In January 1941, sea trials began on the new corvette HMCS Cobalt. She was fitted with ordinary light projectors—neither designed for robustness, nor waterproofed—on temporary supports on one side of the hull; brightness was controlled manually. The trial was sufficiently promising for a better prototype to be developed.
The second version, with blue-green filters over the projectors, was trialled on board the corvette HMCS Chambly in May 1941. This gave better results as the filters removed the reddish bias to the lamps when at low intensity (lower colour temperature). The supports too were retractable, so the delicate projectors could be stowed away for protection when not in use. This second version reduced Chambly‘s visibility by 50% in most conditions, and sometimes by as much as 75%. This was enough to justify development of a more robust version.
The third version featured a photocell to measure the brightness of the night sky and the ship’s side; the projectors’ brightness was automatically controlled to balance out the difference. It was tested in September 1941 on the corvette HMCS Kamloops.
[…]
The British General Electric Company developed a manually operated diffused lighting system, which was trialled on the ocean boarding vessel HMS Largs and the light cruiser HMS Penelope. The Largs surface observation trials were conducted between 25 January and 6 February 1942; air observation trials, using Hudson bombers, took place on the nights of 4/5 February and 25/26 March 1942. They found an average reduction in the range at which the ship could be seen at night from another ship of around 25% using binoculars, 33% using the naked eye. The results from the air were less conclusive.
The best case was on the exceptionally clear moonless night of 29/30 January 1942, when Largs could be seen from a surface ship with the naked eye at 5,250 yards (4,800 m) unlighted, but only 2,250 yards (2,060 m) with her diffused lighting, a 57% reduction. By June 1942, Royal Navy commanders considered that camouflage was largely unnecessary, given that the enemy would be using RDF and submarine hydrophones. In April 1943, the Admiralty decided that diffused lighting was impractical, and development was halted, though discussions continued with the Canadian Navy.
[…]
Because submarines at the surface could see the dark shape of an attacking aircraft against the night sky, the principle of diffused lighting camouflage also applied to aircraft. However, British researchers found that the amount of electrical power required to camouflage an aircraft’s underside in daylight was prohibitive, while externally mounted light projectors disturbed the aircraft’s aerodynamics.
An American version, “Yehudi”, using lamps mounted in the aircraft’s nose and the leading edges of the wings, was trialled in B-24 Liberators, Avenger torpedo bombers and a Navy glide bomb from 1943 to 1945. By directing the light forwards towards an observer (rather than towards the aircraft’s skin), the system provided effective counter-illumination camouflage with an affordable use of energy, more like that of marine animals than the Canadian diffused lighting approach. But the system never entered active service, as radar became the principal means of detecting aircraft.