Energy shielding

Energy shielding is any protective technology that utilizes energy field-based means (such as electromagnetism or gravitics) to deflect incoming damage. Energy shielding can be either passive (always on) or active (activated only upon imminent impact), and may be designed to deflect either projectile- or energy-based strikes, or both.

Forerunner
Forerunner systems mostly use multi-layered active shields paired with sophisticated threat-detection software. In low-priority systems such as Sentinels, the shields may not even be active until a threat is detected, and the shields are only activated long enough to neutralize the threat.

Covenant
While full conformal shielding is rather common in the Covenant as of its more recent ages, this has not always been so. Full shields began as luxuries available only to high nobility, and it were only the military buildups that characterized the Consolidation Period that saw them proliferating en masse even among the lower ranks of Sangheili infantry. Before full conformal shields, the most common application of shielding technology were various single-facet shields such as the shield gauntlets later used by Kig-Yar ground troops or the portable shield emplacements still deployed by the Ninth Age of Reclamation. Even in the earlier ages of the Consolidation Period, lower ranks of Sangheili would deploy with gauntlet-shields rather than full-body shielding. Today, these continue to see ceremonial applications among Sangheili. The lack of full-body shields on the vast majority of non-Sangheili troops was partly a question of expense and resources, but primarily one stemming from the Covenant's feudal hierarchy: the warrior class always needed to maintain their technological edge over the lower-caste species to avoid rendering themselves replaceable or vulnerable to servile rebellions.

Engineering
During the effort to reverse-engineer Covenant shield technology, UNSC scientists discovered that Covenant shields use parallel technologies and many different approaches to generate protective barriers. The different categories of shield technology may operate on similar scientific principles, but the similarities end there.

Conformal shields are often compared to infantry armor of the middle ages. Just as a pikeman might wear a gambeson under a mail coat, and linen under that gambeson, a conformal shield has many flexible layers that overlap. This is necessary because flexible shields are relatively weak. They are far easier to penetrate than single-facet shields. When the layers are overlapped, the lower layers can trap projectiles and fragments that bypass the outer layers. In response to damage, the layers then slide over each other, so that damaged sections of shield are sandwiched by intact sections until the whole shield system can be rebooted.

Conformal shields can be made impervious to laser weaponry. For reasons that are unclear to the UNSC, this is rarely done outside of overshields. The process makes conformal shields glow like a struck match, and therefore opaque, so it is possible that most Covenant warriors prefer to see and be seen on the battlefield.

Single-facet shields are rigid, but constructed in such a manner that damage is distributed throughout the shield. Barring catastrophic and tightly-focused damage, a single-facet shield will not be penetrated until the whole shield collapses. This technology also allows the shield emitters to constantly repair the shield, instead of waiting for a reboot.

While single-facet shield technology can be adapted to allow flexibility, this requires heavy computational power and AI algorithms that are forbidden by Covenant dogma. It is quite possible that the Covenant were not aware that it could be done in the first place.

Polyhedral shields (also known as bubble shields) are an adaptation of both applications, but with novel technology. Each panel of a polyhedral shield is a single-facet shield that must be totally compromised before it is penetrated, but the segmentation of the shields means that damage does not spread from one panel to its neighbors. While this technology is occasionally mounted on vehicles, it is most often used in canopy shields and starship defenses. No applications of infantry defense have been reported in the modern era, though it is possible that some Prophets might integrate them into their thrones.

UNSC
Most UNSC shield systems are active rather than passive, taking advantage of the UNSC's more lenient approach to AI and automation in comparison to the Covenant.

Proto-shields
Proto-shields are a category of electromagnetic impeller systems unrelated to Covenant shield technologies. Some of these have existed for quite some time even before first contact, though the military applications of these early systems were limited. Even prior to the Human-Covenant War, some UNSC ships and stations used localized electromagnetic fields to protect them from incoming radiation. Such systems were far too weak against Covenant plasma weaponry, however. Upscaled versions of the technology were used to shield several colonies without native magnetospheres, such as Mars and the Jovian Moons, from radiation.

Early "plasma windows" used to create pressure barriers in some fringe applications are a related technology.

Passive/semi-active conformal shielding
As soon as the UNSC came into contact with the Covenant, various projects sought to both reverse-engineer Covenant energy shielding as well as develop viable alternatives. The most successful of these was Project AEGIS, which eventually resulted in functional energy shielding by the second half of the 2540s. This came with several limitations, however. The shields were most useful on a small scale, such as on a MJOLNIR battlesuit or small fighter craft. To be viable for sustained operation, they also require an onboard reactor. The larger the object being shielded, the more prohibitive the power consumption of the system. The Covenant solve this problem through their ships' immense power generation capabilities, but it will be some time before UNSC systems reach the same level.

While many examples of conformal energy shielding fell into UNSC hands during the War, the engineering obstacles to replicating the conformal shields were never overcome. Further problems were encountered with trying to adapt captured overshields to the Mark II MJOLNIR. Early tests in the 2540s showed that overshields were "anathema" to the power armor. The shields produced an "EM whine" that degraded circuits, and the energy release of a collapsing shield often triggered the piezoelectric layer. MJOLNIR required re-engineering from the ground up to accommodate energy shields, resulting in the Mark IV. This was then refined into a fully-integrated shield system on the Mark V armor.

Active dispersal fields
Developed during the mid-Covenant War based on old, long-mothballed designs, active dispersal fields use a targeted strong electrogmagnetic field to reduce the energy of incoming plasma weapon attacks; against projectile-based attacks, they are useless. Unlike Covenant-style shields, the dispersal fields are not always on, which makes them more energy-efficient, but also requires a sophisticated threat-detection system to function. This makes them ideal for spacecraft, particularly large warships. Since the mid-war, dispersal field development was a gradient with the later years of the war already seeing fairly successful solutions though the technology would only mature and gain widespread adoption in the post-war period. When used in vehicle armor, they are usually known as shock plates, or jolt plates. By the 2550s, many UNSC vehicles incorporated such armor.

The electromagnetic field generated by the dispersal field must be stronger than that holding the incoming plasma bolt together. Consequently, dispersal fields are generally more effective against plasma torpedoes or beams fired from a ship further away, whereas at close range, the enemy ship's field projectors may be capable of countering the effects of the dispersal field; ultimately, whoever wins comes down to power generation capability. This has led to many hostile Covenant factions favoring close-range passes of UNSC ships in post-war battles as it allows them to more effectively counter the dispersal fields.

Strike shields are essentially dispersal fields that use a battery, rather than a fusion reactor, to function. They fall after a few strikes and require a battery recharge or replacement to replenish. However, they are far cheaper than shielding equipped with an onboard reactor, and increasingly widespread on various infantry armor systems over the 2560s.
 * Strike shielding