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Sept., 1944 REPRODUCTION OF COLOR FILM SOUND RECORDS
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tic MI, Fig. 2 shows the same measurements converted from density to transmission. The distance between the 2 curves at any wave length represents the greatest possible sound modulation at that wave length for this type of track. Therefore, the region between the 2 curves is crosshatched in Fig. 2. If we start from the premise that the spectral sensitivity of the photoelectric cell must be adapted to the region of greatest possible modulation, we should use for this film a photoelectric cell which has a sensitivity only in the visible region, especially at about 650 mju. However, this is not the case in modern photoelectric cells. Their sensitivity maximum lies in the
300
FIG. 3. Relationship between photocell current and wave length
. for Agfacolor sound track and 2 photocells of a different type. The
spectral sensitivity curves of the 2 photocells as reduced to equal
energy input and the spectral characteristic of the exciter lamp are
also shown.
infrared, therefore, in a region in which the maximum density of the sound record is low, their transmission, therefore, is very great. In addition, the sound lamp radiates more strongly in the infrared than in the visible spectrum. If we multiply for each wave length the 3 factors influencing the magnitude at the photoelectric current, that is, film transmission, sensitivity of the cell, and sound lamp radiation, as is done in Fig. 3, we obtain the spectral distribution of the product for these cells as shown by the curves of Fig. 3 represented by the designation caesium oxide cell. The 2 cases of maximum and minimum density are shown. The area lying between such a curve and the abscissa corresponds to the total current flowing through the photoelectric cell. The area between the average of the 2 curves and the abscissa represents, therefore, the average photocell current,