Semi-Powered Infiltration armor

The Semi-Powered Infiltration Armor, or SPI Armor, is an experimental armor system used by the United Nations Space Command that was developed for the Spartan-IIIs.

Description
Though more advanced than the ODST ballistic body armor, it was far less advanced and 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 featured 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" by both Tom-B292 and Dr. Halsey.

SPI Armor 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. It featured full body armor with an enclosed 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 inferior comparing to the Covenant's Active Camouflage technologies.

SPI Armor Mark II
The SPI Armor Mark II was a more advanced version of the SPI Mark I, 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 their camouflaging properties. There were also numerous experimental refits to the gel ballistic layers, upgraded software suites, and other functions. Although the new photo-reactive coating could mimic a wide range of electromagnetic radiation, it was still sensitive to overload, which is why it takes the suit a few moments to reset after a flashbang or plasma hit.

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 were installed into the SPI Armor Mark II.

Advantages

 * 1) The armor is significantly cheaper and easier to mass-produce than MJOLNIR armor.
 * 2) Includes special photo-reactive panels that act as an active camouflage for the wearer.
 * 3) It could be transported easily, because it is not as thick or heavy as the MJOLNIR armor.
 * 4) The armor requires less physical augmentation compared to the MJOLNIR armor, making it more versatile to other would-be users.
 * 5) The armor has an efficient cooling system, and is significantly difficult to detect even with thermal scanners.
 * 6) SPI Armor users does not need a technician to help them don the armor.

Disadvantages

 * 1) Has no energy shield, unlike the SPARTAN-II's MJOLNIR armor.
 * 2) Does not give the SPARTAN-IIIs any enhanced strength, speed or agility, as the MJOLNIR does for the SPARTAN-IIs.
 * 3) It stresses more on teamwork to take down enemy threats.
 * 4) Since the AC plates take time to recover, after explosions such as a flashbang grenade and plasma hits, the SPARTAN-III is visible until the plates recover.
 * 5) Is made of a less dense alloy than MJOLNIR armor as Kelly-087 managed to dent Holly's chest plate with her fist, on the planet Onyx.
 * 6) The SPI armor is not waterproof, unlike MJOLNIR armor.
 * 7) The SPI armor cannot be used for long periods of time without the cooling system failing.
 * 8) The SPI armor has air for only 7 minutes, while the MJOLNIR has air for 90 minutes.
 * 9) The armor is less agile and more clumsy to use.

Trivia

 * Some screenshots released by Bungie led to fans speculating that they would be able to use SPI armor in Halo 3. However, the armor was later determined to be Mark VI MJOLNIR Powered Assault Armor/V variant. The name and changes in appearance are purely cosmetic, however, having no impact on game play. This has been confirmed by Bungie.
 * For all intents and purposes, SPI armor is simply a less powerful version of MJOLNIR armor. It was meant to give the wearer maximum protection at the minimum cost, and so lack many of the more well-known (and much more expensive) features of its predecessor, such as the shields and the ability to support an AI.
 * The helmet bears resemblance to the EVA helmet, the only visible difference being that the faceplate is a bit smaller and the EVA faceplate is a quarter-sphere.

Internal

 * Halo: Ghosts of Onyx

External

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