Science: Photography

Photography is facing a new development almost as far-reaching as the change from tintypes to cellulose film. In the 103 years since Louis Jacques Daguerre invented the daguerreotype the use of light-sensitive silver salts has been the basis of photography. Now silver is being replaced by coal-tar dyes. So far this new dye-treated film has been used only to make duplicates of sound tracks from regular film. But scientists believe that dye-treated film for direct photography is in sight.

In ordinary photography silver bromide, emulsified in gelatin, is spread on a transparent cellulose-acetate film, forming a photosensitive layer on one side. When exposed to light, the bromide becomes activated so that it is easily reduced to metallic silver by a developer. The grains of metallic silver remain as black spots after fixing, so that the image is a “negative”—darkest where the light was strongest.

In the new dye-treated film, there is no surface emulsion and no silver. Instead, a strip of cellophane is thoroughly impregnated with a mixture of “diazo” compounds which are closely related to dyes, but are only a faint yellow in color. Wherever light strikes, the diazo compounds are quickly and effectively bleached. Developing and fixing are combined in a single operation (exposure of the film to ammonia fumes), which may be done in subdued daylight. This exposure does not change the portions of the film that light has reached, but the ammonia turns the unexposed diazo compound red (or some other color, depending on the particular compound used). Result is a positive with the image on the inside instead of on the surface of the film.

For sound-track recording the contrasts are clear and sharp. Since no complicated chemical treatment is involved, the film can be exposed, developed, finished in a continuous operation at 80 feet per minute. Since the image is inside the strip, not on the surface, scratches and dust are no problem. The cost of the dyed film is one-tenth that of silver, and because of its sharpness can carry three times as much sound track per inch.