At the photocathode the incoming light is converted into photoelectrons. As the photocathode is exposed to a high light source, it works at a very high level. Because the photoelectrons are limited, the photocathode will run out of photoelectrons.

The exhausted parts appear to become dark. When this event occurs over a longer time in the form of an image, it appears to be burnt into the photocathode. PHOTONIS Image Intensifiers are less vulnerable for a high light level than any other generations.

That is why PHOTONIS IITs will not be damaged immediately, when others do.