Forgotten Colours of the Past

These early colour photographic processes were based on a fine colour filter, or screen, made up of red, green and blue elements. The screen was placed directly against a panchromatic black-and-white photographic plate or film; in some later materials it became a permanent part of the photographic material itself. The prepared material could be loaded into an ordinary camera and a colour photograph taken in a single exposure. Light first passed through the colour screen and only then formed the black-and-white photograph. Each microscopic area therefore recorded only one colour component. After processing, the black-and-white image was combined again with the corresponding colour screen, and millions of tiny points blended together when viewed to form a single colour photograph.

The dyes used in the screens, however, age. Surviving originals therefore often look very different today from when they were made. Light also accelerates their deterioration, so historical colour transparencies cannot safely be exhibited for long periods. Digital reconstruction allows us to show their original appearance without causing further damage to the originals.

From the 1930s onwards, additive photography was gradually displaced by subtractive colour materials, most notably Kodachrome and Agfacolor Neu. The idea of a colour mosaic did not disappear, however — it returned with digital photography. A digital sensor is essentially monochrome and records colour through a fine Bayer filter: each sensor element records only one colour component, and software constructs the final image. Many modern algorithms have therefore been developed specifically for processing such mosaiced data.

The colour reproductions shown in this exhibition were created using Color-Screen, specialised software that applies these modern methods to reconstruct the colour information in historical photographs as accurately as possible from high-resolution scans. Unlike later subtractive colour materials, this is possible because the colour information is encoded in a remarkably stable black-and-white photographic record. It can be thought of as an analogue counterpart of today’s RAW data. Color-Screen therefore adapts modern RAW-processing algorithms to historical screen photographs. Their original fine colour mosaics are also extremely difficult to reproduce directly by digital means: present-day displays lack the resolution and brightness required to render the screen faithfully.

Color-Screen is an open-source project developed by Jan Hubička of the Šechtl & Voseček Museum of Photography, the Faculty of Mathematics and Physics at Charles University, and SUSE, in collaboration with his students, particularly Linda Kimrová and Melichar Konečný. The source code and descriptions of the methods are publicly available, allowing the results to be verified, the techniques to be further developed, and the software to be applied to other historical collections.