US2608666A - Braun tube for use in television - Google Patents

Description

Aug. 26, 1952 J. DE GIER BRAUN TUBE FOR USE IN TELEVISION Filed Dec. 17, 1949 JOHANNESZD BY I INVHV TOR.

E GIER AZENT Patented Aug. 26, 1952 BRAUNTUBEFOR USE IN TELEVISION V Johannes de. Gier, Eindhoven, Netherlands, assignor to'Hartford National Bank and Trust Company, Hartford, Conn., as trustee 6 Application December 17, 1949, Serial N0. 133,504 In the Netherlands December 21, 1948 This invention relates to Braun tubes for use in television receivers.

In such tubes, inorder to avoida so-called "ion burn" of the image screen, use is frequently made 0119. device operative "to remove from the electron beam the negative ions which, together with electrons, are emitted by the cathode of the tube, orwhich. are produced in the discharge space.

Said device may comprise a cylindrical electrode through which the electron beam passes and which collects a beamof negative ions separated from the electron beam by deflection. This electrode, which may be referred to as a negative ion collector, has a transverse wall (collecting plate) which is provided with an aperture to allow the passage of electrons. The cylindrical part of the ion collector may be a metal body arranged in the tube and is not necessarily of cir-'" cular cross-section; as an alternative, it may be constituted by a conductive coating or a metal part of the tube wall.

The invention relates to Braun tubes for television reception comprising a negative ion collector, in which tubes the negative ions are deilected from their original path and are brought outside the electron beam.

,Braun tubes for use in television receivers are known, which comprise an ion collector, in which the ion beam retains its original direction and the electrons are deflected from this directionby a magnetic field. Such atube is described, for example, in British patent specification No. 538,684. According to said specification, the collecting plate of an ion collector is arranged at or near a point at which the cross-sectional area of the electron beam exhibits a minimum value; the small aperture in the collecting plate is projected by an electron-optical lens onto the image screen. The present invention does not relate to such tubes butconcerns only tubesin which the col-l.

lecting plate lies within the object distance from the projecting lens, suchyfor example, as the tubes towhich Figs. 16 and'17 on pages 375 and 376 of Reviewsof Modern Physics," vol. 18, No. 3.01 July 1946 refer. I. l

The'plane passing through the axes of "the ion collector and the deflected ion beam will be reterredtoas the meridian plane, the plane passing through the axis of the ion collector atright 3 Claims. (01.313-86) angles to the meridian plane will be-referred to as the equatorialplane.

With all the'known tubes of this kind, the aperture in the ion collector for passage or: the electron beam is circular "and-concentric with the axis of the ion collector. According to the invention; a Braun tube for use in television receivers comprising a negative ion collector for intercepting a beam of negative ionswhich is deflected and thus separated. from the electron beam, the collector being constituted by a cylindrical electrodecomprising'a collecting plate, which plate is not located at or near apoint at which the cross-sectional area of the electron beam exhibits a minimum value, ischaracterized in that'the, aperture in the collecting plate for passage of the electronsis not both circular and; concentric with the cylindrical electrode." It the aperture is asymmetrical with respectfito'the equatorial plane, the plane figure it forms has its centre of gravity on that side 01 the equatorial plane opposite thaton which the deflected negative ion beam is located. In this case the aperture may be circular. However; as an alternative; the: aperture may -be symmetrical with respectt'o the equatorial plane, but in this case itis not circular. Naturally, the aperture must not allow passage of the negative ion beam to be intercepted. r I v 5 In order that the invention may be more clear ly understood and readily carried into efiect, it will now be described more fully with reference'to the accompanying. drawing, in which: Fig. 1 is an axial sectional view taken on the meridian plane of an ion collector of known con structionwhich collects a beam of negative ions deflected from itsinitial direction, and 6 M Fig. 2 is a cross-sectional view of the same body taken on the planeIL-II; 6 1 6 r r 6 1 Figs. 3 to 7 are cross-sectional views, similarto that of Fig. 2, of ion collectors for use in a Braun tube according to the invention. Figs. 8, 9 and 10 are graphs themeaning oi which will be apparent from the description.

Referring now to Fig. 1, a negative ion collector comprises a right circular cylindricalpart and a diaphragm 2 which constitutes the collecting plate. The end plane at the left-hand end of I cylinder I is at an angle with the axis 301? the cylinder. At this end, at which the beam 4 constituted by electrons and negative ions enters the cylinder, is produced an electric field which, in the region associated with the beam, has a constant component at right angles to the axis 3, parallel to the plane of the figure. The shaded beam 5 of negative ions is deflected by this field in the direction of the arrow and strikes the col- It is not of importance whether thesethe same part of the path of the electron" beam as the electric deflecting field or.:notraszlongrass the axis of the electron beam 6 emerging from;

the negative ion collector coincides with the ax-is. 3 of the cylinder I.

Fig. 2 shows the circular apertureintlie: dia: n

phragm 2. The radius of this aperture deter mines the diameter of the electron beam Sallowed to pass. The aperture must be so small as to liegwithout the region in which the: ions .-;have an appreciable density. 'Iihewregionwi-thin which the beam fifioif negative. ions strikes .ethecoHector iszindicated-hy'abrokerrline; r The negative:v ion collector; also col-ie'ctsr-elec trons since the electron beam-passing; through the collector is not .sharplyidefi'ned; In. aera-dial direction reckoned" from the axis. .3=,.,the electron densityrgradually diminishea. .However, since the.- decrease inrdensity: isiimateriially smaller close-to thmaxis than: that. with increase in:.distance: therefrom, ai-tcoreof-zgreat density may-be. distinguishedrwithin a. marginal. hal'oz, offless: intensity; The. electrons: in; said, halo; which "nevertheless constitute a. material "percentage of: the: electron current;. are in; part: intercepted. by the diaphragnr: 2;. :This; percentage. increases with-in.- czeas inelectron current;

"The? invention has; forxits: ohjectto reduce the lossgirr electron current, so as to ensure a: greater contrast-between: the darkeryand lighter parts: of.

the mage: .proj ected onto the :screen.

theiaperture- 8 in "Figs 3 is larger thanthat zin "screen,.,;since in this case the image spot would 4 also be increased, or at least exhibit a lack of definition.

The aperture is not necessarily circular. It may extend as far as the cylinder wall. This results in constructions of which Figs. 4 and 5 are examples.

Referring now to Fig. 8, the full line curve illustrates the relationship between the bright-v ness B, measured in mK./cm. of the image spot in a tube having a circular central aperture and the current strength i in ,u amps absorbed by the tube. The broken line curve illustrates the corresponding. relationshipior'fa tubeicorhpi'ising a diaphragm as shown inFig; 41 It will-be seen that the effect of the increase in size of the aperture in the diaphragm increases with increase instrengthoi. thecurrent. A collecting plate as shownih Fig.2 cuts off a greater part of the electron: heam. producing the brighter parts of the image-proiectedpntoa fluorescent screen of a tube -thanbfthe less bright parts, since the core of the. electronbeam increases in size as the current-strength increases. The part of the electron beam intercepted by the collecting plate may amount'to as much as 35% of the total for the brighter parts. Consequently, the brighter parts of the limage'are less brightzthanstheyshouldi'be. The use of the invention, aszisishownzin Fig: 8,. ensures; anincrease; in: brightness.inftherbrighter parts; and thus an: increase: incomrast; The maximum ;intercepted part. of the electron cure rentcan bereduceditcrlfl 31110311655 electrons: are intercepted; byrthe'icolilector. inv a: tuberaccordings. to r the invention-the further advantage: is obtained: that-:with.= a; given total. current strength azsmalier'amountz of heat: iswdeveloped. in. thelacoilector. This-reduces the risk. of 1 the deterioration. of: the: vacuum duet therreleaseiof g-asesafroni'heated-smetal parts;

- Centring the; electron. beam facilitated with: the-tube: according; to :theinvention. This opera-' tionisrequired with; television apparatus, for ex ample; cperative for -the: first: timer The operal tion is carried; out byadjusting the magnetic: fields required: .tcrneutralizez the. 'defi'ectiv'ei eiie'ct; cit the electric field on the. electron beam; Witha central, circular aperture inl the diaphragm; itis notv easy to-zdetenn-ine whether the centre of the electront beam: coincides w-ith the-centre of the aperture r in: the. diaphragm; since? the centrei-oti thebeam. cannot be. observedsdlong as it lies on the collecting plate. Even, when the the centre of 'the:.be8.m'i.iS?SBI-1I1L toe appear, adjustment must becarried out mosticautiously so" as to prevent overadiustment,v the beam. passing the aperture.

If the apertureis asshowninEig; 4", the cen' tring; operation may be carried out" as follows? With; the beam stationary, an unfocused light spot is caused to appear on. the image screen. The brightest partof thetspot isat its centre, but ithasa-largesurface area andit's boundaries are- I'IOt ShaI'PIYidBfiIIBdL In: addition; since the object plane of the. electronxlens ise nearerthe collectingplatefthan. with .a focused beam, an undefined reproduction of the edge :of the col lectingplate iSTObSBITVBdL In".these circumstances,' the-light spot may readily be" caused toiiappear from behind: the. collecting plate and ibexshifted tothe centre of. the'e'dge of the Icoilectin 'plate',= that is to sayntoeth'ev point: of this edgefwhich nearest the axis 3. The latterimovementlis'facil; itatedifithis centre isfmarked-by -an elevationlor (as shown in.- Figs. 4 and 5) by arre'cess. The. increase. in size of the aperture; isia'lso vcon venientii the electron beam. is caused to describe a raster on the-screen, as' often occur in the case of subsequent re-adjustmentj of aitubea With a circular, central aperture, the brightness of the raster, during the move nent ef thetbeam over the aperture, initially increases and immediately afte amaximum has been reached it decreases. Fi t9 llust'rates this-phenomenon by v chrepresentsthe brightries s' lif of th I ate functionof theshift-D o"thebeam r ative totheaxi's 3 of the collector Thus, in order to ensure correct centringj -fthe movement offthe beam-must be ended exactly atthe instant when this: maximum of brightness -'is*observed." lt is very difficult not to overstep this limit.

With a wider aperture, the top of the curve of Fig. 9 is flattened and adjustment is effected more readily. With a collecting plate as shown in Fig. 4 the ascending curve may even terminate in a horizontal part, as shown in Fig. 10. It is not advisable to operate too far into this horizontal part, i. e. the beam must not be shifted too far from the axis 3, since astigmatism would occur. However, less accurate centring of the beam is not so objectionable as with a tube having a conventional circular aperture.

The latter fact results in a further advantage of the tube according to the invention in that the requirement of accurate coincidence of the axis of the electron gun and that of the positive ion collector is less rigid than with the conventional tubes.

Just as the electron beam is not sharply defined so the negative ion beam is not sharply defined and consequently marginal rays of the negative ion beam will escape from the negative ion collector. cular aperture the amount of this marginal radiation cannot be reduced without also reducing the image maximum brightness and contrast. Since as a matter of fact it is only essential in this case to increase the spacing between the edge of the collecting plate and the centre of the ion beam, in a tube according to the invention the device for removing the negative ions may be rendered more effective without incurring a corresponding loss in electrons. It is true that by shifting the edge of the collecting plate to the axis 3 of the negative ion collector, the part cut away from a centrally adjusted electron beam is slightly increased but this occurs only in a restricted sector and not throughout the circumference, as with a diaphragm of the kind shown in Fig. 2. Consequently, as indicated in Fig. 4 by a small bend 9, the depth of the recess may be slightly reduced, so that the minimum spacing p between the edge and the axis of the negative ion beam becomes larger than is possible in Fig. 2, in which this spacing is q, and yet a greater quantity of light may be obtained on the screen than with the known construction. The centre of the electron beam may without objection be With a diaphragm having a central cirbeam than is the case under similar conditions slightly moved away from the edge so as to wholly V avoid the loss of light which might be produced by reduction of the depth of the recess or by provision of a local elevation to mark the centre of the edge. This movement, which is effected by magnetic means, does not result in any shift of the negative ion beam, Instead of collecting fewer negative ions, as might be expected, the use of a collecting plate in the form shown in Fig. 4 yields a better result.

Fig. 6 illustrates that with a tube according to the invention a non-circular aperture arranged symmetrically with respect to the equatorial plane may be provided.

with the other embodiments.

What I claim is:

1. In a cathode-ray tube having an electron gun for directing an electron beam along a longitudinal axis to a point of initial focus, an ion trap for separating negative ions from electrons in said electron beam comprising a cylindrical electrode adapted to receive therein said focused beam, a masking element arranged transverse to said longitudinal axis and within the object distance of said initial focus, and means to deflect the negative ions from said longitudinal axis along an axis intercepted by said masking element and forming with the longitudinal axis a meridian plane, said masking element having a split aperture which is symmetrical about the meridian plane and an equatorial plane perpendicular to the meridian plane and containing said longitudinal axis, said aperture having a dimension in the equatorial plane which is greater than the largest dimension of the aperture in the meridian plane thereby permitting a greater degree of lateral movement of the electrons in said beam about the longitudinal axis in the said equatorial plane than in said meridian plane.

2. In a cathode-ray tube having an electron gun for directing an electron beam along a longitudinal axis to a point of initial focus, an ion trap for separating negative ions from electrons in said electron beam comprising a cylindrical electrode adapted to receive therein said focused beam, a masking element arranged transverse to said longitudinal axis and within the object distance of said initial focus, and means to deflect the negative ions from said longitudinal axis along an axis intercepted by said masking element and forming with the longitudinal axis a meridian plane, said masking element having a rectangular aperture arranged symmetrically about the longitudinal axis, said aperture having a dimension in an equatorial plane perpendicular to the meridian plane and containing the longitudinal axis which is greater than the largest dimension of the aperture in the meridian plane, thereby permitting a greater degree of lateral movement of the electrons in said beam about the longitudinal axis in the said equatorial plane than said meridian plane.

3. In a cathode-ray tube having an electron gun for directing an electron beam along a longitudinal axis to a point of initial focus, an ion ammo;

rfictangular; apegtummrin ihesaidzfiquatorialplanetnanin saiclmeridian 10 2274158 7 25 4 ,4 4: 23.7.2366

ngitudmalaxls; .dimensioniz1. an aqua The following referencessam 011mm in my 8 nmmmcm 12mm file Of #315 Patent:

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