Screen process

The screen process was used by early colour photography systems based on additive colour. The separation of the three primary colours was achieved with a transparent colour filter screen containing a mosaic of grains or a grid of fine lines and/or dots in red, green, and blue.

During exposure in the camera, the screen functioned as a colour-separation filter that exposed the monochrome emulsion through red, green, and blue elements. Light passed through the colour screen before reaching the emulsion. After development into a positive image, the same screen served as a colour filter that additively coloured the transmitted white light. The colour screen, combined with optical mixing in the human eye, produced the perception of a full-colour image.

The basic principles of the screen process were patented by Louis Ducos du Hauron in 1868. The concept was first implemented by John Joly in 1894, and the Joly process was commercially introduced in 1895. Other colour processes using the screen principle include autochrome and omnicolore.