Image intensifier (IMI) - an optical-to-optica/electronic image converter that utilizes a sensitive photocathode as an imaging detector of the incident light and an electron-multiplication stage to amplify the image by several orders of magnitude. The photocathode emits a flux of photoelectrons that whose spatial density mimics the distribution of irradiance at the photocathode. The electron flux, amplified by the multiplication stage is typically converted to an optical image with a phosphor screen or may be converted to an electronic image by a solid state imaging detector capable of responding to irradiation by electrons. This enables the IMI to amplify a very faint optical image.
In more recent designs, the amplification is achieved by using a microchannel plate (MCP), i.e. a glass plate with a dense regular grid of very narrow channels where avalanche amplification of electrons results from impacts on the channel walls of the electrons, accelarated by a potential difference on the order of 1000 V between the two faces of the plate. Two or more MCPs can be cascaded for a greater amplification. The microchannel plate restrict electrons to parallel paths and prevents mixing between electrons emitted at various points of the photocathode. The plate is located between and very close to both the photocathode and the phosphor screen, what is referred to as the proximity focusing. This preserves the spatial structure of the electron flux "image" emitted by the photocathode of the IMI. In a gated IMI, the electron-multiplication stage can be "turned-on" for a very short period of time, on the order of 1 ns, making an IMI a fast gated optical detector/shutter.