International projectionist (Oct 1931-Sept 1933)

September 1933 INTERNATIONAL PROJECTIONIST 13 ABC's OF PHOTO-ELECTRICITY A. J. McMaster G-M LABORATORIES, INC. II. Photo-Conductive Cells PREVIOUSLY it was mentioned that the selenium cell is the best known example of a photo-conductive cell. The element selenium, discovered in 1817 by Berzelius, is known as a semi-conductor. Its light-sensitive properties were discovered by accident in 1873. The usual form of cell consists of one or more parallel pairs of conductors which are bridged by a thin film of grey crystalline metallic selenium. Since light can penetrate but a very small depth of the metal, it is desirable to expose as large an area of the sensitive metal as is possible without increasing the resistance to too large a value. A common construction is a glass plate on which a gold or platinum film is deposited in the form of a grid. Hall of the gold film is separated a short distance from the other half by a long zig-zag path in which the selenium is deposited. When an external voltage is applied, a small current rises to considerably greater value. In general the following characteristics are observed : 1. The sensitivity increases with the applied voltage. 2. The sensitivity of very high resistance cells is usually greater than that of low resistance cells — i.e., the current ratio of light to dark values is larger for small values of current, namely, micro-amperes, than it is for higher values, namely, milliamperes. 3. The net increase in current due to light is proportional to the square root of the light intensity. 4. For rapidly fluctuating light, the current output of the cell is nearly proportional to the light intensity and inversely proportional to the frequency. The static current response to various light intensities is shown in Fig. 9. In Fig. 10 the time rate of response is shown. The lag is apparently due to the fact that as electrons are freed within the metal some of them immediately begin to recombine in their travel to the positive electrode with positive ions. Eventually an equilibrium condition results in which the rate of liberation of electrons from selenium atoms is equal to the rate of recombination. When the incident light is removed, the rate of recombination rapidly reduces the number of free electrons present until the number present becomes so small that the probability of recombination is likewise reduced. In Fig. 11 the dynamic response of a typical selenium cell is shown. As would be expected, the frequency response is poor, although satisfactory results in the audio range are claimed by the use of properly compensated amplifiers. In Fig. 12, the color response of a typical selenium cell is shown. The cell is sensitive throughout the entire visible spectrum. In the physical construction described above the light is generally incident at right angles to the flow of current. A second type of cell has some useful properties. In this form the selenium deposited in a thin film on a metallic plate and semi-transparent metallic conductor is laid on top of the selenium. The incident light is parallel to the flow of current and the resistance of the cell is very much lower than that of the other type. Since the voltage dissipation of any selenium cell is limited to a small value, the voltage applied to these low resistance cells must be low. Xiot E;yoo . other forms of photo-conductive cells have been developed with considerable success. T. W. Case discovered the extremely red-sensitive characteristics of oxidized thallium sulphide. The latter serves to bridge the conductors of a grid as in the selenium cell. F. Michelson has alloyed selenium and tellurium to obtain red sensitivity. Other sulfides and oxides have been used but with less success. Photo-Voltaic Cells Photo-voltaic cells are among the oldest of light sensitive electrical devices. Becquerel in 1839 observed that light on one of two electrodes in an electrolyte produced an e.m.f. T. W. Case and others have developed this effect to a more or less practical degree. Several years ago the Arcturus Company announced their Photolytic cell, which is of this type. Both of the electrodes are covered with crystalline cuprous oxide and a weak conducting electrolyte surrounds them. When one of the electrodes is illuminated, a voltage appears at the terminals of the cell. The cell is unsuited for furnishing continuous current but is intended for use in sound equipment where the fluctuating light intensity produces a varying voltage. The cell is connected in series with a condenser and the primary of a step-up transformer, the secondary of which is connected to the grid of an amplifier tube. The d.c. potential developed at the terminals of the cell is shown in Fig. 13. It is to be noted that the potential approaches a maximum value of ap SELrnlUM CELL. FIG.ll LIBHT FLUX-LUMENS goo jaoo J£oo ^ooo COLOR SENSITIVITY OF SCLC.NIOM C£LL PHOTOLYTIC CELL FIG. 13 PHOTOLYTIC CELL DYNAMIC P ESPOUSE J^/G.14 FOOT-CANDLES FREQUENCY thing to consider is burning rate. If you burned two pairs of carbons of different make but of the same size for an hour, the difference in burning rate might not be very marked, but the slower-burning carbon may well give you an extra two or three-reel run, and leave no appreciable waste end. Naturally, the most accurate check of screen illumination is to be had through use of a photometer, but very few theatres have such a device available. In the absence of such equipment, however, the foregoing information will serve the purpose admirably. Many projectionists try to estimate costs merely by a comparison of carbon prices, without regard for the factors of burning time and "juice" costs, which figures are absolutely necessary for any fair evaluation of costs. proximately 160 millivolts, beyond which increase of light intensity produces very little rise in voltage. When it is used in sound projection, its principal merit is that it can be connected to the amplifier, at some distance by means of a low-impedance line. The frequency response claimed by the manufacturer is shown in Fig. 14. It is customary to use an amplifier which is peaked at the high end. One of the difficulties experienced in the field with this cell has been that the electrolyte