Fogging in photography is the deterioration in the quality of the image or the negative caused either by extraneous light, other electromagnetic radiation, radioactivity or the effects of a processing chemical.
It is seen either as deposition of silver or dyes across all or part of the image unrelated to the original exposure.
Light fogging is where unintended light reaches the photographic material prior to processing[1] is seen as dark areas in the negative which tend to occur over the full width of the film including the margins.
The discovery of X-rays by Wilhelm Röntgen occurred when it was noticed that some fluorescent material lit up at some distance from an experimental cathode ray tube experiment.
Subsequent work showed that a radiation was emitted that fogged covered photographic plates.