Image intensifier

Image intensifier, also called residual light amplifier, are electron tubes which (as a cathode), and a fluorescent phosphor screen in their simplest form consists of a photocathode ( as the anode ).

Photocathode and the phosphor screen are applied as a thin layer on each of a disk-shaped transparent glass substrate. The space between these two planar surfaces is evacuated and contains, depending on the design and generation (see below) further electrodes and possibly a microchannel plate.

Between the cathode and the shield of an electric field generated by a high voltage 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 sensors (eg, CCD image sensors).

If the photocathode is sensitive in a spectral region outside the sensitivity range of the human eye, the image intensifier infrared or ultraviolet radiation can make them visible. In this case is referred to as an image intensifier image sensor.

Image converters and intensifiers are part of night vision devices.

Construction

With a lens, an image is projected similarly to the photocathode, such as a camera. From each illuminated point of the photocathode thus occur due to the external photoelectric effect of photo- electrons and a high voltage ( typically 10 ... 17 kvolts ) over the distance accelerated toward the phosphor screen. There they produce a light spot whose brightness is proportional to the input light at incidence by cathodoluminescence. Thus it can be viewed on the output side of a monochrome, amplified image of the recorded scene.

Close-focus image intensifier

In this simple design, no electrodes are present. The structure resembles a plate capacitor: photocathode and phosphor screen are located on the inside of each a glass plate: the plate with the photocathode is located on the input side, the output side to the fluorescent screen. The intermediate space between two parallel plates ( distance typically 1 mm) is evacuated.

Inverter

The inverter further comprises one or more focusing electrodes in order to increase the distance between the photocathode and the phosphor screen can. In addition to simplifying handling of high voltage, this can be used to reduce the input image to fit 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 optical imaging in a camera.

Microchannel plate (MCP )

Between the photocathode and phosphor screen is a microchannel plate, which can be thought of as a two dimensional array of many secondary electron multiplier. Have image intensifiers with microchannel plates a much higher gain (up to 60,000 -fold) as an inverter and do not return to the image.

Stages of development of image intensifiers

The types of image intensifier or image converter can be divided into the following generations:

• Electron tube
• Optoelectronics