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Photography

Making every photon count

Nature volume 402, pages 856–857 (1999)Cite this article

More than 150 years after the invention of photography, the underlying solid-state photochemistry is still being investigated and improved. One aspect of continual importance is the efficiency of the image-recording stage, which strongly affects the sensitivity of the photographic emulsion. Photons falling on a silver halide microcrystal in the emulsion cause a halide ion (such as chloride or bromide) to lose an electron, which is then captured by a silver ion, converting it into a neutral silver atom. The build-up of silver atoms gives rise to silver clusters: the ‘latent image’, which can be developed after several more steps into a print1. In reality, the halide ions generate pairs of electrons and holes (the positively charged counterparts of electrons), and it is generally accepted that recombination of photogenerated electrons and holes is a major loss process2,3. On page 865 of this issue, Belloni et al.4 present an ingenious way of overcoming this recombination process by doping the silver halide with formate ions, which leads to an increase in the number of silver atoms generated per photon absorbed.

Image-forming reactions in silver halide microcrystals compete with various electron-loss processes, the dominant one being recombination. Overall, many of the photogenerated electrons are lost and the sensitivity of the silver halide emulsion is low. The sensitivity can be greatly improved by chemical sensitization, which is how all modern films and print materials are prepared. During chemical sensitization, reagents containing labile sulphur and gold atoms are used at low concentrations (parts per million or lower) to produce silver–gold sulphide clusters on the surface of the microcrystal. These clusters appear to deepen the existing traps for electrons, thereby reducing recombination5,6,7.

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Figure 1: Colour photography requires three layers of silver halide emulsion sensitive to blue, red and green wavelengths of light.

References

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  1. the Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, 14623, New York, USA

    Richard Hailstone

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Hailstone, R. Making every photon count. Nature 402, 856–857 (1999). https://doi.org/10.1038/47198

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