US2256461A - Electron image tube - Google Patents

Description

Sept. 16, 1941. H. A. lAMs ELECTRON IMAGE TUBE Filed April 23, 1940 2 Sheets-Sheet l I Il s b f,

INVENTOR. HARLEY A. fAMS ATTORNEY.

Sept. 16, 1941.

H. A. IAMS ELECTRON IMAGE TUBE Filed April 25, 1940 2 sheets-sheet 2 INVEN TOR. HARLEY A. [AMS ATTORNEY.

*Patented Sept. 16, 1941 mausi Emc'rnoN maar: ma

Harley A. Iams, Summit, N. J., assignor to Radio Corporation ot America, a corporation o! Deiaware Application April 23. 19x40, Serial No. 331,092

9 Claims.

My invention relates to cathode ray tubes of the television transmitting and image types and more particularly to improved types of cathode ray tubes wherein an electron image is focused on an electron receiving target.

A certain type of distortion known as shear distortion results in many cathode ray television tubes, both of the transmitting and image recreating types when the electrons are directed from their origin such as a photocathode emitter upon a receiving surface disposed at an angle with respect to the electron origin. Thus, it appears that when the electron paths are made to curve from the emitter .to the electron receiving surface or target by the use of a cylindrical magnetic field a shear distortion results. In my copending application, Serial No. 210,952, iiled May 31, 1938, now United States Patent 2,213,548, I have disclosed a television .transmitting tube wherein the electrons are caused to follow semi-circular or arced paths. The principal advantage of such a tube resides in the fact that the light incident upon the photocathode is preventedfrom falling upon the target which may likewise be photosensitive, but I have found that the shear distortion present in such devices produces a final electron image which is somewhat distorted so that the electron image of a rectangular optical image has a rhomboidal cross-section.

It is an object of my invention to provide a cathode ray tube of the type described wherein shear distortion is eliminated. It is a further object to provide a television transmitting tube capable of generating signals having low signal components due to extraneous light and wherein the signals representative of the incident optical image are generated Without the introduction of distortion effects while at the same time retaining the advantageous use of cylindrical magnetic focussing fields. `It is a still further object of my invention to provide a tube of the image recreation type wherein the advantages of the use of a cylindrical magnetic field may be obtained and an optical image may be recreated as an intensified replica without the introduction of shear distortion.

In accordance with my invention, electrons forming an electron image are directed vfrom one electrode to another positioned at opposite ends of an S-shaped path and focused over the length of the path by cylindrical magnetic fields to eliminate shear distortion. More particularly, the S-shaped path comprises two contiguous successive curved paths having nite radii of curved paths to the respective centers of curvay ture so that shear distortion occuring over one path is neutralized =by the shear distortion occuring over the other path.

These and other objects, features, and advantages of my invention will appear and a better understanding of my invention will be obtained from the following description taken in connection with the accompanying drawings in which;

Figure 1 is a cross-sectional longitudinal view of a television transmitting .tube made in accordance with my invention;

Figure 2 isla similar view of an image type cathode ray, tube incorporating the teachings of my invention and;

Figures 3 and 4 are diagrams showing the positioning of certain ofthe electrodes such as may. 'be used in the tubes made in accordance with my invention.

Referring to the drawings and particularly to Figure 1 which shows the application of my invention to a television transmitting tube, the tube comprises a highly evacuated S-shaped envelope I consisting of two abutting torio segments 2 and 3 closed at their una-butting ends by optical transparent windows 4 and 5. Within the envelope and opposite the window 4 is positioned a flat photocathode 6 of the semi-trans-v parent type so that it may be scanned by a beam of light such as may be generated by the conventional cathode ray tube 1 and focused thereon by the conventional lens system shown. At the'opposite end of the envelope I and adjacent the window 5 I provide a mosaic electrode 8 of the semi-transparent type with the photosensitive particles 9 facing away from the window 5 and toward the interior of the envelope I. On the wall of the envelope between the photocathode 6 and mosaic electrode 8 I provide a. conductive wall coating I0 to accelerate electrons from the photocathode 6 and directthem upon 'the mosaic electrode. tive coating I0 may be continuous and extended beyond the photocathode and mosaic electrode, I have found that it may be desirable to provide While the conduc-V separate sections of the .conductive coating over the -length of the tube to aid in focusing the elecnetic field concentric with the toric segments of the envelope and intercepting the photocathode and mosaic electrode normal to their surfaces, to cause the velectrons to follow the curved paths denned by the torio segments. 'I'he coil II is preferably constructed in two sections so that it may be slipped over the toric segments of the envelope from opposite ends. The windings of the coil II or the windings of sections of the coil preferably lie on toric surfaces whose centers of curvature are coincident with those of the toric segments of the envelope. Furthermore, the coil portions enclosing thel toric segments of the envelope may be constructed to produce different magnetic ileld strengths as appears in connection with the specinc operating parameters which I describe later. Since the interior of the tube adjacent the photocathode 6 is subjected to light because of the semi-transparency of the photocathode 6, the conductive ,'wall coating III may be of light absorbing material such as a graphite or "Aquadag coating toffurther reduce ythe light which may be reilected around the torio segments. Notwithstanding the black conductive coating, I prefer to provide a plurality of apertured bames I2 which are amxed to the interior of the tube and extend radially of the toric segments. In addition the length and curvature of the envelope I is such that portions of the envelope intercept a direct optical path between the photocathode 6 and mosaic electrode 8.

It is desirable to make the fluorescent screen of the cathode ray tube 1 of material such as calcium tungstate or othermaterial which has little phosphorescent afterglow upon being scanned by the cathode ray beam developed in the tube 1. 'Ihis tube is conventional in every respect, having the usual cathode and electron beam focusing anodes by the cooperative action of which an electron beam is directed to the fluorescent screen in the usual manner, the resulting light being focused on the photocathode by the lens system between the cathode ray tube I and the photocathode 8. 'I'he mosaic electrode 8 is likewise of the conventional single sided type and comprises a multiplicity of mutually separated and mutually insulated photosensitive particles 9 deposited on an insulating foundation such as a sheet of mica I3 provided with a semi-transparent conductive coating or signal plate Il on the side of the insulating foundation facing the window 5. Such mosaic electrodes together with their method of manufacture are .well known in the art.

As shown by theA drawing, the signal plate Il is connected to ground through the usual load impedance I5 and to the input circuit of a translating device I6 so that its average potential is maintained equal to that of the photocathode 6. In operation an optical image of an object such as represented by the arrow I1 is projected and focused upon the mosaic electrode l by the lens system I8, electrons liberated from the lllllmlassess;

nated surface of the mosaic electrode being accelerated by the conductive-coating Il and passing through the toric segments 2 and I and collected by the photocathode I. Because the photosensitive particles 0 of the mosaic electrode 8 are mutually separated and insulated one from another, a positive electrostatic. image is formed on the mosaicv electrode by the loss'of photo electrons liberated under the influence of the optical image represented by the arrow I l. The fluorescent screen of the cathode ray tube l is scanned by an electron beam which is deflected to form a pattern preferably consisting of horizontal lines. each vdisplaced from the preceding line in a di- .rectlon normal to the lines. An optical image of the pattern is projected upon the photocathode 'I and forms in eifect a scanning light spot on the photocathode l. The word "pattern is here used because the trace of the cathode ray beam in the tube I retraces its path many times per second, or at a frequency sumcient for normal persistence of vision and to meet other operating requirements and merely appears as a pattern because'of the persistence of vision. The electrons liberated from the photocathode 'l by the scanning light spot are focused by cylindrical magnetic fields generated by the coil II. A cylindrical magnetic field may be defined as a field in which the lines of flux are arcs of circles having their centers of curvature on a common axis and in planes normal to that axis. The specific design of the coil II in relation to the tube and the .method of energizing the coil is described following the description of the tube shown in Figure 2. 'I'he electrons liberated from the photocathode are also accelerated by the conductive coating III and directed along arced paths concentric with the axis of the toric segment 2, and continue along further arced paths concentric with the axis of the torio segment 3 under the influence of the magnetic field generated by the coil II. These electrons impinge with low velocity on those areas of the mosaic electrode which have received a positive electrostatic charge due to the loss of photo electrons under the influence of the optical image. Those electrons which are liberated from the surface of the photocathode at the ,points where the magnetic lines of force intercept the photosensitive particles 0 which have zero or negative potentials are deceleratedand accelerated in the opposite direction by the conductive coating I0 and returned to the photocathode 6. .Those photosensitive particles having positive charges by reason of light from the optical image incident thereon collect the electrons from the corresponding areas of the photocathode and in this manner the elemental areas of the electrostatic image representative of light and shade areas of the optical image projected on the mosaic electrode are neutralized by the electrons liberated from the photocathode when scanned by the moving spot of light. Since the photosensitive particles 9 are in capacitive relationship with the signal plate Il and retain an electrostatic charge proportional to the light intensity incident thereon, each time the electrostatic charge is neutralized by the moving light spot generated electrons, a signal will be generated in the impedance I5 which is applied to the translating device Il, whereupon the signal is further amplied and transmitted in the usual manner.

Figure 2 shows a tube somewhat similar to that shown in Figure 1 but designed for purposes of image recreation. The envelope I comprises the two toric segments 2 and 3 enclosing the photocathode 8 adjacent the end having the window l.

In place of the -mosaic electrode at the end of the tube adjacent the window 5. I'provide a fluorescent screen I9 which may be of the conventional type supported on a semi-transparent conductive foundation 20. In the modification of my invention 'as shown in Figure 2 the conductive coating I may be used as shown in Figure 1 but I prefer to replace the conductive coating by mutually insulated sections 2| and 22 of electrically conductive material to provide more uniform acceleration flelds for accelerating the electrons from the photo-cathode B. It is apparent that the baiiles i2 shown in connection with Figure 1 may likewise be used in the image tube of Figure 2 to reduce light incident upon the fluorescent screen I9 which would otherwise decrease the contrast of the fluorescent image. Surrounding the envelope I comprising the two toric segments 2 and 3 I provide a focusing coil as shown in Figure 1 but I have shown this coil as comprising two portions 23 and 24. In the following description and in a number of the claims I will refer to a focusing coil associated with the envelope but it will be understood that this terminology is generic to a coil having a plurality of individual portions whose center lines are contiguous successive reversed curved paths.

In operation the electrons liberated from thephoto-cathode 6 in response to the light such as represented by the arrow 25 focused thereon by the lens system 2B are accelerated along arced paths concentric with the toric segment 2 and along continuing arced paths concentric with the toric segment 3 and impinge at high velocity upon the fluorescent screen i9 since the conductive coatings 2| and 22 and the foundation 20 are maintained at progressively increasing positive potentials with respect to the photo-cathode 6. It will be obvious in the modifications of my invention shown both in Figures 1 and 2 that light incident upon the photo-cathode 6 is absorbed by the conductive coating or coatings on the inner wall of the envelope, and shielded from the mosaic electrode or fluorescent screen by the baffles l2.

- While I have generally described the sections of the envelope l as comprising toric segments in connection with the description of Figures 1 and 2, and have generally described the types of focusing coils which may be used in practicing my invention, it is to be understood that the formation of the toric segments and the provision of suitable focusing coils are directly co-related. I have found that if an electron receiving target is placed at the junction of the two toric segments, that is, in the plane in which the two toric segments 2 and 3 are joined, considerable shear dis- .tortion results. This shear distortion is evidenced by a, change in shape of the electron image so that if the original optical image on the photocathode 6 is rectangular, the resulting electron image incident upon a target positioned intermediate the two toric segments is actually rhomboidal. I have likewise found that an electron image is similarly sheared while passing from the section intermediate the two toric segments toward a target electrode adjacent the window 5, but that this shear distortion is in an opposite direction from that produced upon the electron image in the rst toric segment. Thus, the rhomboidal electron image is further distorted while passing through the second toric segment, but in an opposite direction, so that the resulting electron image returns to a true lrectangular shape. I have found this eect to be true only if the toric segments are constructed in such a manner that their radii of curvature bear a definite relationship with the strength of the magnetic field and the length of the electron paths in the toric segments.

Referring to Figure 3, the photocathode i and target 21, which latter electrode corresponds to either the mosaic electrode l or fluorescent screen i9, are shown in their correspondingpositions as utilized in the tubes of Figures 1 and 2. It will be noted that the center O of the diagram of Figure 3 is the center of curvature of the toric segment 2 and that the center O is the center of curvature of the toric segment 3 and that these centers of curvature lie in the same piane as the center lines 28-28'. The radial distances from O and O' to the center line 20-28 of the two toric segments are finite distances and represented by r and r', respectively. The angles and 4:' represent the angle of the two toric segments and will be referred to as the angles of curvature of the two toric segments. It is therefore obvious that the length of the center line of each toric segment between the photocathode 6 and target 21 is a function of pr and r', respectively. I have found that the shear distortion produced in one toric segment is exactly neutralized by the shear distortion in the second toric segment when the ratio of the magnetic field strengths over the two toricsegments is made equal to the ratio of r' to 4:1'. Expressed mathematically, where H is equal to the magnetic field strength in gausses along the axis of the first toric segment such as the segment 2, and H is the field strength along the axis of the second toric segment such as the segment 3;

. oif

Hf-:r Thus the strength of the magnetic field over the toric segment 2 is made proportional to the angle included between the radii drawn from the center of curvature of each end of the toric segmentk 2 which I term the angle of curvature of the toric segment 2, and also proportional to the radius of the toric segment 3. Similarly, the strength of the magnetic field over the toric segment 3 is made proportional to the radius of the toric seg- Y ment. 2 and to the corresponding angle of curvature of the toric segment 3.

In the operation of the tube shown in Figure 2 the potential applied to the conductive coating 2| may be from zero to 500 volts with respect to that of the photocathode 6, the wall coating 22 preferably being maintained at approximately 500 volts, while the potential applied to the fluorescent screen foundation 20 may be 5000 volts or more, each of these latter potentials being with respect to that of the photocathode. If r #or the current in the coils 23 and 24 of Figure 2 is made unequal by adjusting the resistors 29 and 30. However, with the tubes shown in Figures 1 and 2 and with and r=r, the strength of the magnetic field over the two toricl segments may be equal and may be approximately 50 gausses, and with such a ileld strength, theelectron image from the photocathode may be focused satisfactorily upon the mosaic electrode or fluorescent screen without the introduction of shear distortion.

A tube of practical dimensions for use with reasonable field strengths maybe made of glass tubing of 4 inches inside diameter the angles and radii of curvature being equal and equal to 110 and 6 inches respectively. With these .dimensions lightA from the optical image is prevented from falling on the target and a ileld strength along the tube axis of 50 gausses is sufficient to provide good electron image focus with a potential of 500 volts on the conductive wall coating.

. While I have described m'y invention with reference to tubes havingan envelope including two toric segments joined end to end so that the centers of curvature of the segments lie on opposite sides of the center line of the envelope, I do not wish to limit myself to this particular construction. With reference to Figure 4 which is a diagram similar to that of Figure 3, I have shown the center line of an envelope comprising three toric segments arranged end to end so that the centers of curvature of adjacent toric segments lie in the same plane as, and on opposite sides of, the center line of the envelope. Thus the center line from 3| to 32, from 32 to 33 and 33 to 34 deilnes the three toric segments. kIt will be observed that the centers O, O' and 0" of these toric segments lie on opposite sides of the center line for each of the adjacent segments. With the arrangement of the envelope as shown in Figure 4 the summation of v Hr should equal zero where 4 represents the angle for the respective segments at the center of curvature between radii drawn to the ends of each toric segment, that is, the respective angles of curvature of each toric segment, Hland r representing the respective field strengths and radii of the toric segments. It is obvious that for this summation-to be equal to zero the angles of adjacent segments are of opposite signs. Thus the angle and may be considered as positive and that of c' as negative. It is obvious from the above consideration that for equal magnetic neld intensities and equal radii, the numeral value of a' must equal the numerical sum of and ip" in order to obtain complete absence of shear distortion. The arrangement shownin Figure 4 is of particular advantage where it is desired to construct a tube in which the optical axes at each end, of the tube are coincident. The number of toric segments may be still further increased beyond the three shown in Figure 4, provided the relationship between the ileld` strength, angles and radii given above is considered in the design and operation of the tube.

While I do not wish to be limited to any particular theory underlying the improved operation of my television transmitting or image tubes, I believe that an electron, when liberated from the photocathode in a magnetic field of sumcient strength, tends to follow a line of magnetic flux. If the flux lines are bent, the electron is continually accelerated toward the center of curvature of the line. The force providing this acceleration comes from the motion of the electron in a direction perpendicular to the plane containing the curved flux lines. The distance moved perpendicular to this plane is proportional to the angle throughwhich the electron follows the bent flux line, and' inversely proportional to the radius of curvature and the strength of the ileld. By providing sections of flux having opposite curvature, the ilnal position of an electron can be brought to the plane containing the ux line in which the electron started. When this occurs for all of the electrons in the electron image. shear distortion is eliminated.

In the appended claims I will use the term e-shaped" envelope or coil to denne an' envelope or coil comprising two or more toric segments joined end to end so that the centers of curvature of adjacent toric segments lie on opposite sides of the center line of the envelope or coil.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only two speciilc applications for which my invention may be employed. it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

l. An electron discharge tube comprising an envelope including wall portions which are toric segments joined at their abutting ends and forming an 8-shaped enclosure, a photocathode adjacent one end of said envelope to generate a flow of electrons, a target electrode adjacent the opposite` end of said envelope to receive electrons from said photocathode, and a magnetic coil surrounding-the length of and conforming to the sliape of said envelope-to focus the now of elec' trons from said photocathode on said target electrode.

2. An electron discharge device comprising an envelope including a plurality of abutting toric segments, the center lines of which are contiguous successive curved paths, the centers of curvature of adjacent curved paths lying in the plane of said center lines and on opposite sides of the said adjacent curved paths, an electrode adjacent each of the non-abutting ends of said torio segments and a magnetic coil enveloping and conforming to the shape of said envelope over the length of said toric segments.

3. A television transmitting tube comprising an elongated universaly curved envelope, an electron emitting cathode at one end of said envelope, an electron receiving target adjacent` the opposite end of said envelope optically shielded from said cathode, a magnetic coll extending over the space separating said cathode from said target, said coil comprising at least two portions in the form of toric segments to generate cylindrical magnetic flelds having contiguous successive inversely curved paths extending from said cathode to said target.

4. A te1evlsion transmitting tube as claimed m claim 3 wherein said target comprises a mosaic of mutually insulated particles facing said cathode.

- 5. A tube as claimed in claim 3 wherein said target comprises a screen adapted to become luminescent when bombarded by electrons from said cathode.

6. A television device comprising an envelope having an S-shaped portion, an electron emissive electrode and a target electrode adjacent opposite ends of the S-shaped portion of said envelope portion, a magnetic coil surrounding the .S-shaped portion of said envelope, said coil comprising two toric segment portions of nite radii of curvature with their centers of curvature lying on opposite sides of an S-shaped path between said electrodes to generate magnetic ilelds over said coil portions having a ratio represented by the ratio of r' to 'r where and are the angles formed at the respective centers of curvature of said portions by radii drawn tothe ends of the respective portions, and r and 1" are the radii of the respective toric segment portions of said coil.

7. An electron image tube comprising an enlvelope having two portions, each portion having a curved center line, ythe said portions being joined and forming an elongated enclosure with the center lines forming contiguous successive oppositely curved paths lying in a single plane, a photocathode adjacent one end of said envelope adapted to liberate streams of electrons in response to an optical image focused thereon, a target electrode adjacent the opposite end of said envelope to receive the electrons from said photocathode, and. magnetic means to generate a magnetic iield having lines of force following said contiguous successive curved paths from said photocathode to said target electrode to focus saidfstreams of electrons as an electron image of said optical image on said target eelctrode.

8. An image tube for generating a light and shade replicaof an optical image comprising a photocathode, a uorescent screen separated from said photocathode and adapted to receive electrons from said photocathode, an envelope enclosing said photocathode and said fluorescent screen with portions of said envelope interceptvelope having an inside diameter of approximately four inches the said envelope including two toric segments with equal radii and angles of curvature the centers of curvature being on opposite sides of the center line of said two toric segments each center being approximately six inches from the said center line the angles of curvature being approximately 110 degrees, a

photocathode adjacent one end of said envelope and a fluorescent screen adjacent the opposite end of said envelope to receive electrons from said photocathode, and means to generate a magnetic eld having a eld strength of approximately gausses along the center line of said envelope said means surrounding said envelope over the length of the center line path between said photocathode and said uorescent screen.

HARLEYA.IAMS.

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