The screen process is used by early colour photography processes that were based on additive colour. The separation of three primary colours is effected by a transparent colour filter screen that is covered with a mosaic of grains or a grid of fine lines and/or dots in three primary colours.
During the exposure of a negative with a camera, the screen acts as a colour separation filter and exposes the monochrome emulsion with red, green and blue. The light first passes the colour filter before it reaches the emulsion. After the development to a positive image, the same screen acts as a colour filter to additively colour the white transmitted light in red, green or blue colours. The colour screen and the optical mixing of the human eye creates a full colour image.
The basic principles of the screen process were patented by Louis Ducos du Hauron in 1868. The principles were implemented for the first time by John Joly in 1894 and the Joly process was commercialy introduced in 1895. Other colour processes that make use of the screen process are Autochrome Lumière and Jougla Omnicolore.
Source: Wikimedia Commons, License: public domain