Silver/dyes decoupling and colour restoration of autochromes using multispectral imaging

Abstract

Autochrome glass plates, patented in 1903, are one of the earliest colour photography processes. By overlaying a silver emulsion and a screen of randomly placed minuscule starch granules, coloured with orange-red, green and violet-blue dyes, colour images were obtained. Historically, autochromes were viewed by means of light projection, which led to the alteration of the trichromatic screen due to the low lightfastness of the colour dyes. In this article, we propose two methods for the colour restoration of commercially available autochrome plates. Consisting in virtually separating the colour mosaic layer from the silver emulsion, the first approach uses band subtraction to isolate the individual colour dyes, and the second approach is based on linear spectral unmixing. The restoration approaches showcase the colour revival of autochrome plates that enhance the colours and uncover faded details in these historic photographs. The decoupled silver and dye images were also reproduced as translucent prints and integrated into an interactive museum display, allowing visitors to engage with the restored autochrome photographs in a tangible way.