Photographic Films

Films of highest quality still gain an important part in photography for commercial purposes (photocomposition, X-ray, etc.), although digital methods are reducing the need for the classic physical-chemical processes.

Photographic films are based on photosensitive layers called "Emulsions", which contain silver(I) halides (mainly bromide) suspended in a support of gelatine. Light reduces the silver ions to black particles of metallic silver, the size of which is increased with reducing agents (= "Development"). Then the excess silver ions are complexed with thiosulfate (in the form of sodium thiosulfate solution), and dissolved from the film (= "Fixing"). The result is the "negative" image.

The formation of metallic silver on exposure to light is a highly complex process and can be affected sensitively and specifically by adding other metals to the emulsions on ppm scales.

Apart from silver (Ag), the precious metals gold (Au), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), and iridium (Ir) are essential to the manufacture and the properties of tailor-made emulsions.

The "fundamental" metal silver (Ag) is applied to the emulsion layers mainly in the form of silver(I) nitrate.

Gold (Au) in the emulsion increases the sensitivity of the film, influences the tones, and stabilizes the image.

Iridium (Ir) has influence on the sensitivity of the film.

Rhodium (Rh) serves as a desensitizer, and by this it is a contrast enhancer.

Clusters of palladium (Pd) also sensitize the film.

Ruthenium (Ru) accelerates silver reduction, and thus it makes a faster development rate possible with milder reducing agents.

Platinum (Pt) is a toning agent and stabilizes the image simultaneously.

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