Semi Powered Infiltration Armor

Semi-Powered Infiltration Armor, colloquially called "SPI Armor", was an armor system used by the UNSC. Though more advanced than the ODST Body Suits, it was far less advanced and less powerful than the MJOLNIR Armor. However its main benefit was that it was drastically cheaper to produce than the Mjolnir. It features special photo-reactive panels that mimic the surrounding textures, effectively the UNSC equivalent to the Covenant Active Camouflage systems. It has been described as part legionnaire mail, part tactical body armor, and part chameleon.

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 compared to standard UNSC infantry armor, 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 compared to the MJOLNIR armor endurance of several hours. 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 camouflaging 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 wasn't used by actual SPARTAN-III's until Gamma Company.

Halo 3
The armor permutation in Halo 3 that you can choose for your SPARTAN is not SPI, but EVA (Extra Vehicular Activity) Armor. The name and changes in appearance are purely cosmetic, however, having no impact on gameplay. This has been confirmed by Bungie.

Internal

 * Halo: Ghosts of Onyx

External

 * SPI Modelling Thread (Sci-Fi Meshes.com)