Photographic chemistry – the latent image

It was inevitable that at some point I would turn to making my own developers and photographic paper ..

Why? It comes from a basic need to understand in some depth all of the steps involved in making a picture. This first post of a new series looks at some of the basic chemistry involved in producing a film negative. Later in the series I will look at some of the film/developer combinations that I am beginning to use.


Source: L. M. Slifkin Science Progress (1933-)
Vol. 60, No. 238 (Summer 1972), pp. 151-167

If you are like me, the chemistry I learned for GCSE A level and my first year university biochemistry has long faded. Despite this, I think it’s worth revisiting it to appreciate the magic of what is involved in making a negative.

There are plenty of books on photographic chemistry 1 , many written in the heyday of photographic chemistry in the last century, and of course many journal articles.2

At one level, the photographic film process is simple to describe. Film is made from light sensitive chemicals (silver halides). When film is exposed to light the invisible silver halides are turned to visible silver ( ‘reduced’ to silver) in proportion to the amount of light being reflected from a scene passing through a lens. Special chemicals called developers are used to reduce the silver halide into silver. They are special in the sense that they only convert the silver halides that have been energised by the light. As a film photographer this explanation suffices.

What is interesting, though, is that there are still many things that are not properly understood about the process. Below I look at the first mechanism in the three phase process that constitutes image formation: the latent image phase. In the next post I will look at the other two phases: silver image and, finally, fixed image phases (stabilising it from further change) in line with the following schematic.

Source: M Sahyun, Mechanisms in photographic chemistry
Volume 51, Number 2, February 1974 / 77

Stage 1: the latent image phase

Photographic emulsion is made by combining silver nitrate with a halide (Potassium Chloride or Bromide or Iodide or a combination) to give a silver halide with a nitrate by-product. This is done in high grade gelatin to give the silver halide a substrate, enabling a smooth dispersion of the halide across the acetate of the film (or the paper of photographic paper). It was discovered that suspending the halide in the gelatin was not sufficient. The halide had to be formed in the gelatin. Varying the factors in play such as temperature of addition, halide mix, halide grain size, viscosity of the gelatin etc all make for a diverse range of emulsions with different properties.

The silver halide conforms itself into lattice crystals.

Schematic of structure – but it is more dynamic than this diagram suggests

There are imperfections in the lattice structures, particularly when Silver Iodide is present. These imperfections are important to how these lattices behave when exposed to light 3.

When the lattice crystal is exposed to light (through a camera lens, say) a small cluster of silver ions on the edge of the crystal are primed. The density of primed clusters is proportionate to the amount of light striking the crystals. At this stage the image is latent, awaiting development with a suitable chemical.

But what is a latent image? The mechanism is still not fully understood and competing views exist. A more detailed explanation follows but in essence the simplest way of understanding it is to think of Silver atoms as having different energy levels ( and therefore different structures) depending on the environment they are in. When a silver atom in a halide structure is energised by light, it adopts a state that leans towards breaking free as metallic silver. It is predisposed to being converted to metallic silver by a reducing agent (developer). Diagramatically,

Source: The Chemistry of Photography
by Nichole Marie Witten
University of South Carolina

In more detail: to understand this we need to touch upon the conductive properties of silver halides. The lattice can be thought of as having two energy levels – a valence band in which the electrons are localised around some atoms and a conduction band in which mobile electrons contribute to electrical conductivity. Between the two bands, imperfections in the lattice structure or impurities within it create ‘electron holes’. It is thought that when light energises the lattice, electrons are promoted from the valence band to the conduction band, creating positive holes in the valence band. Some electrons get trapped in the holes between the two bands or even in the valence band and these react with freely moving silver ions to form metallic silver. Metallic silver in its turn becomes an electron trap and the process is repeated. One atom of silver begets two, then four and so on. To understand this one has to give up the notion that a crystal is a fixed structure – rather it is a dynamic structure in which silver ions and electrons flow and in which there are topological differences in sensitivity between the surface of the crystal and its interior.

The process is described in the following diagram from Gurney, R. W., and Mott, N. F., Proc. Roy. Soc. (London), Ser. A, 164, 151 (1938); Mott, N. F., and Gurney, R. W., “Electronic Processes in Ionic Crystals,” Oxford University Press, London, 1948.
4.1: Light splits out an electron and gives a silver ion. 4.2: an electron is trapped in a hole. 4.3: a trapped electron finds a moving interstitial silver ion. 4.4 to 4.7: silver is formed which also acts as a hole. This begets another silver atom in state (0).

The effectiveness of silver halide lattices as photo-sensitive materials depends on the mobility of electrons, interstitial silver ions and also the holes in the valence band (and therefore imperfections in the structure). The state of the silver (Ag0) is such as to produce a latent image that is not yet visible.

Next post

As mentioned, there are a number of different views about how the latent image is formed and how development of the image is triggered. In the next post in this series I will describe the competing claims about the development of the silver image from the latent one and also go into the different types of developing agent.

Show 3 footnotes
  1. the‘ text book is Mees, C. E. Kenneth; “The Theory of the Photographic Process,” The MacMillan Company, New York, 1942
  2. For example: Basic Chemistry of Photography; Baker, T. Thorne; “Photographic Emulsion Technique,” American Photographic Publishing Co., Boston, 1948; * Bullock, E. R. “Chemical Reaction of the Photographic Latent Image,” Eastman Kodak Company, Rochester, N. Y.; D. Van Nostrand Company, New York City, 1927; * Edwal Laboratories; “Modern Developing Methods,” 3rd edition, Edwal Laboratories, Inc., Chicago, 1944; * Jacobson, C. I.; “Developing, The Technique of the Negative,” Focal Press, London and NewYork, 1948; * Lester, Henry M.; “Photo-Lab-Index,” Morgan & Lester, Publishers, New York, (revised quarterly); * Lowe, Edmund W., Ph. D.; “Developers, Fine Grain and Otherwise,” Camera Craft Publishing Company, San Francisco, 1939; * Matthews, Glenn; “The Chemistry of Photog raphy,” Complete Phot. No. 11, pp. 702-718.
  3. Dislocations in Crystals of Silver Halides: J. W. Mitchell Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Jun. 10, 1980, Vol. 371, No. 1744, The Beginnings of Solid State Physics (Jun. 10, 1980), pp. 149-159Published by: Royal SocietyStable URL:

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