Solarisation (or 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, but is technically incorrect when used to refer to prints.
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
In the darkroom
Careful choice of the amount of light used and the precise moment in 
development to provide the additional exposure gives rise to different 
outcomes. However, solarisation is very difficult to manage to yield 
consistent results. 
As a guide, an exposure of 1 second to a 25Watt lamp at 2 metres 
distant at around the end of the first minute of a 2 minute development 
can produce acceptable results. If the exposure is made with the 
developing print still in the tray of developer, it is important to stop
 agitation at least 10 seconds prior to exposure to allow any bubbles on
 the surface to disperse and to ensure that the print is lying flat. 
Solarising colour prints is more difficult because of the more careful 
control of temperature and timing that is required and because most 
amateur processing is undertaken in a processing drum rather than a 
dish.
In colour photography, different coloured lights can be used to 
effect solarisation, but the results become even less predictable.
It is possible to solarise a negative and subsequently solarise the 
print made from that negative. The results of such double solarisations 
are rarely successful, usually producing muddy and poorly defined 
images.
 
Solarisation in digital media
Graphs describing solarisation curves typically place input range of 
tones on the x axis, with black at 0 and white to the right, and the 
output range of tones on the y axis with black at 0 and white up. A 
curve then defines the input to output mapping.
Early video synthesiser
 technologists concerned themselves with achieving arbitrary curves not 
limited by film chemistry. A goal was to extend the range of 
solarisation effects possible to a computer specified curve. They then 
applied the defined solarisation curve to real time video images. A video lookup table
 was often used to implement this. Using this enhanced solarisation 
technology, still photos could also be passed through a grey scale or 
colour lookup table with the advantage that the effect could be 
previewed and progressively improved, instead of a procedure based on 
darkroom exposure calculations applied on a one time basis to a volatile
 light sensitive film or print, as described above. This was an especial
 advantage for creating colour solarisations with 3 primary colours.
Two versions of the same digital photograph, the version on the right is digitally solarised using Corel PHOTO-PAINT 8.