Active camouflage or adaptive camouflage is camouflage that adapts, often rapidly, to the surroundings of an object such as an animal or military vehicle. In theory, active camouflage could provide perfect concealment from visual detection. Active camouflage is used in several groups of animals, including reptiles on land, and cephalopod molluscs and flatfish in the sea. Animals achieve active camouflage both by color change and (among marine animals such as squid) by counter-illumination, with the use of bioluminescence. Military counter-illumination camouflage was first investigated during the Second World War for marine use. More recent research has aimed to achieve crypsis by using cameras to sense the visible background, and by controlling Peltier panels or coatings that can vary their appearance. In animals Active camouflage is used in several groups of animals including cephalopod molluscs, fish, and reptiles. There are two mechanisms of active camouflage in animals: color change and counter-illumination. Counter-illumination Counter-illumination is camouflage using the production of light to blend in against a lit background. In the sea, light comes down from the surface, so when marine animals are seen from below, they appear darker than the background. Some species of cephalopod, such as the eye-flash squid and the firefly squid, produce light in photophores on their undersides to match the background. Bioluminescence is common among marine animals, so counter-illumination may be widespread, though light has other functions, including attracting prey and signaling. Color change Color change permits camouflage against different backgrounds. Many cephalopods including octopuses, cuttlefish, and squids, and some terrestrial amphibians and reptiles including chameleons and anoles can rapidly change color and pattern, though the major reasons for this include signaling, not only camouflage. Cephalopod active camouflage has stimulated military research in the United States. Active camouflage by color change is used by many bottom-living flatfish such as plaice, sole, and flounder that actively copy the patterns and colors of the seafloor below them. For example, the tropical flounder Bothus ocellatus can match its pattern to "a wide range of background textures" in 2–8 seconds. Similarly, the coral reef fish, the seaweed blenny can match its coloration to its surroundings. In research Active camouflage provides concealment by making an object not merely generally similar to its surroundings, but effectively invisible with "illusory transparency" through accurate mimicry, and by changing the appearance of the object as changes occur in its background. Early research Military interest in active camouflage