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.
First generation IIT now are applied for amateur (not professional) applications. All versions of these IIT have common design and as a rule work in condition of high light level that is formed either by moonlight or by IR illuminators. Luminance gain in 1st gen IIT is up to 1000 times.
2nd and 2nd+ gen IIT with micro channel plate (MCP) inside vacuum tube became the result of the next step in the development of IIT. Working principle of IIT with MCP is shown on picture 2.
MCP multiplies electrons produced by the photocathode by several hundreds times, though MCP itself has small size. That made it possible to design high quality NV-devices of a relatively small size. Not very high value of photosensitivity in IR spectrum can be referred to the disadvantages of this type of IIT which is shown on the diagram of picture 3.