Another big secret of the war was disclosed early in 1947 when the operational use of infra-red rays was made known to the public. Scientists who had been studying these rays since the First Great War were, in 1939, given the task of developing and applying for Britain's war use all available knowledge. One problem in connexion with generating the rays was the supply of 3-4,000 volts; but a suitable battery, made of small discs of paper coated on one side with manganese dioxide and on the other with tinfoil, was perfected early in 1941. Britain now had a potent weapon in this light that was invisible to the naked eye.
The infra-red ray detector resembles an ordinary telescope except that inside, between the lenses, is a special device which converts the infra-red rays into green light, which is visible to the user of the detector. The infra-red image converter thus enables an object illuminated by the invisible rays to be seen by the human eye.
First operational use of the new weapon was in the Mediterranean in 1941, when Commandos, equipped with infra-red signalling sets, maintained contact with parent ships offshore. Infra-red binoculars and headlamps were fitted to tanks and transport vehicles to enable them to move freely at night, unobserved by the enemy. As a navigational aid during the Rhine crossing in March 1945, the transport of the British 79th Armoured Division was fitted with this equipment.
An infra-red sight for automatic weapons was also developed, enabling an enemy to be picked off in complete darkness at a range of 30 yards. Night-fighters and bombers of the R.A.F. were equipped with infra-red sets, and the tails of friendly fighters carried infra-red identity lights. Midget submarines, which attacked the German battleship Tirpitz in September 1943, used infra-red rays, and again when they cut the submarine cables at Hongkong and Saigon.
A simple instrument was produced to detect the use of infra-red equipment by the enemy, and extensive reconnaissance was carried out from the air over enemy-occupied territory. Infra-red devices, however, were not used by the Germans on the Western Front, although their limited employment was reported against the Russians.