US2565265A - Stabilized electrooptical system - Google Patents

Description

Aug.` 21, 1951 E. w. PETERSON STABILIZED ELECTROOPTICAL SYSTEM 2 Sheets-Sheet l Filed June 30, 1949 I N VEN TOR. EaGE/vf n4 PE 727.25 0M Wrap/vf V5 GM. NWNH w E. W. PETERSON STABILIZED ELECTROOPTICAL SYSTEM Aug. 21, 1951 2 Sheets-Sheet 2 Filed June 50, l1949 VL TGE REGULFTOR @RME/v7- Mensa/WN@ DEV/CE I N VEN TOR. EUGENE 'l/V. PE 7E/?60A/,

Patented Aug. 21, 1951 STABILIZED ELECTROOPTICAL SYSTEM Eugene W. Peterson, Midland, Mich., assigner to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application June 30, 1949, Serial No. 102,405

(Cl. Z50-207) l Claims.

This invention relates to electron multiplier systems, and more particularly to means for stabilizing such systems against variations in the potential of the voltage sources of such systems, and against variations in the intensity of the exciting light source.

An object of the invention is to provide an improved electron multiplier system for use in photometers, sound reproducers and similar systems for converting light signals into highly accurate electrical signals.

Electron multiplier tubes have Vbeen widely utilized in recent years as light receptors in various electro-optical systems. Such tubes consist essentially of a photocathode, a plurality of secondarily emissive dynodes and an anode. The stream of electrons emitted by the illuminated photocathode is electrostatically focused on the rst dynode.v Upon striking the surface of the rst dynode, the electrons cause emission of a new and greater stream of electrons which are in turn focused on the second dynode. This process is repeated until the multiplied stream is collected at the anode. For a more detailed description of the construction and operation of such electron multipliers reference is made to articles by Zworykin et al. appearing in the Proceedings of the I. R. E., volume 24, for March 1936, at pages 351 to 375 and volume 27 for September 1939, at pages 558 to V566.

The energizing voltages for the electrodes of the electron multiplier tube are supplied from progressively higher potential taps of a voltage dividing resistor network which is connected in series with the primary voltage source. Since the amplication oi the tube is dependent on these exciting voltages, it is obvious that such voltages must be maintained extremely constant in order to insure accurate reproduction of the light signals impressed upon the photocathode.

It is accordingly a special object of this invention to provide simple means for maintaining these voltages constant irrespective of variations in the primary voltage source.

A further diiculty encountered in all lelectrooptical systems is that of maintaining constant the intensity of the exciting light source. Since the electro-optical system is designed to accurately reproduce modulations of the intensity of `lis light, it is essential that the system either be supplied by a constant exciting light source cr that it be compensated for variations thereof. It is dilicult, if not impossible, to obtain alight source of the requisite constant intensity, especially when a source such as a mercury arc or a gas discharge is required; in addition, constant voltage sources and controls for the light source are apt to be cumbersome pieces of apparatus.

It is accordingly an object of this invention to provide a highly accurate and simplied system for automatically compensating for variations in the intensity of such light.

Among the other objects of this invention are: to provide an electron multiplier system in which the output of one electron multiplier tube is utilized to control the amplification of a second electron multiplier; to provide an inverse feedback control system in combination with a plurality of electron multiplier tubes in parallel circuit relation; to provide means for matching the amplication factors of the electron multiplier tubes; and to provide an extremely accurate electro-optical measuring system.

These and other objects of the invention will be more readily apparent from reference to the following detailed description, appended claims and drawings, of which:

Fig. l is a schematic wiring diagram illustrating one embodiment of the invention, and

Fig. 2 is a schematic wiring diagram illustrating a second embodiment of the invention.

Generally speaking, this invention contemplates the use of two or more electron multiplier tubes connected in parallel electrical circuit relation in such a manner that one tube, herein referred to as the monitor, views the light source directly and automatically compensates for variations in light source intensity and in line voltage, while the other tubes, herein referred to as the receivers, receive modulated light from the same source and provide output currents which are measures of the desired light quantities.

The parallel voltage networks for the electrodes of the said tubes are energized thru a thermionic amplifier connected in series circuit relation with said parallel networks and with the rline source of potential. The output of the monitor tube is connected to the control grid of the amplifier in inverse feedback relation. Therefore, any change in the line voltage, or in light source intensity, or in the amplication characteristics of the thermionic amplier will cause a corresponding change in the output of the monitor tube. This change in turn varies the gain of the thermionic ampliiier, whose output is thereby varied to apply a corrected voltage to the voltage divider network supplying the electrodes of the electron multipliers. Since each of .the multiplier tubes has substantially identical gain versus dynode voltage characteristics and. the voltages supplied to each tube are the same, the outputs of the receiver tubes will be accurately compensated. The present system further provides variable resistor means for matching the amplication factors of the electron multiplier tubes.

Referring to Fig. 1 of the drawings, I9 is the monitor electron multiplier tube, and II is the receiver electron multiplier tube. Such tubes might be one of the several commercially available types, such as RCA 931A, 1F21, 1R22 or 1F28, depending on the light level and wave lengths at which they are to be used. The gain of the tube must be sufficiently high to produce a current much larger than dark current when its dynode voltages are somewhat below their maximum safe value with the light source available.

All of the electron multiplier tubes must have the same or nearly the same voltage amplification characteristics if substantially perfect compensation is to be obtained. Means for matching the amplification characteristics is provided by the circuit arrangement of Fig. 2, as described below in more detail.

Photomultiplier I0 includes a photocathode 29, an anode 28 and a series of intermediate dynodes 3i). In like manner, photomultiplier I I includes a photocathode 33, an anode 3| and a series of intermediate dynodes 32.

The operating potentials applied to the electrodes of the tubes I0, II are obtained from a Voltage divider resistance network I8-21 connected in series circuit relation with a source of voltage 9 and a variable impedance I2. The power supply 9 is preferably a substantially constant potential source of direct current, although the circuit will compensate correctly with a power supply having relatively wide variations in potential.

The variable impedance voltage regulator device I2 is illustrated as a pentode amplifier having its plate connected to the positive side of the voltage source, and its cathode I3 connected to the voltage divider network I8 through 21. The tube circuit includes the usual connection between the cathode and suppressor grid. A source of potential I4 is connected between the cathode and the screen grid. This source is indicated in the drawings as a battery, but for some purposes could be a glow discharge tube or other substantially constant source. The value of said po.- tential I4 is dependent on tube choice and current requirements. A grid biasing resistor I5. is connected between the cathode I6 and the control grid I1. To increase circuit sensitivity, a second battery I3 is connected to the control grid I1 to supply a positive bias thereto. It is understood that, While a pentode amplier is shown, variable impedance I2 may be a triode thermionic amplier, or any other suitable device known in the art.

The cathode I8 of amplifier I2 is connected to one side of the voltage divider network which consists of a series of resistances I8 through 21. Resistance IB is preferably of such a value as to maintain the last dynode of each series of dynodes 3B and 32 from 50 to 250 volts negative with respect to anodes 28 and 3l over the operating range of the system. The remaining resistances, I9 through 21, are substantially equal in value so as to provide equal voltage increments, e. g. around 100 volts, between successive dynodes. The values of these resistances are not critical and they need not be exactly matched.

Parallel circuit connections are made from the junction of resistances I8 and I9 to the dynode 38 closest to anode 28 of tube I0 and the corresponding dynode 32 of tube II, and from the junction of resistances I9 and 20 to the second dynode of the tube I0 and the second dynode of tube I I; similar connections are made from each of the remaining taps to corresponding dynodes of each tube. The negative side of the network is connected to photocathodes 29 and 3l and to the negative or B- side of the power supply 9. The overall value of the resistance network should be of such a magnitude that the current through the network is large compared to the current through tubes III and II.

The anode 28 of monitor tube III is connected to the control grid I1 of the voltage regulator amplifier I2, and hence to resistor I5. This resistor, therefore, serves as the main load resistor for the anode current of monitor tube I0. Its value may vary over wide limits, but, to avoid dark current and fatigue effects, should be of such magnitude as to cause an anode current in the range of 10J1 to 10*5 amperes, and this current must produce suiiicient biasing voltage across the resistance to oppose the voltage of battery I3 and drive the control grid I1, preferably within the linear portion of its operating range. Where the tubes are refrigerated, anode currents as low as 10-10 amperes are operable.

The anode output of receiver I I is fed to a current measuring device 40 connected in series with anode 3l and cathode I8 of amplier I2. Hence, current measuring device d, indicates the anodc current of the receiver II. Where the electron multiplier system is a component of a photometer the current responsive device may be a galvanometer or a vacuum tube meter. In the case of a sound reproducer, it would be an audio amplier and speaker.

In .the particular embodiment illustrated, the electron multiplier system is utilized in a photometer. Light from source 34 is reiiected by mirror 3S through variable shutter 38 onto photocathode 29 of monitor lil. Light from the same source 34 is reflected by mirror 35 through variable shutter 31 and through sample 39 onto photocathode 33 of receiver II. The variation in the intensity of the light striking photocathode 33 caused by sample 39 produces a corresponding variation in the anodic current of receiver II, whose magnitude is indicated by current measuring device 40.

Where permissible it is desirable to utilize a monochromatic light source to avoid mismatch of tubes I9 and II due to spectral differences caused by colored samples. Shutters 31 and 38 should be adjusted so that the system is balanced. Since the position of the light beams on the photocathodes greatly influences their gain, the geometrical relationship between the photocathodes and the light beams must be fixed. In some instances it is desirable to place a frosted glass or quartz plate about 1A; inch in front of each tube envelope to scatter the light striking the photocathodes and thus reduce tube positional sensitivity. Of course, focusing lens systems may be used in conjunction with the presentsystem. In any event, it is necessary to protect the entire optical system and the electron multiplier tubes from stray illumination.

In operation, assume a decrease in the intensity of light source 34. The output current of monitor tube I0 will decrease, causing a decrease in the negative bias applied to control grid I1 of amplifier I2. The output of amplifier I2 will, therefore, be increased, increasing the potential across the voltage divider network. This increased voltage .is applied equally to .all of .the dynodes `ill, 32 .of monitor IIJ and receiver II, causing an increase in the amplification factors of both tubes which is suflicient to exactly cornpensate yfor the variation vin light intensity.

In similar fashion, a decrease 'in the voltage of the source of potential will cause a `decreased anodic current output of monitor tube I0. This decreased current will increase `the gain of amplifier .I2 to exactly compensate .the voltages supplied to each of the .electrodes of tubes I and II for the decrease in line potential. The same resulting compensation will follow a variation in the Aamplication characteristic of voltage regulator I2. The outputs .of receiver I2 .and current responsive device 40 are, therefore, maintained substantially independent of light or voltage source fluctuations.

While only one receiver tube has been shown and described in the foregoing discussion, it is obvious that a plurality of receiver tubes may be utilized in the system of the 'present invention. Each additional electron multiplier receiver tube would have each of its electrodes connected in parallel Vrelation with the corresponding electrode of receiver II. Each additional receiver would also have a separate current measuring device connected between its anode and the cathode I6 of vamplifier I2, and would have a separate beam of light from source 34 directed upon its photocathode. Such additional receivers and current measuring devices would be compensated against variations in light intensity and applied potential in a manner exactly corresponding to the above described coinpensation of receiver II.

It is here pointed out that the system of the invention is highly superior to a comparable system employing a simple photocellas a monitor. The amplification response of the simple photocell to variations in light intensity and in applied voltage intensity does not correspond over any wide range with the amplification response of an electron multiplier. Hence a photocell monitor will produce good compensation only over the very narrow range oi light level for which the device is adjusted. Additionally, a simple photocell is not as responsive to minute variations in light intensity and `applied voltage intensity.

The present invention is also superior to comparable systems wherein vthe 'voltage of a single dynode is varied as a means of compensation. For good performance over a wide range of applied voltage and of light intensity, and vfor long tube life, it is necessary that the'potential drop between successive dynodes of the electron multiplier remain substantially equal. Therefore, if compensating voltages are applied to a single dynode, they must either be restricted to a narrow range in value or the resulting tube amplification will be inaccurate, and tube life will be shortened.

For many purposes, the vvoltage'ampliiication characteristics `of the several electron multiplier tubes will be sufiiciently matched providing they are of the same type and manufacture. I-Iowever, where excellent compensation must be obtained, more accurate matching is required. The embodiment of the invention illustrated in Fig. 2 shows a means for accomplishing this matching.

The circuit of Fig. 2 has essentially'the same components as those previously described with regard to Fig. 1. A monitor electron multiplier I0, having an anode 28, a photocathode 29 and a plurality of intermediate dynodes 30 views light directly from source 34. Its output is connected to the grid biasing resistor I5 of amplifier I2 to vary the lgain thereof. Amplier I2 has the identical circuit connections described above. One or more receiver electron multipliers II receives light signals from source 34 and its output current is measured by current measuring device 40.

However, in this embodiment of the invention, the dynodes 39, 32 and photocathodes 29, 33 of the multiplier tubes I0, II are not directly connected. Instead, each tube is supplied from a separate voltage divider resistance network, which networks are connected in parallel circuit relation. Thus, one such network, consisting of resistors I8 through 21 and connected to the cathode I6 of amplifier I2, has taps thereon which are connected to each of the dynodes 30 and to photocathode 29 of monitor I9. A second network, consisting of resistors I8 through 27', and connected in parallel circuit relation with the first network to cathode I6 of amplifier I2, has taps thereon which are connected to each of the dynodes 32 and to photocathode 33 of receiver Il.

The negative side of first network I8-2'I is connected through a variable resistance 4I to the negative or B- terminal of power supply 9. In like manner, the negative side of second network VI-lI-2I is connected through a second Variable resistor 42 to the negative terminal of power supply 9 in parallel relation to first network Iii-21. Resistance I8 may be equal in ohmic value to resistance I8 and resistances I9 through 2l and I8 through 21 may all be substantially equal in ohmic value. Similarly, variable resistance 4I may be equal in ohmic Value to Variable resistance 42. However, the circuit will function correctly if the values of the various resistances are so chosen that the ratios of each resistance in the first network to the corresponding resistance in the second network are all equal, i. e., if the ratio of I8 to I8' is equal to the ratio of I9 to I9', etc. Preferably, resistances I9 through 2l are all equal in ohmic value, and resistances I9 through 21 are all equal in ohmic value. Variable resistors 4I and 42 serve as a means for adjusting the relative voltages applied to the electrodes of tubes I 9 and I I and hence for matching the amplification factors thereof.

As previously described, monitor tube I9 is part of a feedback loop, and, by designing the loop to have a high gain, a relatively small change in its anode current will cause a large change in its amplification factor, providing excellent compensation.

Even if this is attained, the system still might not provide good compensation of the receiver tubes. In order to achieve as good compensation of the receiver as of the monitor, the voltage amplication characteristics of the tubes must resemble each other over their entire operating range so that AAio AAM All] All where A10 and A11 are the amplication factors of thetubes and AAio and AAii are changes therein. This relation will never be strictly true for all operating voltages. However, by selecting tubes of the same type and manufacture, the relation will be reasonably satisfied.

Resistances 4I and 42 are adjusted so that' (2) V10=OV11 where V10 and V11 are the operating voltages applied to tubes l0 and Il, and 1c is a constant slightly greater or less than unity. By proper adjustment of the resistances, Equation 1 may be satisfied at a particular voltage. By selecting this voltage as the middle voltage of the system operating range, extremely close matching of the amplification factors of tubes l and Il will be accomplished. This selected voltage will be of a value such that the slopes of the amplification curves of the tubes at this voltage are equal, and preferably will be at a point in the center of the linear portions of these curves. This insures highly accurate compensation of the receiver tube.

While only one receiver tube is shown and described, it is obvious that the system may include a plurality of such tubes. Each additional tube would be provided with a separate network and variable resistance identical with network IBL-21 and resistance 42. Each additional network and variable resistance would be connected in series relation, and this combination would be connected in parallel circuit relation with the series combination of network I8'-21 and resistance 42.

For many purposes, either resistance 4l or resistance i2 may be fixed in value, variation of one of these resistances being suiiicient to accomplish the desired tube matching.

I claim:

i. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes each having an anode, a photocathode and one or more intermediate dynodes; a source of unidirectional voltage; a variable impedance; parallel connections between said electron multiplier tubes, said parallel tube circuit being connected in series circuit relation with said source and said impedance; and a connection between the output of said rst electron multiplier tube and said impedance to vary said impedance, whereby the output of said remaining electron multiplier tubes is maintained constant irrespective of variations of the intensity of said light source and of said voltage source.

2. An electro-optical system comprising, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a source of unidirectional voltage; a variable impedance; parallel circuit connections between said electron multiplier tubes, said parallel tube circuit being connected in series circuit relation with said source and said impedance; means to substantially match the amplification factors of said tubes; a connection between the output of said rst electron multiplier tube and said impedance to vary said impedance, whereby the output of said remaining electron multiplier tubes is maintained constant irrespective of variations of the intensity of the light source and of the voltage source.

3. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more dynodes; separate variable resistors in series circuit relation with one or more of said multiplier tubes; parallel circuit connections between said electron multiplier tubes; a source of unidirectional voltage; a second variable impedance; said parallel tube Yremaining networks and variable resistors;

circuit being connected in series circuit relation with said voltage source and with said second variable impedance; a connection between the output of said first electron multiplier tube and said second impedance to vary said impedance, whereby the output of said remaining electron multiplier tubes is maintained constant irrespective of variations of the intensity of said light source and oi said voltage source.

4l. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said source, each of said tubesA having an anode, a photocathode and one or more intermediate dynodes; a separate voltage dividing network for each of said tubes having connections to each of the electrodes of said tube, said networks being connected in parallel circuit relation; a source of potential; va variable impedance; series circuit connections between said source, said impedance, and said parallel circuit; and a connection between the output of said rst tube and said variable impedance to vary said impedance, whereby the output of the remaining electron multiplier tubes is automatically compensated for variations in the intensity of said light source and of said voltage source.

5. An electro-optical system comprising, in

combination, a light source; a plurality or" electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a separate Voltage dividing network for each of said tubes having connections to each of said electrodes of said tube; a variable resistor in series circuit relation with each of said networks, said first network and said first variable resistor being connected in parallel circuit relation with the series circuits of each of said a source of potential; a variable impedance; said voltage source, said variable impedance and said parallel circuit being connected in series circuit relation; and a connection between the output of said first tube and said variable impedance to vary said impedance, whereby the output of the remaining electron multiplier tubes is automatically compensated for variations in the intensity of said light source and of said voltage source.

6. An electro-optical system comprising, in combination, a light source; two electron multiplier tubes each having an anode, a photocathode and one or more intermediate dynodes; a separate voltage dividing network for each of said tubes having connections to each of said electrodes of said tube; a'variable impedance in series circuit relation with one of said networks; parallel circuit connections between said iirst network and said variable impedance, and said second network; a source of potential; a second variable impedance; said parallel circuit being connected in series relation with said voltage source and said second Variable impedance; and a connection'between the output'of said i'lrst tube and said second variable impedance to vary said second impedance, whereby the output of said second electron'multiplier tube is automatically compensated for variations in the intensity of said light source and of said voltage source.

7. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes each having an anode, a photocathode, and one or more intermediate dynodes; a voltage dividing resistance network having taps. thereon; separate parallel connections between each of said tapsand a corresponding eleotrodeof each of said tubes; a source of potential; a variable impedance; said source of potential, said variable. impedance, and said network being connected in series circuit relation; and a connection between the output of said rst electron multiplier tube and said variable impedance, whereby the outputs of said remaining electron multiplier tubes are automatically compensated forr variations in the intensity of said light source and of said voltage source.

8. An. electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said electron multiplier tubes having an anode, a photocathode andA one or more intermediate dynodes; a voltage dividing resistance network having taps thereon; connections beu tween said iirst of said taps and the rst dynode of each of said tubes; similar separate connections between each of said remaining taps and a corresponding electrode of each of said electron multiplier tubes; a source of potential; a variable impedance; said source of potential, said variable impedance and said resistance network being connected in series circuit relation; and a connection between the output of said first electron multiplier tube and said variable impedance, whereby the outputs of said remaining electron multiplier tubes are maintained substantially independent of variations in the intensity of said light source and of said source of potential.

9. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a separate voltage dividing network for each of said tubes having connections to each of said electrodes of said tube, said networks being connected in parallel circuit relation; a source of potential; a thermionic ampliiier having its output connected in series circuit relation with said source of potential and said parallel circuit; and a connection between the output of said first electron multiplier tube and said thermionic amplier to Vary the ampliiication of said amplifier, whereby the outputs of the remaining electron multiplier tubes are maintained constant irrespective of variations in the intensity of said light source and of said source of potential.

10. An electro-optical system comprising, in combination, a light source; two electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a separate voltage dividing network for each of said tubes having connections to each of said electrodes of said tubes; a variable resistor in series circuit relation with said first network, said rst network and said variable resistor being connected in parallel circuit relation with said second network; a source of potential; a thermionic amplifier having its output connected in series circuit relation with said source of potential and said parallel circuit; and a connection between the output of said first electron multiplier tube and said thermionic amplifier to Vary the gain of said thermionic amplifier, whereby the output of said second electron multiplier tube is automatically compensated for variations in the intensity of said light source and of said voltage source.

ll. An electro-optical system comprising, in

combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said electron multiplier tubes having an anode, a photocathode and one or more intermediate dynodes; a voltage dividing resistance network having taps thereon; connections between the iirst of said taps and the rst dynode of each of said tubes; similar separate connections between each of said remaining taps and a corresponding electrode of each of said electron multiplier tubes; a source of potential; a thermionic amplifier having its output connected in series circuit relation with said source of potential and said' network; and a connection between the output of said rst electron multiplier tube and said thermionic amplier to vary the gain of said amplifier, whereby the outputs of saidl remaining electron muitiplier tubes are automatically compensated for variations` in the intensity of said light source and of said voltage source.

l2. An electronic measuring system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dy nodes; a source of unidirectional voltage; a variable impedance; parallel connections between said electron multiplier tubes, said parallel tube circuit being connected in series circuit relation with said voltage source and said impedance; a connection between the output of said rst electron multiplier and said impedance te vary said impedance; one or more current responsive devices connected to the outputs of said remaining electron multiplier tubes, whereby the outputs of said current responsive devices are maintained substantially independent of variations of the intensity of said light source and of said voltage source.

13. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a source of unidirectional voltage; a variable impedance; parallel circuit connections between said electron multiplier tubes, said parallel tube circuit being connected in series circuit relation with said source and said impedance; means to match the amplification factors of said tubes; a connection between the output of said rst electron multiplier tube and said impedance to vary said impedance, whereby the outputs of said remaining electron multiplier tubes are maintained constant irrespective of variations of the intensity of the light source and of the voltage source; and a separate current responsive device connected to the output of each of said remaining electron multiplier tubes.

14. An electronic measuring circuit comprising, in combination, a light source; two electron multiplier tubes each having an anode, a photocathode and one or more intermediate dynodes; a separate voltage dividing network for each of said tubes having connections to each of said electrodes of said tube; a variable impedance in series circuit relation with one of said networks; parallel circuit connections between said rst network and said variable impedance, and said second network; a source of potential; a second variable impedance; said parallel circuit being connected in series circuit relation with said voltage source and said second variable impedance; a connection between the output of said rst tube and said second variable impedance to vary said sec- 11' ond impedance, whereby the output of said second electron multiplier tube is automatically compensated for variations in the intensity of said light source and of said voltage source and a current measuring device connected t0 the output of said second electron multiplier.

15. An electro-optical system comprising, in combination, a light source; a plurality of electron multiplier tubes excited by said light source, each of said tubes having an anode, a photocathode and one or more intermediate dynodes; a voltage dividing resistance network having taps thereon; connections between one of said taps and the photocathodes of each of said tubes; connections between a second tap and the rst dynode of each of said tubes; similar connections between each of the remaining taps and the remaining correspending dynodes of each of said tubes; a source of unidirectional voltage; a thermionic amplifier having its plate connected to the positive side of said voltage source and its cathode connected to the positive side of said resistance network; a biasing resistor connected between said cathode and the control grid of said tube; a source of constant biasing potential for said tube; a connection between said control grid Yand the anode of said rst electron multiplier tube; a separate current responsive device connected between said cathode and each of the anodes of said remaining electron multiplier tubes; and a connection between the negative side of said voltage Source and said photocathodes and the negative side of said resistance network, whereby the outputs of said remaining electron multiplier tubes are automatically compensated for variations in the intensity of said light source and of said voltage source.

EUGENE W. PETERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,198,233 Snyder, Jr Apr. 23, 1940 2,412,423 Rajchman et al Dec. 10, 1946

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