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1949 LEAD-SULFIDE PHOTOCONDUCTIVE CELLS 701
subsequent paper will treat this situation more thoroughly than is possible here, where our main interest lies hi the characteristics of this one particular type of cell.
No direct measurements have been made, or at least none have been published, of the transmission of the composite dye track in the wavelength region beyond 1 micron. Transmission curves have been published7 for the region below 1 micron. In the near infrared, all gelatin dyes are known to be transparent. It is not surprising, then, that there has been concern over the use of the lead-sulfide cell with this track. On the other hand, it is practically impossible to secure samples of the track on the open market, and there is a strong indication that the film companies do not consider their present dyeimage tracks commercially feasible. However, the collection of data on the use of this cell with color film of all types was considered important.
There are four types of sound track in use at the present time with color film: (1) Metallic silver. In this case the sound is printed first, then the emulsion is cleared, and the color picture printed by the regular three-layer process. (2) Silver sulfide, which may be applied by a rather complicated edge treatment after the picture is printed. (3) Ferri-ferrocyanide, which is the cyan layer of the twocolor processes. (4) Dye image, as it is called, which is usually a composite of the three layers in the familiar tripack color film.
It is extremely difficult to make accurate comparative alternatingcurrent measurements of the performance of any phototube with different types of sound track, since standard recordings are available only in the case of the silver track. One is faced with the problem of comparing the level from a few bars of music, for example, with that from the 400-cycle test loop! It is possible, however, to make directcurrent measurements which theoretically give the information one is after. Such measurements consist essentially in measuring the effective total percentage transmission of the sound-track material. Consider an area of the track equal to the area covered by the scanning beam in a projector. The peak-to-peak alternating signal current (corresponding to 100 per cent area modulation) is equal to the difference between the direct photocurrent obtained with clear base, and that obtained with an equal area of the printed track material, superimposed on the base. Direct photocurrent measurements of this type are appealingly simple and accurate. The results are shown hi Fig. 7. The solid lines represent the signal output relative to an arbitrary reference level, as computed from direct-current