Night vision device

There is more information available on this subject at Night vision device on the English Wikipedia.

A night vision device, abbreviated as NVD, is an optical instrument that allows images to be produced in levels of light approaching total darkness. They are most often used by the military and law enforcement agencies, but are available to civilian users.

Operation

Night vision can work in two very different ways, depending on the technology used.

Image enhancement: This works by collecting the tiny amounts of light, including the lower portion of the infra-red light spectrum, that are present but may be imperceptible to the naked eye, and amplifying it to the point that one can easily observe the image.
Thermal imaging: This technology operates by capturing the upper portion of the infra-red light spectrum, which is emitted as heat by objects instead of simply reflected as light. Hotter objects, such as warm bodies, emit more of this light than cooler objects like trees or buildings.

Use

The SRS99 series sniper rifles are equipped with night vision scopes. However, the night vision feature is only accessible to players in Halo: Combat Evolved and Halo: Reach.

All versions of the MJOLNIR Powered Assault Armor possess night vision and infrared enhancements. However, they are unavailable in the course of most of the games, with the exception of Halo: Reach.^[1]

Sangheili combat armor also has built-in night vision, albeit with a purple color rather than the usual green found in UNSC versions.

While living things are outlined in dark green by the night vision, corpses are considered as the background (not outlined). In Halo: Reach, a dead Spartan-III's combat knife is outlined in dark green (which means alive); this is a small mistake made by Bungie.

Gallery

The night vision effect can be seen on the scope of the Sniper Rifle System 99 Anti-Matériel. Note the light green hue.
A Spartan using night vision.
An Elite using night vision.
A Spartan using night vision in Halo: Combat Evolved.