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Sept., 1944 REPRODUCTION OF COLOR FILM SOUND RECORDS 211
the color sound track is very high and, consequently, when this cell is used a very strong background noise must be expected, which is even increased as the reproducer gain is raised on account of the low modulation.
Therefore, the two phenomena of low sound modulation and high ground noise are connected with each other, and both have their cause in the improper adaptation of the photocell to the absorption of the dyes composing the sound record. It is clear that the use of other photocells will give much better results. The right column of the table contains the values for a blue-sensitive photocell. It is seen that the modulation is much greater than that of the caesium cell and that the average transmission is decreased. Practice has shown that color-developed sound records can be reproduced well with these cells.
These explanations show why the external photoelectric effect was chosen for sound reproduction from the group of photoelectric phenomena. In addition to the advantage of the great internal resistance of these cells6 they have the further advantage neglected for many years, that the spectral sensitivity of the cell can be changed according to the purpose and depending on the choice of the cathode material or the cathode layer. The caesium oxide cell generally fulfilled the requirements of sensitivity and spectral distribution for the reproduction of silver sound tracks. We disregard here the frequent desire to shift the spectral distribution of these cells beyond 1200 m/x toward the infrared in order to be able to use the red rays of the sound lamps better.
Research in the field of high-sensitive oxidized alkali cathodes, among which is the caesium oxide cathode, was promoted mostly by Asao, de Boer, Kluge, and Gorlich and, referred to the caesium oxide cathode, gave approximately the following picture:7 The caesium oxide layer coated on a silver support contains caesium atoms. Adsorbed caesium atoms are on the surface and decrease the energy necessary to liberate electrons so that the degree of coverage plays an important part. The photoelectric sensitivity may be increased by additional introduction of foreign metal atoms.
The requirement of the photocell with respect to colored sound track, namely, a spectral distribution in the blue and violet part of the visible spectral region, is fulfilled not only by potassium oxide cells, which were studied mainly by Kluge,8 but also by potassium hydride cells, studied bv Elster and Geitel.9 With respect to the