The theoretical basis for camouflage is the underlying methodology used in by camouflage, whether natural or human-made. The definition of camouflage involves concealment and obscurity, whether applied to the natural coloration of animals, or the paint schemes used on military vehicles. The methods by which concealment or obscurity are attained share a common set of strategies intended to deceive the observer. Camouflage is not limited to the commonly encountered visual camouflage, but encompasses other senses as well.
The use of camouflage, no matter what the specific application, has certain basic requirements that must be met. First, the camouflage must be tailored to the observer. Second, the camouflage must deceive the observer into making a false judgment about the camouflaged object. The strategies of camouflage can be broken down into up to four categories, which are cryptic, disruptive, mimicry, and countershading. Each of these deals with a different method of deceiving the observer, and often strategies are combined increase their effectiveness. A tiger's distinctive coloration, for example, uses a combination of countershading and cryptic colors and patterns to blend into its environment.
Humans have been practicing the art of camouflage since prehistoric times, as evidenced by depictions of camouflage use in cave paintings. Among humans, camouflage is of particular importance to the military, where the art of camouflage is as important as marksmanship. United States Army field manual FM 5-20 states in chapter 1: Camouflage uses concealment and deception to promote our offensive action, to surprise, to mislead the enemy, and to prevent him from inflicting damage upon us. Concealment includes hiding from view, making hard to see clearly, arranging obstructions to vision, deceiving and disguising, and deception involving sound.
Painters and other visual artists are sometimes employed in the development of camouflage, because they often deal with human visual perception, and it was an American artist, Abbott Handerson Thayer, who first published a scientific paper in 1892 describing the countershading technique (see below) often found in natural camouflage. While camouflage has been used for hunting, from when prehistoric hunters dressed in animal skins to mimic their prey, to the Scottish gamekeepers' ghillie suits still worn by hunters and snipers today, camouflage in a military context was considered effeminate and greeted with scorn in the late 19th century.
Military camouflage did not achieve widespread use until World War I, after the introduction of airplanes for observation made its use far more important. French gun crews wore smocks smeared with brown and green paint to break up their outlines and provide concealment from the aerial observers; it is likely that the first soldiers to try this were artists serving in the infantry. Upon seeing a camouflaged cannon for the first time, artist Pablo Picasso is reported to have said, C'est nous qui avons fait ca, or it is we who have created that. This early camouflage was not restricted to just coloring, it applied to shape as well; at an early demonstration by the American Camouflage Corps, President Woodrow Wilson approached within ten feet of a soldier without noticing him. The soldier was in a foxhole covered by a papier-mâché cover camouflaged as a rock.
Many plants and animals possess some form of camouflage, for many reasons. Natural camouflage can involve many different senses, including sight, smell, touch, and sound. The reasons for camouflage are primarily to deceive predators or prey, but camouflage can serve to aid reproduction. Some animals are even capable of adapting their appearance to match their environment. A few examples of varied strategies in camouflage are:
Camouflage is an attempt to avoid observation, and as such, it is tailored to the vision of the expected observer. In military contexts, in particular, this is not limited to normal biological vision, but applies to night vision systems, radar, and arguably sonar as well. Camouflage strategies can be broken down into four categories: cryptic (or blending), disruptive (or dazzle), mimicry, and countershading. These may be applied individually, or in combination with one another to provide an overall camouflage strategy. For example, the blending of disruptive and cryptic coloration is called coincident disruption.
Cryptic camouflage is an attempt to blend into the environment and become effectively imperceptible. To do this, the camouflaged object must minimize observable differences between itself and the background with respect to the senses of the target observer. The definitive example of camouflage is a cryptic camouflage designed to match the visual appearance of the expected background, generally using a mottled pattern of greens and browns to match ground and foliage color and break up the outline of the camouflaged object.
Blending into the background is a complex task in cases where the camouflage is to be effective in a wide range of environments; camouflage that works in the arctic, for example, is unlikely to work in the jungle, or in the desert. Cryptic camouflage must match the colors and spatial frequencies of the background to be effective, and may have to suppress non-visual cues as well.
Nature provides some extremely effective examples of cryptic camouflage, one example of which is the class of insects generally referred to as stick insects or walking sticks. They are of the order Phasmatodea, which derives from the Greek phasma, meaning phantom or appirition. Their cryptic camouflage matches both the coloration and shape of the plants in their environment, and they take the form of twigs, bark, leaves or lichen, depending on species.
A good example of cryptic camouflage applied to another sense is the insect repellant DEET. DEET is believed to work by blocking insect olfactory receptors for 1-octen-3-ol, a secondary alcohol produced by humans and emitted by perspiration and respiration. By blocking the receptors, it makes the wearer effectively undetectable to mosquitoes, ticks, and many other insects that rely on olfactory cues to find their targets.
Disruptive camouflage is not intended to blend into the environment, and in fact often involves bright, eye-catching colors that would be the antithesis of cryptic camouflage. Disruptive camouflage seeks to confuse the observer, by providing visual cues that override the camouflaged object's features. This prevents the observer from accurately identifying characteristics of the camouflaged object(s), such as shape, size, orientation, and number of objects in a group.
Another form of disruptive camouflage is ink used by octopuses, or smoke used by the military to obscure movement. The cloud of ink or smoke is readily visible, but obscures what is within or behind it. The likely root of the word camouflage is camouflet, a French term meaning smoke blown in someone's face as a practical joke. In the case of the octopus ink, the resulting cloud also provides disruption of the sense of smell.
Mimicry is the attempt by the camouflaged object to be observed as some other type of object. In some cases this can be quite a complex process, such as in the case of the Mimic Octopus, which can change shape to resemble lionfish or other poisonous fish in its habitat. Mimicry is perhaps the broadest example of camouflage; though the most obvious form to humans is visual mimicry, other senses, such as olfaction (smell) or hearing may be involved as well.
Decoys and fishing lures also utilize mimicry to deceive and lure an observer. Some animals, such as the anglerfish, use part of their body to lure prey into range, while human hunters use decoys to lure targets into a killing field. Decoys are also used by the military, typically to draw enemy fire to the decoy, and away from the real target.
Also called Thayer's Law, a tribute to the artist who first described it, countershading is the process of using lighter colors on normally dark areas, and darker colors on normally light areas. This removes some of the visual cues used for depth perception, causing the countershaded object to appear flat, rather than as an object with depth. This is the same effect used in the hollow-face illusion, where a concave object is shaded in such a way as to appear convex. Military doctrine also applies this technique to camouflage paint. The standard method for applying camouflage paint to exposed skin is to use dark paint for bright areas of the face, light paint for shadowed areas, and combining this with a disruptive pattern over large areas of skin. 
Eyes differ widely across the animal kingdom. Some animals see in monochrome, perceiving only brightness, while other see two or, like humans, three types of color; some species of birds and reptiles are capable of distinguishing four different colors. Even in animals with the same degree of color perception, the pigments respond to different wavelengths, meaning what would appear to be a metamer to one species would be a completely different color to another species.
In addition to differences in how color is perceived, the nervous system does a significant amount of processing to the image, finding edges, adjusting contrast, and determining spatial frequency.
Normal humans (excluding individuals who have a form of colorblindness) see though the actions of four types of receptors in the eye; the rods, which are highly sensitive, perceive brightness, while three types of cones perceive red, green, and blue colors. Humans, and most primates, are trichromats, able to perceive three types of color. The density and sensitivity of these rods and cones determine how sensitive the eye is to color. Brightness is the most important factor, followed by green, red, and blue colors, in that order. Sometimes unexpected colors provide good cryptic coloration, such as pink. A shade called Mountbatten pink was used during World War II for ships when it was discovered that it disappeared against the morning and evening skies, and the Special Air Service used pink as the primary color on the desert camouflaged Land Rover Series IIA patrol vehicles, leading to the nickname The Pink Panthers.
Color perception in animals varies by species; most mammals are dichromats, perceiving two colors. Deer, for example, see blue and yellow, but not red, and their perception of blue is far broader and more acute than humans. A camouflage garment laundered in typical laundry detergent laced with ultraviolet dye, added to brighten colors, may blend into the background when seen with the human eye, but the deer's eyes are sensitive into the near ultraviolet portion of the spectrum, and the garment will stand out as a brightly colored object. Orange and red, however, are far enough from the deer's yellow receptor range that they will appear as dim colors, so what a human sees as a bright orange element in a camouflage pattern will appear as a dark color to the deer's eye. Similarly, the rods of deer are sensitive further into the near ultraviolet than are human eyes (though still lacking in response at the red end of the spectrum), and provide a greater response. This gives deer superior vision in low light conditions. Birds, on the other hand, do see red, and near ultraviolet; both ultraviolet dyes and blaze orange will be visible to them.
Spatial frequency perceptors in the eyes of many animals allow easy identification of mismatched spatial frequencies. By matching the spatial frequency of the background, such as a tiger's stripes, the camouflaged object can blend in; by not matching it, such as with the zebra's stripes, the camouflaged object can stand out.
Vision, in animals from flies to humans, has special processing mechanisms for detection of motion. Motion will destroy the illusion of even otherwise perfect cryptic camouflage, so objects relying on cryptic camouflage must remain very nearly stationary with respect to their environment. This does not apply to other camouflage strategies; the disruptive form of camouflage, for example, was applied to ships in the form of dazzle camouflage, providing a more effective camouflage for the moving ships than a cryptic strategy. Octopus ink and smoke screens are other forms of camouflage that can effectively be used to conceal movement.
Many animals rely far more on senses other than sight, so when attempting to deceive these animals, it is important to consider other senses. An example of this is the wide range of products available to the hunter to deal with the issue of scent. These can range from scent blocking clothing, which serves as a cryptic form of camouflage, to scents that mimic plant, animal, and food scents, designed to mask the hunter's scent or lure the game in.
Clothing can also provide a cryptic camouflage with respect to sound. Certain fabrics, such as cotton/nylon blends, create a significant amount of sound when rubbed together, or against other objects. Other fabrics, such as polar fleece and many all natural fiber fabrics, are much quieter, and will reduce the amount of noise generated by movement. Game calls are used as a form of mimicry, with calls existing that mimic the sounds of everything from ducks and geese to bears and coyotes. These calls range from simple reed instruments that mimic a duck's quack to remote controlled electronic playback devices that provide the user with a menu-driven selection of digitally recorded sounds.
Radar, sonar, night vision, and thermal imaging are all common forms of observation, and just like natural senses, camouflage exists in many forms to deceive these technological senses. Camouflage may be implemented by cryptic means, such as stealth technology applied to military vehicles, by disruptive means, such as radar jamming and deception, or mimicry means, such as the decoys like the AN/SLQ-25 Nixie and anti-missile flares.
In the more traditional sense of camouflage, there has even been camouflage clothing designed to help defeat technological detection. The US military issued a pattern that consisted of a grid of darker green lines on a spotted, dark green background, that was designed to confuse early night vision devices. There is also a thermal camouflage cream under development, that reduces thermal infrared emissions from bare skin. Tests have shown a five times or greater reduction in detection range when this cream is applied to exposed skin.
Chameleons and some octopuses have the ability to alter their color through the use of chromatophores which allow the animal to change the type and amount of light reflected. This is a form of adaptive or active camouflage. Research is being done into military adaptations of adaptive cryptic camouflage, using sensors and adaptive coloration to sense and adapt the coloration of an object to better match its background.