Active camouflage

Active camouflage or adaptive camouflage is a category of stealth technologies designed to conceal an object from visual detection by adapting to its surroundings. This can be accomplished by a wide range of techniques, from light-sensitive smart paints which mimic surrounding textures and ambient lighting to exotic light-bending lensing fields.

UNSC
The United Nations Space Command, and human militaries in general, have long utilized basic forms of adaptive camouflage. The Insurrection and the Human-Covenant War have seen a number of advances in these technologies, from early experiments with adaptative camouflage fabrics to the sophisticated photoreactive coating used in the Semi-Powered Infiltration armor.

Semi-adaptive camouflage
The simplest form of adaptive camouflage has classically taken the form of programmable nanoprismatic paints capable of cycling between a select set of variant camouflage textures, and primitive photoreactive materials that adjust their brightness to the ambient light level. By the mid-26th century, such systems are standard in most UNSC gear, including infantry armor plating. Newer models of UNSC combat fatigues are also made out of a photosensitive smart fabric capable of changing its color and camouflage texture. Similar combat uniforms are used by some of the better-equipped non-UNSC military forces, including a handful of colonial military organizations, though due to their expense, many groups forgo them for traditional static camouflage fabrics and coating solutions.

Most Covenant nanolaminate materials incorporate a multichromatic capability, being able to change their surface color at will albeit usually retaining their signature iridescent sheen. This eliminates the need to repaint the surface of a suit of personal armor, vehicle or starship as they change ranks or unit affiliations. UNSC researchers have noted that it seems to be well within the Covenant's technological capability to use this technology to manufacture highly-advanced adaptive camouflage coatings, but for cultural reasons they instead op for high-visibility primary colors on most of their gear while using an entirely unrelated technology to provide visual concealment. In the decades since the war, the procurement of manufacturing equipment for small-scale nanolaminate sheets has enabled the UNSC to begin testing the use of semi-adaptive camouflage coatings derived from Covenant materials.

Adaptive-texture coatings
The conventional option for active camouflage, adaptive-texture coatings, colloquially known as chameleoflage, use malleable OLED displays or electrophoretic ink in conjunction with light-sensitive cells or miniaturized cameras to project an image of an object's background. While superior to conventional camouflage, they are only truly effective in rendering an object invisible if the camouflaging effect can be oriented relative to observers, specifically enemies. Older chameleoflage systems also have a lag time, with the coating taking some time to adapt to its background; as such, these systems are most effective when the user is immobile. This still makes them useful for snipers and scouts, however, who often use cloaks with a chameleoflage coating and IR masking. The different ways different species perceive light and color showed early adaptive camouflage fabrics to be ineffective, with the Kig-Yar, for example, being easily able to spot UNSC combatants in such camouflage due to the ways their electrophoretic pigments render colors. Still, types of adaptive camouflage retain a niche in the UNSC military, mostly among special forces. In addition, UNSC Prowlers use a type of adaptive-texture coating to conceal themselves from view, though on stealth spacecraft, visual detection is generally a secondary concern to other means of cloaking such as thermal emissions masking.

Photoreactive coatings
Photoreactive coating is an advanced metamaterial composed of a mutable nanocrystalline lattice that guides light and other electromagnetic radiation around an object, effectively concealing it from view. A powerful central computer linked to the coating is used to control the geometry of the lattice and further compensate for the distortions relative to observers. While highly effective, photoreactive coating remained prohibitively expensive due to the lack of mass-manufacturing infrastructure until well into the 2560s. Similar photoreactive coatings have since been adopted by a number of other armor systems, including the UNSC Army's Project LOKI infiltration suits. Limited tests of applying the material on UNSC Prowlers in lieu of conventional chameleoflage cloaking have also shown promise.

The main advantage of photoreactive coatings over Covenant active camouflage systems is their comparative energy-efficiency, enabling their use over a much longer duration with minimal power drain. As a result, most UNSC research and development since the war has been geared toward improving native photoreactive technologies over reverse-engineering the power-hungry Covenant active camo.

Covenant
Based on Forerunner technology, the Covenant's preeminent active camouflage or cloaking technologies are not based on special surface coatings at all, but rather the highly advanced manipulation of gravitic lensing fields used to bend light around the object. Due to the immense energy requirements of the process, most Covenant active camouflage can only be used for a short duration. Its main advantage over the UNSC's material-based active camouflage is its ability to cloak almost any object without having to alter its surface materials.