Solarization is a photographic phenomenon in which the tonal values of a photographic image undergo a partial or complete reversal as the direct result of extreme, massive overexposure - exposure levels so vastly in excess of the normal working range of the emulsion that the characteristic curve of the film or paper turns back on itself, causing the densest, most heavily exposed areas to produce less density in the developed image than areas of somewhat lower but still extreme exposure. True solarization is therefore a consequence of the physical and chemical behaviour of the photographic emulsion at the very extreme upper limit of its exposure response, where the relationship between exposure and resulting density breaks down and reverses, rather than continuing the expected pattern of increasing density with increasing exposure that characterises normal photographic operation.
The underlying mechanism of true solarization is related to the phenomenon known as the Herschel effect, in which extremely intense exposure to long wavelength light can cause the partial destruction or desensitisation of the latent image centres formed during initial exposure. At the extreme overexposure levels required to produce solarization, the enormous quantity of photons striking individual silver halide crystals can cause the silver clusters forming the latent image to grow so large that they begin to lose their effectiveness as development nuclei, or can cause a chemical breakdown of the latent image that reduces its developability. The result is a reversal of the expected density gradient at the extreme highlight end of the tonal scale, with the most massively overexposed areas producing less developed silver density than areas of extreme but slightly less catastrophic overexposure.
True solarization is rarely encountered in normal photographic practice, requiring exposure levels many thousands of times greater than those used in conventional photography - such as might occur if a film were accidentally exposed directly to the sun at very close range or subjected to an extremely powerful flash discharge at point blank range. The visual effect it produces in the developed image is a partial tonal reversal confined to the extreme highlight areas, where the expected maximum density is replaced by a reduced density that gives those areas a lighter, more mid-toned appearance in the final image.
The term solarization is very commonly - and incorrectly - used in popular photographic literature and practice to describe a quite different and much more easily and controllably achieved tonal reversal effect known correctly as the Sabattier effect, named after the French scientist Armand Sabattier who first described it in 1862. The Sabattier effect is produced by briefly re-exposing a partially developed photographic film or print to a uniform light source during the development process, causing a partial reversal of the tones in the areas that receive this secondary exposure and the formation of characteristic Mackie lines - thin lines of increased or decreased density - at the boundaries between areas of different tonal value. Unlike true solarization, which requires extreme physical overexposure before development and is essentially an accident, the Sabattier effect is a controllable and deliberately exploited darkroom technique that can be applied to any partially developed photographic material with a brief flash of light during development.
The confusion between solarization and the Sabattier effect arises partly from a historical misidentification of the two phenomena in early photographic literature, where the visually similar partial tonal reversals produced by both effects led to them being conflated under the single name solarization. The celebrated Man Ray and Lee Miller - who popularised and extensively explored the Sabattier effect as a creative darkroom technique in the surrealist art movement of the 1920s and 1930s - contributed to the entrenchment of the term solarization as a description of their deliberately produced Sabattier effect work, and the misnomer has persisted in common usage ever since despite being technically incorrect. Photographers and writers who value precision in their terminology use the name Sabattier effect for the fogging-during-development technique and reserve the term solarization for the true extreme overexposure phenomenon, but in everyday photographic parlance the two terms continue to be used interchangeably with little distinction.