US2452619A - Cathode-ray tube - Google Patents

Description

Nov. 2, 1948. WEIMER 2,452,619

CATHODE-RAY TUBE Filed Feb. 7, 1946 INVENTOR.

} 2270/ K nl mer Patented Nov. 2, 1948 CATHODE-RAY TUBE Paul K. Weimer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 7, 1946, Serial No. 646,076

2 Claims.

'This invention relates to cathode ray tubes for television and other purposes of the orthicon type disclosed in my application, filed November 28,

1945, Serial No. 631,440.

means is disclosed for preventing helical motion In that application scanning coil in respect to the decelerating ring around the space immediately in front of the target. This adjustment varied the relative phases of the spiraling effect of the coil and decelerating ring so that they could be brought into section, partly diagrammatic, of a cathode ray tube involving the improvement.

Referring to the drawing, the tube has an evacuated container l usually of glass, with the usual indirectly heated cathode 2, grid 3 and first anode l constituting the gun. The end 5 of the anode also constitutes the first dynode of the multiplier. This may be a thin sheet of silver-magnesium alloy clamped over or otherwise fastened to the end of the anode, which may be of non-magnetic *stainless steel. Around the gun in spaced or insulated relation are positioned the other multipliers consisting, as shown, of the second stage 1, third stage 8,;iourth stage 9 and fifth and last stage l0. "Between the fourth and fifth stages is located a fine mesh screen H constituting the collector electrode for the electrons bombarded from the last electrode. The second, third and fourth stages have a dynode having radial slanting blades I2, somewhat resembling an electric fan. Each of these three stages has a. fine mesh screen I3 attached to the rim of the support for the dynodes and spaced a small distance from the dynode blades, which may be made of silver magnesium alloy, for example. The particular form of multiplier is not claimed herein, but is fully described and claimed in my co-pending application, filed September 16, 1944, Serial No. 554,494, now U. S. Patent No. 2,433,941, issued June 6, 1948, and

opposition. The result was the helical motion reference is made thereto for adetail explanation produced by the two sources was made to cancel, oi the multiplier stages,

so that the beam had no helical motion when it Before describing the particular improvement arrived at the target. In that application it was claimed in this application, it may be said that also disclosed that the decelerating lens could be the return beam bombards secondary electrons madeto have less spiraling effect on the electrons 5 from the dynode o h fi Stage 5 and these by using a fine mesh screen close to the target are directed into the field of the dynode and on the gun side thereof. The present application screen of the second stage, passing through the is directed to the latter modification of that prior fine .mesh screen and bombarding secondary application. electrons from the dynode blades. These sec- I It is an object of this invention to provide a 71 ondaries are attracted to the higher potential cathode ray tube decelerating means that will field of the next stage 8, and so on. The secgive less scanning of the return beam over the ondaries bombarded from the dynode blades of first dynode of the multiplier, which reduces mul the fourth stage are attracted through the meshes p Shadingof the collector screen i l and bombard secondaries It is another object of the invention to construct from the dynode of the fifth stage III, which is a a tube with the decelerating electrode adjacent fiat metal disc of silver magnesium. The secthe scanned surface of the target to provide for ondaries emittedby this last dynode are attracted more uniform landing of the electrons from the to the collector screen and pass out of the tube beam across the raster. to the pre-amplifier or any other desired device. Another object of the invention is to provide The tube has an electrode I4, commonly called a higher electric field at the target surface to the persuader, which establishes a certain field improve the resolution. that directs the secondaries from the first dynode Other objects of the invention will appear in 5 into the second stage. the following specification, reference being had j The oa d Wall coating y, in fact. be a to the drawing, in which the single figure is a 40 m t l coating on h inside f the env lope l, but

I prefer to make it of non-magnetic metal of relatively poor conductivity, such as Nichrome cylinder l5. Opposite the end of cylinder l5 remote fromthe gun is placed a target I6 consisting, of a thin semi-conducting sheet of glass, described and claimed in the application of Albert Rose, filed November 28, 1945, Serial No. 631,441. Adjacent the unscanned surface of the glass target IE is a fine mesh screen I! which may be contact spaced from the target, that is, touching the same, or it may be spaced to 500 mils orniorefas "disclosed in'the application of Harold B. Iiaw-,-filed April 19, 1945,Serial No. 589,241. The target 16 and screen I! are suitably mounted "in a metal ring 'or cylinder frame IB.

thereto. .sulated relation to the other electrodes,.but I find On the inside surface of the end of the envelope remote from the gun is positioned the photocathode l9, marked PC in the drawing, which may be a thin metal coating on the inside wall that is substantially transparent to light and yet is sufficiently conducting for movement of electrons thereover. Adjacent the photocathode i9 is an electrode "20, .called a photocathode ring, marked "PCRX in-the drawing, and positioned. between ring 20 and the target frame I8 is another ring 2| conductively joined to the frame Hi. All

the parts and electrodes inside the tube, except photocathode H], as shown, may be supported in any way known in the art, forexample, as disclosed in the applications {of Stanley V. .Forgue, filed February 7, 1946, Serial No. 646,075, and file-d January 28, 1946, Serial No. 643,925, and now D. S.

Patent No. 2,441,315, issued May '11, 1948. My invention is not dependent upon the particular supporting means and hence it is not shown.

Around the outside of the tube, is arranged a .solenoid coil .22 adapted to produce a uniform direct current magneticfocusing field for the out- ;going and returning beam electrons, as well as for the photoelectrons emittedby the photocathode 19. Outside the tube is also arranged a com- ;pensating coil 23, as disclosed and claimed in the application of .Albert Rose, filed April 11, 1942,

Serial No. 438,562, nowU- S. Patent No. 2,407,905,

.issued-;September 17, 1946. Thiscoil produces a :fieldat a-right angle to the axis of the tube and, upon adjustment around the axis, it eliminates helical motion of the beam electrons due to the unavoidable slight misalignment of the gun in factory construction. Inside the coil 22 is positioned the horizontal and vertical deflecting coils, shown as a unit. As is well known, one-coil of the unit produces a deflecting .field at a right .angle to the-axis of the tube to produce line scan- .sion of the target by the beam and the other produces a field ata right angle to the axis and .also to the field .of the other deflecting coil to produce frame scansion. The line scansion coil will beconnectedto a saw-tooth generator ofline .scansion frequency and-the other one will be connected to a saw-tooth generator of frame scanning frequency. These saw-tooth generators are .well known in theart and are not shown.

-To provide the more uniform fieldadjacent the raster surface of the target l6, afine mesh screen 2.5 isarranged in front of the target and parallel This screen vmay 'be supported in inthatpexcellent results are obtained when it is mounted in a metal framefitting conductively inside the cylinder electrode 15, corresponding to target isimportant and should satisfy the follow- .ing. conditions:

.1. Itf-Should be out of focus for the. incident and :alsothe return beam.

, ..2. It should not introducespurious patterns.

The first condition is not critical. The screen vis atits most out-of-focus position when the distanceof the screen jfromthe. target in centimeters is equal approximately to :where E is the screen potential in volts andis the :magnetic field -in gausses produced .by the magnetic focusing field of coil. 22.. The second condition is more critical It 11.

found experimentally that occasionally an apparent screen pattern can be seen even when the first condition is completely satisfied. This is due to an interference between the screen shadow pattern impressed on the out-of-focus incident beam and the similar pattern impressed on the out-of-focus return beam. The type of spurious screen pattern 'seen depends upon. the transit time of the electrons .between their first and second .when mounted directly on thewall cylinder [5 and operated at wall potential, it is slightly advanitageous to have the screen a few volts positive with respect to the wall, so that secondary electrons knocked out of the screen by the beam will return thereto and will not reach the multiplier. In this way the noise introduced by the screen can be minimized.

.The screen 25 should have at least transmission. It should be free ofimperfections and should be ,fine mesh of at least 200 wires per inch. Screens are available to meet-these conditions.

The tube may be operated at various voltages ,in the electrodes, but I have indicated on the drawingsatisfactory voltages, positive and negative with respect to ground. I

Briefly, the operation is as follows:

Alight image focusedon the ph otocathode 1.9 liberates electrons therefrom and these are focused on the glass target by the fieldof coil .22.under the acceleration force of the potential of ring 20 and screen ll. Secondary electrons are bombarded from the rear (photocathode-side) sideofthe glass and are collected by screen H, which establishes the maximum potential to which the target may rise. Thus, a positive charge image is formed on the rear surface of the glass that exactly corresponds to the light image focused on thephotocathode I9. As the beam from the gun scans the front side-0f. the target,.it is decelerated substantially'to zero electron velocity and only enough electrons land on the front side of the target to deposit electrons equivalent to those on-the rear side, the potential pattern set up on the front side :by-the charge image being substantially the same as that "on the rearside, due to the extremethinnessofthe .glass film, which is about 0.2 milor less. As explained in the said Albert Rose application, Serial No. 631,441, the conductivity of the glass is such that theelectrons-landed pass through and discharge'the electrostatic image-on the rear side Within a frame time. The remaining electrons of the beam return towards "the gununder the accelerating force of the screen 25 and wall cylinder l5. These returning electrons are focused by the field of coil 22 and land on an area of dynode 5.0utside the aperture through which the beam was shot from the gun. The returning beam is modulated by loss of electrons to the target l6 and the secondary electrons bombarded from the dynode 5 are produced in proportion thereto in multiplied numbers. These are-directed by persuader into the "field of-the second multiplier stage and after multiplication by the succeeding stages a greatly multiplied signal is fed into the output terminal, as already referred to.

The field established at the raster surface of the target is uniform throughout the raster, except at the perimeter. This can be balanced out by adjustment of the deflecting coils 24, as explained in detail in my first-mentioned application. The adjustment is indicated by arrows 26 and the adjusting means of that application is diagrammatically indicated by the manually adjustable rod 21. Due to the reduced helical effect of screen 25 as compared to a decelerating ring, the coils of deflecting unit 24 should be designed in size and ampere turns to have a helical effect as small as possible, so that the helical effects will balance out to produce uniform landing of the electrons on the target. Since the beam lands electrons on the target in proportion to the varying potentials established on the elemental areas by the charge image, the expression uniform landing is explained as meaning that the number of the electron-s landing on the raster at potential +e will always be the same, no matter whether that area is at the center of the raster or is at the outside or perimeter thereof. To accomplish this there must be minimum loss of energy of the electrons due to spiraling and my invention accomplishes the purpose better than decelerators heretofore used.

I claim:

1. A cathode ray tube of the Orthicon type comprising a cathode and an anode for forming a beam of electrons, a semi-conducting target, a metal screen in front of the target, a photocathode and electric field producing means for forming a charge image on one surface of the target, field producing means for scanning electrons from said beam through said screen and over the other surface of said target to discharge said image, said screen being spaced from the target at such distance as to preclude the superimposing of screen image signals on the beam returning from the target through said screen.

2. In cathode ray tubes of the Orthicon type, means comprising a cathode and an anode for forming a beam of electrons, a semi-conducting target, a fine mesh metal screen in front of the target, a, photo-cathode and electric field producing means for forming a charge image on one surface of the target, field producing means for scanning electrons from said beam through said screen and over the other surface of said target to discharge said image, said screen being spaced from the target at such distance as to preclude the superimposing of screen image signals on the beam going toward and returning from the target through said screen.

PAUL K. WEIMER.

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

UNITED STATES PATENTS Number Name Date 2,021,907 Zworykin Nov. 26, 1935

Images