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The
'New Look'
in
Photocells
By RAYMOND A. DUSAULT, JR.
Radiant Lamp Corporation
THE commonplace observation that good things come in small packages might very well apply to the newest and most recent development in the photoelectric cell field, the lead-sulfide photocell. A mite of a tube in size,, this photocell promises to do big things for the motion picture industry. Since this cell was announced last Spring, considerable interest in it has been shown by projector manufacturers.
Of importance to the entire projection field is the need to know more about these cells and what they will do to improve performance. Rather than go into a detailed technical study of the cells, the writer feels that a broad general discussion of the unit and its advantages would be of most interest to IP readers.
Called the "PbS cell" for short (a chemical term abbreviation), the leadsulfide cell is of the photo-conductive type. This means that the resistance of the cell decreases under the influence of
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LOCAL NO. 070 I. A. T. S. E.
Laredo Texas
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light. Although this property of photoconductive lead-sulfide (in the form of Galena) was known as early as 1900. it has been only within the last few years that improvements in sensitivity have been made to the point where it is now practical to apply production techniques to the commercial manufacture of these cells.
The PbS cells differ from the conventional photocells in use today in the manner in which they convert light into electrical energy. The standard cell now in general use is known as the photoemissive type and depends on a different phenomena for its operation. In either case, however, the end results are the same in that light from the exciter lamp, modulated (varied in intensity) by the film soundtrack, strikes the photocell where the light fluctuations are converted into electrical current. This feeble current is amplified and then piped to the loudspeakers.
The lead-sulfide photocell has remarkably stable electrical characteristics. This feature will result in the cell finding applications in numerous other fields such as burglar alarms, phonograph pickups, tabulating machines, etc. However, since we in the motion picture industry are primarily concerned with its use for sound reproduction, we will consider some of the properties and adv^ntages of the PbS cell as compared with the conventional photo-emissive type.
PbS Cell Physical Characteristics
The PbS cells are the smallest commercial photocells developed to date for the sound projector industry. The cell is available in several sizes, the smallest of which is only a little over one inch long by x/± inch in diameter. The extremely small size of these cells is of primary importance to the manufacturers of 16-mm equipment, resulting in space economy and permitting the cell to be mounted in hitherto inaccessible places. Fig. 1 shows the outline and dimensions of a typical PbS cell.
The frequency response of the PbS cell is about the same as that of the photo-emissive gas photocell, being fairly flat to 10,000 cycles and then dropping off rapidly at higher frequencies.
Whereas photo-emissive cells tend to change their sensitivity with age and with exposure to strong radiation, experience to date with the lead-sulphide cells seems to indicate stable electrical characteristics and sensitivity both with age and with prolonged exposure to strong radiation. The PbS cells are not limited by exposure to ultra-violet light, as are the photo-emissive types with SI surfaces.
The PbS cell is completely non-microphonic, as contrasted with conventional types. The lead-sulfide coating is applied inside the cell directly on the glass.
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FIG. 1. Dimensions of a typical lead-sulphide
cell, which is inserted directly into a socket
without the need for a base.
There is practically no way that vibration might cause microphonics in these cells. This is not true of the photo-emissive units, however, due to their type of construction. In the latter type the anode and cathode are usually supported on stiff wires attached to the bottom of the cell. Although these are made as rigid as possible, it is not unusual for vibration to vary the displacement of the cell elements enough to cause unwanted modulation of the sound.
Spectral Characteristics
The PbS cell has an extended spectral response in the infra-red region up to
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PROJECTIONISTS LOCAL NO. 364
AKRON OHIO
LOCAL NO. 360 LUFKIN, TEXAS
Al Garrard
Business Agent — Secretary
INTERNATIONAL PROJECTIONIST • December 1948
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