Type-3 active camouflage
From Halopedia, the Halo wiki
The Type-3 Refraction Dissonance Modifier/Camouflage (T-3 RDM/C), more commonly known as active camouflage or active camo, is a refraction dissonance modifier and an armor plug-in that can bend light around the surface of an individual or structure. Designed to make the wearer invisible to visual scanning for a short time, its user is only noticeable by the diffraction of light around the wearer, making them appear as a "shimmer". It also generates false radar signatures to help conceal the movement of its user. However, these contacts are a double-edged sword; the presence of them will instantly alert any observant enemy to the fact that there is a cloaked enemy in the vicinity.
Even before they pooled their knowledge of sensor masking and optical camouflage techniques with the establishment of the Writ of Union in 852 BCE, the Sangheili and the San'Shyuum experimented with various active camouflage systems. While active camouflage devices eventually incorporated some reverse-engineered Forerunner technology, Covenant devices utilize a distinct branch of "baffler" systems.
The device was deemed an essential piece of Covenant technology early on during the Human-Covenant War, and extensively researched by the Office of Naval Intelligence. Over the course of the war, the active camouflage device was wielded by many Sangheili—usually those found in Special Operations or Zealot classes. However, ONI's research breakthroughs led to ad hoc compatibility with particular UNSC armor and BDU types, the most prominent and notable being that of the MJOLNIR Powered Assault Armor, as worn by Spartans.
It is most effective while motionless, as movement disturbs the effect and makes the operator much more visible. An excellent combination is a long-range weapon (like a sniper) to pick off far away enemies with little chance of return fire as they cannot find you, or a close-range weapon (like a Shotgun or Scattershot), and a closed hall with little cover, where it will give the operator the split-second chance they need to kill their opponent first.
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