US2219872A - Television reproducer apparatus - Google Patents

Description

Oct. 29, 1940. L, M, MYERS 2,219,872

TELEVISION REPRODUCER APPARATUS Filed April l5, 1938 Fig .1

INVENTOR 1 LEONARD 1mm/ER;

ATTO R N EY Patented Oct. 29, 1940 UNITED STATES 2,219,872 TELEVISION nEPRonUcEn APPARATUS Leonard Morris Myers, Middlesbrough, England, y assignor to Radio Corporation of America, a corporation of Delaware Application April 1 3, 1938, Serial No. 201,700 In Great Britain March 8, 1938 s claims. (c1. 25o-iso) 'Ihis invention relates to television reproducer apparatus and has for its object to provide improved television reproducer apparatus capable of producing powerful pictures suitable for pro- 5 jection. Great difliculties have been experienced hitherto in obtaining reproduction of television pictures of sufhcient power for projection. The usual form of cathode .ray tube in which `the reproduced pictures are built up by scanning a fluorescent screen has serious limitations as regards the light power which can be obtained from the fluorescent screen. The present invention seeks to avoid the limitations of the usual known form of cathode ray reproducer by providing an improved form of reproducer of the kind wherein the reproduced pictures are obtained, not by fluorescent eiect in a fluorescent screen, but by heating a picture reproducing electrode to incandescence and thus obtaining pictures by light due to heat.

According to this invention a picture reproducing cathode ray tube having a screen which is to reproduce pictures by light due to heating is characterised in that the said heating is eiected at least in part by electron bombardment by a thermionic emitting electrode which is in turnv heated to effect thermionic emission, said last mentioned heating being effected, at least in part, by an exploring cathode ray which bombards .30 and thus heats the said thermionic emitting electrode. Thus in carrying out the invention the thermionic emitting electrode, which will hereinafter be referred to as the secondary cathode is scanned by a picture signal modulated cathode ray which accordingly causes localised heating (which is preferably additional to general preheating) and temperature increase in the secondary cathode and therefore produces from each individual point of its area, thermionic emission corresponding to the light or shade in the corresponding point in the subject of transmission. The thermionic electrons thus emitted proceed to and are focussed upon the final picture reproducing screen (which may be termed an incandescent anode) to heat it correspondingly and thus produce the nal picture. The modulation of the cathode ray may be of any known type but intensity modulation is preferred.

'I'he invention is illustrated in the accompanying drawing in which Fig, 1 shows a cathode ray tube, together with deflecting coil systems and optical system embodying my invention;

Fig. 2 shows in plane View the heating element of the thermionic emitting cathode embodied in 65 my invention; while Fig. 3 shows in more detail in `cross-section the structure of a thermionic emitting cathode in accordance with my invention.

Referring to Figure 1 which shows schematically one form of television reproducer cathode ray 5 tubeembodying the invention,` the tube therein shown has an envelope with two neck portions l, 2, and a central bulb portion 3 the diameter of which is at least four times that of the final anode or screen on which pictures are to be l0 reproduced, said bulb portion being made of the glass known under the registered trade-mark Pyrex or of some similar hard glass. In the neck portion lis an electron gun structure 4, of

" any form known per se, having associated therel5 with mutually perpendicular ray deecting means .constituted by mutually perpendicular deflecting coils. For simplicity in drawing these coils are represented merely by the rectangles 5 but it will be appreciated that there are two mutually per- 20 pendicular coils for line and framing deflection' asin ythe well known way. The gun includes the usual control electrodes by which the intensity of the ray may be varied in accordance with picture signals and a short'shielded coil 6 is provided for 25 magnetically focussing the ray. When the ray enters the bulb portion 3 its path is bent. circularly (as indicated) by a magnetic eld generated by a pair of coils (of which only one, l, is shown) so that it enters the neck portion 2 and 30 scans a secondary cathode 8 capable of thermionic emission which is mounted in saidneck portion 2. This cathode 8, whose .preferred construction will be described later herein is subjected to general heating from a source 9 and also to sup- 35 plementary localised heating by the bombardment of the'cathode ray and since the instantaneous value of the thermionic emission from any point in said cathode .is dependent upon its instantaneous temperature' there will be produced, 40 as it were, a thermionic emission image of the subject of transmission. Electrons thermionically emitted from the secondary cathode 8 are drawn off by the actionof a suitable positive electrode l0 forming part of an electron-optical sys- 45 tem I0, Il, and entering the bulb portion follow curved paths as indicated to fall upon the incandescent anode I2. A coil I3 is provided for electron focussing purposes. A

It will be seen that in the tube just generally 50 described the scanning cathode ray from the gun 4 forms what may be termed a temperature image on the secondary cathode 8 and this in turn produces a thermionic emission image from which the final light image on the incandescent anode I2 is obtained. I4 is a lens system for optical projection of the images formed on anode I2 on to a viewing screen (not shown).

l To give a practical example-to which how- 5 ever the invention is not limited-the electron gun may be of the high beam current high potential type with the cathode at about 30,000 volts and the electrode nearest thereto (the Wehnelt cylinder) at about 20,000 volts relative to the potential of the'secondary cathode 8. vThe rating may be about 40 to 60 watts. The intermediate cathode 8 is earthed. The final cathode or screen, which is preferably made as described later herein, may be maintained 'at about +5,000

electrode II is earthed and the electrode I0 may be maintained at about 2,000 volts'positive in relation thereto. n

There Will now be described a preferred way of making the final anode or incandescent screen I2. As will be appreciated the requirement here to be satisfied for good definition is that the temperature rise shall be closely localised and there shall be a sufliciently steep temperature gradient to ensure that the heat spread (to incandescence) is not greater than about half the Width of one scanning line. This requirement involves that the material which is to be heated to incandescence shall be slow to conduct heat away from a locally heated point and this in turninvolves that the said material shall be very thin. Hitherto it has been found very diiiicult, if not impossible, to construct lan incandescent type of screen sufciently thin to satisfy these difficult requirements, and an important feature of the invention resides in solving this diiiiculty and thus obtaining improved results as regards fidelity and definition by providing the incandescent screen with a thin layer of lamp black under a .10 protecting surface iilm of finely divided tungsten or other suitable material. In more detail, one

way of making the screen I2 is as follows: A

sheet of nickel or other convenient conductor is first coated with a thin deposition of lamp black in the form of a film about 0.004 thick. This coating may be convenientlyeffected by smoking the metal with a candle. Finely divided tungsten powder is then shaken through one or more thin meshes on to the surface of the lamp black film. If the mesh is iine enough the particles of tungsten appear to adhere to the surface of the lamp black so that even violent shaking will not disturb the surface film. The metal backing, with its lamp black layer and protecting surface film of finely divided tungsten, is then mounted in the tube envelope and is subjected to a de-gassing operation which is made as complete as possible.

Owing to the low thermal conductivity of the lamp black screen such de-gassing cannot be satisfactorily accomplished as in the ordinary way with the aid of eddy current heating, and accordingly a filament for use in the de-gassingoperation is introduced -into the tube. This filament (not shown) which may consist of a suit- 65 able length of tungsten wire about 0.001" thick is supported about 1 cm,-in front of the screen, and the said screen is de-gassed by using it as the anode of a diode whose cathode is constituted by the filament. The lamp black screen should receive about 100 watts per square centimetre in order to heat itl up to and above the maximum working temperature to which it may be heated in subsequent use. When de-gassing is completed the lament can,` if desired, be burnt out, but even if the fllament is left in position it will not volts relative to the secondary cathode 8. The

seriously impair the picture since its width is substantially less than the normal width of a scanning line in picture reproduction.

The thermal conductivity of lamp black is very low as compared to that of tantalum, tungsten, or molybdenum-being less than vone ten thousandth (it is of the order of 10-5 calories/cm. degree`sec.)and it has been found that if a thin film of lamp black of 0.004" in thickness or less is deposited on a metal plate (for example of nickel or tantalum) a temperature of over 2,000 C. on the plate is not appreciably conducted through the film. Furthermore, with a screen I2 made as just described, a picture scanning line of white hot intensity can be obtained on the'surface with very little penetration of heat to the ,metal backing and very little heat spread The protecting iilm of iinely divided tungsten or other suitable metal is for the purpose of protecting the lamp black against evaporation in use for lamp black will evaporate in a high vacuum at temperatures higher than about 2,000 C.

As regards the secondary cathode 8 it will be appreciated that each point in the temperature image thereon will persist at an elevated though falling temperature for a substantial time (which must, of course, be less than the picture repetition period) after the scanning cathode ray which has caused its rise of temperature has passed on. Accordingly care must be taken 4to minimise spread of heat from point to point through the secondary cathode 8 so as to ensure that the heat spread will not be such as to spoil the detail.

A preferred form `of secondary cathode 8 is made as follows: An independently heated nickel (or other suitable vacuum metal) lm whose area is slightly greater than that of the electron image to be formed thereon is taken and is coated to a thickness of about microns with a coating of barium and strontium carbonates on the side which is to be bombarded by the gun 4. The coating ensures that in actual operation the surface can be 100 C. (or thereabouts) hotter than the nickel carrier. The carbonates are then reduced to oxide by heating for a short time at about 950 C. in accordance with known technique- Then after 4activation of the cathode coating, the cathode is ready for use. In use the coating is heated by the independently heated nickel backing to about 800 C. and localised variations in temperature (to produce the electron image) about this temperature are occasioned by the bombarding picture modulated cathode ray. The preferred construction of independently heated secondary cathode 8 consists of a tungsten wire heating grid clamped between two thin nickel plates and insulated therefrom by mica discs. Figure 2 shows the heating grid 8a and one of the mica discs 8b. Figure 3 represents in schematic section the cathode assembly 8a being the heater, 8b the mica discs, 8c the nickel plates (the clamps are not shown) and 8d the coating on one of these plates. One end of the heating grid 8a is connected to the coated nickel plate, i. e. the cathode proper. The whole assembly is mounted in an annular nickel or other suitable carrier mount (not shown). Preferably, before the coating 8d is deposited the cathode structure is fully outgassed by,heating for a prolonged period in a separate bulb; it is then removed from this bulb, coated with the barium and strontium carbonate and sealed into the envelope n which it is to be employed, the coating being thenl reduced-to oxide and finally activated by operating with anode potentialas lknown per It is possible, though it-is not preferred, to .constitute the cathode -8 by a metal foil of a thickness of about 10-.4 cms. the metalA being of low specific heat and low density such as tungsten, nickel, molybdenum, tantalum or palladium.v ,In order to secure the required thermionic` emission from thel electron gun with a thermionically sensitive layer, for example a mono-molecular layer of ycaesium or barium. In the case of a tungsten foil secondary cathode, the foil should charge through oxygen at a pressure of about 2 mm. of mercury until the face of the foil acquires a-suita'ble thickness of metal oxide as indicated by its observed colour. 'Then either barium or caesium is deposited on the film of oxidised metal by `liberating the barium. of caesium (as the case may be) from a suitable compound such as barium oxidev or caesi-um chloride. A foil cathode so constructed can be subjected to direct general heating by a heating current passed through it to pre-heat it to about 800 C.. Where a foilcathodeis used the foil may beobtained by evaporatingv tantalum or other metal from a body of the metal which is positioned on a tungsten Vor similar electrically heated filament, the metal evaporated being evaporated upon a metal carrierso as to form a deposit thereon, the said metal carrier j beingv afterwards removed to leave a foil. .Examples ofI suitable carrier metals which mayfbe used in manufacturing tantalum foils by this method 'are nickel, zinc` and copper, nickel being preferred'.

One way tif-carrying' outthis method of obtaining the foilwll now be describedLThere is employed 40 a glass envelope withheavyfcurrentseals'at each end and about 25 cms. in' length. vvIn this. envelope is mounted a lrectilinear vtungsten filament to which-electrical connection is madethrough the seals, the connection at least vat one end being made through a zig-zag tungsten or other' spring support so as to simplify glass blowing operations. The tungsten filament-may be about 20 thousandths of an inch in diameter. A strip of tantalum, the metal from which the foil is to be formed, about 1 mm. in Width and 2 thousandths of an inch in thickness is closely. wrapped after the manner of a puttee about the tungsten. Coaxially surrounding the filament and suitably supported from the seals4 is a cylinder of nickel which constitutes the carrier metal upon which.

the tantalum is to be deposited. Assuming that a foil 10 cms. square isv required, thediameter of the cylinder may be about 4 cms. The cylinder yis so supported and insulated, e. g. by means-of glass beads, as to prevent current being passed through it.

When the tungsten filament v75 temperature is, of course, below the melting point from the metal foil itis coated on theface remote.

be rst oxidised, for example by passing" a distantalum foil.

with its ltantalum wrapping and the cylinder have been positioned of tungsten and slightly below that of tantalum. For the figures already given it will be found that a filament current of about 3i) amps; and a power consumption of the order of 100 watts per inch length of filament will be required. 'I'he power radiated will be to a large extent absorbed by the surrounding nickel cylinder whose temperature will consequently rise untilit glows a dull red. This nickel .cylinder incidentally protects the glass walls of the envelope against collapse.

By occasionally switching voff the filament current it is possible to observe the appearance of the inside of the nickel cylinder. gins to appear on this inside surface the tantalum is starting to evaporate and at this stage the current through the filament should be controlled with caution. The amount of tantalum originally wrapped about the filament is chosen (by weight) to give the weight of foil finally required. Accordingly, the process above described is continued until all, or practically all, the tantalum is evaporated from the filament. 'If the lament current is carefully controlled, there is little fear of the tungsten` evaporating. It has been found that a practical figure to adopt is 20.4 milligrams weight of tantalum per cm. of length of filament.

When the evaporation process has been corn- 'pleted the envelope is opened, and the nickel cylinder with the tantalum on the inside thereof is taken out, cut, and then rolled out hat into a sheet. The rolled out flat sheet is then immersed `the length of time taken for this dissolving step in the process, itis advisable not to employ a nickel cylinder of too great thicknessa thickness of one half thousandth of an inch is sufficient. The acid solution employed should not be strong, for if it is too strong the tantalum foil may be ruptured due to too violent liberation of gas from the nickel.

Instead of removing the nickel by a chemicall process as above described, the nickel cylinder with the' tantalum deposit may be operated as an anode in another evacuated envelope having a filament Which"provides electronic emission. This filament is heated and electron discharge set up, the discharge device being so operated that the anode temperature becomes high enough to evaporate the nickel thus leaving behind the Obviously in this meth'od the anode (nickel-tantalum) should be so carried from .its supports that whenvthe nickel is evaporated, the remaining tantalum foil is 'still adequately supported.. These supports may constitute .the supports which will ultimately be required to carry the foil in vthe cathode ray tube `vfor which it is intended.

It is advisable that supports be welded to the foil before the nickel is removed irrespective of the method adopted for removal. The supports should be of tantalum o r molybdenum wire, preferably the former, and the weld between the supports and the foil should, of course, be made directly' between the said foil and the said s'upports `so that the removal or destruction oi.' the nlckelwillnot affect the said supports.

Itis possible, but again it is not preferred, .to constitute thefinal incandescent anode I2 by metal foil, for example, tungsten, tantalum or 'molybdenum made as above described and about 10-4 cms. thick.

It will be appreciated that the total quantity of light given off from the incandescent anode I2 and therefore available for projection,`is greatly increased by reason of the lag eiIect in the secondary cathode 8 from which the bombarding electrons are thermionically derived; that is to say, by reason of the fact that the temperature rise brought about at any picture velement in the secondary cathode subsists for a substantial time after the causative scanning beamhas passed.

If desired, the incandescent anode I2 may also be pre-heated either bypassing a current therethrough or by arranging for there to be a constant component of electron bombardment from the secondary cathode.

Having now particularly described and ascertained the nature of my said invention and in what manner the same 'is to be performed, I declare that what I claim is: Y

1. A television picture reproducing cathode ray tube comprising an electron gun for projecting a beam of electrons, a planar thermionic electron emitting cathode positioned obliquely with respect to the electron gun and adapted to be bombarded by the projected beam of electrons, and a thermo-luminescent screen positioned obliquely with respect to said cathode and adapted to be bombarded by thermionic electrons'released from said cathode by the impact of the electrons from the electron gun.

2. A television picture reproducing cathode ray tube comprising an electron gun for projecting a beam of electrons, a planar thermionic electron emitting cathode positioned obliquely with respect to the electron gun and adapted to be bombarded by the projected beam of electrons, a thermo-luminescent screen positioned obliquely with respect to said cathode and adapted to be bombarded by thermionic electrons released from said .cathode by the impact of the electrons from the electron gun, and cylindrical electrodes positioned intermediate said cathode and said screen for focusing thermionically'emitted electrons upon said screen.

3. A television picture reproducing cathode ray tube comprising an electron gun for projecting a beam of electrons, a planar thermionic electron emitting cathode positioned obliquely with respect to the electron gun and adapted to be bombarded byl the projected beam of electrons, means for heating the thermionic emitting cathode uniformly across the emitting surface thereof, and a thermo-luminescent screen positioned obliquely with respect to said cathode and adapted to be bombarded by thermionic electrons released from said cathode by the impact of the electrons from the electron gun.

4. A television picture reproducing cathode ray tube comprising an axially aligned electron gun for projecting a beam of electrons, a planar thermionic emitting cathode of extended area, said cathode being positioned at an acute angle with respect to the axis of the electron gun. a luminescent screen positioned at right angles with respect to the axis of the electron gun and obliquely with respect to the thermionic electron emitting cathode, and electron focusing means positioned intermediate the cathodey and the screen.

5. A television picture reproducing cathode ray tube comprising an axially aligned electron gun for projecting a beam of electrons, a planar thermionic emitting cathode of extended area, said.

cathode being positioned at an acute angle with respect to the axis of the electron gun, a luminescent screen positioned at right angles with respect to the axis of the electron gun and obliquely with respect to the thermionic electron emitting cathode, electron focusing means positioned intermediate the cathode and the screen, and means for heating uniformly the thermionic emitting surface of the cathode.

6. A television picture reproducing cathode ray tube comprising an axially aligned electron gun for projecting a beam of electrons, a thermionic emitting electrode positioned obliquely with respect to the axis of the electron gun and adapted to be scanned by the projected beam of electrons and to emit electrons from the same surface scanned by the beam of electrons, a thermo-luminescent screen adapted to be impacted by electrons released from said cathode by the impact of the electrons from the electron gun, electron focusing means intermediate the cathode and the screen, and means for heating the emitting surface of the cathode uniformly.

LEONARD MORRIS MYERS.

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