Image intensifier tube with photocathode protective circuit

An image intensifier tube comprising:

• a housing having an entrance window with a a photocathode at one end thereof, and having an exit window at the other end thereof;
• have an entrance face facing the photocathode,
• a channel multiplier plate, mounted in the housing between the photocathode and the exit window;
• an input electrode attached to the channel plate, said input electrode being cylindrically prolonged in the direction of the photocathode;
• an entrance electrode on the entrance face of the channel multiplier plate, electrically connected to the input electrode;
• a cylindrical intermediate electrode, mounted in the housing between the photocathode and the input electrode, said intermediate electrode having a frusto-conical portion with the narrower end nearer to the photocathode is
• means to apply a given potential to said intermediate electrode;
• and
• means to apply to said input electrode a potential lower than said given potential;
• such that substantially all electrons reflected from the input electrode are intercepted by the intermediate electrode and substantially no ions can reach the photocathode from the channel plate, thereby reducing the formation of ion spots on the photocathode.