US2213175A - Television transmitting tube and system - Google Patents

Description

Aug. 27, 19443. H'. A. IAMS ET AL TELEVISION TRANSMITTING TUBE AND lSYSTEM Filed May 27, 1939 c@ A TTORNE Y.

.Paieme'd Aug. 27, todo UNITED STATES' PATENT 2,213,175 v TELEVISION Talsisivn'r'rmo TUBEAND Application May 27, 1939, Serial No. 276,106

13 Claims.

Our invention relates to -televisiontransmitting tubes and is particularly directed to tubes utilizing low velocity electron scanning beams.

cordance with-one teaching of our invention the Television transmitting tubes of the low velocity electron beam scanning type have been made in which a combination magnetic-electrostatic deflection system including electrostatic plates wholly immersed in a uniform axial magnetic field shifts the beam in a plane parallel to the plates. Tubes of this type produce a distortion or devfocusing eect evidenced in the image recreated on a viewing tube by unequal resolution at the center and at the edges of the recreated television image. The resolution at the center can be made very high, but decreases with increasing distance from the center. The cause of this distortion or defocusing effect was unknown until we found byv exhaustive research, mathematical analysis and the construction of mechanical models representing the motion of electrons in such a combination field, that this defocusing effect was due to an interaction of the electro static and magnetic elds which increases the amplitude of the helical paths of the electrons while under the influence of the two fields, and

only after our discovery of the cause of this disr tortion could \a satisfactory solution be approached.

The principal object of our invention is to provide a television transmitting tube having a combination electrostatic-magnetic deilection eld which will transmit an optical replica of a picture without the introduction of electronic defocusing diiculties It is a further object of our invention to provide a tube of the type described for the transmission of an optical image replica wherein the replica can be transmitted with equal delity over its entire area. Another object of our invention is to provide low velocity electron beam scanning tube having substantially the same resolution over the entire picture area.

In accordance with our invention, an electrostatic image corresponding in electrostatic energy distribution to the light distribution of an optical image is formed on a target or mosaic electrode which is scanned by a low Velocity electron beam from an electron gun, the scanning being accomplished in a combination magnetic-.electrostatic eld without introducing large amplitude components of deflection transverse to the normal path of the beam. Moreparticularly in acelectron beam is directed through an electrostatic eld which uniformly increases over a portion of the beam path with increasing distance from (Cl. Z-158) mersed in an axial magnetic eld, the axial distance traveled by the beam in the constant eld being of predetermined length.

A better understanding of our invention will be obtained and other objects, features and advantages will appear from the following description taken in connection with the vaccompanying drawing in which:

Fig. 1 is a longitudinal sectional view of a tele- Vision transmitting tube embodying our invention,

Fig. 2 is a cross section of the tube shown in Fig.A 1 taken along the line 2-2,

Fig. 3a is a longitudinal cross section of certain structure shown in Fig. 1, I

Fig. 3b is a graph showing the electrostatic field distribution of the structure of Fig. 3a.,

Fig. 4a is a modification of the electrostatic deflection structure shown in Fig. Land Fig. 4b is a graph showing the electrostatic field distribution of the structure of Fig. 4a.

Considered broadly, the apparatus of our'invention comprises an evacuated envelope having a target preferably of the mosaic type at one end l and anelectron gun surrounded by an electron collecting electrode or electrodes at the opposite end of the tube. 'I'he target, if of the mosaic type, is provided on'its front surface with an extremely large number of mutually insulated photoelectrically sensitized particles, and it is so positioned that it may be scanned by an electron beam from the gun and that itmay have focused thereon an optical image of the object of which a picture is to be transmitted. The potential between the electron gun and target is so adjusted that the electron beam is projected at relatively 10W velocity and directed upon the target at extremely low or substantially zero velocity, that is, with a velocity approaching zero at the point of impact therewith. In operation, elemental areas of the mosaic electrodeacquire electrostatic potentials proportional tothe intensity of the light incident thereon; thus, particles of the mosaic which are more highly illuminated acquire the more positive electrostatic charges with respect to the unil-` luminated particles, and these positive charges separated photosensitive particles I I.

ing the mosaic electrode we make the mica sheet.

ture to be transmitted are neutralized by the low velocity electron scanning beam. Intermediate and extending wholly between the electron gun and target electrode we provide a uniform axial magnetic fleld and means Within the axial field' to generate an electrostatic field to scan the beam over the target without producing undesirable deflection or distortion effects.

Referring specically to our tube structure shown in the drawing, the tube comprises an tured cold electrode, such as a control electrode,

4 connected to the usual biasing battery and a flrstanode 5 maintained positive with respect to the cathode 3. The electron stream leaving the first anode 5 is accelerated at' relatively low velocity and concentrated into an electron scanning beam focused on the front surface of the target or mosaic electrode by a second anode 6 which is preferably an apertured tubular member partially surrounding the first anode 5. The first anode 5 and the second anode are maintained at the desired positive potentials with respect to the cathode by a battery I. Closely adjacent the electron gun and between the anode 6 and the target we provided'a centrally apertured electron collecting or shield electrode 8 to which the electrons of the beam not reaching the target are directed and collected, the aperture being provided to allow an lunimpeded electron flow between the electron gun and target.

The mosaic electrode 2 which faces the electron gun preferably comprises a substantially transparent sheet of insulation such as the mica sheet 9 having on its rear surface a translucent or semitransparent electrically conducting film I0, thel lopposite surface of the mica sheet being provided with an exceedingly great number of mutually In mak- 9 of the desired area having a uniform thickness of approximately .002 inch and as a first step coat one side of the sheet with a film of metal of suffi cient thinness as to be substantially transparent so that an optical image suchas represented by the arrow I2 may be focused on the photosensitive particles I I by a lens system I 3. The mosaic of mutually separated particles may be made by depositing on the front surface of the mica sheet.

9 finely divided silver oxide which is reduced to. provide a surface of individually separated 'silver particles or globules .II which are subsequently oxidized and sensitized with caesium or other alkali metal during the evacuation process.' Such an electrode structure and a methodof sensitization is disclosed by S. F. Essig in U. S. Patents in t e circuit of tlie collecting electrode, 8 tov grund.

In accordance with our invention we provide between the collecting electrode 8 and the mosaic electrode deflection plates such as the plates IB-I'I having a particular formation, as later described, which are connected to a source of dei flection potential and to ground through a center tapped resistance of l to 10 megohms. The electrostatic field produced between the plates in combination' with an axial magnetic field deflects the electron beam over the mosaic electrode 2 in a direction perpendicular to the plane of the drawing of Fig. 1. The axial magnetic fleld is preferably generated by a magnetic coil I8 which is of slightly larger diameter than the envelope I and extends over and beyond the space between the collecting electrode 8 and the mosaic electrode 2. Deflection of the electron beam in a direction normal to that produced by the plates |6-I'I in the field produced by the coil I8 is accomplishedby a pair of deflection coils lil-26. This deflection is preferably the frame or vertical deflection, since in standard television systems the frame deflection is of, lower frequency than the horizontal line deflection and the magnetic deflection coils I 9-20 should preferably be oper.

ated at the lower of the two frequencies.

We have found, as a result of considerable research and by the construction and test of a 'large number of tubes that with the use of an electrostatic deflection field in combination with a uniform axial magnetic field it is desirable to have the electron beam leave or emerge from the deflection plates With only its, original axial velocity and with no components of velocity transverse to the axisfother than that resulting from the initial transversel .emission velocity from the cathode. The axial magnetic field causes the electrons which are emitted by the cathode with initial transverse velocities to follow helical paths of small amplitude. The length ofthe path of a single helix or the pitch of the helix along the velocity of' the electrons, that is by varying the potential difference applied between the cathode flection plates upon entering the combinationV magnetic-electrostatic field is such that its helical path projectedv on aplane normal to the axis of the tube is a. series of cycloicis. The axial length of these cycloids is equal to the length or pitch of the individual helices as defined above. The average transverse velocity of -drift ofv the electrons produced by suddenly subjecting the electrons to the electrostatic field is proportional to Where E is the strength of the electrostatic field and I-I the strength of' the magnetic field, and

the instantaneous transverse velocity ofthe electrons varies from zero to and decrease gradually. The curvature of the. end portions may therefore vary from that of a true hyperbola- Without materially increasing the i amarre is Em the electron drift tends to assume the average velocity of with no fluctuation between zero and We have found that the reverse action takes place when the electrons emerge from an electrostatic field which gradually decreases along a portion of the beam path with increasing distance from the electron gun so that no increased amplitude is imparted to the emerging electrons other than that possessed by reason of the initial emission transverse velocity.

Therefore, in accordance with one teaching of our invention, the deflection plates used in combination with an axial magnetic field are so shaped as to provide, when energized by deflection potentials,gradually increasing and decreasing entrance and exit electrostatic elds. Refer'- ring to Figs. 3a and 3b, the deflection plates lli-l1, as also shown in Fig. 1, are flared outwardly-from the axis of the tube'at the opposite ends of the plates so that the electrostatic deection eld at the leading ledges-of the plates, such as at point A, produces substantially no electrostatic eld, the shape of the plates being such that the eld gradually increases as the beam proceeds from the electron gun and toward the central portion of the plates, point B. Withfurther travelof the beam through the plates i6-H,

such as from points vC to D, it passes through a gradually decreasing field. With such an arrangement, substantially none of the transverse velocityacquired by the electrons of the beam while under the influence of the plates is retained by the beam upon leaving the plates. This mode of action is true only in a combination magnetic- 'electrostatic field because if the magnetic eld is absent, the1 desired function of the electrostatic plates as used in conventional electrostatically deflected tubes is to introduce transverse velocity of the electrons of the beam, which is retained by the electrons upon leavingthe electrostatic eld. In contrast to the above and in accordance with our invention, the transverse velocity acquired by the electrons in the region of the plates is reduced to substantially zero upon leaving the plates.

We have foundthat the end portions of the plates; that is, from points A to B and points C to D should have the prole of a hyperbola, the plates from points C to D likewise being of hyperbolic curvature. We have founduhowever, as the result of tests made on tubes constructed in accordance with our invention that the conditions of operation may be closely approximatedif the curvature of these plates is not truly hyperbolic, as long as the entrance and exit fieldsA increase transverse velocity of the electrons in the emergent beam.

It is not necessary to have the curvature of the plates shown in Fig. 3 continuous; thus, the points Band C may be separated by any desired dis-1 tande depending upon the other parameters of the tube such as the size of the target, the velocity of the beam through the deflection plates and the maximum .defiecting potentials applied to thev plates. Furthermore, the length ofj the clurvedlector electrode 8 of Fig. 1. The two pairs of portions ofthe plates, such as frompoints A to B and C to D, are preferably maintained equal to an integral number of cycloids or pitch distances in the path of the beam between these respective points. For this condition the length from A to B and from C to D along the axis of the tube" would be equal so as to encompass an integral number of cycloids or pitch distances. The length of the plates over which they are fiat, that is, from points B to C, is not critical. Since the field generated in -this portion of the system is uniform, this lengthmay be varied irrespective of the number of cycloids over this portion of the beam. The length-or pitch of the helices in the axial4 magnetic field and the length of the cycloids in the combination field may be determined by the expression where K is a constant equal to approximately 21,

when V represents the axial velocity of the elec-`- tron beam in volts, and H represents the strength of the iield in gausses. We have found that the desired separation between the plates iii-Il at Ytheir point or points of closest approach to each other, such as at all points` between B and C, is a function of accuracy of alignment of the axis of the electron gun and plates with the tube axis, but it is not critical. The Iwidth of the plates |B--l'i, that is, the dimension in a direction perpendicular to theplane of the drawing should be at least as wide asth'e mosaic electrode 2 taken in the same direction if the entire width of the mosaic electrode is to be utilized. Thus, the

width of the plates should equal or exceed thel Width of the optical image focused on the mosaic eelctrode;

As pointed out above, the vinstantaneous trans-v lecting the'initial transverse velocity received at their. points of origin. vrI'hus, the entrance Yand exit fields may be so located with respect to the cycloidal phase of motionoi Vthe electrons that the electrons upon emerging from the electrostatic field have no appreciable transverse velocity, and this condition may be made uniform for any position of the beam at which it leaves the electrostatic eld.

Referring to Figs. 4a' and 4b which show`a cross section and the eld distribution of a deflection system constructed in accordance 'with this second teaching of our invention, .the plates lB-l'lmay be similar to those shown in Figs. 1 and 3a.. Within the space separating the plates IBf-ll of Fig. 3a we provide two pairs of plates 2|-22 and 2 3--24 which are maintained at the average "potential of the deflection plates ISL-l1 which is preferably ground potential or the same potential which is applied to the anode 6 and the colto the target plates 2I-22 and 23-24 are in telescopic relationwith the plates IB-I'I and are separated along the tube axis by a predetermined distance a', as shown in the drawing, and by a distance .b normal to the axis. Furthermore, the plates. 2| and 23 and the plates 22 and 24 are preferably coplanar. Since in accordance with the further teaching of our invention it is desired to subject the electrons of the lbeam -to the electrostatic eld of the plates |6-I1 at a time when the instantaneous transverse velocity of the electrons is zero and to remove the electrons from this deflection eld under similar conditions of zero instantaneous transverse velocity, the separation a along the axis should contain an integral number of cycloids. If this condition is fulfilled, the electrons may be made to enter and leave the electrostatic deflection cld at a time when their transverse velocities are zero by adjusting the potential applied to the cathode by the battery 1. The separation b of the plates 2|-22 and 213-24 normal to the tube axis determines the fringing elds existing adjacent the points E and F, which elds extend slightly within the plates 2I-22 and 23-24. The separation b should therefore be made as small as possible, and we have found that this separation is preferably no greater than lotlie length of the cycloids or spirals.l If these conditions are fulfilled, the electrons of the beam enter the electrostatic eld produced by the plates IS-l'l ,at a point at which this iield is a maximum such as indicated in Fig. 4b at E. Similarly, the electrons suddenly Aleave the electrostatic eld produced between the plates lG-H as shown at point F, Fig. 4b.

It will be clear from the above that our construction of electrostatic deflection plates as shown. 1n Fig. 4, when used in combination with an axial magnetic field,- will impart no transverse velocity to the electron beam which is retained by the beam after it passes out of the velectrostatic eld between the plates It-I'L As" a consequence, the electron beam will proceed or mosaic electrode 2 along spiral paths whose amplitude is dependent only upon the electrons of 'the 'beam at their source of origin. We have found that it is very difficult, if not impossible, to provide an. emission surface or source of electrons in which all of the electrons are liberated in an axial direction. As a consequence, the electron beam, 4while under the influence of the laxial magnetic field, will follow spiralpaths, but since the transverse velocity of emission is very low, the amount of defocusing or fdistortion caused by the spiralV motion of electrons is negligible.

In order to provide a static free space, and by static we mean the condition under zero deflection, in the region of the plates llil1 of Fig. 1 and between all of the plates shown in the structure of Fig. 4a, We provide an electrode 25 between the plates IG-I and the mosaic electrode 2. The electrode 25 is' provided w1th a slet 26 having a width equal to or slightly greaterthan the minimum separation of the plates llil1 slightly greater than the 'separation b as shown -in the structure of Fig. gi. The electrode 25 isl connected to the rsame potential as the collecting. electrode 8 or to ground as shown in the drawing. The .electrode 25, as in thecas of the ,electrode 8, may begtermed a shield electrode since it provides a shield to prevent the electro.- static field generated by the deflection potentials electrostatic field initial transverse velocity acquired by the y of Figs. 1 and 3 or equal to or applied to the plates l6--l1' from fringing out toward the mosaic electrode. To obviate any 'collection of electrons on the wall of the tube between the collecting electrode 8 and the electrode 25 or between the electrode 25 and the mosaic electrode 2, which would introduce an undesirable eld in this space, we have found it desirable to coat the inner wall between these electrodes with a lm 21 of electrically conducting material which is likewise connected to the electrodes 8, 25 or to ground.

In operation of tubes made in accordance with our invention we have found that the operating parameters such as the size of the electrodes, the separation of electrodes and operating potentials, may be varied over wide limits. However, to limit the structure to practical proportions we have found that the potential applied between the cathode 3 and the anode 6 is preferably between 50 and 250 volts. Tubes operating with a potential below 50 voltsA have shown decreased beam focusingz properties, and the use of poten- Itials below this value necessitates greater accuracy in the alignment of the electron gun electrodes and the deflection plates. Certain tubes operating with potentials above 250 volts have given unstable operation. The axial magneticI field is preferably between 50 and 150 gausses Below 50 gausses some distortion due to stray electrostatic fields may be produced, and the use of a field stronger than 150 gausses necessitates increased deflecting power and use of heavier coils and results in poorer focus of the electron beam near the edges of the mosaic electrode. The width of the target or mosaic electrode is preferably made between and 7 centimeters, since the width of the plates must be of the same or greater width and would necessitate a tube of larger overall diameter. The length 0f the deflection plates is preferably` between and centimeters, for when using a length less than 10 centimeters, some distortion and defocusing of the beam is produced. Lengths of the deflection plates above 20 centimeters unduly increase the tube length and the length of the focusing coil.

While we have indicated the preferred embodiments of our -invention of which we are now aware and have indicated the specific application as directed to cathode ray transmitting tubes having target electrodes of the photosensitive mosaic type, it will be apparent that our invention is by no means limited to the purpose of television transmission, to the exact forms illustrated or toits use in cathode ray tubes incorporating target electrodes of the mosaic type, but that many variations .may be made in the particular structure used,'such as by replacing the mosaic electrode with target electrodes of the uoreswcent, photoconductive, or photovoltaic type, without departing from the 'scope of the invention as set forth in the appended claims.

We claim: 4

1. A cathode ray tube having electron beam generating means, a target in the path of the electron beam, beams extending over the space Abetw'een said beam generating means and said with beam deflecting potentials, an electrostatic field which increases uniformly in intensity along electrode structure wholly immersed in said field aportion .of the beam path with increasing distance from ,said beam generating means and which decreases uniformly inintensity along another portion of the beampath with increasing distance from said beam generating means.

2. A cathode ray tube Ahaving an electron source, an oppositely disposed electron receiving target, an apertured cold electrode adjacent said source through which the electrons fiow as an electron beam toward said target, means to generate a'magn'etc field having lines of forceparallel with the axis between said 'cold electrode and said target and extending over thespace separating said cold electrodev and said target, a pairof apertured shield electrodes in planes substantially normal to said axis and separated one from the other along the length of said axis and. electrostatic means wholly immersed in -said field and between said shield electrodes to generate a deiiection field which increases uniformly in intensity with increased distance from said cold electrede over a portion of the beam path and decreases uniformly with increased distance from said cold electrode over another portion of the beam path.

3. A cathode ray` tube having a thermionic cathode to emit electrons, an apertured cold electrode adjacent saidcathode through which the emitted electrons flow to. form a beam, a target electrode in the path of said beam, means extending over the space between said anode and said target to generate a'substantially uniform axial magnetic field, means including a pair of apertured shield electrodes between said anode and said target to provide an electrostatic field free space anda pair of electrostatic deflection plates wholly immersed in said axial magnetic field and within said electrostatic field free space, said platesbeing shaped so that their separation is greater at theends thereof adjacent said anode and said target than at points intermediate said ends to generate an electrostatic field which uniformlyincreases anddecreases in intensity betweensaid plates'with increased' distance from said anode.

f4. A cathode ray tube envelope, an electron gun and an oppositely disposed target in said envelope, means surrounding the envelope to generate a substantially uniform magnetic field along the longitudinal axis and extending between said gun and said target,v

two apertured shield electrodes spaced one from 'the other in planes normal tothe longitudinal whereinboth of the said deflection plates have.

end portions in the direction of the target whose profile is hyperb o1ic 6. In combination-.with a vcathode ray tube having means to generate a uniform magnetic eld, an electrostatic deflection structurefwholly gun and immersed in said field comprising a pair of platesl having a uniform separation from one to the other' over a predetermined length of said field and outwardly ared portions on either side of said predetermined length.

In combination, a` cathode ray tube having means rto generate a beam of electrons "and means `to generates uniform magnetic field, an

having an evacuatedand partially surrounding the path of said beam posed and separated plates, a second pair of oppositely disposed plates extending within the space separating said first-mentioned pair and a third pair of oppositely disposed plates extending within said first-mentioned pair -and separated from said second pair of plates, the plates of said second pair and the corresponding plates of said third pair being coplanar, said second land third pairs of plates providing a field-free space over the path of said beam exposed to said first pair of plates inthe -absence of beam deection potentials applied between said first pair of plates.

8. In combination, acathode ray tube having a target of the mosaic type, means for developing of plates, the plates of one pair being coplanarv with the corresponding plates of the other pair, the two pairs of plates being separated by a predetermined distance along the path of the beam said predetermined distance being less than lthe length of said first pair of plates along the path of the beam..

9. A cathode ray tube having an envelope, an

electron gun and an'oppositely disposed target electrode within said envelope, a magnetic coil surrounding the space between said gun and said target electrode to generate a magnetic field parallel with the axis 'between said gun and said target electrode to direct electrons emitted from said electron gun with components of velocity transverse .to said axis along helical vpaths toward I said target, a pair lof oppositely disposed deflection plates, one ony either side of the said axis to deflect the beam in a plane parallel with the plates, two pairs of plates symmetrically disposed with respect to said rst pair, each pair extending within the space separating the plates of said rst-mentioned pair `and spaced apart along the said axis by a distance equivalent to the length of an integral number of pitch revolutions of the beam along the helical paths'.

10. In a cathode ray tube system, a tube having acathode to emit electrons, an anode to accelerate the electrons emitted from said cathode, a target in the path of said electrons, a pair of plates, the two pairs of plates being separatedv along the said axis by a predetermined distance, means to generate a magnetic field extending over thefspace separating said anode and said target having lines of force parallel with the said axis to cause electrons emitted from said cath-v ode with velocities transverse to said axis to follow helical paths toward said target and means :to apply a difference of potential between said cathode and said anode such that the electrons ofthe beam describe an integral number of pitch revolutions along the, helical paths while passing between the predetermined distance separating the said two pairs of plates.

11. In a cathode ray tube system, a tube having a cathode to emit electrons, an anode to accelerate and form the electrons into a beam, a target oppositely disposed from said cathode and anode to receive electrons, a pair of oppositely disposed deflection plates, one on either side of the longitudinal axis between said cathode, anode and target, and two additional pa-irs of plates in telescopic relation withsaid rst pair of plates separated one from the other by a predetermined distance along the said axis, means to apply a diierence of potential to said cathode with respect to said anode and target and means to generate a uniform axial magnetic field between said anode and said target of sutcient intensity to cause electrons emitted by said cathode with velocities transverse to Said axis to follow a helical path and an integral number of pitch revolutions along the helical path between said pre` the said two pairs Patent No. 2,215,175.`

HARLEY A. uns, ET AL.-v

CERTIFICATE 0F CORRECTION.

.to cause the electrons of the beam to follow a helical path toward sa'id target and a source 01'v electric potential connected between said cathode and said pla-tes sucient to cause the pitch of the helical path Within said two pairs of plates to be no greater than 10 times the distance b.

13. A cathode ray tube as claimed in claim 4 wherein the aperture of said shield electrode nearer said target is elongated and having a width in the direction of the separation of said deflection plates substantially equal to the miniA mum separation between said deection plates.

HARLEY A. IAMS. ALBERT ROSE.

August a?, 191m.

It is hereby certified that error appears in the printed specification ofthe above numbered patent requiring correction as follows: Pagelb second column, line 66, claim 1, for lthe word "beams" read -r-lm-eans--gand that the said Letters Patent should be. read with this correction therein that the same may conform to the -record. of the case in th` Patent Qffice.

4Signed and sealed this A121:11 'day of. November, A. D. 191m.

(seal)l Henry Van Arsdale, Acting Commissioner of Patents.

CERTIFICATE 0E CORRECTION. Patent No. 2,215,175.` ,August 27, 191m.

HARIEY A. uns, ET AL.'

It is hereby certified that error eppears in tb' printed specification.. ofthe above numbered patent requiring correction as follower-Page secord column, line 66, claim 1, for .the wozd "beams" read mea.'ns- Yand that the said Letters Patent shou1d be. reno. with this correctioljhere'l- :lx1-- that the same may conform to the record of the case inth I atent )f-f'ice.l signed and sealed this 12th 'day of. November, A. D. 3,914.0.

Henry Van Arsdale, (seal) Acting' Commissioner of Patents.

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