Gating

Gating is a feature of a tube's power supply 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 back to the photocathode.

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.