Projection engineering (Sept 1929-Nov 1930)

Page 16 Projection Engineering, April, 1930 Circuit suitable for measurements in the ultra-violet range. gas-filled cell, as this is not the case. Nevertheless, in cases where extreme accuracy is desired, the highly evacuated cell is to be preferred. The action of the photocell of either type is practically independent of the frequency of light variation and no time lag is encountered in operation up to variations of 108 second in frequency. Conditions Under Which Cells Operate The photocell may be required to operate under three conditions : d-c. relay operation, d-c. linear operation, or a-c. linear operation. Respectively, the employment is in the first case as each definition implies : for the operation of a sensitive relay, for the indication of light intensity variations as in photometric measurements, or in conjunction with an amplifier for the translation of light impulses as in television and sound reproduction. Circuit arrangements covering the three phases of use appear in Figs. 2, 3 and 4 in the order of definition. Photoelectric cells are employed as an element of counting mechanisms, operating relays at the entrances of vehicular tunnels. Special cells in highly sensitive circuits are employed in the measurement of therapeutic light and in astronomical observations. The circuit shown in Pig. 5 is extremely sensitive to minute unbalances and in conjunction with a quartz envelope cell and a specially chosen photoelectric medium is eminently suited to measurement in the ultra-violet range. Study of the curves in Fig. 1 and reference to the characteristic of color filters are self-indicative of the possibilities of photoelectric devices in the analysis of spectra. Certain substances heated to a degree just short of visible red are rich in infra-red radiation and in connection with redsensitive photoelectric devices, Baird has utilized the phenomenon in television and in fog-piercing devices without the use of visible light. Photoelectric cells are utilized for controlling high power equipment in conjunction with the newly developed "Thyratron" of the General Electric Company. Two circuits for controlling power in this manner are shown in Figs. 6 and 7. Although the circuits are simple in appearance the balances involved are of a delicate nature. The circuit equations are given by Hull in the reference given in the bibliography. ■ \~~c .1 w T\ LOAD <JjSiii!JS!iliLr\ uuuj, QQQJ fffuMu'OOoOOOOOO" A.C. SUPPLY FIG.6 Employment of a turning on a hot cathode photoelectric cell for a Thyratron. Quantitative measurements on photoelectric cells may be undertaken with less equipment than would be imagined. A standard source of illumination and several sensitive measuringmeters are all that are required. The cell and the illuminating lamp should be mounted in a box excluding all extraneous light. The distance between the source and the photocell under measurement should be variable over a calibrated range. If a source of light of undetermined nature is employed, experimental determination of its characteristics should be undertaken if accurate results are desired. The schematic diagram of the experimental set-up is shown in Fig. 8. The standard of light used in these measurements is the Lumen which is FIG.8 CALIBRATED SCALE . j i i i i i i — i — i — i — i — i 9 ©M sasm&b Jtflflflflflgy (WOWO'OOOOOOoff A.C. SUPPLY FIG.7 Circuit for turning off a Thyratron by means of a photocell. equivalent to one foot-candle incident upon one square foot of surface, or in metric units one centimeter-candle upon one centimeter square. The calculations are in the form l=CxA/D2 where l=The light flux in Lumens, C=The candle power of the source. A=The area of photo-sensitive surface. D=The distance. The light conductance per Lumen (G) may be taken as the figure of merit among tubes of a given type. This is the slope of the anode voltageanode current curve divided by the light flux in Lumens. Thus, L dia 1 dea G: Current Outputs With Various Voltages The curves in the Figs. 9, 10, and 11 illustrate the results to be obtained. Even with the light source but roughly known the accurately calibrated scale allows us to obtain accurate curves. p— FIG ..9 =* z "in ^ ^ / Q.10 a. rf5 o Q ° / VACUUM CELL 2 < / 20 40 60 80 ANODE VOLTAGE 100 Schematic of laboratory set for photoelectric measurements. Anode current-voltage curve of a typical photocell. Fig. 9 shows the current through a vacuum cell with a fixed illumination and variable voltage. The marked saturation point similar to the filament emission curve of a three electrode tube is evident. The second curve in the figure shows the effect of ionization of gas present at low pressure in a gas-filled cell.