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20 PRINCIPLES OF CINEMATOGRAPHY
The point which concerns us at the moment is that some chemical action must have taken place in the film emulsion whilst it was exposed to the rays of light passing through the lens. The film emulsion carries a suspension of thousands of minute silver bromide crystaline grains which, on exposure to light for any appreciable period would automatically blacken (this is readily proved by leaving a waste piece of film in a lighted room for some time). The amount of exposure given to a film by rapidly opening and closing the shutter is not, of itself, sufficient to produce a visual change, but merely produces a 'latent' image. If, after such an exposure, the film is then taken into a dark-room and placed in a suitable solution, called the developer, the silver bromide grains which were affected by the light will be reduced, by the action of the developer, to metallic silver and will, of course, become black.
Since only those grains which receive light will be so affected, and the density and concentration of the blackening will depend upon the intensity of the light, a negative image of the scene will be formed and, if the amount of exposure has been correct, the distribution of the densities will bear a true relationship to the amount of light reflected by the various sections of the scene.
So much for the actual developing process. It must now be remembered that other sections of the film, which did not receive any illumination, still contain silver bromide grains which, if left in their present state, will blacken when the film is taken out into daylight. Because of this the film must first be immersed in a bath of water, to remove any developing solution which may still be on its surface, and then be placed in a 'fixing9 bath. This is basically a solution of Sodium Thiosulphate, and is capable of dissolving only those sections of the emulsion which have not been reduced to metallic silver. After this process the film will consist of areas of transparent film base plus areas of varying degrees of density which, together, form the image of the scene. It now only remains to wash and carefully dry the film.
When the film is dry it may be placed in contact with a piece of printing paper, that is, a piece of paper on which has been coated a light sensitive emulsion, similar to that on the film. The emulsion surfaces of both the film and paper should be facing each other. Light is then directed at the film side of this combination so that the paper is exposed through the film and by amounts inversely proportional to the densities of the film emulsion. By processing the paper in a manner similar to that undergone by the film, a positive image of the scene will be produced.