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Solarisation 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.
The Sabattier effect
Initially, the term solarisation was used to describe the effect observed in cases of extreme overexposure of the negative in the camera. The effect generated in the dark room was then called pseudo-solarisation. This fine distinction is not made in the jargon of contemporary photography.
The effect was first described in print by H. de la Blanchere in 1859 in L’Art du Photographe. It was described again in 1860 by L.M. Rutherford and C.A. Seely, separately, in successive issues of The American Journal of Photography, and in the same year by Count Schouwaloff in the French publication Cosmos. The phenomenon should have been christened the Blanchere Effect, for it was not described by Sabattier until later in 1860 in Cosmos, and another paper published in 1862 in the Bulletin de Societe Francaise de Photographie.
The effect was usually caused by inadvertent severe over-exposure or occasionally by accidentally exposing an exposed plate or film to light before processing. Artist Man Ray perfected the technique which was accidentally discovered in his darkroom by his assistant Berenice Abbott.  It is evident from publications in the 19th century that this phenomenon was invented very many times by many photographers as it tends to occur whenever a light is switched on inadvertently in the darkroom whilst a film or print is being developed.
In modern film photography, this effect can be emulated for artistic effect by briefly exposing the film to actinic light during chemical development. However, because of the speed of modern films, the effect is much more commonly seen in printing.
In solarisation, not only are parts of the image reversed in tone but a thin line is generated around areas of contrasting tone, called a Mackie line. If a film is treated the line is light, which produces a dark line in the print; when the print itself is processed it produces a white or light line around areas of high contrast. It is therefore always possible to determine whether a film or print has been used to produce the solarisation.
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.
Solarization abstracted mathematically and extended to digital media
Graphs describing solarization 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 synthesizer 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 gray scale or color 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 color solarisations with 3 primary colors. Some commercial modern digital cameras and some photo manipulation software can generate solarisation type effects but these are often very coarse (due to use of low resolution or undersampling) in comparison to carefully managed solarisation performed on a large, well-made, chemically processed print. Such a print attains resolution limited only by the grain size of the film.The above copied from: