Semi-Powered Infiltration armor

Semi-Powered Infiltration Armor, colloquially called "SPI Armor", was an armor system used by the United Nations Space Command. Though more advanced than the ODST Body Suits, it was far less advanced and less powerful than the MJOLNIR Armor used by the Spartan-IIs. Its main benefit was that it was drastically cheaper to produce than the MJOLNIR armor. This armor was designed with an emphasis on stealth rather than raw power. It features special photo-reactive panels that mimic the surrounding textures, effectively being 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
The SPI armor Mark I was used by the Spartan-IIIs of Alpha Company. 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. The armor featured full body armor with a helmet and HUD, and a supply of breathable atmosphere; though only enough for 7 minutes, compared to the MJOLNIR armor's endurance of 90 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. The featured photo-reactive panels, gave the Spartans of Alpha Company active camouflage. The camouflage patterns however shifted irregularly and imperfectly, which made the photo-reactive panels unmatched against the Covenant elites Camouflage technologies. This was because the SPARTAN III program didn't allow enough time to get the photo-reactive panels working properly.

SPI Armor Mark II
The SPI armor Mark II was a more advanced version of the armor, first used by the drill instructors of Beta Company and then by the Spartans of Beta Company. 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. Although the new photo-reactive coating could mimic a wide range of EM(Electro Magnetic) radiation, it was still sensitive to overload, which is why it takes the suit a few moments to reset after a flash-bang for example.

It was proposed in 2537 for Alpha Company, but the request was denied because the armor was over budget. After the "successful" operation of Alpha Company, the Mark II armor was approved for mass production.

After the augmentation of Gamma Company, small adjustments where installed into the SPI Armor Mark II.

Advantages vs. MJOLNIR Armor

 * 1) The armor is a lot cheaper and easier to mass-produce.
 * 2) Includes special photo-reactive panels that act as active camoflage.
 * 3) It could be transported easier, because it is not as thick or heavy as the MJOLNIR armor.

Disadvantages vs. MJOLNIR Armor

 * 1) Has no energy shield that the SPARTAN-IIIs require considering the amount of danger they face.
 * 2) Does not give the SPARTAN-IIIs the same amount of enhanced strength, speed and agility as the MJOLNIR does for SPARTAN-IIs.
 * 3) After long use, tends to heat up and can become very uncomfortable.
 * 4) Since the AC plates take time to recover after explosions such as a flash bang grenade, the SPARTAN-III is visible until the plates recover.
 * 5) It isn't environmentally sealed, and thus can't be used in a vacuum or underwater.

Internal

 * Halo: Ghosts of Onyx

External

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