Automatic brightness control
From Night Vision Wiki
Automatic brightness control is a feature of the tube's power supply 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 phosphor screen (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.
