US2537250A - Electronic tube - Google Patents

Description

P. K. WEIMER ELECTRONIC TUBE Jan. 9, 1951 3 Sheets-Sheet 1 Filed Aug. 5, 1945 I ENTOR.

BY w/m azw/ Jan. 9, 1951 P. K. WEIMER ELECTRONIC TUBE Filed Au 5, 1945 3 Sheets-Sheet 2 I'll INVENTOR. ME

Wail/la firm/Mi) landing on the target remains the .same.

Patented Jan. 9, 1951 ELECTRONIC TUBE Paul K. Weimer, Princeton, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application August 3, 1945, Serial No. 608,663

1 12 Claims.

This invention relates to television pick-up tubes, particularly those having two-sided targets. A suitable type of two-sided target has previously been developed which consists of a thin sheet of semi-conducting glass having such thinness and conductivity that a charge image formed on one side will be discharged'within a frame time by electrons landing from a beam scanning the other side. The glass, on the other hand, is sufiicienty non-conducting to prevent the spreadng of the charge image to a material extent within a frame time. A television pick-up tube having such a target is disclosed in the application of Albert Rose, filed November 28, 1945. Serial No. 631,441, which is now U. S. Patent 2,506,741, granted May 9, 1950 and an improved type is disclosed in' the applicat on of Harold B. Law, filed April 19, 1945, Serial No. 589,241, new U. S. Patent 2,460,093 issued January 25. 1949. This type of tube is generally known as the image orthicon.

The image orthicon is particularly suited for electron multiplication of the modu ated cathode beam returning from the region of the target and I have developed an efficient electron multiplier for use in this type of tube, .on which application was filed September 16, 1944, Serial No.

554,494,.now U. S. Patent 2,433 ,941, issued January 6,1948.

In the image orthicon, as disclosed in the foregoing applications, a charge pattern is formed on the unscanned side of a thin glass target, which sets up a potential pattern on the scanned side of the target. As the beam is scanned at substantially zero velocity over the target in an electro-magnetic focusing field, sufficient electrons land to bring the elemental areas down to cathode potential as a potential datum and the remainder of the beam returns to the multipliers.

This manner of discharging the target image has certain disadvantages, an important one .being the limitation of signal-to-noise ratio. The signal resulting from this type of discharge is proportional to the electrons landing on the tar-'- get from the beam for discharge of theimage; The beam must have sufiicient intensity to furnish suflicient electrons vior this purpose, but increas ing the intensity above this value does notin crease the signal, as the number of electrons On the other hand, noise in apick-up tube is proportional to the square root of the beam current; therefore, low signalto-noise ratio in the prior types of image orthicon is a necessary .consequence of small beam current.

Another object is to provide a pick-up tube in which more charge can he landed from the beam than is stored on the target.

Another object of the invention is to provide an image orthicon in which thecharge image on the target is discharged by means other than the scanning beam.

Another object of the invention is to provide an image orthicon in which the potential controllng the landing of the electrons from the cathode beam adjacent the charge image is constant while an elemental area is being scanned.

Another object .of the invention is to provide a tube with a two-sided target that is discharged independently of the cathode beam.

Another object of the invention is to provide a target and associated elements so that signals proport onal to the charge image may be generated without removing the charge image there-'- by Further objects of the invention will appear in the following description, reference being had to the drawings, in which:

Fig. 1 is a .diagrammatical illustration of an image orthicon containing my improved pick-up tube with its operating'circuit.

Fig. 2 is an enlarged section through a very small area of the control screen indicating the equipotential lines of the electrostatic field in the vicinity of the scanned side of the target screen;

Fig. 3 is graph of current produced by the electrons landing on the discharge screen vs. the

potentials of the target.

Fig. 4 illustrates how my invention may be used to produce a charge-image amplifier.

Fig. 5 is a modification of the target of Figure 4.

Referring to Fig. 1, the improved image orthicon consists of an evacuated envelope l containing a gun 2 at one end thereof with a glass anode are partially sectioned to illustrate their tubular construction. The grid and first anode have minute orifices, as indicated, through which the beam 9 is projected in the usual Way to reach the target with approximately zero velocity. Tubular electrode I is axially placed around the front part of the gun adjacent thereto. This is often referred to as a persuader, because its function is to direct the secondary electrons I emitted by the dynode 8 into the succeeding multiplier dynode, as will be hereinafter referred to. Wall coating II may be applied to the inner Wall of envelope I in known manner. 7

Outside the envelope I is placed the deflecting unit I2, consisting of a frame having two electromagnetic coils with their field axes perpendicular to each other and to the longitudinal axis of the tube. One of these coils deflects the beam in a vertical direction in Fig. l and the other deflects it at right angles to the plane of the drawing. These coils are of well-known construction and hence have not been individually shown. It will be understood that the deflection coils will have varying voltages applied thereto, say by saw-tooth generators of suitable frequencies, to produce line and field scansion. The deflecting unit I2 is made manually adjustable by suitable means, such as a small rod I3, for balancing the electrostatic and magnetic forces for uniform landing on the target, as disclosed in my copending application, filed November 28, 1945, Serial No. 631,440, which is now U. S. Patent 2,533,073, granted December 5, 1950.

Outside of the envelope I is compensating coil I4 having a' field'perpendicular to the axis of the tube. By adjusting this coil around the tube axis, any slight miscenteringof the beam due to mechanical imperfections can be eliminated. Also, outside the coils I2 and I4 is placed coil l5, which produces a strong magnetic focusing field parallel to the axis of the tube on both sides of the target.

Around the gun is placed a plurality of ad ditional multiplier dynodes I6, I1, I8 and I9 and a coll cting electrode 20, which is a screen. This is connected to the desired utilization device indicated as an amplifier tube 20. The multiplier dynodes and the other parts of Fig. 1 thus far referred to are particularly described in my said application and it will be sufiicient' to say respecting the dynodes I6, I! and I8 that'each consists of angular radial blades 2!, somewhat like an electrical fan, held in a suitable annular frame, having a suificient axial opening to pass in non-conducting relation over the first anode l. Multipliers of the type disclosed in my copending application, filed August 21, 1945, Serial No. 611,720, now U. S. Patent 2,443,547 issued June 15, 1948 or any other type, may also be used, as my invention is not limited 'to any par ticular type of multiplier.

In front of the blades of the dynodes shown are screens 22 secured to the annular frames so as to be in electrical contact with the associated multiplier blades 2|, from which they are suitably spaced. The multiplier dynodes I6, I? and I8 are partially broken away toshow the blades 2I and screens 22. The multiplier dynode I9 is the final multiplier stage and consists of a flat annulus spaced from and surrounding the first anode, similar to the other dynodes. The multiplier dynodes 8, I6, I'I, I8 and I9 may have any desired coating of active material to produce suitable emission of secondary electrons upon 4 bombardment by primary electrons. Various voltages may be applied to the dynodes and other electrodes of the tube. By way of example I have indicated in the drawing suitable voltages.

At the opposite end of the tube, the photocathode 4 may be a semi-transparent coating of light-sensitive emissive material on the inside of the end of the envelope I. A frame 23 of metal supports the thin glass target 3, such as disclosed in the said Rose application, and also supports the mesh collecting screen 24 spaced from the'tanget, preferably as disclosed in the said Law application, but it may be spaced and supported in any other desired way. The various electrodes would be supported in well-known ways, not illustrated.

Adjacent the glass target sheet 3 is arranged the decelerating ring 25, which is grounded to gun cathode potential or thereabouts. Between the frame 23 and the photocathode 4 is electrode '26, which is connected to a potential more negative than ground, but having sufiicient positive potential relative to that of the photocathode I to accelerate at high electron velocity the photo-electrons onto the glass target sheet 3 along the magnetic lines of the focusing coil I5. Suitable voltages for electrode 26 and photocathode 4 are indicated in the drawing.

The parts thus far described in Fig. 1 are disclosed in my said application 554,494 now Patent 2,433,941 and are not, per se, claimed herein.

In the orthicon of my said application, the charge pattern formed on the unscanned side of target 3 is discharged by the electrons landed from beam 9 on the scanned side of the target, which brings it to the potential of the cathode as a datum potential. In the invention of this application, the electrons from the beam do not land in substantial quantity on the target 3. The potential pattern thereon does, however, control the landing of beam electrons on adjacent metal mesh screen 21 and hence modulates it in'accordance with the potential pattern. This is accomplished without discharge of the chargeimage on the unscanned surface of target 3. The discharge screen 21 is substantially on the surface of glass target 3 but spaced therefrom on the gun side' sufiiciently to prevent conductive contact. This mesh screen 21 has the potential of the cathode of the gun or slightly positive thereto. The collecting screen 24 controls the average potential of the glass target 3 and may be biased from 2 to 6 volts or more negative to the discharge screen 21.

Fig. 3 shows the average potential of the collecting electrode 24 relative to the current produced by the electrons landing on the mesh discharge screen 21. For one method of operation, the negative bias of mesh screen 24 is adjusted so that the variation in potential due to the charge image may fall on the part A--B of the curve of this figure and for another method, operation may be had on the portion of the curve CD.

The theory of operation will first be explained when the bias voltage of mesh screen 24 is such that operation is had on the part A-B of the curve of Fig. 3.

' Suppose a moving scene is focused on the photocathode I (Fig. 1). Photo-electrons are emitted and are projected at high velocity along the magnetic focusing lines of coil I5 through mesh screen 24 onto the unscanned side of the glass target 3. A charge pattern is formed on this surface of the glass 3, due to the emission of secondary electrons, whichare collected by mesh screen 24, leaving the target surface varyingly positive in proportion to the light density of the picture projected onto photocathode 4. The target has capacitative relation with discharge screen 21 and the charge pattern sets up a potential near and around the discharge screen. The bias voltage field applied to the collectin screen 24 sets up a voltage around the screen 21, the equipotential lines of which are indicated :in Fig. 2. The charge-image on the photocathode side of the glass target 3 varies these potentials in proportion to the charge-image.

It will be seen from Fig. 2 that the negative bias voltage applied to the collecting mesh screen 24, makes the path in front (gunside) of the open meshs 28 of discharge screen 21 more negative than the pathln front-of the wires 29 of screen 21, which have zero potential difierence in respect to the gun cathode. The electrons from the beam, as they approach the glass target '3, find in their path the +0.4V potential line, the zero potential line, the -0.4V, -;6V and -1.0V negative potential lines. The electrons will be deflected to less negative potentials and 29) that has a definite. potential/though they I reach that electrode by passing through a potential field controlled by the charge pattern. In my said prior application and in other prior art structures, as far as I am aware, the electrode on which the beam electrons land (glass target-3 or equivalent) changes its potential as the electrons land.

It will be appreciated that the electrons landing on screen wires 29 do not discharge the charge pattern on the photocathode side of the target 3. However, with a moving scene in the next one or more frames, as the light changes, the secondary electrons emitted by photo-electron bombardment of the glass target will not all go to the collecting screen 24, but enough will rain down on the elemental areas of the previous chargepattern to discharge the scanned elemental areas. Thus, in my improved pick-up tube, the landing of photo-electrons produces the discharge of the charge pattern by bombarding secondary electrons from the glass target that land on adjacent scanned areas in a frame. The discharge of the image by means other than the landing of beam electrons on the target areashas the important advantage that urface conditions on the screen electrode 21 may bevaried to control the effects of the landing thereon without varying the effect of the charge-image on the beam.

With stationary scenes the glass target will, of course, notbe discharged at all, as the photoelectrons bring the potential up on the elemental areas and counteract the raining down of Sec ondaries from adjacentjareas, With stationary pictures the illumination therefore builds up to saturation, the charges in successive frames being added to those of previous frames. Thus, in stationary pictures my invention is an improvement over the usual tube, in which the beam discharges the scannedv areas, in that greater charge-image is obtained 'through'build-up. An-

important advantage of 'this' 'action is that, in taking stationary television pictures with very low lighting, the full storage time of the target may be many times greater than a frame time. This permits one to construct a television pickup tube of extremely high sensitivity and time e posure may be taken, as in photography, while continuously transmitting the picture.

Incase moving pictures are taken where the average light intensity suddenly decreases, the secondaries produced by the photo-electrons may not rain down in the surface of target 3 sufiiciently to discharge the charge-image in the desired time. In that case, the negative potential bias of screen 24 can be reduced so that some of the electrons of the beam will land and help discharge the image.

The beam modulated by the potential-pattern of the charge-image returns toward the gun to the first dynode 8, where the secondaries bombarded therefrom are directed through screen 22 into the second dynode l6 under the influence of the potentials in dynode l6 and persua'der it. The secondarie emitted by the vanes 2| of dynode l6 pass through screen 22 to the plates of the third dynode I1 and a similar operation is carried out in dynodes I 8 and i9. The greatly multiplied secondaries from the last dynode i9 are collected by screen electrode 20 and utilized in the circuit typified by amplifier tube 29.

If desired, the signal may be obtained from the discharge screen 21 without multiplication and the multiplier structure omitted. This will be particularly feasible in my improved tube, as the intensity of the beam current can be increased 'tobring up the signal value, as the signal-to-nois ratio is expressed in the ratio a L k f as already explained.

By decreasing the negative bias of screen 24 relative to screen .21, operation may be had on portion CD of the curve of Fig. 3. In thi operation, the charge-image is less negative to the screen 21 and the light regions of the picture tend to attract beam electrons more directly toward the openings 28 ofthe screen. These, however, do not land but return to the multiplied dynodes. In the dark areas of the picture, the electrons tend not to enter the openings 28, due to the :lower potential established by the charge-image and have a greater tendency to be defiected to the wires 29. Less beam current would therefore return to the multiplier for the dark regions. With this method of operation, maximum beam current would reach the multiplier for white regions and minimum current for dark regions. This reversal of polarity produces an important result. It is well known that, in the usual image orthicon, noise is most noticeable and objectionable in the dark regions of a picture and myimprovement reduces the signal-tonoise ratio in the dark regions by reducing the beam current going to the multipliers. Another important result in this method of operation is that the reduced beam current reaching the first dynode of the multiplier will reduce the spurious signal produced by the non-uniformity of the 7 surface or other characteristics of the dynode,

7 on the glass target 3 which is a target for the photo-electrons.

A modified method of operation may also be had on the portion CD of the curve by biasing the discharge screen 21 2 to volts positive relative to the gun cathode (ground), so that secondary electrons are emitted therefrom under the impact of the beam electrons. These secondary electrons will escape and accelerate to the multipliers in greater number when the beam is over the bright areas because the glass swings less negative, that is, in the direction D, and less secondary electrons will escape from the screen indark areas when the potential of the glass target swings more negative, that is, toward C. In this modified operation, substantially no beam electrons land on the relatively negative glass target.

In Fig. 4 I have indicated how image amplification may be obtained with a modification of my improved target. The image produced on glass target 3 would be produced by collecting on a collector screen 24" the secondary electrons bombarded from the surface of glass target 3 by the photoelectrons from photo-cathode 4. Discharge screen 21" would be sensitized and light rays 30 would produce photo-electrons therefrom. Focusing coil l5 may be arranged in sections to permit this entrance of light. The photo-electrons leaving the sensitized screen 2'! will be greater in number than the photo-electrons landing on the other opposite surface of the glass target 3 and will be proportional to the potential pattern of the charge-image produced by the lastmentioned photo-electrons on target 3. Thus, the screen-excited arrangement constitutes an amplifier stage. The amplified photo-electrons emitted by screen target 21 will be accelerated and focused onto another glass target 3' through collecting screen 24' to. produce an amplified charge pattern thereon by emission of secondary electrons collected by screen 24'. This will control' the photo-electrons emitted by sensitized screen 21' and the photo-electrons from this second stage will be accelerated and focused on a third glass target 3" through collecting screen 24 and produce a further amplified charge-image. The operation from this point on will be the same as already described. Thus, an amplified image can be produced before scanning the beam 9 over the target.

The discharge screens 21 and 21" may be caused to emit electrons by thermal treatment under control of the charge-image and the action would be the same as just described. For example, the screens 2'! and 21" may consist of strands 3| arranged so that a heating current from a source 32 may be passed through them in parallel, as indicated in Fig. 5, and cause them to emit electrons under control of the potential pattern of the adjacent glass target. In this case, the focusing coil may be of standard construction instead of the form shown in'Fig. 4. Obviously, the strands of the screen may be arranged in series instead of in parallel. In the image amplifier of Fig. 4, it would be preferable to make the glass target opaque to prevent scattering of light and reduction of contrast.

Various modifications may be made without departing from the scope of the invention.

Having described my invention, what I claim is:

1. An electron tube system comprising a tube having a target, a screen electrode closely adjacent said target, a multiplier dynode relatively remote from said screen electrode, means including'a photo-cathode for producing achargeimage and a potential pattern on said target, a potential source biasing said target negative to said screen electrode, means for supplying electrons to a region closely adjacent elemental areas of said screen electrode, the potential of said pattern opposite each of said elemental areas causing one current of electrons to move from said region toward said screen electrode and another current thereof to move toward said dynode, one of said currents being directly proportional to said potential and the other being inversely proportional thereto and means for producing a focusing field between the screen electrode and the multiplier dynode.

2. An electron tube system comprising a tube having a target sheet, a screen electrode closely adjacent one surface of said image screen, a multiplier dynode relatively remote from said surface, means including a photo-cathode for producing a charge-image on the other surface of said target sheet and producing potential pattern on the first-mentioned surface, a potential source biasing said target sheet negative to said screen electrode, means for supplying electrons to a region closely adjacent elemental areas of said screen electrode, the potential of said pattern opposite each of said elemental areas causing one current of electrons to move toward said screen electrode and another current to move toward said dynode, one of said currents varying directly with said potential and the other one varying inversely therewith, and means for producing a focusing field between the screen electrode and the dynode.

3. A pick-up tube system comprising a tube having a cathode ray beam gun, a target sheet, an electrode adjacent one side of said target sheet, and means for forming a charge-image on the other side of said target sheet, a potential source applying substantially the same'potential to said electrode and the cathode of said gun, means for scanning the beam of said gun over said electrode, and a. potential source biasing said target sheet sufiiciently negative to said electrode to prevent substantial landing of beam electrons thereon.

4. A pick-up tube system, comprising a tube having a cathode ray beam gun, a target sheet, a mesh screen adjacent one side of said targetsheet, a photo-cathode spaced from the other side of said target sheet, a potential source applying substantially the same potential to said screen and the cathode of said gun, electric field producing means for projecting photo-electrons from said photo-cathode onto said other side of said target sheet to bombard secondary electrons therefrom, an electrode between the photo-cathode and the target for collecting said secondary electrons, a potential. source connected to said electrode having a potential adapted to produce a bias on the targetsheet sufiiciently negative to the screen to prevent beam electrons landing on said target sheet and electric field producing means for scanning said beam over the screen for landing electrons thereon under control of the potential set up by the emission of said secondary electrons.

5. A pick-up tube system comprising a.tube having a cathode ray beam gun, a dielectric target, a first mesh screen adjacent one side of said target connected to the cathode of said gun to have substantially its potential, a photo-cathode on the other side of said dielectric target, field producing means for projecting photo-electrons to said first screen to prevent substantial landing of beam electrons on said target through the meshes of said first screen without blocking their landing on the strands thereof under control of said potential pattern, and field producing means for scanning said beam over said first screen.

6. A pick-up tube system comprising a tube having a cathode ray beam gun, a multiplier dynode positioned around the axis of said gun and adjacent the front end thereof, a dielectric target, a mesh screen at one of the sides of said dielectric target connected to the cathode of said gun to have substantially its potential, a photocathode and field producing means on the other side of said target for projecting photo-electrons thereonto at sufficient velocity to bombard secondary electrons therefrom and produce a potential pattern on the first-mentioned side, an electrode between the photo-cathode and said dielectric target for collecting said secondary electrons, a potential source connected to said electrode adapted to set up a sufl'iciently negative potential in the meshes of the screen to prevent substantial landing of beam electrons on said target and permit their landing on said screen under control of said charge-image, field producing means for scanning said beam at substantially zero electron velocity over said mesh screen, field producing means forming a uniform electromagnetic focusing field axially of said gun, said gun accelerating the electrons of the'beam not landing on said screen along lines of said focusing field to land on said dynode with suflicient electron velocity to bombard'secondary electrons therefrom.

7. A pick-up tubesystem comprising a tube having a cathode ray beam gun, dielectric target, a mesh screen at one of the sides of said dielectric target connected to the cathode of said gun to have approximately its potential, a photocathode and field producing means for forming a charge-image on the other one of said sides of said dielectric target, said charge-image setting up a potential pattern on the first-mentioned side, field producing means for scanning said beam in front of the elemental areas of said target, an electrode, a potential source negatively biasing said target relatively to said mesh screen for landing beam electrons on the mesh screen at a rate varying inversely with the decrease of said negative bias by said potential pattern.

8. A pick-up tube system comprising a tube having a cathode ray beam gun, a target structure including a glass sheet and a mesh screen at one of the sides of said glass sheet, means connecting said screen to the cathode of said gun to have approximately its potential, a photo cathode and field producing means for forming a charge-image on the other one of said sides of said glass sheet setting up a potential pattern on the first-mentioned side, field producing means for scanning said beam in front of the elemental areas of said screen, an electrode, a potential source negatively biasing said glass sheet relatively to said mesh screen for landing beam electrons on the mesh screen at a rate varying directlywith the decrease of said negative bias by said potential pattern.

9. The method of operating a cathode ray beam pick-up tube having a gun, a dielectric target, a mesh screen adjacent the gun side of said dielectric target, said method comprising the steps of applying a potential to the mesh screen substantially equal to the potential of the cathode of the gun, biasing the dielectric target sufficiently negative to the mesh screen to prevent the electrons of the beam from landing on the dielectric target and producing a charge-image on the dielectric target to varyingly decrease said negative bias to land electrons of the beam on the mesh screen under'control of the potentials set up by said charge-image.

10. The method of operating a cathode ray beam pick-up tube having a gun, a dielectric target sheet and a mesh screen closely adjacent one side of the dielectric target sheet, said method comprising the steps of applying a potential to the mesh screen substantially equal to the potential of the cathode of the gun, producing a charge-image on the dielectric sheet, scanning the beam over the mesh screen, biasing the dielectric sheet sufficiently negative to prevent electrons landing thereon and to cause them to land on the mesh screen in proportion to the charge-image.

11. The method of operating a discharge tube having a dielectric target sheet and a mesh screen closely spaced from and overlying one surface of said dielectric sheet, said method comprising the steps of establishing a charge pattern on said dielectric sheet, establishing a potential on the mesh screen, providing a flow of electrons away from the mesh screen, and biasing the dielectric sheet sufficiently negative relative to the potential of the mesh screen to control the electron flow away from areas of the mesh screen in proportion to the charge on the underlying portion of the dielectric sheet.

12. The method of operating a discharge tube having a dielectric target sheet and a mesh screen closely spaced from and overlying one surface of said dielectric sheet, said method comprising the steps of, establishing a charge pattern on said REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,176,190 Schroter Oct. 17, 1939 2,254,617 McGee Sept 2, 1941 2,407,906 Rose Sept. 17, 1946 2,433,941

Weimer Jan. 6, 1948

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