US2570665A - Method and means for correcting sensitivity drift of amplifiers - Google Patents

Description

N. R. GUNDERSON 2,570,665 METHOD AND MEANS FDR CDRRECTING sENsITIvITY DRIFT 0F AMPLIFIERS 5 SheetsSheet l Oct. 9, 1951 Filed oct. 18, 1948 95o; l SR... lus

IN V EN TOR.

A 7" T ORNE' Y Non/VAN R. Ga/voERso/v Oct 9, 1951 N. R. GUNDERsoN 2,570,665

METHOD AND MEANS FOR- CORRECTING SENSITIVITY DRIFT oF AMPLIFIERS Filed Oct. 18, 1948 i 3 Sheets-Sheet 2 i +550 vous 4e /40 Rf/Easmr 8 VOUS lll 40 AMPL/F/ER z Maron [/0 VOLT$ AC.

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A T TRNE Y Oct- 9, l95l N. R. GuNDERsoN 2,570,665

` METHOD AND MEANS FOR CORRECTING SENSITIVITY DRIFT OF' AMPLIFIERS Filed Oct. 18, 1948 3 Sheets-Sheet 3 NORMA/v l?. Gu/VDERsO/V INVENTOR.

Tram/Ey Patented Oct. 9, 1951 TENT OFFICE METHOD AND MEANS FOR CORRECTING SENSITIVITY DRIFT OF AMPLIFIERS Norman R. Gunderson, Pasadena, Calif. Application October 18, 1948, Serial N o. 55,033

19 Claims.

This invention relates generally to a method and system for compensating for changes in sensitivity in amplifier systems. It relates more particularly to an intermittently effective feedback system connected in an inverse or negative feedback sense, and primarily, though not necessarily, intended for use in correcting or compensating for sensitivity drift in an electron multiplier amplifier with-or without a phototube input forming an electro-optical system.

The present invention is-primarily, though not necessarily, intended for use in` automatically compensating for changes in phototube sensitivity in a scanning process, especially in a process in which photographic negatives, positives or other images are scanned. The present invention is described and illustrated in a form employing electron multiplier phototubes of the electrostatically focused type comprising a photosensitive cathode and output anode and a plurality of electrostatic focusing, secondary emission electrodes (commonly known as dynodes). The R. C. A. 931 electron multiplier is exemplary of suc tubes, although the invention is applicable when other various multiplier and even ordinary phototubes are employed.

Electron, multiplier phototubes such as the R. C. A. 931 are subject to a very considerable degree of sensitivity drift primarily during continued operation of the tube although there is some sensitivity drift due to aging. The sensitivity drift due to continued operation is probably caused by a number of cooperating factors such as temperature changes occurring in various elements of the phototube during continued operation thereof, such changes being either self caused or caused by variations in ambient temperature, and an inherent instability arising from the mode of operation of such an electron multiplier phototube wherein saturation current flows and relatively little space charge limiting and stabilizing effect is employed. l Y

My invention provides an electron multiplier amplifier capable of operating in a stable manner (Cl. Z50-205) case the time factor limits the sensitivity drift to a relatively small value. A sensitivity drift compensated electron multiplier phototube amplifier of the type I have invented will find eX- tremely wide application in the art, and indeed will probably replace most ordinary phototubes heretofore used.

GenerallyA speaking, an illustrative preferred form of the present invention comprises an amplier system including a phototube and an amplifier effectively connected to the output of the phototube, and inverse feedback means responsive to amplified phototube output signal arranged to modify the light input to the phototube in a negative feedback sense in accordance with the value of the amplified phototube output signal whereby sensitivity drift of the system will be minimized. In the preferred form of the present invention means are provided for intermittently, alternately, effectively rendering the inverse feedback adjustment means effectively responsive and nonresponsive to the amplied phototube output signal whereby the light input to the phototube will be intermittently modified in a negative feedback sense in accordance with the amplified phototube output signalduring the corresponding intermittent responsive periods.

The present invention may also include refer- -encing means for intermittently rendering the modulation of the input to the system (which may be electric input or light input) to be of a selected reference value during the simultanei ous, intermittent periods when the inverse feedback means is effectivly responsive to the amplied phototube output signal. The inverse feedback means may be arranged (whenever during a'responsive period the amplified phototube output signal differs from a selected standard value) to modify the light input to the photo- 'tube in a manner tending to return the amplified .a plurality of secondary emission electrodes, and

the system input is the light input to the photosensitive cathode. The referencing means may also be arranged to intermittently and frequently apply a short-duration electric signal of a selected reference value across various or all of the secondary emission electrodes of the electron multiplier phototube during the simultaneous, ini termittent periods when the inverse feedback the amplified phototube output signal from the selected standard value and when the referencing means causes the modulation of the light input to the phototube to be of a selected reference value. In another preferred general form of the present invention, an electron multiplier phototube of the type just described is employed, and the system input is an electric input 'signal applied to the secondary emission electrodes of the electron multiplier photot'ube.

The preferred form of the invention standardizes system input conditions momentarily during each intermittent peri-odrof .adjustment by negative feedback means. During 4this intermittent period of adjustment the phototube current output regulates phototube light input in an inverse feedback manneruntil the phototube current output falls Within ia narrow predetermined reference range. This virtually eliminates any change in effective sensitivity of the photoelectric scanning unit. Thus the Vpresent invention provides virtually complete Asensitivity back system .is employed it is extremely difficult 1^ to avoid the production of oscillation or instability in the system. Most prior art negative feedback amplifiers and servomechanisms inj tended for use with a rapidly nuctuating input signal are therefore compromises so arranged as to avoid instability and Voscillation and yet have the least possible .error in sensitivity drift correction compatible with the instability and oscillations avoidance requirements.

Through the use of the present invention, instability and oscillation are avoided and yet during each negative feedback period the sensitivity drift of the amplifier is virtually completely corrected without error in a manner never before attainable.

Since the sensitivity drift in an amplifier, whether of the electron multiplier phototube type or not, is a relatively slow thing, the intermittent correction thereof virtually without error is much more desirable than the continuous relatively inaccurate sensitivity drift correction of an amplifier attainable through prior art systems and methods.

With the above points in mind:

It is an object of the invention to disclose and provide a simple and efficient method of compensating for changes in response characteristics of a phototube, such as an electron multiplier, at frequent intervals during the use of the circuit in which such tube is employed.

It is an object of the present invention to provide an improved negative feedback system for virtually completely correcting sensitivity drift of an amplifier system.

It is a further object of the present invention to provide an improved negative feedback system for use with an amplier to intermittently apply negative feedback to the input to the amplifier in a manner tending to correct amplifier sensitivity drift intermittently.

It is a further object'of the present invention to provide an improved negative feedback system for use with an amplifier to intermittently apply a selected reference input modulation to the amplifier and simultaneously to apply negative feedback to the input to the amplifier in a manner tending to return the amplifier output to a selected standard Value having a fixed relationship to the vinput to the amplifier whereby to intermittently compensate for amplifier sensitivity drift.

Other and allied objects will be apparent to those skilled in the art from a careful study of the illustrations, specification and appended claims.

To facilitate understanding reference will be made to the following drawings in which:

Fig. l is a Vdiagrammatic electrical schematic drawing of one illustrative embodiment of the present invention applied to a logarithmic amplilier circuit.

Fig. 2 is a schematic diagram illustrating a modified feedback compensating system which may be used with a logarithmic amplifier system of the character shown in Fig. 1.

Fig. 3 is a diagrammatic electrical schematic drawing of a third form illustrating the application of the method to a system employing an antilogarithmic amplifier.

Fig. 4 is a schematic electrical diagram, together withran isometric view cfa scanning drum, showing means for periodically energizing the adjusting system of my invention. j

Fig. l illustrates one embodiment of the present invention employed in conjunction with a logarithmic amplifier system of the general type more particularly described in my co-pending application, Serial No.V 702,172, filed October 9, 1946, now Patent No. 2,454,871, and also described =v and illustrated in my Patent No. 2,413,706 in conjunction with a mechanically driven scanning drum adapted to carry a photographic negative or positive, or other image to be scanned by the photocathode of the electron multiplier phototube forming part of the logarithmic amplier.

In the example illustrated in Fig. l, an electron multiplier amplifier provided with m'eans for introducing an electronic input thereinto and secondary emission controlling means, is shown. The specific form of electron multiplier amplifier shown in Fig. l is of the type wherein the electronic input is provided by a photosensitve cathode 2 and wherein the secondary emission controlling means comprises a plurality of secondary electron emission controlling velectrodes (commonly known as dynodes) indicated generally at 97. The electronmultiplier` phototube l also includes an output anode 3. It should be noted that the plurality ofsecondary electron emission electrodes 91 are connected at spaced points along a resistor 4 through which acontrolled electric current passes, thus applying the proper poy tential to each of the secondary emission electrodes 97 for controllably producing secondary electron emission from each of said electrodes and electrostatically focusing the emitted electrons upon the succeeding secondary emission electrode. The originalsource of electrons is the photosensitve cathode which emits electrons in response to light input thereto, which are then electrostatically focused upon the adjacent, succeeding secondary emission electrode, this con- 75A tinuing until the last emission electrode focuses .the electrons emitted therefrom upon the output anode 3.

The resistor 4 -is connected in series with a rectifier tube 9 and terminals 5 and 6 of a regulated, constant voltage power supply 1, the purpose-of which will be more fully described hereinafter. The photocathode 2 of the electron multiplier tube is also connected to the lower end ofthe resistor 4 and is also connected to the negative terminal of a power supply 38.

`It should be noted that the terminal 5 of the .power supply is at a negative potential with respect Ato ground and that the positive terminal 8 of the power supply 1 is grounded, thus connect- 4ing the resistor 4 in series with an electron tube '26 through the cathode 21 which is also grounded. The anodev 28 of the tube 26 is connected to the positive terminal 29 of the power supply 38. Since the power supply 38, in the example illustrated, is of 1400 volts potential which is considerably greater than the potential supplied by power supply 1, normally speaking the anode of the rectifier tube 9 is negative with respect to the cathode thereof, thus allowing no current to pass through the circuit .comprising rectifier tube 9, the resistor 4 and the power supply 1. The only current which normally flows through the resistor 4 is that which flows from the power supply 38 through the resistor 4 to the terminal 5 of the power supply 1 to the positive terminal 8 thereof, which is grounded, through the cathode 21 of tube 26 to the anode 28 of the tube 26 and back to the positive terminal 29 of the power supply 38. .Since this current flows through the tube 26,

Yit can be seen that the current .normally flowing through the-resistor 4 (and therefore the potentials applied to the secondary emission controlling. electrodes 91) is controlled by the bias of the grid 25 of the electron tube 26.

The effective over-all amplificaton'of `an electron multiplier tube of the type illustrated at I is controlled by the potentials applied to the sec- ;ondary emission controlling electrodes 91 and thereforefthe amplification of the electron multiplier phototube I is controlled by the bias of the grid 25 of the electron tube 26. Potentials here given are illustrative only and not limiting.

The output anode 3 of the electron multiplier `phototube I is connected through a resistance I8 Ato the positive terminal 3| of a power supply 28',

which is grounded at 32. The terminal 3| of power supply 28', in the example, is at a 300 volt Vpositive potential with respect to ground. The

anode 3 is also connected to the grid II' of an electron tube I2, the cathode I3 of which is .grounded and the anode I4 of which is connected through a resistance I5 to the terminal 3| of the power supply 28. The anode I4 of electron tube 'I2 is also connected through a resistance I6 to the grid I8 of an electron tube I9, the cathode 28 `of which is grounded and the anode 2I of which is connected through a resistance 22 to the positive terminal 3| of the power supply 28. 'Ihe lower end of the resistor I6 and the grid I8 are 'connected through a resistance I1 to the lower terminal 21 of the power supply 28 which, in the example illustrated, is at a negative potential of inverse feedback means which will be more particularly described hereafter. The anode 2| of 4the electron tube I9 is connected through a re- .Sistance 23 to the grid 25 of the electron tube 26 Velectron 28|.

also connected through a resistance 285 to av 2I5 closes the switch 2| 6.

6 and through a resistor 24, to the negative terminal` 21' of the power supply 28. The lower end of resistor 22 is also connected to the anode 200 of an electron tube 28 I, the cathode 282 of which is grounded and also connected to a switch 283 arranged to selectively make contact with an electric contact 284 connected to the grid of the The grid and the contact 284 are terminal 286 which is connected to the negative terminal of a low voltage power supply (such as V--15 volts) sufcient to bias said tube beyond its cut-off point.

From the above description it can be seen that the output of the electron multiplier phototube I is resistively coupled to the succeeding ain-pliier stage including the electron tube I2 which is resistively coupled to the succeeding amplifier stage including tube I9 which is resistivelyrcoupled-to the succeeding amplifier stage including tube 26, and that the output of tube 26 (which corresponds to the output of the multiplier tube I), controls the current flow through the resister 4 and consequently the potential across the dynodes of the electron multipler tube I, thus controlling the amplification constant thereof in an inverse feedback manner and tending to maintain the anode output current of multiplier tube I virtually at a constant value, irrespective of light input to the photocathode 2 of the multiplier phototube. The voltage applied across the bleeder resister 4 (which can be picked offY at the output terminal 49) has a virtually logarithmic relationship to the light input to the photocathode 2. The operation of this logarithmic amplier is more fully described in the hereinabove mentioned co-pending allowed patent application Serial No. 702,172, filed October 9, 1946, now Patent No. 2,454,871, and it will not be described in detail herein.

The input to the system in the example illustrated in Fig. 1 is understood to be light modulated by a photographic positive or negative or other image by means of a scanning process. One form of scanning process which may be used for this purpose is illustrated in Fig. 4. As there shown, a photographic positive 281 is mounted on transparent cylinder 288. Light from light source 41 is focused on photographic positive 281 by means of lens 289, and the light which passes through photographic positive 281 and transparent cylinder 288 falls on phototube I. Phototube I is connected to the amplifier 2I8 by means of conductors 2| I, 2I2, and 2 I3.

Only so much of the scanning means has been shown as necessary to the understanding of the invention. The scanning unit consisting of light source 41, lens 289, and phototube I is a struc- Vtural unit which, as is well known in the art, is

driven in synchronisrn with the rotation of cylinder 288 so that the scanning unit slowly moves longitudinally of the cylinder.

A portion of the photographic positive 28T shown on Fig. 4 as shaded portion 2I4, is of uniform optical density for use in adjusting the system during the intermittent adjusting period. A cam 2 I5- is mounted on the same shaft as transparent cylinder 288 and operates the switch indicated generally at 2I6. During the intermittent adjusting period when the scanning beam is scanning the area of uniform density 2I4, cam Connection is then made between contacts 43 and 44 and between contacts 283 and 284. Contacts 283 and 284 are connected to the amplifier 2 I8 by means of wires 2li-,and 21.8. Contacts 43 and 44 ofl switch 2.I6 lare connected to motor 42 and the 110 volt A. C. power supply by means of wires 21.9 and 22D..

The Operation of the system is such that during :the intermittent adjusting period the .scanning ,beamv scans the area of uniform density 2I4 and ,atthe same time the two sections of switch 2I6 are closed. This causes the intensity of light from the light source 4'! to be adjusted in an `inverse feedback manner by the rheostat 46 which; is in turn controlled by motor 42, amplifier 2 I il' and phototube i until the anode current of phototube I falls within a narrow predetermined 'Referenoing means are provided to intermittently apply a selected reference Voltage or signal to the secondary emission electrodes 91. In the yexample illustrated this includes the electron `tube 2Q! which is connected in parallel with the looi'itrolv of the flow of current through resister 4. lthus applying a constant pre-selected reference signal to the secondary emission electrodes 91, during thev intermittentl periods when the switch 203 is closed under the control of the scanning drum 233 carrying the image 213i; beingl scanned.

The scanning drum 2381' is also arranged to close the switch 43. simultaneously with the closure of switch 233, and theV image 23:? carried by the scanning drum or the scanning drum itself, is ,provided with a strip 254 of constant optical 'density in a position such as to simultaneously modulate the light originally emitted by lamp 4l' rby a selected reference density during the period -of time when the switches 23 and 43 are closed, :whereby the modulation of the light input to the photocathode 2 will be of a selected reference =value. intermittently at precisely the same instant that a selected xed reference value poten- -tial or signal is applied to the secondary' electrodes 9? andV at precisely the same instant that the switch 43 is closed to render the inverse feedback means responsive to Variations in the amplied phototube output current.

The inverse feedback means used during the intermittent adjusting period includes an electron tube 35, the grid 34 of which is connected through a junction i! to the output of electron tube I2. In this way the output Voltage of tube I2 controls the current through tube 35 and through coil 3S of a relay indicated. generally at 38. The relay coil 38 controls the position of switch arm 39 positioned between two electric contacts 43 and 4i. If the current through tube 35 and relay coil 33 is greater than the closing current of the relay, then connection will be made between contacts 39 and 4I and the motor will be caused to rotate in one direction. If the `current through tube 35- and relaycoil 38 is less than the opening current of the relay then connection will be made between contacts 39` and 4l! and the motor will be caused to rotate in the the relay armature.- During the K put signal.

vvoltage drop. in resistor lo v'slightly greaterY than the positive potential above ground of kthe ter.- mirialrSI of DOWelsuDpl-y V28?. In the example Ygiven the voltage ofv terminalv 3| is +300 volts with respect toV ground. If the current from anode 3 of phototubeI decreases by a very small percentage from the mean value for correct adjustment, then the grid of tube I2 Will be made more Vpositive than the mean value for correct adjustment; VThe anode I4 of tube I2 and the grid 3 4 of tube 35 will in turn be made more negative than their respective mean values. The relay 38,',will be devenergized and connection will be; made between contacts 39 and 4D. This causes the motorv42to rotate in such a direction that'rheostat 46 increases the `current to scanning lamp 4l. The opposite action takes place if the current from anode -3 of phototube; I is tenter-than the normal mean, value during the automatic adjusting. period. It Should be noted that the switch 43 which is closed only during the intermittent checking periods under the control oi the mechanically driven scanning drum 26S, ina-kes it' impossible for the motor 42 to be energized for rotation in either direction except during the intermittent checking periods. The shaft 45 ofthe motor 42`is arranged to .control the 'positionA of a variable rheostat 43 connected to the scanning-lamp` 41' and through leads 48 to a suitableY source of` potential, in the vexample illustrated, 8. volts D. The motor 42 is connected through leads 44 and switch 43 to a Silitable source of potential.V

Operation orthe above embodiment of the present invention can be vsummarized as follows. Periodically during the scanning of the image 23? carried by theY driven scanning drum 28,fthe modulation of light from the lamp 4l' is momentarily change by a selectedV xed reference den,- sity, in the; example described by means oi a constant optical' density strip 2I'4 along'theedge of the image. At thesame time that the light ismodiiied by the constant density optical strip (or by other. suitablezmeans), the switches 203 and 43 are momentarily closed. The closure of the switchy 203 interrupts the control of the multiplier by the amplier and a xed reference value signal is applied to the secondary electron emission controlling means, in this case the dynodes 9T, thus standardizing the amplification conditions or" the electron multiplier I. The simultaneous closure of switch 43 places motor 42 in condition` for energization in either direction under the controlfoff the relay 38', which acts to energize` the motor in either of two directions whenever the anode current ofphototube I- deviates from a selected standard value. The energization of the motor 42 takes place in a direction such as to rotate the variable rheostat 46 in a manner whereby tocontrol the electric current ow through the lamp 47 in a negative feedback sense., Thus the system provides for a standard input modulation during the checking periods, a

standard voltage applied Vto the secondary. emissiony controlling means duringl theV checking iperiod, and provides for modif-ying the light input tothe phototube in a negative'feedback sense in response-to deviation' of the outputy signal from a standard' value during they checking period, until such standard Value is reached by the out- Thusv during each checking period, the sensitivity drift of vthe electron multiplier amplifier is Virtually completely compensated,

.The checking periods may. talee place atv the-'fend logarithmic amplier system shown in Fig. 1 at the junction 50 in lieu of the inverse feedback means of Fig. 1 shown below junction 50. The lead from the junction 50 may be connected to the grid of an electron tube 52, the cathode 53 of which is grounded and the anode 54 of which is connected'through a resistance 55 to a terminal 56 adapted to be connected to suitable source of positive potential. The anode 54 of the electron tube 52 is also connected through a resistance 51 to one end of a switch 60 and also through a resistance 58 to a terminal 59'adapted to be connected to a suitable source of negative potential.

The switch 60 is arranged to close a circuit to grid 6| of an electron tube 62, the anode 64 of which is connected `to a terminal 65 adapted to be connected to a suitable source of positive potential and the cathode 66 of which is connected through a resistance 61 to a terminal 68 adapted to be connected to a suitable source of negative potential.

to ground. It should be noted that the switch 60 corresponds in function to the switch 43 shown in Fig. 1 and is arranged to be intermittently closed by the scanning drum 208 (or other timing means) in the same manner and at the same time as switch 203. The cathode 66 of the electron tube 62 is connected in parallel through resistances 69 and 18 to grids 10 and 19 of electron tubes 1| and 80. which form the output stage of an oscillator, the cathodes of which are connected through a resister 201 to ground and the anodes 88 and 89 of which are connected to opposite ends of a primary coil 90 of an output transformer, the center tap of which is connected lto aterminal 9| adapted to be connected to a suitable source of positive potential. The secondary winding 92 of the output transformer is connected to the scanning lamo 93. which corresponds tothe scanning lamp 41 of Fig. 1.

An oscillation generator indicated generally at 81 includes electron tubes 11 and 86 4coupled to the tubes 1| and'fl through leads connecting the anodes 16 and 85 through resistances 15 and 84 to points 13 and 82 connected through capacitors 12 and 8| to the grids 10 and 19` of the tubes 1| and .80. The points 13 and 82 are also connected through resistances 14 and 83 to the terminal 9| and tbe source of positive potential.

VDeviation of thevphototube output signal is fed through the switch 60 to the grid 6| offelectron tube 62 modifying the bias thereof and current therethrough, thus modifying the grids and 19 -of tubes 1| and 80 and modifying the output oscillations of the tubes 1| and 80 which are fed by th'e oscillation generator 81. The transformer secondary winding 92 feeds oscillations to the 'lamp 93 which have been modied as herein- The grid 6| of the electron vtube 62 is also connected through a capacitor 63 Anegative feedback sense.

10 current to the grid 6| and preventing capacitor 63 from becoming charged or discharged between adjusting periods.

The embodiment of the present invention illustrated in Fig. 3 is a modication intended for use with an antilogarithmic amplifier system of the type more specifically described in my co-pending application Serial No. 702,172, now Patent No. 2,454,871. In this modification of the invention, the electron multiplier phototube 94 is provided with a photocathode 95, anode 96, and a plurality of secondary emission electrodes 91, connected across a resistor 98, the upper terminal 99 of which is connected to a source of negative potential and the lower terminal |00 of 'which is arranged to be connected to receive the system input signal. The anode 96 of multiplier tube 94 is connected through a resistance |0| to terminal |02 of a power supply |03 which,in the example shown, Vis at approximately volts positive potential with respect to ground. The anode 9S is also connected to grid |04 of an electron tube |05, the cathode |06 of which is connected through a resistance |01 to ground. The anode of tube |05 is` connected to lights ||2 and ||3 which are connected to terminal 4 of the power supply |03, which may be at approximately 350 Volts positive potential with respect to ground. Light source 2 (as modulated by vane |40) is directed upon the cathode of tube 94, whereas light source ||3 is the scanning light.

Cathode |06 of the output tube |05, is coupled through resistance |08 to the grid |09 of electron tube ||0, the cathode 5 of which is grounded and the anode ||1 of which is connected through a resistance |4| to the positive terminal |4 of the power supply |03. Grid |09 of'tube ||0 is connected through a resistance ||9 to the negative terminal |25 of a power supply |26. Said negative terminal |25 in the example illustrated, is at approximately 350 volts negative potential with respect to ground. Anode ||1 of tube ||0 is connected through the resister |20 to grid |22 of an electron tube |50, the cathode ||6 of which is grounded and the anode ||8 of which is connected through a resistance |42 to the positive terminal ||4 of the power supply |03. Grid, |22 of tube |50 is also connected through a resistance |2| to the negative terminal |25 of the power supply |26. Anode ||8 of tube |50 is also connected through a resistance |23 to the grid |21 of an electron tube |5|, the cathode |28 of which is grounded and the anode |29 of which is connected through a relay coil |30 to the positive terminal ||4 of the power supply |03. VThe grid |21 of tube |5| is also connected through a resistance |24 to the negative terminal |25 of the power supply |26.

A control switch having amovable element |32 positioned between opposed electric contacts |33 and |34 is arranged to be actuated by the relay coil |30, thus closing either of two motor circuits of thereversible motor |35 in a manner similar to that previously described in connection with motor 42 in Fig. 1, thus rotating the motor shaft |39 in either of two directions', positioning a vane or light valve |40 of suitable configuration between the light source and thev photocathode of multiplier 94. Whenever Vswitch |36 is closed during a'checking period, vane |40 will-modify the light impinging on the photocathode 95 in a Switch |36 may be actuated in any suitable manner andresembles switch 43 in characteristics. v

It should be noted,lthat in this case the input 11 to the-system is an electric input signal applied to the-secondary-emission controlling means and that the output from the system corresponds to the phototube output signal and can be taken `from-the system at any suitable point. It may comprise the light emitted from either or both ofthe light `sources lf2-U3 or it may comprise the current iiowing therethrough or the amplifled-signal correspondingthereto. It will be seen that in Y.this case the relationship of output to input is virtually antilogarithmic, Whereas the relationshipbetween-system output and svstem input in the logarithmic circuit employed in conjunction with the apparatus of the present invention and illustrated in 1, is virtually logarithmic.

While the present-invention has been described in connection with an electron multiplier amplifier havingr va photocatbode arranged to provide an electronic input to the secondary emission controlling means in Vresponse: to light imningincr the photocathode. it is to be 1'nd^rstood that the inventionis not limited to this arrangement, Vsince `any type of electronic input to lan electron -multiplier tube cold be substituted .for

the photoelectric input. For example, the -l^c tronic input to the secondarv emission controlling means .might be of a tberrpionic type. or va rious other tyres loi" electronic input may be emploved with` the present invention. While the present invention is described as employing secondary emission electrodes, it is not limited to such. an arrangement, since any type of secondary emission controlling means may be employed.

For example, vmagnetic electron focusing and/or secondarv emission controlling means maybe employed. Vsingle or in conjunction with electrostatic secondary ,electron emission controlling and/or'focusing means.

The referencing `means mayalso be modified Within wide limits Without departing Vfrom the spirit of the present invention. Any means Vfor providing intermittent iixed reference modulation of system input and simultaneous inverse feedback to the amplifier for causing the modiiication of the-ampliner output to a xedstandardbearing a certain preselected relationship to the 'iixed selected Vreference input may be employed. Numerous types of inverse feedback means may he employed. The means for .modifying thelight input may also be modiiied .within Wide limits.

Those skilled inthe art will, from .the description given, understand that both a constant density stripand a vane may be used in modulating the light supply to the phototube simultaneously, or sequentially. Y

The 'inverse Vfeedback may be applied to the secondary emission controlling means rather than'to the electronic input to the electron multiplier tube if desired. Thisis more particularly described, illustrated andclaimed in my copending application, SerialNo. 55,034, filed concurrently herewith, which is now U. S. Patent No. 2,534,668, granted December 19. 1,950.

The examples described ,and illustrated herein are 'exemplary only, andare not intended to limit lthe-scope of the present invention, which is to be interpreted inthe light of the appended claims only.

Iclaim: 1. A method of compensating for changes Yin response characteristics of a multiplier phototube and vampliiier controlling said multiplier phototube during a scanning operation, which comprises: periodically interrupting the control,

lil

of said multiplier phototube by said amplifier and simultaneously subjecting the multiplier `photo'tube both to scanning light modulated "by a standard density and to a predetermined potential and adjusting the intensity of the scanning light -in accordance With the response of `the phototube and amplier While under the'inuence of vsaid light and predetermined potential.

2. A'method of compensating for changes in response characteristics `of va multiplier phototube in .an operating circuit of a scanning .device which comprises: periodically interrupting va scanning operation andsubjecting the multiplier phototube to a .predetermined'potentiatand a light modulated by a standard density and aidjusting the' lightin accordance with the response 'of-thephototube While under the 'influence of .the predetermined Ypotential andk .modulated j light; andcontinuing the scanning operation withjthe lightinits adjusted state.

`3. In a method of automatically .compensating for changesin response characteristics oanam- ,.plirler including a .multiplier phototube .having Ya .plurality of secondary .emission electrodes, .the steps of: intermittently and frequently interrupt- -ingthe operation of an amplifier Vfor a periodpf Vshort durationV and subiecting the .multiplier phototuhe .to a 'predetermined .uniform potential and .a light .of `a yalue `expecteri to produce .a selected, fixed response from thephototube; .and adjusting thelight input to Athe phototuloe dur.- ingsaid period in a` negative feedback mannerin accordance with deviation Vcf 'thephototdbeeresponse, while under .said L.predetermined potential, Vfrom the selected response; vand.continuing the operation `0l" the amplifier with the lightinits adjusted state until -.the .succeeding .checking period. Y l

4.1m a method of .correcting forlchanges ,in .sensitivity .of -a scanning system, including .a scanning `light and .an amplifier provided .with a phototube, 1the steps .of -periodically and .intermittently interrupting the scanning .operation and subjecting .the phototube .to .a predetermined, uniformpotential and .a .lightnia value Aexpected to produce a `selected response from ithephototube; adjusting the lightinput .to the-.photocell in accordance with. deviatior-i-of .the phototube response, while under said Ypredetermined .potential., from theselected response; and continuing .the scanning .operation with .thelightin gitsad- .justedstate thereof: means for supplying light; an inverse .feedbackmeans` operably connected to ythe-phototube .and amplifier, said Yfeedback means being 4responsive Vto phototube output `to modify Jthe light to Ythe vphototube rin accordance -with the 'output of the phototube; means for periodically and intermittently restoring the modulation :of the Isystem input to a selected, fixed reference value; Tand means 'fori-rendering the inverse 4feedback means responsive -only during said intermittent checking periods, said feedback means being arranged to modifythe light input in a direction tending to return the output -signal-to a selected standard value whenever, during such checking periods, the -phototube output differs from a desired value. Y

6. A system of the characterstatedin claim 5 wherein the phototube is an electron .multiplier phototube amplier .of the type including .-.a

if Pmioenive Cathode., an Output anode and a plurality of secondary emission electrodes, and wherein the modulated system input is the light input to the photosensitive cathode of the phototube.

'7. A system of the characterstated in claim wherein the means for intermittently restoring the system input toa selectdvalue include means for applying an electric signal of a selected fixed reference value across the secondary emission electrodes of the electron multiplier phototube amplifier during the simultaneous periods when the inverse feedback means is responsive to deviation of the amplified phototube output signal from the selected standard value, and when4 the light input to the phototube is modulated by a selected reference fixed density.

8. A system of the character stated in claim 5 wherein the means for intermittently restoring the modulation of the system input comprises a mechanically driven scanning system arranged to carry an image to be scanned by the phototube, said image being provided with a strip of constant modulating effectiveness arranged to intermittently modify the modulation of the light input to the phototube to a selected fixed reference value during scanning of the image.

l9. A scanning system of the character stated in claim 5 wherein the phototube is a multiplier including a photosensitive cathode, an output anode, and a plurality of secondary emission electrodes; and the means for intermittently restoring the modulation of the system input comprise a mechanically driven scanning drum, a strip of constant modulating effectiveness arranged to intermittently modify the modulation of the light input to the phototube and switch means correlated with the scanning drum to intermittently apply an electric signal of desired reference value across the secondary emission electrodes of the phototube simultaneously with the modulation of the light input to the phototube by the strip.

10. A scanning system of the character stated in claim 5 wherein the inverse feedback means include a reversible motor, variable rheostat means operably connected to the motor, a light source under the control of the rheostat, reversing switch means for energizing the motor for rotation in either direction, and relay means responsive to deviation of the amplified phototube output for actuating the reversing switch means.

11. A scanning system of the character stated in claim 5 wherein the inverse `feedback means include a reversible motor, variable rheostat means operably connected to the motor, a light source under the control of the rheostat, reversing switch means for energizing the motor for rotation in either direction, and relay means responsive to deviation of the amplified phototu'oe output for actuating the reversing switch means, the means for intermittently rendering the feedback means responsive including switch means arranged to energize and de-energize the motor in timed relation.

12. A scanning system of the character stated in claim 5 wherein the inverse feedback means includes reversible motor means, light valve means operably connected to the motor and arranged to modify light input to the phototube in accordance with the direction of rotation of the motor, reversing switch means for energizingthe motor for rotation in either direction, and relay means responsive to deviation of th'e phototube output for actuating the reversing switchmeans.A 13. A system of the character stated in claim 5 wherein the inverse feedback means comprises electric oscillation generating means; and including modulator means arranged to modulate the output oscillations produced by the electric oscillation generator in accordance with the amplified deviation of phototube output current from a-fixed reference value; and scanning lamp means arranged to be effectively energized by the output electric oscillations and to supply light input to the phototube.

14. A system of the character stated in claim 5 wherein the inverse feedback means comprises electric oscillation generating means; and including modulator means arranged to modulate the output oscillations produced by the electric oscillation generator in accordance with the amplified deviation of phototube output current from a fixed reference value; and scanning lamp means arranged to be effectively energized by the output electric oscillations and to supply light input to the phototube, the means for rendering the inverse feedback means responsive to the phototube output including switch means arranged to connect and disconnect the amplified phototube output to the modulator means in the inverse feedback means in timed relation to the scanning of an image and only during checking periods when the modulation of the light input into the phototube is of fixed reference value.

15. A system of the character stated in claim 13 wherein the electric oscillation Vgenerating means is of the square wave generating type.

16. In an electron multiplier amplifier system arranged to receive a modulated system input, the provision of: means for supplying electronic input to the electron multiplier amplifier; and inverse feedback means effectively responsive to the electron multiplier output signal arranged to modify the electronic input to the electron multiplierin a negative feedback sense in accordance with the electron multiplier output signal, whereby sensitivity drift of the amplifier will be minimized.

1'7. A system of the character stated in claim 16 including: means for intermittently, alternately, effectively rendering the inverse feedback means effectively responsive and non-responsive to the electron multiplier output signal, whereby the electronic input to the electron multiplier amplifier will be modified intermittently in a negative feedback sense in accordance with the electron multiplier output signal during the corresponding intermittent responsive periods.

18. In an electro-optical amplifier system arranged to receive a modulated system input rand including a phototube and an amplier connected to the output thereof to produce an amplified phototube output signal, the provision of means for supplying light input to the phototube; inverse feedback means responsive to the amplified phototube output signal arranged to modify the light input to the phototube in a negative feedback sense in accordance with the amplified phototube output signal; and means for periodically and repetitively rendering the inverse feedback means effectively responsive fora period of short duration to the amplified phototube output signal, whereby the light input to the phototube will be modified periodically duringl the corresponding intermittent responsive periods and sensitivity drift of the system will be minimized.

19. A system of the character stated in claim 19 gummi;

I5 16 means for intermittentlyrestoring the UNITED? SFIMYISES PATENTS modulationsof theesystem nputmo a selectedxed. Number Name Date reference value during the simultaneous, inter- 20.12.573 Long. Aug. 27 1935 mittenuperiods when the inverse feedback means 2153193' Morse"`.`""`"` Mag; 16 1939 is: responsive to the ampled Phototube Output 6 213472015 Wolosc-n-nmu Apr. 18-1 1944 Slanal 2,41-1f,44,o LeY Page Nov. 19, 1946 NORMAN R- GUNDERSON 2,412,423.. Rajcnman et a1. Dec.- 10, 194e REFERENCES CITED The following references are of record in the 10 fue ofthis patent:

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