The Action of Light and Halide Sensitivity

Light is always one of the reactants in a photochemical system so an understanding of the properties of light and its behaviour is needed.

Photography relies on the dual nature of light, behaving as both a particle and a wave because photographic film varies in its sensitivity to the wavelength of light hitting its surface. Light striking the photographic fim's surface ejects an electron, a particle, from a light-sensitive chemical on this surface. This photochemical process iniciates a series of oxidation-reduction (electron-transfer) reactions, producing a permanent image.

Ag⁺Br^-(crystal) + hν (radiation) → Ag⁺ + Br + e^-

The silver ion can now combine with the electron producing a silver atom.

Ag⁺ + e^-  → Ag

The light sensitivity/radiation of a silver halide film (ie the film speed/ASA) is linked to the size of the grain and to the compostion of the halide. Generally, as the grain size in the emulsion increases so to does the effective light sensitivity. However this is only up to a point. An optimum grain size exists for a given sensitivity because the same amount of silver atoms are needed for the developer to start reduction of the whole grain despite the size of the grain, so that producing larger grains reaches a point when the sensitivity does not increase significantly enough to warrent an increase in the size.

All photographic emulsions contain various sizes of crystals, but in a given emulsion the size range is from less than 0.1micron in slow emulsions like paper prints to a few microns in fast negative emulsions.

The action of light means that development of negatives and positives must be done in the absence of light. The processes involving the positives are in fact done under low intensity red light with the development of negatives in totally darkess. Safe Light is low intensity red light used in dark rooms. The absorption of silver halides drops off sharply at about 500nm and red light is 700+ nm so this longer wavelength allows a certain amount of time in which the positive will not be affected.

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