Diffused lighting camouflage

uses bioluminescence to counter-illuminate its underside to match the brighter sea surface above. Diffused lighting camouflage was explored by the Royal Canadian Navy (RCN) and tested at sea on corvettes during World War II, and later in the armed forces of the UK and the US. 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. == Prototyping ==

Royal Canadian Navy began the secret sea trials of diffused lighting camouflage in January 1941. Burr was quickly called to Canada's Naval Services Headquarters to discuss how to apply diffused lighting camouflage. Simple tests in the laboratory served as proof of concept. 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 Admiralty informed the prime minister, Winston Churchill, at the end of that month, stating that the "results seem quite promising". Churchill replied the next day, suggesting that "Surely all this business should be pressed forward on a broader front than the one ship?" Accordingly, in April 1941 the Admiralty ordered further development work for "full scale trials". 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 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 unlighted, but only 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. File:Diffused Lighting fitting for USS Hamul short.jpg|Diffused Lighting fitting for USS Hamul, short type File:Diffused Lighting fitting for USS Hamul.jpg|Diffused Lighting fitting for USS Hamul, long type File:USS Hamul plan for Diffused Lighting.jpg|USS Hamul plan for Diffused Lighting camouflage fittings after the sea trials held on 3 January 1942 ==Active service==

used its diffused lighting camouflage to attack a U-boat. Both Edmundston and Rimouski were fitted with about 60 light projectors: those on the hull were on retractable supports; those on the superstructure were on fixed supports. Each ship's diffused lighting system was tested systematically in St Margaret's Bay, and then trialled when actually escorting Atlantic convoys in 1943. Experimentally, the diffused lighting reduced the ships' visibility by up to 70%, but at sea the electrical equipment proved too delicate, and frequently malfunctioned. Worse, the system was slow to respond to changes in background lighting, and the Canadian Navy considered the lighting too green. In September 1943, Rimouski, using her diffused lighting system, but also some navigation lights, approached in the Baie des Chaleurs. The intention was to make Rimouski appear as "a small and inoffensive ship" in an operation to trap the submarine, and this appears to have worked as the U-boat did not detect her. However the attack failed, as a wrong signal sent from shore alerted the submarine's commander, Kapitänleutnant Rolf Schauenburg; U-536 dived and escaped. Following the Allied victory in the Battle of the Atlantic – through long-range aircraft, radar, code decryption, and better escort tactics – the need to camouflage ships from submarines greatly decreased, and diffused lighting research became a low priority. The work was halted when the war ended. ==In aircraft==

'' counter-illumination system of automatically adjusted forward-pointing lights was trialled in B-24 Liberators from 1943 onwards. 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. But the system never entered active service, as radar became the principal means of detecting aircraft. ==See also==