Both active and passive night-vision systems detect infrared energy that is invisible to the human eye. Each system helps people see in the dark -- even when there is an absence of light, infrared radiation is still there.
Like the human eye, passive night-vision systems receive wavelengths of energy that are reflected off objects.
But passive systems detect far-infrared wavelengths, or heat energy.
Most inanimate objects such as rocks, pavements and trees are close to the same temperature, so they appear in the display only slightly distinct from each other.
To drivers, those objects can be largely ignored because they typically are unmoving.
But hotter objects that generate their own heat -- people and animals -- appear as a bright light in passive night-vision systems.
Active systems use near-infrared light.
Humans can't see the beam. But like searching with a flashlight, the car uses lamps that illuminate objects ahead of the car.
Sensors mounted on the car detect reflected light from those lamps. The system's display simply translates the infrared radiation into lightwaves that humans can see.