View source for Power Supply Unit (PSU)

[[File:Psu_and_core.png|Modern PSU (left) and Core (right) |alt=|thumb|300x300px]]
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|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. all those functions use the same detection mechanism: They measure the current to either the [[Phosphor Screen|phosphor screen]] or the [[Microchannel Plate (MCP)|MCP]], if these currents exceed a certain limit the respective function is triggered.

=== Automatic brightness control ===
Automatic brightness control is a feature which controls the gain according to the brightness over the whole image. This is usually determined by measuring the current consumption of the photocathode. It controls either the voltage of the phosphorscreen (in case of Gen. 0 or 1) or the voltage across the MCP (in case of Gen. 2 or above). Automatic brightness control was first used on larger devices for vehicles or crew served weapons in the late 1960s.

=== Bright source protection ===
Bright source 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 feature where the photocathode is switched between an operative and an inoperative state. 

In the operative 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 the tube a focus ring). 

In the inoperative state the potential is around +50V. This positive potential causes the electrons (created by the photocathode) to get accelerated backwards to the photocatode. 

There are two variants of gating, manual gating and autogating:

During manual gating, the tube switches between both states according to an 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 the image, between both states with high frequency. The aim is to get better contrast in mixed (urban) light conditions.

[[Category:Technology]]