Solarization (photography) different from pseudosolarisation
Pseudo-solarisation (or pseudo-solarization) is a phenomenon in photography in which the image recorded on a negative or on a photographic print is wholly or partially reversed in tone. Dark areas appear light or light areas appear dark. The term is synonymous with the Sabattier-effect when referring to negatives. Solarisation and pseudo-solarisation are quite distinct effects.
In short, the mechanism is due to halogen ions released within the halide grain by exposure diffusing to the grain surface in amounts sufficient to destroy the latent image.