Photography reinvented Principles of photography overturned by a new one-step process. 9 August 2002 Reported by Philip Ball in 'Nature' Scientists have reinvented the 200-year-old process of photography. A new one-step method could make high-resolution colour prints from digital images. Since its invention in the early 1800s, photography has involved basically the same process - using light to convert silver salts into dark particles of silver metal on film. John Marshall and colleagues at the Polaroid Corporation in Waltham, Massachusetts, have effectively replaced silver by acid. Exposure of the photographic film to light produces acid, which then converts colourless dye molecules into coloured forms1. The whole process, called acid-amplified imaging (AAI), takes place in a single sheet of film. It requires none of the 'wet' processing conventionally used to develop and fix photographic images. At this stage, AAI film isn't sensitive enough for snaps in ordinary daylight, say the team. But it can be triggered by strong light sources such as lasers and light-emitting diodes. It would be ideal for applications such as digital printing, producing a colour image directly from an electronically controlled light source. One of AAI's biggest potential advantages is that it can generate extremely sharp, high-resolution colour images. In conventional photography, the resolution limit is set by the size of the silver grains. Reprinting history For years, researchers have been trying to eliminate the messy wet chemistry of conventional darkroom processing. In the early years of the science, photographers had to carry with them an entire miniature chemical laboratory. But despite impressive advances in photographic technology, today's photographers still use roughly the same technique as at its conception - when Frenchman Joseph Nicéphore Niépce captured the view from his window near Chalon-sur-SaôÔne in 1816. Light falls on a film impregnated with a silver salt, transforming the silver ions into neutral silver atoms. These then aggregate into tiny particles of silver. The developing process 'amplifies' the exposed silver particles, growing them into grains big enough to appear dark. In colour film there are three layers of light-sensitive material, each containing a coloured 'sensitizing' dye that absorbs light in the red, blue or green region of the spectrum, which make up nearly all colours. A colour negative is then made by depositing coloured dyes within each layer, which either stick to the silver particles or lodge in the unexposed regions. The silver is removed, leaving the dyes behind. These various stages involve wet chemical treatment of the film - even in instant Polaroid cameras, where it all happens in situ. The Polaroid team's new process uses the same three-layer principle - but the dyes that form the colour image are switched on by acid. First, light exposure splits apart a compound called an iodonium salt to make a 'primary acid' in the AAI film. Sensitizing dyes ensure that the salt is split only by red light in the red-sensitized layer, and so on. Next, the small amount of primary acid is increased as it catalyses the formation of a 'secondary acid'. Once amplified, the acid converts colourless dyes dispersed throughout the film into coloured forms, a different colour for each of the three layers. This all happens automatically once the film is exposed.
