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PROCESSING MOTION PICTURE FILM 125
by the surrounding mirror of "G\ Thus an observer at T will see the central spot which illuminates the film to be measured, and the brightness of which may be varied by rotating the wedge 'W, and also a surrounding comparison beam of constant brightness with which the central spot must be matched. Mirror 'D\ glass 'F\ mirror *G' and eyepiece T are all mounted in a casting hinged to the main body of the instrument so that the entire head may be raised to permit the film to be inserted between it and the wedge. Since the foregoing arrangement depends entirely upon the brightness of the lamp 'A', it is necessary to adjust this brightness before a series of measurements can be made. This operation is performed by first rotating the wedge ' W until the setting for zero density appears in the lower right-hand window as shown in the figure. If, under these conditions, the central spot is not of equal intensity with the surrounding field as viewed through the eyepiece, the knurled screw 'K' should be turned until balance is obtained. It will be obvious from the diagram that this action moves the lightsource either nearer to or further from the comparison head and therefore adjusts the brightness of the central spot without altering the surrounding field. The piece of film to be measured is then placed in the centre of the opal bed, with the emulsion side in contact with it. The moving head of the densitometer is then lowered into contact with the upper surface of the film and the circular wedge is rotated until the spot in the centre of the field is neither darker or lighter than the surrounding area and, therefore, disappears. The density of this spot on the film is then shown under the hair-line on the small window seen at the bottom right-hand corner in the illustration.
Sensitometry Applied to Processing
Having described the machines used to produce sensitometric data, it now remains to apply these instruments to laboratory practice. To observe any relationship which may exist between Exposure and Density it is most convenient to express the results graphically as shown in Figure 55.
It has been previously stated that the Kodak Sensitometer makes a series of exposures along a strip of film in such a manner that each exposure step is greater by \/2 than that of its predecessor. If, for the purpose of illustration, we assume that the first step, shown to the left-hand end of the horizontal axis of the graph in Figure 55, is of one-second duration, the second step will be V2 or 1-4142 times greater, the third step will be 1-4142 times greater than the second, and so on throughout the entire