Semi Powered Infiltration Armor

Semi-Powered Infiltration Armor, colloquially called "SPI Armor", was a armor system used by the UNSC. Though far more advanced than the ODST Body Suits, it was far less advanced and less powerful than the MJOLNIR Armor. However it's main benefit was that it was also significantly cheaper to produce than the MJOLNIR armor. It was used by the Spartan-III supersoldiers. It features special photo-reactive panels that mimic the surrounding textures, effectively the UNSC equivalent to the Covenant Active Camoflague systems.

SPI Mark I
SPI Mark I was used by the Spartan-IIIs of Alpha and Beta Companies from 2531 to 2545. It featured hardened plates and photo-reactive panels. Though advanced, it could only take a few glancing blows before failing. It featured full body armor with a helmet and HUD, and a supply of breathable atmosphere, though only for a few minutes. It is also equipped with a cushioning layer of liquid nanocrystals that provided more ballistic protection than three centimeters of Kevlar diamond weave without the bulk. It was worn by the Spartan-IIIs of Alpha and Beta Companies, as well as Gamma Company for a short time.

SPI Mark II
SPI Mark II was a more advanced version of the armor, with numerous upgrades. They had new photo-reactive coating to boost camoflauging properties. There were also numerous experimental refits to gel ballistic layers, upgraded software suites, and other functions. It was proposed in 2537, but the request was denied because the armor was over budget. It is weakest at the knees.

SPI Mark II was used at first by the Drill instructors for Beta Company, but weren't used by actual SPARTAN-III's until Gamma Company.