An image intensifier tube is a vacuum tube device designed to increase the intensity of available light in an optical system. This allows use under low light conditions, it can also be used to facilitate visual imaging of low-light processes such as fluorescence of materials when they are bombarded with X-rays or gamma rays.

The device below takes in X-rays and gives out visible light.

Fluorescent screen, A converts X rays to visible photons.

Fluorescent screen, B converts electron kinetic energy into light photons. 

The photocathode converts photons to emission of electrons via the photoelectric effect.

The anodes increase the kinetic energy of electrons travelling from cathode to anode by accelerating the negative particles towards them as they are positive and opposite charges attract.They also serve to focus the rays of electrons.

The tube is evacuated so that the electrons can travel unimpeded by matter.