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PROCESSING MOTION PICTURE FILM 119
which such light is permitted to operate. At one time it was thought that photographic exposure obeyed the law of reciprocity and that a high intensity, operating for a short period, would produce a density equal to that obtained when a correspondingly low intensity was in operation for a long period. This has since been proved to be incorrect and it is now established that, although in both cases the product of Intensity and Time may be equal, the resultant densities will not be identical.
In attempting to expose successive pieces of film so that a series of densities may be produced it is therefore necessary to decide the precise manner of exposure to be employed. Obviously, an exposure may be of long duration with low illumination, or short duration and with high illumination and, therefore, the type of exposure given when testing film and processing conditions should be related to the exposures the film is likely to receive under practical conditions. Once the type of exposure has been fixed it is necessary to decide which of the two factors shall be maintained constant and which shall be varied by known amounts. For example, it is possible to expose film so that successive strips receive increasing exposures either by maintaining the light intensity at a constant value and varying the time of exposure or by varying the intensity of the light and maintaining the time factor constant.
Instruments designed to give precise exposures to successive small areas on a length of film are known as 'sensitometers' and have been arranged to operate either by varying the light intensity when they are known as 'Intensity Scale Sensitometers' or by varying the time of exposure when they are known as 'Time Scale Sensitometers'. As is well known, the "colour temperature" of a tungsten light source varies according to its intensity and, since monochrome film is not sensitive to all colours to an equal degree, it is therefore an advantage to employ a neutral density wedge to adjust the exposure in intensity-scale sensitometers rather than to vary the current passing through the lamp. Here again it is necessary to employ lamps running at different voltages when films used for different purposes are being tested. It is also necessary to insert filters in the light beam to produce the most advantageous conditions for films of different colour sensitivity.
To obtain a series of increasing exposures is, of itself, of little value unless the amount by which each exposure is greater than that which precedes it is related to the response of the film. As has been shown previously, linear increments in Density are actually Logarithmic increases in Opacity and, therefore, it follows that