Hard light is a Forerunner technology that allows light to be transformed into a solid state, capable of bearing weight and performing a variety of tasks. This involves an as-yet unexplained interplay between coherent, high-energy light and gas particles, such as air.
Hard light was used extensively by the Forerunners for a variety of purposes. The control panels and interfaces found in many Forerunner installations are composed entirely of hard light, and can react to the touch of a Forerunner or Human despite their utter lack of mechanical components. Hard light was also commonly used in the composition of Forerunner structures and ships, either for aesthetic purposes or as a covering for mechanical and technological mechanisms, such as on the arms of a War Sphinx. Hard light is also used in the locking mechanisms of Forerunner doors. Light bridges made of hard light are common throughout Forerunner installations; these bridges are strong enough to bear the load of heavy objects such as a M808B Main Battle Tank, yet can be switched on or off as easily as a normal beam of light.
Forerunner design seeds also made use of hard light. When activated, the rotating pillars of the machine would emit "scaffolding" of hard light that would fabricate a machine or ship using raw materials obtained on-site.
Hard light was also used in the firing mechanisms of the Halo installations, and also as a protective structural reinforcement to stabilize and protect the rings against tidal and gravitational forces from other objects.
Many Forerunner ships use hard light technology to form its internal divisions and compartments. This allows a Forerunner ship to easily reconfigure its layout, furnishings or decoration at will.
Most Forerunner structures project a hard light decorative shell over the structural support elements of the building. Almost all Forerunner buildings seen on the Halo installations or the Ark lack this covering; the Forerunners did not install hard-light generators on the Halos' structures, as they were meant to be purely functional. Additionally, any structures that possessed hard light decor only activated in the presence of Forerunners, although the decoration in the Menachite Forerunner Complex activated when a group of reclaimers entered.
Various Forerunner weapons, particularly ones used by the Prometheans in Halo 4, use hard light as ammunition. These weapons are particle dilators that use an unexplained method to convert light mass into hard light for use as a projectile. This would presumably provide both the kinetic energy as with solid projectiles like those used by the UNSC as well as the high energy output of the weaponry used by the Covenant. Such weapons are the Binary Rifle, Light Rifle, and Boltshot.
It is also believed to be able to protect an individual from a nuclear explosion at point-blank range as shown when Master Chief manually detonated a HAVOK Tactical Nuclear Weapon but was protected by a barrier of hard light surrounding him thanks to intervention by Cortana as she was dying, along with manifesting a hard light, life-sized holographic avatar to be able to touch John's armor before disintegrating completely upon her death.
- Halo: Combat Evolved (First appearance)
- Halo: The Flood
- Halo: First Strike
- Halo Graphic Novel
- Halo 3
- Halo Wars
- Halo: Blood Line
- Halo: Evolutions - Essential Tales of the Halo Universe
- Wages of Sin (First identified as "Hard light")
- Halo: Cryptum
- Halo: Primordium
- Halo 4
- Halo: Spartan Assault
- Halo 5: Guardians