Hello biscuits! In this blog series, I (Chris) am going to take you through some of the fundamental science of film photography. By trade I’m a chemist, and I want to share the glorious science behind analogue image making.

Part I - The Latent Image

Sound of silver talk to me, makes you want to feel like a teenager

A wee refresher if you’ve not read about atoms, electrons and bonds since high school!

• Atoms are the smallest pieces of matter possible via chemical reactions. They are the matter that make up the known universe, and are made up protons (positive charge), neutrons (no charge), and electrons (negative charge). I won’t be going into subatomic particles here!

• An element is a substance made up of a single type of atom. For example, things like iron (Fe), neon gas (Ne) or silver (Ag).

The single fundamental elemental property exploited to make images is the light sensitivity of silver salts (more commonly called silver halides). In chemistry a salt is defined as a compound consisting of oppositely charged ions — most commonly, a metal (positive) and a non-metal (negative).

Silver halides refers to the group of silver compounds made up of silver (herein referred to by its chemical symbol Ag) and halogens -- bromide (Br), chloride (Cl), and iodide (I).

These three halides, AgBr, AgCl, and AgI are the compounds that form images on film. A very simple way of describing film is an emulsion of silver halides sandwiched between gelatine and coated onto a plastic base. Here’s a schematic showing a cross-section of black and white film. 

We’ll focus on silver bromide (AgBr), as it makes up the bulk of the silver halides used in the photographic process, and explore just how light causes an image to form.

Silver bromide crystals are structured as a cubic lattice structure (shown below). These individual crystals are called grains (see, we’re getting there!). The following diagram shows a perfect crystal example, which does not readily occur in reality. It is actually the faults (specifically, the Frenkel defect) that allow the photographic process to occur. 

The Frenkel defect describes the occurrence of areas of the grain with higher negative (Ag-) and positive (Ag+) charges. 

Now, when struck by photons (i.e. light) the grain will absorb one or more photons. Via energy transfer, this results in a free, fast-moving electron (remember, electrons have a net negative charge) that gets trapped in one of the Frenkel defect spaces mentioned earlier. Think of the Frenkel spaces as a crack or gap in the floor, with the electrons being the dust pushed over it, and the broom being your camera shutter.

The neutral Br atom is ejected and absorbed by the surrounding gelatine. What we have now is a neutral silver atom on the surface of a grain. This now forms what we call the latent image, and this is what is present on unprocessed film -- that is, the film that you give us to process and scan!

The amount of photons it takes to form this metallic silver depends on the makeup of the emulsion (hence, different film sensitivities). The minimum number of metallic silver it takes to form a latent image in a grain is between 5 and 50 silver atoms, in a single grain made up of ~10^12 atoms. 

Quite incredible to think that so few atoms can end up being a photographic image!

That’s all for this instalment. In the next part I’ll cover the development process, and the science behind turning the latent image into a negative! 

Any questions, criticisms or feedback, just hit the comments section.

Bibliography

Gee, Phil. “Phil G's Photo Workshop Notes.” Photography a Vehicle For Learning, http://philipgee.com/indexP.html. Accessed 25 8 2021.

Malinowski, J. “The Role of Holes in the Photographic Process.” The Journal of Photographic Science, vol. 16, no. 2, 1968, pp. 57-62, https://doi.org/10.1080/00223638.1968.11737436.

Meredith, W. J., and J. B. Massey. Fundamental Physics of Radiology. Butterworth-Heinemann, 1977, https://doi.org/10.1016/C2013-0-13302-8.