From Halopedia, the Halo wiki
Photoreactive panels are a form of active camouflage developed by the United Nations Space Command. The outer layer of the Semi-Powered Infiltration armor is coated with photoreactive panels to hide the wearer from sight. While not as sophisticated as Covenant active camouflage, the technology advanced during the Human-Covenant War, with the newer SPI generation being fitted with more efficient photoreactive panels in 2551.
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