The traditional silver halide camera

Silver halide photography, from its invention in the 1830s, relied almost exclusively on finely ground silver bromide crystals suspended in a gelatine emulsion. These, it was discovered, were altered by the absorption of light so that they could be reduced chemically to black, colloidal silver by a variety of sensitive reducing agents to produce a negative image in the emulsion. This could then be changed, usually by copying to positive images in transparent emulsion or on sensitized paper. The sensitivity was restricted at first to light of short wavelengths in the blue to ultra-violet region of the spectrum until a chemist, H. W. Vogel, at the University of Berlin, discovered the technique of dye-sensitization to make orthochromatic emulsions sensitive to yellow and orange light. Later improvements in this technique eventually made red-sensitive panchromatic emulsions available – at a price – in the early years of the twentieth century. S. M. Prokudin–Gorskii, in particular, led the way by making simultaneous exposures through red, green and blue filters to give negatives, from which diapositives could be made for simultaneous projection with three projectors and the same colour filters, on to a screen, thus producing the first colour photography slide shows.