US2295536A

US2295536A - Light sensitive circuit

Info

Publication number: US2295536A
Application number: US414083A
Authority: US
Inventors: Walter J Albersheim
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1941-10-08
Filing date: 1941-10-08
Publication date: 1942-09-15
Anticipated expiration: 1959-09-15

Description

p 1942- w. J. ALBRSHEIM 2,295,536

LIGHT SENSITIVE CIRCUIT 2 Sheets-Sheet 1 Filed Oct. 8, 1941 IN [/5 N TOR W. J. ALBERS/vf/M TM. Au br. A7 TORNE V p 5, 1942- w. J. ALBERSHElM 2,295,536

LIGHT SENSITIVE CIRCUIT Filed Oct. 8, 1941 2 Sheets-Sheet 2 All! .//V V: N TOR W. J. .A L BERSHE/M A 7' TORNE V Patented Sept. 15, 1 942 UNITED STATES PATENT} OFFICE- LIGHT SENSITIVE CIRCUIT Walter J. Albersheim, Great Neck, N. Y., assignor to Western Electric Company, Incorporated, a corporation of New York Application October 8, 1941, Serial No. 414,083

This invention relates to improvements in 16 Claims.

two anodes and a common cathode in the same glass envelope.

In a photocell of this type it is desired that no light intended for one portion only of the cathode shall reach the remaining cathode portion and further that no electrons or other charged par-- ticles belonging to the current conduction from one anode to its corresponding cathode portion shall stray over to take part in the current conduction from the other anode to the other portion of the cathode. In practice it is found that neither of the foregoing requirements is fully met. In the Western Electric 9A cell, for example, optical and electronic crosstalk is greatly reduced by the cell construction shown in U. S.

Patent 2,198,650 above referred to, but enough crosstalk remains to bring it about that if the two currents corresponding to the separate anode-cathode paths are equal, a small fraction, say 5 per cent, of each such current is due to crosstalk from the other conducting path. Optical crosstalk arises from direct incidence of light on the cathode portion for which it is not intended and also from scattered reflections of light within the cell. Electronic crosstalk arises in part from electrons which escape their approtion in volume of a few tenths of a decibel. Again where the two film tracks are sound records corresponding to the same sound source but with different microphone positions such as stereophonic records, the only effect of crosstalk will be i a slight loss in effective localization, whereas the quality is not impaired because, as in the pushpull case, the two records are of the same acoustic character.

However, it has been proposed to use such a f photocell for the reproduction of two parallel film tracks of which one is a sound track and the other is a record related to the sound record but distinct therefrom, such as a volume control track for regulation of the gain of the reproducing amplifier. In this case, since the control track is usually a record of a high frequency modulated by the envelope of the sound signal, crosstalk from it into the sound record part of the cell introduces noises fatal to good sound reproduction. Crosstalk from the sound record into the control'track reproduction has the effect of confusing the gain controls.

The object of this invention is the substan- Q tially complete compensation of undesired currents such as the crosstalk above described in the separate outputs of photoelectric cells having two or more anodes anda cathode commonthereto, for the types of sound film reproduction above enumerated or for any other application in which it may be proposed to use such photocells. This object is attained by providing a method and circuits adapted to eliminate the undesired voltage priate anode to reach the other anode, in part" from scattering of conducting particles released from the glass envelope by electronic bombardment, but chiefly from secondary electronsdue to collisions between photoelectrons and gas molecules.

There are various purposes for which such photocells with plural anodes and common cathode may be used in the reproduction of sound films. The cells in practical use are provided with two anodes and a common cathode and are customarily called push-pull cells because they have been principally used for the reproduction of two sound records side by side on the same film, both being records of the same sound source, but everywhere difiering in phase by 180 degrees. In such a case the existence of crosstalk of the low value characteristic of the 9A cell results in no perceptible damage to the reproduced sound, the efiect being only a reduccomponents from the grids of amplifying tubes appropriately connected each to one of the photocell outputs while leaving the desired voltage components eflective onsaid grids at nearly the same values as if crosstalk were wholly absent.

Fig. 4 is an extension of the circuit of Fig. 3, enabling this invention to be applied to a photocell having three anodes and a common cathode.

In all figures corresponding parts are identified by the same reference numerals.

Referring to Fig. 1 two light beams L1 and L2, which may be transmitted from a single light source through two separate film tracks, are incident on the photosensitive surface of cathode 4 of photocell l which cell is provided with two anodes to which cathode 4 is common. These light beams are intended each to illuminate one portion only of surface 4, as L1 for portion 2, and L2 for portion 3. however, some of L1 reaches portion 3 and some of L2 reaches portion 4. A voltage of, say 90 volts from battery 5 is applied through polarizing resistor 1 to the photocell anode 6 facing cathode portion 2, and through polarizing resistor 9 to anode 8 facing cathode portion 3. Under the influence of the incident beams L1 and'L2, photoelectric currents flow in the two portionsof the cell incompletely separated by cusp I of cathode 4 and as previously explained, each of these. curcents contains a small component properly belonging to the other.

The grids H and I2 of amplifying tubes VI and V2 respectively are connected to

cell anodes

6 and 8, grid H to, anode 6 through capacitor 13, grid l2 to anode 8 through capacitor I4. Voltage to plates. l and N5 of the amplifying tubes may be supplied as in Fig. 1 from the positive terminal of.battery 5 through the primaries. of; transformers TI and T2, respectively, or maybe supplied from a separate voltage source. Cathodes' and [8 of tubes VI and V2, respectively, areconnected to ground and the negative terminal ofbattery 5 (orother voltage supply) through the conventional biasing resistors and by-pass condensers as indicated by 19 and 20 in Fig. 1.. The filament heating circuits are not shown.. The tube outputs are taken from the secondary terminals of transformers TI and T2. In the conventional circuit mesh connecting grid and cathode of an amplifying tube to the output terminals of the photocell, the photocell cathode is connected to ground, to the negative battery terminal and to the end of the tube grid leak remote from the grid. According to this invention, however, a potentiometer 23 is inserted between cathode 4 and the ground line G. Grid leaks 2| and 22 are connectedas shown-in Fig. 1 to separately

adjustable taps

24 and 25, respectively, on potentiometer 23 instead of directly to ground line G. The insertion of potentiometer 23 between cathode 4 and ground line G and the separate connection of grid leaks 2| and 22 to adjustable taps on potentiometer 23 constitute the circuit improvement of this invention as applied in solving the problem of amplifying separately and without mutual contamination modulated photocell currents corresponding to modulations of light flux in the incident beams L1 and L2. Beams L1 and L2 may be modulated, for example, by a sound record and by a volume control record, respectively, on a photographic film. Such a film andthe mechanism for feeding it past. an exciting light source, together with the optical system for illuminating the film and effecting the separation of beams L1 and L2 are not shown, inasmuch as they form no part of this invention;

How the method of the invention attains the desired object will'be apparent from the following discussion with references therein to Fig. 2. If, for example, photocell l is a Western Electric As indicated in Fig. 1,

rent in the potentiometer inserted between the cell cathode and ground. In each anode-cathode path the ratio of desired to undesired current is 95 to 5, but the currents to be separated appear in the cathode-to-ground potentiometer 24, each equal to the total current in either cell path.

The same distributions are characteristic of the alternating currents provoked in the photocell by modulation of the light beams L1 and L2 provided these beams are both modulated to the same extent. Therefore if amplifying tubes, which are suitably Western Electric 2623 tubes, are connected one each to one of the photocell paths in the conventional manner the grid voltage of each tube will contain desired and undesired components also in the ratio 95 to 5. The current in the returnconductor from cathode 4 to ground line G will consist of two approximately equal currents each corresponding to modulation of one of the light beams L1 and L2. The voltage drop across potentiometer 23 inserted according to this invention, therefore also consists of two approximately equal components. It will be understood that potentiometer 23 may comprise two parallel resistorsprovided one with tap 24, the other with tap 25.

Fig. 2, a circuit symbolically equivalent to that of Fig. 1, exhibits instantaneous relationships of the alternating currents therein. For simplicity, battery 5, capacitors l3 and 14 are replaced by short-circuit connections since Fig. 2 concerns only alternating currents. It is an object of this invention to impress upon grid 11- a voltage proportional only to L1 and upon grid l2 a voltage proportional only to L2.

In Fig. 2 the right-hand anode-cathode path of photo-cell l is represented by current source i1 connected to the external circuits through a very high internal resistance 28 of the order of 100 megohms. Similarly, 22 represents the lefthand path of photocell 4 as a current source in series with internal resistance 29 also of the order of 100 megohms.

'In the following analysis, let in and i 2 be the photoelectric efiiciency of cathode portions 2 and 3respectively, S1 and S2 be the fractions of light beams L1 and L2 respectively, which fall undesirably S1 on cathode portion 3 and S2 on oathode portion 2. Then the cathode emission will be Most of the current i1 will flow to anode 6, most of 2 to anode 8' in cell 1, but the portion of each current will be diverted by electrical crosstalk represented in Fig. 2 by

resistances

26 and 21. In the case of 5 per

cent crosstalk resistances

26 and 21 are each of the order of 2000 megohms or twenty

times resistance

28 or 29. In the diagram of Fig. 2 2'21 and iaZ are the currents actually reaching

anodes

6 and 8, respec- 9A cell and if light beams L1 and L2 are of equal tively. Equations 1 and 2 apply to the steady currents corresponding to unmodulated light beams L1 and L2. These equations apply equally to the alternating currents which correspond to modulation of light, it having been assumed that L1 and L2 are modulated to the same extent. In What follows, i1 and 2'2 will be taken to be alternating currents, L1 and L2 modulated light amplitudes.

In the equations below, each resistance value is ildenigfized by the appropriate numerals in Figs.

Considering the mesh of Fig. 2, one finds the total current to anode 6:

R27 R28 RT1+'R29 BEER Multiplying Equations 1 and 3, we find The approximation involves neglecting resistances R1 and R9, which are of the order of 1 megohm, and so negligible in comparison with the internal cell resistances R28 and R29 and with the still greater crosstalk resistances R26 and R21.

Simplifying Equation 5, one may write al l In evaluating the voltage e1 between grid II and ground, it is convenient to consider as positive current flowing toward grid H in grid leak 2|, as negative current flowing toward ground in potentiometer 23. The voltage on grid ll contributed by 2'21 is ea1=ia1Z1 where Z1 is the actual resistance between grid ll and ground G. Approximately, R1( 21+R24) R7121 R7+R21+R24 R7+R21 since R24, the resistance included between grid and tap 24 on potentiometer 23, is much smaller than R7 and R21.

With this approximation,

R7R21 R7+R21 (9) The voltage contributed by current is in potentiometer 23 is to the same approximation as (9). Combining (9) and (10) one finds e1=e21+e3, or

R l= T ai zi s z4) It is desired that e1 be independent of L2. By

substituting in (11) the components of 7:21 and is which depend on L2,

Equations

6 and 8, one finds that L2 disappears from (11) if that is, if

Q2R21=R24 (13) From (1 1) and (13) one finds ei=,fj isozia) (14) Substituting in (14) th L1 components of (6) and (8), one finds Adjusting in the same fashion tap 25 on pctentiometer 23, one obtains by similar reasoning for the voltage on grid I2 The described adjustments of

taps

24 and 25 permit the electrical reproduction of light signals L1 and L2 independently of each other. In practice, these adjustments are preferably made as follows, since Q1, Q2 are usually unknown:

Light L1 is obscured, and tap 24 is adjusted by trial to minimize the current (now solely due to light L2) in the output circuit of VI. After this adjustment of tap 24, L1 is restored, L2 is obscured and tap 25 is adjusted to minimize the current due to L1 in the output circuit of V2.

Having made these adjustments of

taps

24 and 25, one obtains voltages e1 and e2 on grids H and I2, respectively, independent of each other. Each of the voltages e1 and 62 is less than it would be were there no crosstalk, in the ratio 1-Q1Q2 to 1. Thus, the object of this invention is attained.

Those skilled in the art will recognize that grids II and [2 may be coupled conductively to

anodes

6 and 8, respectively, and capacitatively to grid leaks 2| and 22, respectively, provided appropriate changes are made in the supply circuits to tubes VI and V2, without departing from the spirit of this invention.

In the circuit of Fig. 1 and its symbolic equivalent Fig. 2, the two signal currents which are to be separated are superimposed in greatly differing ratios in difierent circuit elements. Under the conditions assumed to illustrate the application of this invention, the two currents appear with approximately equal values in potentiometer 23, but in either anode-cathode path of cell I these currents are in the ratio of about to 5. According to the method of this invention, the separation is effected by introducing in series with the normal grid-to-ground voltage of each amplifying tube a voltage of opposing phase and of suitable magnitude derived from potentiometer 23, to effect cancellation of the undesired component at the expense of cancelling at the same time a small fraction of the desired component.

As a numerical example, assume:

L1=L2=0.01 lumen (direct current component) 1 I 2=100 microamperes per lumen S1=S2=0.02

R21=R22=0.5 megohm With the above constants, Equation 5 gives ia1=ia2=0.936 L1+0.064 L2 (direct current component) In this case, Equation 13 gives R24=R25=32,000 ohms Equation 15 gives 61:0.29 M1 Volt e2=0.29 M2 volt ode and the ground line.

where M1 and M2 .arethe modulationiactors of light fluxes L1 and L2, respectively; c1 and 62 are the maximum instantaneous values of the alternating voltages on grids II and I2, respectively. Were there no crossstalk, enand eg -would:

each be greater than the values given in the ratio 1/1(2 .0.064) or 1.15.

The method of this invention isapplicable not solely where the undesired voltages'are compensatedby voltagesof thesame kind and of Terminal 24 of grid leak 2 is connected to a tap ion-potentiometer 35; :terminal 25015 grid leak 22 is connected correspondingly :to potentiom- .eter;32. Other circuit-elements are identically as in :Fig. 1. Sincetheg-rid and :plate voltages of each Itubeare opposite -in phase, the desired and'undes'rred voltage components on the grids have-counterparts-of opposite phase .in the plate circuits amplified inthe same ratio.

'Inthe-description of Figs. 1 'and'2, it was stated that the resistance of potentiometer 23 should Tbeapproxima'tely 2Q times R21. In Fig. 3 the resistance :o'f potentiometers 32 and 35 'may be suit-ably chosen as follows:

The 'grid voltages, proportional approximately to R21 and R22, are amplified to be proportional .to these resistances multiplied :by

where is the amplification factor of the .tube *and Rexa, Rm. are the load resistance .(through theoutput transformer) and the internal resistance of the tube, respectively. Therefore, to obtain a compensating voltage proportional to 2Q R21, the resistance of potentiometer 32 should bear to thetotal resistance Rs1+Rs2 the ratio In these formulas R32 and R35 may be neglected, since they are small compared to R31 and R34, respectively.

Resistors R31 and Rai arechosen of much (say 100 times) higher resistance than the load resistances. As an example, the load resistance of each tube may be 110,000 ohms and the internal tube resistance"20,000 ohms. In this case R31 and R34 are each suitably 1 megohm. If now ,u:16 and 2Q=0.10,

Accordingly, 25,000 ohm potentiometersiare suitable for R32 and R35.

Condensers and must ibe .o'f negligible Such an application 5 ill] 46 and. 48.

impedance compared to Bar and R34-at the lowest frequency to be transmitted; :for the ;present purpose they are suitably chosen 0.1 microfarad. each.

If the load impedance of either tube is not a pure resistance,compensation .for its reactive component must be provided in the shunt 30--3|32 (or -33-34-35-). This may be done by shunting R31(or R3 with an impedance of the same character as the load impedance, bearing to the latter the same ratio that R31 (or R34) bears to the internal tube resistance; other expedients for this purpose will occur to those employing the invention.

The impedance through which voltage is supplied from battery '5 to the tubeplates l5 and I6 and to the

cell anodes

6 and 8 are readily :chosen by-one skilled in the 'art'who has before him the characteristics of the amplifying tubes and of the photocell he has elected to use. The grid leaks 2| and '22 are likewise readily chosen; in the illustration (Fig. 1) of the present invention, using the Western Electric 9A photocell and 2623 tubes, suitable values are as follows:

and the biasing resistors with by-pass condensers l9 and 20, suitably-comprise each a resistance of 1000 ohms in shunt with 'l if. It may be desired to .take the amplified photocell voltages from plates l5 and I6 of tubes VI and V2 to subsequent amplifiers; in this case transformers TI and T2 may suitably be replaced byresistances of about 10,000 ohms. For the circuit of Fig. 3 the same resistors and capacitors are appropriate as for the circuit of'Fig. 1. It has already been stated how those employing this invention may choose the resistances of potentiometer 23 of Fig. 1 and of potentiometers32 and 350i Fig. 3.

The alternative circuit of Fig. 3 permits the application of the invention to the case of photocells having three or more anodes and a common cathode. How this is accomplished is'shown in Fig. 4, where cell I has a third anode 36 and an additional cusp l0. Voltage to anode 36 is supplied from battery '5'through resistor 31. Amplifying tube V3 is connected through capacitor 4| to cell anode 36. Voltage to plate 39 of tube V3 is supplied from battery 5 through the primary of transformer T3, the secondary of which serves to deliver the output of tube V3 to a subsequent circuit, in the samemanner as the secondaries-of transformers TI and T2.

Plate 39 of V3 is shunted to ground line G through capacitor 43 in series with resistor 44 and potentiometer 45. The circuit connections of tubes VI and V2 are identically as in Fig. 3, except that the grid of each of these tubes is provided with an additional grid leak resistor, grid II with resistor 52 brought to tap 53 of potentiometer 45, grid l2 with resistor 50 brought to tap 5| of potentiometer 45. Likewise grid 38 of tube V3 is provided with two grid leak resistors, Resistor 46 is brought to tap 41 of potentiometer 35, resistor 48 to tap 49 of potentiometer 32.

Thus in Fig. 4, the gridof each amplifying-tube is enabled to obtain a compensating voltage of proper phase and magnitude from the plate circuits of the other tubes. Each grid separately would, without the compensation provided :by this invention, be afiected by the desired .and undesired voltage components, the .latteraris'ins from crosstalk in cell l and being of much lower value than the desired component.

Potentiometers

32, 35 and 45 are doubly tappedto supply compensating voltages: potentiometer 32 supplies such voltages to grids I2 and 38; potentiometer '35, to grids II and 38; potentiometer 45, to grids I l and [2. While the separate potentiometer settings are not wholly independent, appropriate settings of the taps on each of the three potentiometers can readily be found, since each grid voltage has a desired component greatly predominant over the undesired components.

The resistances of

potentiometers

32, 35 and 45 may be determined by the rule already given in the description of Fig. 3. The two grid leak resistors shown in Fig. 4 paralleled on the grids of each tube are, however, each 1 megohm, so that they combine to make the impedance of the grid-to-ground connection of each tube 0.5 megohm as in Figs. 1 and 3. v 7

While the invention has been described with reference to photoelectric cells having in the same glass envelope plural anodes and a cathode common thereto, it will be understood that the method of the invention is not limited to the devices illustrated in the foregoing description. The method of the invention may be applied in the case of multiple electrodes of one polarity, whether positive or negative, with a common electrode of opposite polarity or with physically separate electrodes of opposite polarity connected to a com mon terminal. The electrodes in question may be those of any type of photosensitive device, and may or may not be housed in a common envelope.

What is claimed is:

1. In a light sensitive circuit including a photoelectric cell having dual anodes and a cathode common to said anodes, a source of light signals associated with the first of said anodes, a second source of light signals associated with the second of said anodes, an amplifying tube individual to each of said anodes, a common ground connection to the cathode circuits of said amplifying tubes, conductive coupling between said common ground connection and the common cathode of said photoelectric cell, couplin between the grid of the first of said amplifying tubes and the first anode of said photoelectric cell, and coupling between the grid of the second of said amplifying tubes and the second anode of said photoelectric cell, means for electrically reproducing said light signals independently of each other, including a grounded potentiometer traversed by currents corresponding to the second of said light signals and a resistive coupling between the grid of the first of said amplifying tubes and a selected tap on said potentiometer.

2. In a light sensitive circuit including a photosensitive device having dual electrodes of one polarity and an electrode of opposite polarity common to said dual electrodes, a source of light signals associated with the first of said dual electrodes, a second source of light signals associated with the second of said dual electrodes, an amplifying tube individual to each of said dual electrodes, a common ground connection to the cathode circuits of said amplifying tubes, conduct ve coupling between said common ground connection and the common electrode of said photosensitive device, coupling between the grid of the first of said amplifying tubes and the first of said dual electrodes, and coupling between the grid of the second of said amplifying tubes and the second of said dual electrodes, meansfor electrically reproducing said light signals independently'off each other,- including a grounded potentiometer traversed by currents corresponding to the second of said light signals and a resistive coupling between the grid of the first of saidamplifying tubes and a selected tap on said potentiometer.

3. In a light sensitive circuit including a photoelectric cell having dual anodes and a cathode common to said anodes, a source of light signals individually associated with each of said anodes, means for electrically reproducing said light signals independently of each other, including an amplifying tube individual to each of said anodes, coupling between the grid of each of said tubes and the corresponding one of said anodes, a common ground connection to the cathode circuits of said tubes, a potentiometer connected between said common ground connection and the common cathode of said photoelectric cell, and a resistive coupling between the grid of each of said tubes and a selected tap on said otentiometer.

4. In a light sensitive circuit including a photoelectric cell having dual anodes and a cathode common to said anodes, a source of light signals individually associated with each of said anodes, means as in claim 3 for electrically reproducing said light signals independently of each other, wherein the potentiometerconnected'between the common ground connection and the common cathode of said photoelectric cell has a resistance of the order of 10 per cent of that of the resistive coupling between the grid of either amplifying tube and the corresponding tap on saidpotentiometer. c r

5. In a light sensitive circuit including a photoelectric cell having dual electrodes of one polarity and a common electrode of opposite polarity, a source of light signals individually associated with each of said dual electrodes,'means for electrically reproducing said light signals independently of each other, including an amplifying tube individual to each of said duel electrodes, coupling between the grid of each of. said tubes and the corresponding one of said dual electrodes, a common ground connection to the cathode circuits of said tubes, 2. potentiometer connected between said common ground connection and the common electrode of said photoelectric cell, and a resistive coupling between the rid of each of said tubes and a selected tap on said potentiometer.

6. In a light sensitive circuit including dual photosensitive devices having electrically independent electrodes of one polarity and electrically connected electrodes of opposite polarity, a source of light signals individually associated with each of said electrically independent electrodes, means for electrically reproducing said light signals independently of each other, including an amplifyingtube individualto' each of said electrically independent electrodes, coupling between the grid of each of said tubes and the'corresponding one of said electrically independent electrodes, a common ground connection to the cathode circuits of said tubes, a potentiometer connected between said common ground connection and the electrically connected electrodes of said photosensitive devices, and a resistive coupling between the grid of each of said tubes and a selected tap on said potentiometer. 7. In a light sensitive circuit including aphotoelectric cell having dual anodes and a cathode common to said anodes, a source of light signals individually associated with each of said anodes, means for electrically reproducing said light signals independently of each other, including an amplifying tube individual to each of said anodes, coupling between the grid of each of said tubes and the corresponding one of said anodes, a common ground connection to the cathode circuitsof said tubes and to the common cathode of said photoelectric cell, a shunt circuit comprising capacitance and resistance in series individually connecting the anode of each of said amplifying tubes to said common ground connection, and a resistive coupling between the grid of each of said tubes and a selected tap on the resistance included in the shunt circuit connecting the anode of the other of said tubes to said common ground connection.

8. In a light sensitive circuit including a photoelectric cell having dual anodes and a cathode commonto'said anodes, a source of light signals individually associated with each of said anodes, means as in claim '7 for electrically reproducing said light signals independently ofeach other, wherein each shunt circuit connecting the anode of an amplifying tube to the common ground connection includes a resistor of which the resistance bears to that of the tube load a ratio of the order of 100 to 1, in series with a condenser of which the capacity in microfarads is of the order of onetenth the resistancein megohms of said resistor.

9. In a light sensitive circuit including a photoelectric cell having dual anodes-and a cathode common to said anodes, a; source of light signals individually associated with eachof said anodes, means as in claim 7 for electrically reproducing said light signals independently'of each other, wherein each shunt circuit connecting the anode of an amplifying tube to the common ground connection comprises in series a resistor of which the resistance bears to the resistance of the tube load a ratio of the order of 100 to 1, a condenser of which the capacity in microfarads is of the order of one-tenth the resistance inmegohms of said resistor, and a potentiometer of which the resistance is of the order of per cent of that of said resistor.

10. In a light sensitive circuit including aphotoelectric cell having plural anodes and a cathode common to said anodes, a source of light signals individually associated with each of said anodes, means for electrically reproducing said light signals independently of each other, including an amplifying tube individual to each of said anodes, coupling between the grid of each of said amplifying tubes and the corresponding one of said anodes, a common ground connection to the cathode circuits of said tubes and to the common cathode of said photoelectric cell, a shunt circuit comprising capacitance and resistance in series individually connecting the anode of each of said tubes to said common ground connection, and resistive coupling between the grid of each of said tubes and a selected tap on the resistance included in each shunt circuit connecting the anode of another of said tubes to said common ground connection.

11. In 'a light sensitive circuit including a photo-sensitive device having plural electrodes of one polarity and an electrode of opposite polaritycommon to said plural electrodes, a source of light signals individually associated with each of said plural electrodes, means for electrically reproducing said light signals independently of each other, including amplifying tube pling between the gridof each of said amplifying tubes and the corresponding one of said plural electrodes, a common ground connection to the" cathode circuits of said" tubes and to the common electrode of said photosensitive device, a shunt circuit comprising capacitance and resistance in series individually connecting the anode of each ofsaid tubes to said common ground connection, and resistive coupling between the grid of eachofsaid tubes and a selected tap on the resistance including in'e'ach shunt circuit connecting the anode of another of said tubes to saidcommon ground connection.

12. In a light sensitive circuit including a plurality of photosensitive-devices having electrically independent electrodes of one polarity and electrically connected electrodes of opposite polarity, a source of light signals individually associated with each of said electrically independent electrodes, means for electrically reproducing said light signals independently of each other, including an'amplifying tube individual to each of said electrically independent electrodes, coupling be-' tween the grid ofeach of said tubes and the corresponding one of said electrically independent electrodes, at common ground connection to the cathode circuits of said tubes and to the electrically connected electrodes of said photosensitive devices, a shunt circuit comprising capacitance and resistance in series individually connecting the anode of each of said amplifying tubes to said common ground connection, and resistive coupling between the grid of each of said tubes and a selected tap on the resistance included in each shunt circuit connecting the anode of another of saidtubes to said common ground connection.

13. In a light sensitive circuit including a photoelectric cell having dualanodes and a cathode common to said anodes, together with an amplifying tube individual to each of said anodes, the the method of compensating crosstalk between the two anode-cathode paths of said photoelectric cell which comprises introducing into the grid circuit of each of said amplifying tubes a controlled voltage derived from the currents generated in said photoelectric cell opposite in phase but corresponding in magnitude to the crosstalk voltage normally present in said grid circuit.

14. In a light sensitive circuit including a photoelectric cell having plural anodes and a cathode common tosaid anodes, together with an amplifying tube individual to each of said anodes, the method of compensating crosstalk among the several anode-cathode paths of said photoelectric cell which comprises introducing into the grid circuit of each of said amplifying tubes a controlled voltage derived from the currents generatedin said photoelectric cell individually opposite in phase but corresponding in magnitude to each of the crosstalk voltage normally present in said grid circuit.

15. In a light sensitive circuit including aphotosensitive device having plural electrodes of one polarity and an electrode of opposite polarity common to said plural electrodes, together with an amplifying tube individual to each of said plural electrodes, the method of compensating crosstalk among the several anode-cathode paths ofcsaidphotosensitive device which comprises introducing into the grid circuit of each of said amplifying tubes a, controlled voltage derived from thecurrentsgenerated .in said photosensitive device individually opposite in phase but corresponding in magnitude to each of the crosstalk voltages normally present in said grid circuit.

16. In a light sensitive circuit including a photosensitive device having plural electrodes of one polarity and an electrode of opposite polarity common to said plural electrodes, the method of compensating crosstalk among the several anodecathode paths of said photosensitive device which consists in neutralizing each crosstalk voltage derived from the currents generated in said light sensitive circuit by a voltage comprising plural components at least one of which is of corresponding magnitude but opposite phase to said crosstalk voltage.

WALTER J. ALBERSHEIM.