From Halopedia, the Halo wiki
Signal flares are a type of pyrotechnic that produce a brilliant light or intense heat without an explosion. Flares are used for signaling, illumination, or defensive countermeasures in civilian and military applications. Flares may be ground pyrotechnics, projectile pyrotechnics, or parachute-suspended to provide maximum illumination time over a large area. Projectile pyrotechnics may be dropped from aircraft, fired from rocket or artillery, or deployed by flare guns or handheld percussive tubes.
Flares produce their light through the combustion of a pyrotechnic composition. The ingredients are varied, but often based on strontium nitrate, potassium nitrate, or potassium perchlorate and mixed with a fuel such as charcoal, sulfur, sawdust, aluminium, magnesium, or a suitable polymeric resin. Flares may be colored by the inclusion of pyrotechnic colorants.
Signal flares seem to be standard equipment for UNSC servicemen. During the Battle of Trove, John Forge and his men from the UNSC Spirit of Fire used Recon Team Echo's signal flare to find the missing reconnaissance team's M312 HRV. During the Battle of Sigma Octanus IV, Corporal Harland used green signal flares to mark his squad's location to a Pelican carrying Spartan-II Blue Team. During the Fall of Reach, Noble Team used blue and red signal flares to attract the attention of incoming dropships. During the Battle for Zeta Halo some UNSC servicemen carried flare guns in chest holsters. The Liang-Dortmund Corporation manufactures their own emergency flare.
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