Flashlight

Flashlights are small, usually portable devices, also attachable on weapons, which project a beam of light to aid vision in darkened areas.

Flashlights are quite common in the UNSC for combat in lowly-lit environments. The ODST troopers and Spartan-IIIs are the only known soldiers to not need the flashlights because ODSTs have the VISR system, and Spartan-IIIs use night vision. As such, many UNSC weapons have under-mounted flashlights below the barrel. In addition, most variants of MJOLNIR armor have a built-in flashlight on the helmet. Some Covenant armors have flashlights as well, such as the special flashlight integrated into the armor of the Arbiter.

Halo: Combat Evolved
In Halo: Combat Evolved, the flashlight was originally intended to be only usable while carrying the MA5B Assault Rifle or M90 Shotgun, both of which have built in flashlights mounted beneath the barrel. However, this was eventually changed. The only flashlight usable in the game is that mounted on the right side of the MJOLNIR Mark V helmet, which can be used at any time. This change was incomplete at release, as the flashlight's beam will still swing with the gun's movement while melee-attacking or reloading. Marines can be seen to use the gun-mounted flashlights on the level Truth and Reconciliation. The MJOLNIR Mark V's flashlight has a limited battery pack which will slowly deplete while in use and recharge while turned off. The flashlight turns off when it has been activated for around a minute. Pressing the flashlight button while zooming in with a Sniper Rifle activates the rifle's night vision feature instead.

Halo 2
The MJOLNIR Mark VI Armor also contained a built in flashlight. However, this model was more advanced and drew its power directly from the suit's power source. Because of this, it did not have a battery and could be used for any length of time. However, it had a light-sensor which caused it to turn off automatically in well-lit areas. Although the Assault Rifle was absent from the game, the Shotgun still possessed its flashlight. Despite this, it could not be used and no Marines were ever seen doing so.

Halo 3
The flashlight functions in much the same way in Halo 3 as it did in Halo 2. Like in Halo: CE, both the Assault Rifle and Shotgun possess inbuilt flashlights. However, the Shotgun's flashlight is now mounted on the weapon's left side rather than underneath the barrel. As before, neither are usable and only the light built into the armor functions. Unlike Halo 2, however, the flashlight in Halo 3 does not turn itself off in well-lit conditions, and can be left on indefinitely without any negative side-effects. The Arbiter, controlled by Player 2 in Co-Op mode, has a light source built into the armor over his left shoulder and it functions identically to the Master Chief's light. N'tho 'Sraom and Usze 'Taham, Player 3 and 4's characters, respectively, do not appear to possess any light-emitting feature at all, however the players can still toggle a light on and off, and this light functions identically to those used by the Master Chief and the Arbiter.

Trivia

 * The light of the flashlight can be used to easily spot opponents using Active Camouflage, since the light reflects off of them. This is most noticeable in Halo: Combat Evolved, and when the contrast is turned up high, Stealth Elites can be clearly seen. The same concept works for Multiplayer.
 * In Halo: Combat Evolved, if you acquire Active Camouflauge and have the flashlight turned on, not only does the flashlight have no visual effect, it also negates the effect of the camouflauge to enemies. So when using camouflauge always remember to turn off the flashlight.
 * The flashlight was only usable in Multiplayer in Halo: CE and its PC counterpart. In Halo 2, the same button is used for voice chat in Multiplayer and the flashlight cannot be used. In Halo 3, the controls moved to the D-pad and in Multiplayer activates chat or forge mode. Halo 3: ODST's replacement of the Flashlight, known as VISR mode, is usable in the Firefight multiplayer game-type, being the only flashlight-derivative that can be used in multiplayer.
 * In Halo 3: ODST, Orbital Drop Shock Troopers are seen not to use flashlights. Instead, their helmets' heads-up displays have the Visual Intelligence System, Reconnaissance built in. This is the first example ever seen of a regular soldier having more advanced technology than a Spartan-II. The flashlights on the shotgun and the assault rifle remain, but they are not usable. Mickey has a helmet-mounted flashlight, but it is never seen being used. It is probably used for specific functions such as bomb defusing, although this is not shown in-game.
 * In Monitor mode in Forge, looking at a surface may reveal a built-in light, shining out of the Monitor's single eye, which may have been created to shine in front of shadows to see better. It is not present if you set the monitor camouflage to 'good'.