We have already discussed the ASA and DIN methods for characterizing the sensitivity (or "speed") of photographic film. In this topic we describe some of the chemical processes that are involved in photography.
Almost all films use light-sensitive chemicals based on silver halides that are coated on an inert base material such as glass or plastic. (A silver halide is a compound that includes silver combined with one of the four common halogens: fluorine, chlorine, bromine or iodine. Silver fluoride has very little light sensitivity, and many films use either silver chloride or silver bromide.) Most modern films use coatings based on gelatin emulsions: relatively viscous substances with tiny grains of the silver halide dispersed throughout the material. (Think of something like mustard.) The size of the silver grains varies somewhat among different films, but they are typically a few microns in diameter (1 micron= 10-6 m= 0.000 04 inch). Given this size, there are something like 10 million grains in a typical 35 mm negative.
Silver halides by themselves are not very sensitive to the longer wavelength portion of the spectrum (reds and yellows), and the original halide-only films were sensitive only to blue. The emulsion in modern films also contains additional chemicals to make the film sensitive to the longer wavelengths as well. This type of film is called panchromatic (sensitive to all colors). All general-purpose black and white film is of this type. The older orthochromatic films, which are sensitive to blue and green but not to red, are still used for special applications. Even some panchromatic films may be somewhat more sensitive to blue than to red, so that objects with lots of blue in them (like the sky) appear to be too bright. For this reason, many photographers use orange or yellow filters in front of the lens when using black and white film. These filters remove some of the blue, and balance the exposure of the blue sky and the non-blue ground.
All of the silver halides are unstable to some degree. They will decompose spontaneously liberating free silver, so that any film that uses these chemicals has a finite shelf life. Cooling the film decreases the rate of spontaneous decomposition, so that many photographers keep film in a refrigerator or freezer until it is used. The silver halide molecules are decomposed when the molecules are struck by light, liberating free silver atoms. The number of grains which have atoms of silver formed in this way is proportional to the intensity of the incident light. (Recall that the proportionality is logarithmic rather than linear.) In general, only a relatively small number of atoms of free silver are formed in this way, so that there is no visible image on the film at this point. Since the incident light is divided among all of the grains, there is an inverse relationship between grain size and sensitivity (as measured by the ASA number, for example). Other things being equal, a film with smaller grains will have greater resolution (because the resolution in the image is directly related to the size of the grains) but a lower ASA value, since the intensity striking any one grain will be less.
The film is "developed" by placing it in a solution that would eventually decompose all of the halide molecules, liberating free silver. The rate at which this decomposition proceeds depends on how many free silver atoms are already present in each grain, so that the developer tends to produce more free silver at those locations where the exposure to light had already started the process. If the development process is stopped at the proper moment, only the silver halide in those grains that initially had atoms of free silver (as a result of having been exposed to light) is converted to metallic silver. Grains that had no free silver initially are not altered. If the developer and the residual unaltered silver halide are removed, the result is a "negative" silver image. This negative image has lots of opaque free silver where the incident light was strong, some free silver at points where the light was weaker and only the transparent backing material where there was no incident light to begin with. The negative is "printed" by shining a light through it onto a second piece of photographic paper. The development process is repeated, the image is inverted again, and a positive image results. The gray scale both in the negative and in the final print is produced by varying the amount of opaque free silver.
"Reversal" films can produce a positive image in one step. These films are used for preparing slides and transparencies for projection. The reversal to produce the positive image is accomplished as the film is developed. The process of developing the film starts the same way as for a negative image described above. After the developer has converted the latent image to one containing free silver at each point where light struck the film initially, the silver is washed away (the process is often called “bleaching”) leaving only unmodified silver halide where there was no light initially. This remaining unmodified silver halide is exposed to a second flash of light. The process of developing the film is repeated, and now free silver is formed at these locations where there was no light initially. The development process is stopped at the appropriate point and the remaining steps of washing and drying are the same as above. When this film is placed in front of a projection lamp, the free silver formed during the second exposure blocks the light and produces a dark point on the screen. The points which had free silver formed during the first exposure are transparent because that silver has been washed away. The result is a positive image.
Black and white reversal films exist, but they are not very common any more. They were used to produce slides of black and white subjects – the kinds of things that are now done by electrostatic copying onto transparent plastic sheets which can be displayed by view-graph machines. The same sort of reversal process is used to produce color slides, as we will discuss in the next topic.