From Halopedia, the Halo wiki
"Hologram" redirects here. For the Armor ability, see Type-27 hologram.
Holography is a technological system that forms a visual representation of an object in three physical dimensions, as opposed to the planar image of traditional screens that simulate depth through a number of different visual effects. Holography is employed by the humans, the Covenant and the Forerunners in a variety of functions - communications, battle and sensor displays, and as distractions and decoys. Devices used to project holograms are commonly referred to as holotanks.
The UNSC uses holographic technology in a variety of ways. Artificial intelligence constructs, such as Cortana, project holographic representations of themselves for personal communications with humans, with each AI taking a unique appearance of their own choice. The UNSC also uses holograms for communications between individual humans. The commanding officer of one ship may "appear" on another to deliver a message when normally access between the two vessels is restricted. Many UNSC warships, bases and other facilities are equipped with holographic tables, used to project topographical maps or other images for tactical purposes.
Holography is frequently used in human computer terminals, which, though not actually solid, respond accordingly to physical input without the need for a separate device such as a keyboard. Holography also has many civilian uses. For example, some nightclubs such as Club Errera make extensive use of holograms. Humans also use holographic technology to display holographic "photographs", referred to as holo-stills. Holographic tape is used to display information on the sides of civilian vehicles.
The GEN3 MJOLNIR Powered Assault Armor is capable of displaying the hologram of an integrated AI over its hand to communicate directly with outside forces rather than using the armor's speakers. The AI is also capable of transferring into systems through these holographic emitters, the emitters having not been a feature of previous generations of the armor.
The Covenant's holographic technology is also based primarily on that of the Forerunners, and though superior to UNSC technology it still pales in comparison. The Covenant use holograms for visual displays of space and ground combat, allowing a better general view of the action and better coordination. Holograms are also used as decoys, through devices known as holo-drones, able to generate their own shields and weapons to give the illusion of reality. One of these was used by the heretic leader Sesa 'Refumee before his death at the hands of the Arbiter. The Covenant also utilize less sophisticated holographic decoys in the field, a technology which has also been replicated by the UNSC.
As shown by the ability of holo-drones to use weaponry, certain Covenant holograms are somehow capable of physically interacting with their environment. The Prophets, in particular, make extensive use of holography in their governance of the Covenant. Religious sermons, such as those by the Prophet of Truth  and Prophet of Regret, are transmitted holographically via portable projectors, allowing the Prophets to preach from the relative safety of their point of transmission while protected by armed guards.
Consistent with their high level of technological advancement, the long-vanished Forerunners were true masters of holography. Their projections were accurate far beyond UNSC or even Covenant capabilities, able to appear indistinguishable from physical objects and simulate solidity via the use of sophisticated hard light technology.
On the various facilities of the Halo Array as well as other Forerunner installations, holograms are used to convey information on the installation, from general visual representations to more specific details. Virtually all Forerunner control panels and terminals are based upon solid, hard light holography.
The Forerunners also used their mastery of holography to produce realistic decoration to interior spaces; for example, the Forerunner ruins underneath CASTLE Base, despite eons of neglect, were still capable of projecting myriad environments from hundreds of worlds.
What is generally, and incorrectly, referred to as a "hologram" in most science fiction, including the Halo series, is actually a volumetric display. True holograms are not three-dimensional images. More specifically, Halo "holograms" seem to be of the static volume type, creating an actual three-dimensional image rather than creating the illusion of it.
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