Power Supply Unit (PSU)
The power supply unit (PSU) converts low voltage supplied by the battery of the housing (commonly between 2 and 12V, depending on the unit) into the high voltage required by the tube (up to 60 kV for cascade designs). In most formats, the power supply is integrated into the image intensifier tube, with the housing simply supplying battery voltage directly.
Most PSUs are a two stage design with a lower voltage part (often 2kv) with all the control logic in one stage and the high voltage stage which creates the necessary voltage for the phosphor screen. With older designs each stage is often a seperate component.
The logic inside the first stage is responsible for functions like automatic brightness control, bright source protection, gating or manual gain.
Automatic brightness control
Automatic brightness control is a feature which controls the gain according to the brightness over the whole image. The messurement is often done via measuring the current consumption of the photocathode and controls either the voltage of the phosphorscreen (in case of Gen 0 or 1) or the voltage across the MCP (in case of Gen2 or above). Automatic brightness control was first used on larger devices for vehicles or crew served weapons in the late 1960s.
Brightsource protection
Brightsource protection is aimed to protect the tube against bright flashes and strong lights. If the current flow inside the tube is great enough the BSP shuts the Unit down.
Gating
Gating is a function where the photocathode is switched between a working and a non working state.
In the working state the photocathode has a negative potential of around -200V compared to the MCP input side or in case of a gated Gen 0 or 1 tube a focus ring.
And around +50V in the non working state. This positive potential causes the electrons (created by the photocathode) to get accelerated backwards to the Photocatode.
There are 2 variants of gating, a manual gating and a auto gating:
In manual gatin, th
he tube switches between both state according a Input signal. This method is often used for scientific purposes, like ultra high-speed photography.
The second and more common method is autogating. The tube switches automatically, according to the brightness of image, between both states with a high frequency. The aim is to get a better contrast in mixed (urban) light conditions.
Manual gain
Manual gain is a feature where the user can lower the Gain with a potentiometer. There commonly 2 diferent tube formats that can have manual gain:
