US2115093A - Cathode ray tube - Google Patents

Description

April 1938. M. VON ARDENNE CATHODE RAY TUBE Filed May 24, 1934 INVENTOR Von fir denne anfred HTTORHEY Patented Apr. 26, 1938 UNITED STATES PATENT OFFICE CATHODE RAY TUBE Application May 24, 1934, Serial No. 727,207 In Germany June 10, 1933 13 Claims.

My invention relates to electron discharge devices and more particularly to cathode ray tubes as used for oscillographs and in television transmission systems or similar electronic devices using a concentrated electron beam.

The general form of cathode ray tube as known in the art and used for oscillographs and in the cathode ray television systems, contains essentially the three parts: a source for producing a thin stream or pencil of electrons traveling at very high velocity, a fluorescent target or luminous screen for the electrons to strike against to produce a luminescent spot, and some mechanism for deflecting the path of the electron pencil in any direction and for varying the intensity of the electron beam to produce any desired pattern or image upon the fluorescent screen.

While my invention is in particular useful for cathode ray tubes including a gaseous atmosphere for focusing the electron beam, it is understood that the novel features of the invention as will be set forth are equally applicable to tubes operating with a partial or a high vacuum.

., Cathode ray tubes of known type utilizing a gas for focusing the electron beam are characterized by their very great simplicity of both construction and operation.

One of the major disadvantages of cathode ray tubes of this type is the fact that the brightness of the luminous spot on the fluorescent screen is insuflicient for many uses, such as for direct projection of a television image on a screen similar as in a moving picture or lantern projector.

It has not been possible to increase the anode voltage beyond several thousand volts for increasing the brightness of the luminous spot on the screen due to undesired interference with the concentrating action of the gas as the anode 40 voltage increases, and it has furthermore not been possible to materially increase the degree of luminescence by measures and improvements applied to the source of the cathode ray.

Accordingly, it is one of the main objects of my invention to provide means in connection with a cathode ray tube for substantially increasing the degree of brightness of the fluorescent spot on the luminous screen as compared with tubes of this type heretofore known. in the art.

50 There are several reasons for the limited luminosity of the fluorescent spot in cathode ray tubes hitherto known in which a translucent fluorescent screen is applied to the inside wall of the tube whereby the viewing direction is against the outer 55 or rear side of the screen.

One disadvantage of a screen of this type is the fact that the luminescent energy produced by the impact of the electrons is distributed over two sides of the screen; that is, the front side and the rear side, resulting in substantial reduction 5 of the useful luminescence, as is obvious.

Accordingly, it is another object of my invention to provide a luminous screen for use in cathode ray tubes in which the luminous energy is applied to the front side of the screen only im- 0 pinged upon by the electron beam for increasing the degree of luminescence obtained.

Another disadvantage of screens used in tubes heretofore known is due to the fact that the screen has to be very thin to produce sufficient lumi- 15 nosity at the rear side which in turn entails a reduction of the luminosity by absorption by the screen itself.

Accordingly, a further object of my invention consists in the provision of a fluorescent screen for 20 use in cathode ray tubes in which the thickness of the screen is not limited, enabling the use of a screen of increased thickness as compared to hitherto known tubes for increasing the degree of fluorescence obtained.

Another disadvantage inherent in cathode ray tubes of known construction is the fact that a large amount of light passing the glass wall of the tube to which the screen is applied, is lost by total reflection from the outer wall of the tube with the added disadvantage of disturbing halo efiects being produced which in turn greatly impair the detail or definition of the pattern or image recorded on the screen.

Accordingly it is another object of my invention to provide a fluorescent screen for use in cathode ray tubes in which losses by total reflection and a blurring of the picture by halo efiects due to total reflection from theouter wall of the base to which the screen is applied is substantial- 1y eliminated.

Still a further disadvantage of luminous screens used in cathode ray tubes heretofore known is the fact that on accountof the bad heat conductivity of the glass wall upon which the screen is applied, 5 the particles of the screen impacted by the impinging electron pencil became excessively heated, resulting in impairment of the luminescence, causing in turn a decrease of brightness of the luminous spot and a blurring or loss of detail of the pattern or image produced.

Accordingly it is another object of my invention to provide a luminous screen for use in cathode ray tubes in which heat produced by the impact of the electron pencil upon the screen is angles and furthermore the irregularities of the glass of the tube result in great optical distortions, as pointed out.

I overcome these disadvantages in accordance with a further feature of the invention by inclining the plane of the screen relative to the axis of the tube preferably at an angle of 45 with the axis of one of the two pairs of deflecting plates. In this manner, it is possible to View the screen at right angle to its surface or to reproduce or project the pattern or image by means of an optical system having its axis arranged at right angle to the screen surface. It is furthermore possible by such an arrangement to mount a high speed lens system of short focal length at a minimum distance from the screen enabling the use of inexpensive lenses of high power or relative aperture as the price of a lens of given speed or relative aperture decreases, the shorter the focal length, as is well known. For this purpose, I provide a spherical end portion of the tube in which the luminous screen is arranged which has the further advantage that the glass wall may be manufactured free from any defects or surface irregularities, resulting in an elimination of optical distortions.

Referring more particularly to Figure 1, I have shown a cathode ray tube construction according to the invention comprising a glass bulb I consisting of a cylindrical neck portion housing the source producing the electron ray and the deflecting and controlling mechanism and a spherical end portion enclosing the luminous screen 1 arranged at an angle of 45 to the axis of the tube. The tube is mounted upon a base 2 and provided with a press 3 acting as a support for the cathode and the well known cylindrical or concentration electrode 5. The cathode 4 may be either a directly heated'cathode as shown,

or it may be an indirectly heated cathode Well a known in the art. The cathode terminals are connected to prongs 4 and the concentration cylinder 5 is connected to a prong terminal 5 mounted at the bottom of the base 2 for connection to the proper operating potentials. I have furthermore shown at 6 an anode in the form of a disc with a central aperture for passing the electron pencil produced by the cathode 4 and the concentration cylinder 5. Numerals 8, 8 and 9, 9' represent the usual pairs of deflecting plates arranged at right angle to each other for both horizontal and vertical deflection of the cathode ray. I have furthermore shown at II] and II terminals for applying the deflecting or sweep potential to the plates 8 and 8', respectively, and imilar terminals may be provided for the plates 9 and 9' as is understood.

The luminous screen 1 consists of a base plate such as a metal plate upon which there is applied a coating of luminous material I, such as of zinc silicate or the like well known in the art. I have furthermore shown the screen I connected to an outside terminal l2 through lead I! for applying a potential to a screen equal to the anode potential, as shown, to prevent disturbing back current from the screen to the anode electrode. I have furthermore shown a constricted portion of the tube close to the spherical end portion, enabling further the close mounting of an inexpensive optical lens system of short focal length, as will be seen more clearly from Figure 4. The diameter of the constricted portion is limited by the dimension of the screen 1 and should be such as to allow a maximum sweeping angle of the electron pencil to cover the entire surface of the screen 1. In order to prevent the effect of disturbing wall charges, I have shown a further electrode l3 applied to the outside Wall of the constricted portion maintained at anode potential by the connection, as shown.

The inclined mounting of the screen involves various added distortions of the luminous spot or the pattern or image produced on the screen I. These distortions increase as the length of the tube or of the electron ray, respectively, decreases, a procedure which I have found advantageous, as will be pointed out hereinafter.

In general, especially in the case of television, it is required that the sensitivity of deflection is substantially equal for both pairs of deflecting plates. By the provision of an inclined screen and assuming that the distance between the deflecting plates in both deflecting systems is the same, it is seen that the deflecting plate system arranged close to the anode electrode which is furthermore at a greater distance from the screen than the other plate system, has a considerably increasedsensitivity of deflection. I compensate for this difference of sensitivity by making the sensitivity of the plates arranged close to the anode less than the sensitivity of the plates arranged close to the screen. This may be obtained, for instance, as shown by Figures 1 and 2, by varying the distance between the deflecting plates; that is, by using a larger distance between the plates 8 and 8' arranged close to the anode 6 as compared with the distance between the plates 9 and 9'.

A further distortion is produced by the deflection caused by plates 9 and 9' in a vertical direction due to the inclined position of the screen I. This has the effect, as is seen, that the portions of the image or pattern at the part of the screen which is at closer distance to the deflecting system are concentrated while the portions of the image or pattern at larger distance from the deflecting system are expanded. Thus in the case of equal deflecting or sweep voltages applied to both pairs of deflecting plates, which under normal conditions would result in a luminous surface of substantially square shape, the distortion produced due to the inclination of the screen by the deflecting plates 9 and 9' will result in a surface of trapezoidal shape as compared to a square when no distortion would be present. I avoid this distortion in accordance with my invention by arranging plates 9 and 9 at an angle such as shown in Figure 3 in such a manner that within the range of shorter length of the cathode ray the sensitivity of deflection is increased on account of the greater electric field strength between the plates while for the range of longer length of the cathode ray the sensitivity of deflection is less, due to the decreased field on account of the larger distance between the plates 9 and 9'- A further effect produced by the inclined position of the screen 1 results in a distortion of the shape of the luminous spot produced by the impinging electron pencil. Thus, it is seen that if the cross-section of the pencil under normal conditions is substantially circular producing a circular spot on the screen, in the case of an inclined screen as shown the circular spot will be distorted into an oval or elipsoidal shape. I overcame this distortion in accordance with a further feature of the invention by providing a cathode producing a pencil of elipsoidal cross-section, such as by making the emitting surface of the cathode of oval shape. The same effect may be obtained by using a loop-shaped filament which, as is known, produces'an oval electron stream. Thus,

1 itis only possible to properly orientate'the stream stood.

clination of the screen 1 in a tube as described is r produced by the cathode so as to compensate the oval distortion to obtain a substantially circular .spot'on the inclined screen. For'this purpose the large axisof the oval stream should be in a direction of the deflecting plate .system'mounted close to the fluorescent screen ,(9 and 9), as is under- Still a further disadvantage caused'b'y. the indue to ,a' decreaserof definition of theluminous spotzat' certain portions onthe screen; that is, at

those parts on the screen which are at a fartherdistancefromuthe cathodethan the remaining parts. 'I'his'resultsin a substantial lack'of-de tail or "definition especially'in thecase of televi: sion images at those portions of a pattern or picture lying atthe'outer portionsof the screen; that is, for the larger deflecting angles.

I avoid the aforementioned defect and disad- V vantageof an inclined screen by the provision of means for applying different potentials. to the ance material such "as shown at I 1 in Figure 5,

. like.

various points on the luminous screen in sucha r manner as to substantially compensate .for' the varying length of the impinging electron pencil individual elementary areas on; the

upon .the screen.

In accordance with one embodiment of the invention, I provide' a'screen consisting of resistor 'alternatively'I may use a base plate upon which a coating of resistance material is applied as by means of any well known method, such as by cathodefsputtering or precipitation and. the

nected to metallic connecting elements or electrodes l8 and I8" mounted along the upper and lower 'edge of the screen I! and in contact with the resistance :surface I! coated'upon a base metal or itself forming the support for the 1u.

minous'sc'reen; This is shown iniurther detail byflFigure' 6. In' this manner a gradual voltage increase from the upper to thelower edge on the screen is obtained which is added to a normal positive voltage 20 applied to the screen as shown in Figure 5. By properly choosing the voltage l9 it'is seen that by an arrangement of this sort a gradually increasing potentialis'applied to the' 7 screen with increasing length of the impinging electron pencil, thus' compensating and eliminate; J ing the difference. in detail or'definition .of' the "luminous spot for different ideflecting angles,fas

the fluorescent screen; that is in other words, .by

adjusting the conductivity at different zones on the screen, any deviations or distorting efiects other than those described due to any cause may be'compensated in this manner, or alternatively any modification of the picture or pattern or its definition may be adjusted as desired. Thus for instance, it is possible to produce additional acceleration of the electron pencil between the I have furthermore shown a battery l9 con-,

screen and a preceding accelerating electrode, especiallyfor large deflecting angles in which case celeration of the-electron pencil to secure a desired luminosity ordegree of definition, as may be desired, of the fluorescent spot'produced by the electron impact. I

the screen itself may act as a means for final ac- In all the aforementioned cases where a resistance material is used in connection with the luminescent screen, care should be taken that the resistance of the support for the screen between the connecting'electrodes is sufiiciently low so as to prevent any substantial potential drop due to currentflow caused by the impinging electron pencil and causing undesired. modification and distortion of the electric field. I have found that in the case of tubes operating with compar atively.low voltages, the voltage drop produced by the ray current along the support of the 'lumi nescent screen shouldbe less than 10 volts. As with the usual tubes the intensity of the ray cur rent is about 1O "amperes, this value would coraprecipitating method, as known in the art.

By using atub e as described hereinbefore with the" additional means for compensating the various, distortions and errors introduced by the inclinationof the screen, I have been able to secure records of patterns or images on the fluores cent screen absolutely free from any distortion;

When an optical lenssystem is desired, such frespond to a resistance of about 100,000 ohms whichcan easily be obtained by a sputtering or as for reproduction or 'projection'of the screen pattern or image, the size and cost of'the lens will be the less, the smaller the size of the fluorescent screen, as is obvious. .However, a de-- crease of the dimensions of the fluorescent screen is only possible if the diameter of the luminous spot can be decreased at the same time. While in. tubes as heretor'orc known in the art in which the luminous screen is directly applied to the inside wall of the tube, the diameter of the screen is from 12 to 20 centimeters,

it has been possible by using. a construction according to my invention to decrease'the diameter of the luminous screen to 7.5 centimeters or less. Despite this small dimension'of the flu crescent screen, I have been able to secure an exceedingly sharp definition or detailof the pic- 'ture by decreasing the length of the ray or of the tubeyrespectively. I have found that with decreasing length of theray, especially in the case of gas filled cathode ray tubes, the sharpness ofthe luminous" spot increases considerably. As is known, the electron-optical repre sentation of the cathode in the tubesas known in'the art takes place with considerableenlargement depending, as pointed out, to a large extent upon the length of'the ray. Investigations have furthermore'shown that a decrease of the tube' is advantageous not only on account of the decrease of the diameter of the luminous. spot but also'on account of the increase of the current at the top of the ray, resulting in a considerable increase of the luminescence produced by impact of the ray upon the luminous screen.

I have .found by measurements that in the usual cathode ray tubes provided with a gaseous filling, the ray current at the top is only about 20% of the entire emission current, the rest being lost by stray electrons leaving the ray in a lateral direction.

ably decreased and the current intensity at the end or the recording placeis increased by 50% By decreasing the 'ray' length, the loss by stray electrons is considerand more of the total emission current. This not only results in an increase of brightness to about the double value, but simultaneously is accompanied with a decrease of the interfering and disturbing side luminescence caused by the stray electrons to about half the value as compared to cathode ray tubes of standard design.

By means of cathode ray tubes constructed in this manner, images of exceedingly sharp contrast are obtained due also to the fact that the above-mentioned disturbing halo effects have been eliminated. It is furthermore advisable, as pointed out, to connect the metal screen with the anode to prevent a back flow of the electrons through the gaseous space in the tube, thus eliminating the number of disturbances caused by such back currents.

I have furthermore found it advisable, as pointed out, to prevent wall charges at the constricted portion of the tube by providing an outer metallic coating also connected to the anode or ground, respectively, as shown.

Referring more particularly to Figure 4, I have shown a cathode ray tube as illustrated by the previous figures, mounted in a housing 14 upon a displaceable support H for adjusting the tube relative to an optical system as indicated at l5, such as a projecting apparatus for direct projection of the screen image similar to a lantern slide or moving picture projector. The tube is mounted in an inclined position as shown and the objective system l5 arranged with its axis at right angle to the surface on the screen I. I have furthermore shown at IS an opening or window for controlling and observing the operation, especially the cathode heating of the tube.

If the image produced by the optical system i5 is reversed as in the case of an ordinary projector, this may be compensated by optical or electrical means so that a final upright and right-sided image is obtained on the projection screen. Thus for instance, the image may be reversed by the use of prisms or mirrors as is well known, or alternatively, in accordance with a further feature of my invention, the orientation of the image may be controlled by properly choosing the polarity of the deflecting voltages applied to the deflecting plates 8, 8 and 9, 9', respectively. Thus, in the case of optical projection whereby the image is reversed in the projector, the polarity of the sweep voltage moving the beam in the vertical direction should be such that the picture is being scanned from the bottom to the top so as to obtain a reversed picture on the luminous screen, which will then appear as an upright picture on the projection screen. Similarly, if the relationship of the sides of the picture is reversed this may be compensated by proper choice of the polarity of the deflecting sweep voltage moving the beam across the image, as is understood. Alternatively, the tube may be mounted in an upside-down position to obtain a reversed picture on the luminous screen which will appear as an upright picture on the projection screen. According to a further feature, I may provide reversing switches for reversing the polarity of the sweep voltages in any desired manner, such as for either direct observation of the picture through a magnifying lens or for projection by means of a projection lens, as described in more detail in my copending application entitled Television system, filed June 13, 1934, and bearing the Serial No. 730,408.

By using a tube and an arrangement as described, operating with an anode voltage of the order of 4000 volts and a gaseous filling consisting of a hydrogen gas, I was able to obtain a bright screen picture enabling a direct projection upon a surface of 2 x 2 meters in a darkened space. By the same apparatus, oscillographic records were possible with a recording speed up to- 20 kilometers per second using a high speed lens system.

I claim:

l. A cathode ray tube for recording purposes comprising an envelope; means for producing a concentrated, electron beam therein; means for deflecting said beam in two mutually perpendicular directions; an opaque screen impinged by the electrons to produce a light spot thereon, said screen being mounted at an angle to the line of direction of said beam; and electrostatic means for compensating for recording distortions produced by the inclination of saidscreen with respect to a viewing direction at right angle to said screen. 2. A cathode ray tube comprising an envelope; means for producing a concentrated electron beam therein; two pairs of deflecting plates arranged at right angle to each other for deflecting said beam in conjugate rectangular coordinates; an opaque screen covered with luminescent material for the electrons to strike against mounted with one coordinate at right angle and with the other coordinate inclined tothe line of direction of said beam; and means for compensating image distortions produced by the inclination of said screen.

3. In a cathode ray tube as claimed in claim 2 in which the portion of said envelope enclosing said screen is of substantial spherical shape.

4. A cathode ray tube as claimed in claim 2 in which one pair of said deflecting plates is arranged with its'axls parallel toand at closer distance from said screen and said other pair of deflecting plates is arranged at a further distance from said screen with its axis at an angle of 45 with said screen, and in which the distance between the plates of said latter deflecting system is greater as compared to the distance between the deflecting plates of said former system. 5. In a cathode ray tube as claimed in claim 2 including said screen being adapted to have biasing potentials applied thereto to secure a predetermined potential distribution of different zones of the screen area for compensating the effect of varying length of the cathode ray due to the inclination of said screen.

6. In a cathode ray tube as claimed in claim 2 including means for varying the cross-sectional shape of said cathode ray for compensating distortions of the shape of the luminous spot caused by the inclination of said screen.

'7. In a cathode ray tube as claimed in claim 2 including means to secure an oval cross-sectional shape of said electron beam for compensating distortions of the luminous spot on said screen caused by the inclination of said screen.

8. In a cathode ray tube as claimed in claim 2 in which said luminous screen is coated upon a base of resistance material, and adapted to have 9. Acathode ray tube comprising an envelope,

means for producing a beam therein, a luminous screen in thepath of said beam and at an angle thereto, said screen comprising an opaque base mirrored upon the side facing said beam, and the concentrated electron beam therein, means {or luminous material positioned on said mirrored face of the base. 7 7 10, A cathode ray tube comprising an envelope,

'means forproducing a beam therein, a luminous screen in the path of said beam, said screen comprising a'highly polished opaque aluminum base;

facing saidbeam and having the luminous mate rialpositioned on said polished face.

11. A cathode ray/tube for recording purposes,

comprising an envelope, meansfor producing a deflecting said beam in two mutually perpendicular directions, an opaque screen covered with luminescent material for the electrons, to strike against to produce a light spotthereon, means to support the screen at an angle to the direction of said beam, said screen being adapted to have electric biasing potentials applied theretorto prevent I the flow of back current from the screen.

12; A cathode ray tube for recording purposes,

comprising an envelope/means for producing a concentrated electron beam therein, electrostatic means for deflecting said beam in 'two mutually perpendicular directions, an opaque screen covered with luminescent material for the electrons to strike against to produce a light spot thereon, means to support the screen'at an angle to the direction of said beam, said screen being adapted to have electric biasing potentials applied thereto to prevent the flow of back current from the screen.

'13. A cathode ray tube comprising an envelope, means for producing a concentrated electron beam therein, means for deflecting said beam in mutually perpendicular directions, a target eleck trode, means for supporting the target electrode at an angle to the line of direction of the electron beam, and means to develop a potential difference between the beam producing means and the target electrode, said potential difierence being of a magnitude progressively changing in accordance with the distance between the elemental areas or the target and the beam producing means.

- MANFRED VQN V ARDENNE.

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