Why lm was in fact digital, electronic photography is analogue, and how it might turn digital again Back to the future
The rst usable digital photography technology was developed by Louis Daguerre in 1839. The sensor was a silver-coated plate exposed to vapour of the halogen gases. Each droplet of the vapour would form a tiny crystal of silver halide on contact with the silver, to form the picture element or ‘pixel’. These pixels utilised the photoelectric effect, whereby the energy of a photon incident on an atom is dissipated to release an electron. If this atom is part of a silver halide crystal then the electron replaces a halide ion, resulting in an atom of silver.
At this stage the image was ‘latent’ or invisible. To make it visible, the sensor was exposed to mercury vapour, which on contact with silver forms a dark mercury-silver amalgam. The size of an amalgam particle formed depends on the mix of silver and amalgam. If it favours mercury, large particles are formed. Where silver predominates, they are small. The outcome is that dark areas with little silver become covered with dark amalgam, whilst light areas are bright. In effect, the mercury amalgam has counted the number of pixels exposed to light, which is why this is a digital process.
Subsequently, alternative processes were developed. The vapour-formed pixels of silver halides were replaced by crystals suspended as an emulsion in gelatin, which became known as ‘grains’. The hazardous mercury was replaced by organic agents which reduced the whole grain to silver. This resulted in an image where bright parts of the scene would be light and light parts dark, known as a ‘negative’. This was inverted by a projection system to produce a positive. But the process was still digital: each grain was either exposed or not.
This technology went through enormous enhancements in the following century and a half. The technology of the halide grains was improved, providing smaller grains, which were more sensitive to the photoelectric effect. The chemistry of development was improved, which allowed grains with smaller amounts of free silver to be converted to silver.
Ways were found to link dyes to the silver grains, resulting in them being colour sensitive, and then coating three emulsion layers to produce colour images.
The rise of analogue
However, after over 150 years of success, in the early 21st century this digital technology began to be replaced by a new analogue technology. It was also based on the photoelectric effect but operating in silicon rather than silver halide. Free electrons released in the silicon resulted in an electrical charge. The sensor was divided into pixels arranged in a regular grid, unlike the random organisation of the digital sensor. This was possible because the charge in each pixel could be measured electronically, meaning that one pixel could register a range of exposure values. This made it an analogue process.
But now, after 20 years or so of analogue photography we might soon see a return to digital. The inventor of the CMOS sensor, Professor Eric Fossum, has for some time been promoting a system in which the size of the pixels is reduced to the point where they become single-bit in nature, that is, either exposed or not exposed. A company called ‘Gigajot’ has been formed to develop this idea, and has recently published papers detailing some initial prototypes, which allow the counting of individual photoelectrons.