Devices that convert an invisible infrared image into a visible image. Infrared radiation is usually considered to span the wavelengths from about 0.8 or 0.9 micrometer to several hundred micrometers; however, most infrared imaging devices are designed to operate within broad wavelength regions of atmospheric transparency, that is, the atmospheric windows. At sea level, for horizontal paths of a few kilometers' length, these are approximately at 8–14 μm, 3–5 μm, 2–2.5 μm, 1.5–1.9 μm, and wavelengths shorter than 1.4 μm. The radiation available for imaging may be emitted from objects in the scene of interest (usually at the longer wavelengths called thermal radiation) or reflected. Reflected radiation may be dominated by sunlight or may be from controlled sources such as lasers used specifically as illuminators for the imaging device. The latter systems are called active, while those relying largely on emitted radiation are called passive.

Active optical imaging systems were developed to achieve a nighttime aerial photographic capability, and work during World War II pushed such systems into the near-infrared spectral region. Development of passive infrared imaging systems came after the war, but only the advent of lasers allowed creation of active infrared imagers at wavelengths much longer than those of the photographic region. Striking advances have been made in active infrared systems which utilize the coherence available from lasers, and hybrid active-passive systems have been studied intensively.