The term solarization in photography is used to describe the effect of tone reversal observed in cases of extreme overexposure of the photographic film in the camera. Most likely, the effect was first observed in scenery photographs including the sun. The sun, instead of being the whitest spot in the image, turned black or grey. When a photographic layer, suitable for solarization, is exposed to actinic radiation, the resulting darkening after development will not increase steadily, but reaches a maximum which decreases under more intense exposure. In general is the phenomenon only then called solarization if the exposure has been produced in one "shot", meaning no pauses or double-exposures. The exposure to achieve solarization can be increased by exposure time or by increasing the light intensity
Man Ray
Hands (1922)