Why are certain chemicals photosensitive

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I'm trying to investigate why certain chemicals are photosensitive and create a latent image which may be developed ( even good old fashioned silver halides!!!) and why the developer only acts on the crystals which have been sensitised. Any explanations would be gratefuly received.

-- Nigel garbett (nigel garbett@3com.com), October 27, 1999


What is your background. I can give you some very simplified and short explanation myself, or I can try to guide you to some literature which explains the process on a more advanced level.

-- Thomas Wollstein (thomas_wollstein@web.de), November 22, 1999.


Photosensitive metalic salts like silver halides are composed of metal atoms like silver with either bromine, chlorine or iodine atoms. When the molecule is struct by photons of sufficient energy, free electons are released and the molecule breaks down to its base elements, leaving free silver atoms in the emulsion. This is the latent image.

Development works as a chemical catalyst. developing agents like metol, hydroquinon, Glycin, Amidal, etc. react with the free metal in the latent image and cause a type of chain reaction with the surrounding halides. The developer "reduces" or breaks down the surrounding halides to their base elements. The level of this action is proportional to the amount free silver in the latent image and the particular characteristics of the developing formula. Almost all developing formulas contain a restraining compound that hinders the catalyst action. Potassium bromide is a common restrainer. Without the restrainer, the catalystic action could run out iof control and reduce all silver in the emulsion regardless of latent image or not.

Hope this helps


-- Don Sigl (donsigl@hotmail.com), May 30, 2002.

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