Printing on silver halide (AgX) based Colour Negative (CN) media is frequently thought of as old-fashioned. Coated paper dates back about a hundred years – a respectable history, during which some of the biggest and best-known global brands (Kodak, FujiFilm, Dainippon) have spent ££-millions and many man-years developing a huge range of media for different applications. Compared with paper, which has been around for maybe 1000 years, photo printing is positively new technology!
Silver halide (AgX) printing with Colour Negative (CN) photographic media works by subtracting colours from white light (subtractive colour system).
Silver halide (AgX) media has three layers of light-sensitive coatings, one each for Red, Green and Blue, one on top of the other like transparency foils, on a backing sheet (white paper for photographic paper, clear substrate for film).
The three coating layers are a gelatine mix, containing grains of dye-sensitised silver halide (∆ AgX) that reacts to Red, Green and Blue wavelengths, surrounded by a large number of colourless “colour couplers” (o).
When the paper is exposed to light, the sensitized silver halide grains in the appropriate layer form a “latent image”, or a small activation of Ag+ ions along the grain edge (●).
Imagine an RGB image with a single pixel of solid Green (R=0, G=100, B=0). As the paper is colour negative, this is inverted before exposure (R=100, G=0, B=100). Or in other words, there is maximum exposure in Red and Blue layers, and none in the Green layer.
This reaction oxidises the Developer, which in turn reacts with the colourless colour couplers around each exposed grain to produce a cloud of coloured dye. The couplers are the complimentary colour to the exposing light – Cyan for Red, Magenta for Green and Yellow for Blue.
Then the print is washed in clean water, removing any residue of the coatings, including the silver, which is recovered via the recycling process.
The resulting print has coloured dyes in the three layers, in amounts proportional to the exposure level in those layers, which act like coloured filters. What we see is the result of the coloured filters removing (subtracting) their complimentary colours.
In this example, Red and Blue exposures create Cyan and Yellow dyes respectively. Cyan dye removes Red from white light, and Yellow removes Blue, leaving just Green – the colour of the original pixel.