The photographic process

by M. Feuerbacher 2003.

Photography is defined as a technique to produce permanent images of objects on suitably prepared light sensitive plates or films by means of an optical system. The optical system can be a lens, a mirror or a hole. It focuses (visible) light and produces real images of the object on the plates or films. Photography, i.e. the making of the permanent image, is based on a photochemical process. The most well known process is the halogenide-silver process, which is based on the light sensitivity of silver halogenides (AgBr, AgJ, AgCl). The plates or  films used today consist of a carrier covered with an emulsion (suspension) of AgBr with 1% AgJ in gelatine. The halogenides are suspended microscopically homogeneous in the gelatine in the form of small crystals of the size of 1µm or less. These small crystals are usually referred to as the grains.

Defects in the crystal lattice of the silver bromides are very important for the photochemical process. These defects are foreign atoms (sulphur, heavy metals) in the lattice, which origine from additives in the gelatine. The elementary photographical process has firstly been described by R.W Gurney and N.F. Mott.

• Light quantums are absorbed by silver bromide crystals. One valence electron per light quantum is emitted from a Br atom (inner photoeffect).
• The electrons move freely through the AgBr crystal lattice. If no recombination (capture by Br atoms) takes place, the electrons (e-) are trapped by the lattice defects. The lattice defects become negatively charged and capture interstitial Ag+ Ions.
• If a number of Ag+ Ions is captured a local area with Ag+ e- excess is manifested, leading to a breakdown of the cation lattice. Locally the AgBr lattice is replaced by metallic silver.
  

These areas are  seeds in the latent image. The latent image can be made visible by the development process. Developing is a chemical reduction, which continues the action of the light in the elementary photographic process, and transforms full grains carrying areas of metallic silver fully from AgBr to metallic silver. The developer acts as electron donator. Electrons are captured by Ag+ Ions the AgBr lattice, which are then discharged to metallig Ag. This process  starts to take place at the seeds in the latent image, where the developer is preferentially absorbed.

Developers consist of a number of chemicals. Firstly they contain a suited reduction substance, e.g. hydrochinon. The effectivity as electron donator is increased by eg. sodiumcarbonate, kaliumcarbonate or borax. Another essential ingredient is kaliumbromide. This substance avoids the development of those grains, which have not been hit by the light. The lifetime of the developer is increased by addition of e.g. sodiumsulfite, which reduces the oxidation by air.

After the development, those grains which have been hit by the light consist of metallic silver, the others still consist of AgBr. The fixation process removes the remaining AgBr grains. For this purpose normally a sodiumthiosulfate solution is used, which transforms the AgBr into a easily soluble silver salt. This is removed from the emulsion by water (washing).