Autochrome

The autochrome process was the first commercially successful colour photographic process and was introduced in 1907. It produced a colour image directly on a glass plate and marked a major milestone in the history of colour photography.

Autochromes are direct positive transparencies. The process is based on a glass plate coated with a mosaic of microscopic potato starch grains dyed red-orange, green, and blue-violet. This colour screen is overlaid with a panchromatic silver halide emulsion. During exposure, light passes through the coloured starch grains before reaching the emulsion, recording colour information through additive colour synthesis.

After exposure, the plate is developed using a reversal process that converts the negative image into a positive. The result is a colour transparency that must be viewed using transmitted light. Because the colour information is embedded in the starch grain screen, autochromes have a characteristic soft, grainy appearance.

Autochromes required long exposure times compared to black-and-white processes and were therefore mainly used for landscapes, still lifes, and carefully posed portraits. Despite these limitations, the process was widely adopted by amateurs and professionals until the 1930s.

In stereoscopy, autochromes were used to create colour stereoviews of exceptional visual richness. Their thickness and viewing requirements led to the development of specialised stereoscopes, designed to accommodate autochrome plates and provide transmitted illumination.

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