What is this?
Sunlight reflected from the Moon, greater and minor planets, light from a congestion of stars and fogs, the luminescence of oxygen and nitrogen in the top layers of an atmosphere serve the sources of radiation creating natural night light level. For example, night light level equals to ~0,3 lux in a full moon and ~0,002 lux in a starlit night without the Moon. When the sun sits down horizon, not only the level of light decreases, but also the spectral structure of radiation changes and displaces to the nearest infra-red area. To detect the object on the place confidently itís important to take into consideration not only natural night light level but also the contrast of the object that depends on its background (green grass, dried up grass, black soil etc.).
You can see on picture 1 that the objects have the maximum contrast with a background in near IR areas while the human eye has its maximum of sensitivity in a greenish-yellow range. To create an NV-device itís necessary to reach repeated strengthening and transformation into a visible range of reflected light from the objects in visible and nearest IR area of a light spectrum.
The principle of action of NV-devices is based on transformation of the photons reflected from the object to electrons, the subsequent multiplication of electrons and return transformation to the photons perceived by a human eye. To provide such transformation and strengthening of light a device named image intensifier tube (IIT) is used. According to their design they belong to three generations: 1st, 2nd+ and 3rd.