Night vision technology captures available electromagnetic radiation outside the portion of the spectrum visible to the human eye.
There are 2 primary technologies that are used for night vision devices: image intensification and thermal imaging.
Image intensification (I2) is best for seeing details like maps, because it works by intensifying the existing light spectrum. Low levels of ambient light pass through a photocathode that converts the light photons to electrons, then amplifies them. Sensitivity levels to various infrared, ultraviolet and visible spectrum wavelengths vary with the exact device. They then hit a phosphor screen (read: “TV screen”) where they are converted into visible light (read: “picture”).
The phosphor screen is colored green because the human eye can differentiate more shades of green than other phosphor colors. Like cameras, night vision devices have various image magnifications. The distance at which a human-sized figure can be clearly recognized under normal conditions (moon and star light, with no haze or fog) depends on both the magnifying power of the objective lens and the strength of the image intensifier.
Infrared (IR) or thermal imaging uses heat sources (aka. “deep infrared” spectrum) instead. Because infrared is actively emitted and not just reflected, and isn’t blocked as easily as visible light, this form of “infravision” works in no-light conditions that may prevail underground and inside dark buildings, or in conditions like dust storms, fog, etc.
