Photochemical Photography (Film)

A widespread technology for recording the information at the focal plane of a lens is something that everyone is extremely familiar with - film. Although it may be a common, everyday item, its roots stem directly from various, typically complicated chemical processes. However, in order to understand how film "records photons", it is important to understand certain aspects of the underlying architecture of film.

Each roll of film that would be typically purchased on the market today has dozens of extremely thin photochemical layers stacked on top of a layer of a base material such as celluloid or polyester (The Photographic Process 1, How Photographic Film Works 4). The base is essentially the component that all the other layers and chemicals are adhered to. This base element only exists because otherwise the film would be so fragile that it would tear apart inside the camera due to mechanical strain. It has no effect on the overall photochemical processes. The second component of film is a rather surprising one - gelatin. This component is used to bind all of the individual photochemical layers together and, ultimately, to bind these layers to the base material (How Photographic Film Works 4). An illustration of these layers can be seen below in figure one.

Figure One (Illustration of the typical layering scheme present in photographic film)

(Image courtesy of The Photographic Process (Dr. Gambhir))

In the figure above, the supercoat is typically a protective coating that keeps the film from being damaged during development of the film. The emulsion layer consists of the few dozen layers that are key to the photochemical processes. The adhesive is the gelatin, and the base is the celluloid.

The most interesting part of the film, of course, is what lies within the layers contained in the emulsion. Within a few of these one to two dozen layers are small silver-halide crystal grains, which are typically created from bromide, chloride, and iodide (the number of layers typically varies between different manufacturers/types of film). These crystals are the workhorses of the photochemical world, and are directly analogous to the pixels on CCD chips. Like CCD's, some of the crystals need to be sensitive to red, green, and blue. However, Silver-halide crystals are only naturally sensitive to blue light, so special organic molecules called "spectral sensitizes" are added to the surfaces of the crystals. These organic molecules increase the silver halide crystal sensitivity to red, green, and blue light, depending on which organic molecule is added. These molecules must then release an electron to the silver halide crystal when it is struck with red, green, or blue light (How Photographic Film Works 4).

In order to understand just how this electron is freed and where they are freed from, consider a typical silver bromide crystal lattice as seen below in figure two.

Figure Two (Crystal lattice of a typical silver bromide crystal)

(Image courtesy of from Influences of Silver Halide Crystal..)

This lattice contains ions of bromine and silver (Br- and Ag+). When incoming light hits this crystal lattice, the extra electron contained on the bromide ion is released. This free electron then jumps from the now bromine atom to the positively charged silver ion. As a consequence of this electron coming into contact and binding to the silver ion, the silver ion is transformed into metallic silver (Ag). This creates a small region of silver metal (CS39J Session Seven 1). When this is occurring all over the film at different regions of the focal plane and at different intensities, a latent image is produced out of silver atoms (essentially creating an extremely faint black and white image). A diagram of this electron becoming dislodged can be seen below in figure three.

Figure Three (Illustration of a photon hitting a bromide ion and freeing an electron)

(Image courtesy of CS39J Session Seven)

Once this silver atom has been left behind, development of the film enhances this latent image and produces the colors from the different layers of film. Color is produced from development from the fact that one layer contains silver halide crystals that have been organically sensitized to red, another to blue, and another to green. Development of the film takes into account the concentrations of silver atoms at each location everywhere on each layer. A region with a higher concentration of silver atoms in the red plane as opposed to blue and green will turn up as a red color when developed. As an example of the concept, imagine three of these crystals stacked on top of one another within the different planes of the film (let's say the top one is sensitive to red, the middle to green, and the bottom to blue). If a photon of red light (i.e. if a photon that contains the energy level of that found in red light, related to E=hf) hits the stack of three crystals, the crystal most sensitive to red will be activated and will thus create a silver atom in that location of the plane. When this region is then developed, the red will be present at that location (i.e. a pixel location).

As stated previously, there are other varieties of crystal structures found in common films. Another such crystal lattice (silver iodide) can be seen below in figure four.

Figure Four (Crystal lattice of a typical silver iodide crystal)

(Image courtesy of from Influences of Silver Halide Crystal..)

There are some interesting properties that are naturally inherent to the lattices seen in figure four and figure two. For instance, these lattices can be made larger or smaller, depending on the will of the film manufacturer. Larger crystals will allow for the film to be exposed more quickly, but at the cost of decreased resolution (i.e. pixel density, as a larger crystal will be absorbing more surface area on the focal plane and hence the pixel/surface area ratio will be smaller) while smaller crystals expose more slowly, but offer higher resolution (higher pixel/surface area ratio) (The Photographic Process 1). However, these exposure times are relative, as small crystals with certain chemicals fused into their lattice will expose faster than a larger crystal without such a chemical fused into it.