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210 R. GORISCH AND P. GORLICH Vol 43, No. 3
the area between the 2 curves the photocell modulation. For the caesium oxide cell we cannot expect good results on account of the poor relation between the 2 areas.
If we substitute for the infrared-sensitive photoelectric cell, a cell which has its sensitivity maximum in the region of the blue light (Cs, Sb cell), entirely different curves are obtained. These curves are also recorded in Fig. 3, and the sound lamp radiation is considered as before. It is seen that for these cells a much more favorable relation exists between the photocell rest current and the modulation alternating current.
In order to extend these results to the practice, a sound strip was photometered with the same cells. The following values were obtained:
Caesium Oxide Cell Cs, Sb Cell
Per Cent Per Cent
Tmax. 88 60
Tmin. 70 7
AT 18 53
TR 79 33.5
These values clearly explain the low sound modulation with the use of the customary cells. Kuster5 compared in his work the color reversal film with a silver reversal film. He found a lower modular tion for the color film which revealed scattered values even if photocells of one type were used so that, apparently, small differences in individual cells have a considerable effect. However, only caesium cells were studied.
On the question of background noise it has been found previously1 that in the conversion of a silver image to a dye image a change of the background noise occurs. This may be calculated by determining the change in the transmission of the layer. However, this is merely noise from grain, or that part of the background noise which is based on the grain structure and which is heard only with entirely undamaged film. This is less important in practice because the socalled scratch noise, including all the noises due to dust particles, scratches, dirt spots, etc., is normally stronger and increases considerably after the film has been used for some time. This scratch noise is proportional to the average transmission of the sound track, because all dust particles and other irregularities cause a much greater light impulse, if the film is more transparent. The last line of the table shows that for normal photocells the average transmission TR of