The Joly colour process is an early additive colour photography process devised by Dublin physicist John Joly in 1894.
Based on a method proposed in 1869 by Louis Ducos du Hauron in Les Couleurs en Photographie – Solution du Probleme, the Joly colour process used a glass photographic plate with fine vertical red, green and blue lines less than 0.1 mm wide printed on them. The plate acted as a series of very fine filters, in a similar way to the later Paget process.[1]
To take a photograph, the filter screen was placed in the camera in front of an orthochromatic photographic plate, so that the light passed through the filter before striking the emulsion. After exposure, the plate was processed and contact-printed on another plate to make a positive black-and-white transparency. This was then placed in register with a viewing screen of the same type as used for exposure, to produce a limited-colour transparency that could be viewed by transmitted light.
The Joly process was introduced commercially in 1895 and remained on the market for a few years. However, it was expensive and the commercially available emulsions of the time were not sensitive to the full range of the spectrum, so the final colour image could not achieve the look of "natural colour".[1]
A large collection of colour slides by John Joly, mainly of botanical subjects, are held by the National Library of Ireland.