Infrared radiation is radiation
at a longer wavelength than visible light, which means it cannot be
seen by the naked human eye. As infrared radiation can be detected as
heat, this can be shown on a screen or captured by a digital camera,
with hotter objects showing up brighter against colder surroundings
(e.g. a human body against a colder background).
color cameras can “see” infrared radiation as well as visible light,
these cameras are equipped with an IR-cut filter, to prevent distortion
of the colors the human eye can see. To use the camera in very dark
locations or at night, this filter can be removed, to allow infrared
radiation to hit the image sensor and thus produce images.
An infrared lamp can be used for improved illumination for night surveillance, whilst not producing any extra visible light.