Image intensifiers work by focusing an image on a photocathode. The photocathode emits electrons when struck by light. The electrons are accelerated by an applied voltage to a phosphor screen which then emits more light. This can be done up to three times, that is an image intensifier can have up to three stages, (photocathode - phosphor screen sections), with each stage amplifiying the amount of light received many times. A three stage system has a maximum gain of about 125,000X. Three stages are the current limit due to distortion setting in.

Image intensifier tube are classified as being one of three generations:

First generation tubes, provide high resolution, dynamic range and low noise. Unfortunally they have a low light gain.

Second generation tubes provide greater light amplification by multiplying the number of electrons from the photocathode tube by using a microchannel plate to increase the number of electrons that hit the phosphor screen, which unfortunally decreases the image resolution and dynamic range while increasing the light gain.

Third generation tubes use a different photocathode material, ones which use Gallium-Arsenide and have a higher sensitity in the infrared range than the older image intensifier tubes.