Image intensifier
Image intensifiers , also known as residual light intensifiers , are electron tubes which, in their simplest form, consist of a photocathode (as cathode ) and a fluorescent screen (as anode ).
Photocathode and the phosphor screen are a thin layer on a respective disc-shaped transparent glass - substrate applied. The space between these two flat surfaces is evacuated and, depending on the design or generation (see below), contains further electrodes and possibly a microchannel plate .
A high voltage generates an electric field between the cathode and the screen and accelerates the emitted electrons.
The image intensifier amplifies small amounts of light so that they can be registered with the eye or electronic image recorders (e.g. CCD sensors ).
If the photocathode is sensitive in a spectral range outside the sensitivity range of the human eye, the image intensifier can make infrared or ultraviolet radiation visible. In this case the image intensifier is called an image converter .
Image converters and residual light intensifiers are part of night vision devices .
construction
With a lens is similar to a camera an image on the photocathode projected. From each illuminated point of the photocathode so occur due to the external photoeffect photoelectrons from and by a high electrical voltage (typically 10 to 17 k volts accelerated) over the distance toward the phosphor screen. There they generate a light spot through cathodoluminescence at the point of impact , the brightness of which is proportional to the entrance lighting. A monochrome, enhanced image of the recorded scene can be viewed on the output side.
Close focus image intensifier
In this simple design there are no additional electrodes. The structure is similar to a plate capacitor : the photocathode and luminescent screen are each on the inside of a glass plate: the pane with the photocathode is on the input side, the one with the luminescent screen is on the output side. The space between the two parallel plates (distance typically 1 mm) is evacuated.
Inverter
An inverter also contains one or more focusing electrodes in order to be able to increase the distance between the photocathode and the fluorescent screen. In addition to making it easier to handle the high voltage , this can be used to reduce the size of the input image in order to adapt it to the size of the subsequent optical components. The name "inverter" is derived from the fact that the output image is upside down - it is an electron-optical image of the photocathode, similar to the optical image in a camera.
Microchannel plate (MCP)
Between the photocathode and the fluorescent screen there is a microchannel plate , which can be imagined as a two-dimensional array of many secondary electron multipliers. Image intensifiers with microchannel plates have a much higher gain (up to 60,000 times) than inverters and do not reverse the image.
Development stages of image intensifiers
The types of residual light intensifiers or image converters can be divided into the following generations:
Generation "zero"
Simple light converter (near focus image intensifier) that converts infrared radiation into visible. Development around 1940.
First generation
Inverter; Light amplification 1,000 to 8,000 times depending on the design and acceleration voltage; Lifespan about 2,000 hours; Introduction around 1950.
Second generation as well as "two plus" and "two super plus"
A microchannel plate is located between the photocathode and the fluorescent screen , which works like a two-dimensional secondary electron multiplier. Light amplification 15,000 to 60,000 times. Lifespan 2,000 to 15,000 hours. Introduction around 1960.
Third generation
Further development of the second generation: photocathode based on GaAs , ion barrier on the microchannel plate to increase the service life. Light amplification> 40,000 times. Lifespan> 10,000 hours. Development from the beginning of the 1970s in the USA. First deployed in 1978. Introduced into the US Army in 1982.