Without chemistry, photography, as it is known, would not exist. Photography is a complex array of chemical reactions. Each step, from the manufacture of the film through the final prints or slides being viewed, has its own particular chemical processes.
The film that is used in most cameras is a thin base of gelatin (peptides of collagen) which is bonded to a polymeric plastic sheet cut into strips. The gelatin is hardened during manufacture by cross-linkage of the polypeptides. This is so the gelatin matrix is not damaged by mechanical movement through the camera. The gelatin acts as a support for the light-sensitive material that is used to capture an image. The gelatin protects the light sensitive material and it also enhances its sensitivity. The light-sensitive material used is a silverhalide. With modern camera film silver bromide generally is used. Other photographic emulsions may use other silver halides; for example silver chloride is used for any photography using infrared light. All film is a sandwich with a scratch-resistant coating, an emulsion layer (containing the silver halide), a gelatin support layer, and an anti-halation layer. When light strikes the film the silver halide decomposes to give silver. This is indistinguishable from the rest of the film and is called a latent image.
In order to see the latent image in the film, the image first must be treated or developed. The chemical that does this is called a developer. When the developer is added to the film it converts the silver ions into black, colloidal silver. This is a reduction process. The silver halide not exposed to light remains unaffected at this point. Next, an acidic chemical bath, called a stop bath, is added to halt any further change or development in the silver. The stop bath's acidic nature rapidly neutralizes any alkaline developer which has not been removed. A solution of ethanoic acid is commonly used for this purpose. The next stage of the process is the addition of chemical called a fixer, because it fixes the image permanently in place. The fixer reacts with the unconverted silver halide to produce a water soluble compound. One fixative used commonly is sodium thiosulfate (or hypo). A complex of soluble silver thiosulfate is produced. When the film is washed the last remains of the unexposed silver halides are washed away. This whole process produces a black-and-white negative image of the subject photographed.
Producing color negatives requires a similar process but there are three layers of light sensitive emulsion. Each layer is sensitive to a different wavelength range of light. There is a blue-sensitive layer, a yellow-sensitive layer, and a green-sensitive layer. Within each of these layers there are dyes which are released when the film is developed.
The third type of film, transparency film, produces slides (a positive image on the film). A similar process of development occurs here but there is a two-stage development. First, the black- and-white latent image is developed and then the rest of the unexposed material is chemically fogged. The color dyes then are developed. Afterwards the silver is bleached out using the fixer as before.
These processes provide an image on a piece of film. With the slide film this is the final product. With the negative types of film a positive print is required. The negative is projected onto light-sensitive paper and the paper is then developed using similar processes to those previously described. This gives a positive image of the original subject on a permanently fixed sheet of silver halide-impregnated paper.
There are many variations on the basic pattern above but all photography employing film uses some form of the basic processes outlined. The photographic process was first shown by Louis-Jacgues-Mandé Daguerre (1789-1851) in 1838 and many others have adapted it over the years to give the process that is now used.
This is the complete article, containing 635 words
(approx. 2 pages at 300 words per page).
