US2096986A

US2096986A - Braun tube

Info

Publication number: US2096986A
Application number: US666554A
Authority: US
Inventors: Ardenne Manfred Von
Original assignee: Loewe Opta GmbH
Current assignee: Loewe Opta GmbH
Priority date: 1931-03-28
Filing date: 1933-04-17
Publication date: 1937-10-26
Anticipated expiration: 1954-10-26
Also published as: GB388623A; FR734239A; DE755947C; GB394496A; BE387442A; GB387536A

Description

Oct. 26, 1937. M VON ARDENNE 2,096,986

BRAUN TUBE Original Filed May 16, 1932 l' l/ H913;

Fig 4 lnvemor:

Maw/M Patented Oct. 26, 1937 .1 I

UNITED STATES PATENT OFFICE BRAUN TUBE Manfred von Ardenne, Berlin, Germany, assignmto Radioaktiengesellschaft D. S. Loewe, Berlin- Steglitz, Germany, a company of Germany Original application May 16, 1932, Serial No. 611,568. Divided and this application April 1'7, Serial No. 666,554. In Germany March 7 Claims. ((1250-7875) The invention relates to improvements for why the same has not been observed heretofore. fluorescent screens preferably for screens in The light phosphorescence covers the weak Braun tubes for television devices. fluorescence, as will be more clearly described be- It will be appreciated that as regards the conlow. In order to make the invention fully understruction of scanning transmitters employing stood, reference will be had to the accompanying 5 Braun tubes it is of fundamental importance that drawing, all elements reveal a minimum sluggishness in Fig. 1 of which shows the cathode ray excitation operation. In the case of the television transof an element of the luminous screen as dependent mission means heretofore employed having on time, whereas 10 cathode ray tubes use has been made of the Fig. 2 shows the corresponding fluorescence l0 phosphorescent effect, which in itself supplies and phosphorescence phenomena. considerable amounts of light energy. It is well In Fig. 3 the amplification degree of a photocell known that by phosphorescent effect in the case amplifier preferably employed according to the of screens erroneously termed fluorescent screens invention is shown as dependent on frequency.

there is. meant a luminosity, which does not in- Fig. 4 illustrates a complete television system I stantaneously follow on the excitation of the according tothe invention. cathode ray, and also requires a certain time to In Fi 1 e Sh c tat o 0f th C t ode die down. In the case of certain silicates, such ray of a scr en-surf e lement is sh wn p n for example as zine silicate, these cessation ent on time. It is assumed, for example, that this periods, so-ealied fading periods, are in the order exc p e eIIIOlII'ItSv 0 One-millionth Of a 20 of one-tenth of a second, and in some cases are second. There then occurs instantaneous excitaeven higher in order. tion of the screen-surface element to the extent As already mentioned above, it is necessary for of fluorescence and a phosphorescent effect which the purpose of reproduction without distortion to commences slowly, but proceeds much more proemploy at the transmission end only those elenouncedly and dies down very gradually. This is 25 ments which are void of sluggishness. The matemOre C a y shown in Fig. 2 Which there is rials above referred to do, therefore, not enter illustrated the amount of light dependent on time, into the question as screen material for television im, the one hand in respect of the fluorescent eftransrnitters. As experiments have shown, propfeet and on the other hand in respect of the phos erties of a much more favourable kind as regard phorescent efiect. the object concerned are disclosed by calcium and Accordin OW to t e nvent o it s the intencadmium tungstate. The lighting-in and fading tion, for the purpose of image reproduction free periods of these materials are in the order of 10-5 of distortion, to employ the screen fluorescence seconds. Even the sluggishness of these matealone, or at least for the greater p in the telerials, however, is too great for television rich in vision transmitter for the very rapidly occurring 35 detail. An ideal material for the screen of a variations in the excitation of the screen. In frecathode-ray television transmitter would be a quency ranges below 100,000 periods the strong screen material which utilizes solely the fluoresphosphorescent effect is not of a detrimental kind, cent effect, whereby there is understood by as it is possible with the assistance of present-day fluorescent in its usual meaning a luminous efdistortion-correcting methods in the amplifierv to 40 feet, which commences instantaneously with the provide this with a frequency characteristic of excitation and terminates instantaneously with such nature that the strong phosphorescent efcessation of the excitation. feet in the case of low frequencies results in the According to the existing state of scientific same output potentials as the weak fluorescent knowledge it is assumed that a cathode-ray effect in the case of higher frequencies. The 45 fluorescence does not exist at all. Now with the separation of phosphorescent and ffuorescent efassistance of special measuring means, which fects in the case of relatively low frequencies is cover the quantitative comprehension of highmore diflicult, and is also not essential,, whereas frequency fluctuations in light, the applicant has in the case of high frequencies this results autobeen successful in establishing that, in contramatically, the phosphorescent effect being of 50 distinction to the previous assumption, a cathodesuch a sluggish nature that the same supplies a ray fluorescence does in point of fact exist. Cerconstant. intensity amplitude as compared with tainly, the fluorescence supplies approximately the rapidly occurring variations in the excitaone to two tenth powers less light than the phostion.

phorescence. This is apparently also the reason Accordingly,.there is preferably employed for 55 the photo-cell potential an amplifier such as illustrated in Fig. 3. In the latter the degree of amplification V is shown as being dependent on f (in periods).

In order to cause the weaker fluorescent effect to also produce in the photo-cell amplifier of the transmitter potentials which are situated above the shot mark (Schrotpegel), very appreciable beam energies may possibly require to be converted in the cathode ray tube of the transmitter, for example beam currents of -1 milliampere at 3-10 kv speed.

To avoid necessity for excessive differences in the frequency characteristic of the transmission amplifier, materials are preferably employed in which the phosphorescent effect remains as small as possible in comparison with the fluorescent efiect. Under certain circumstances the use of a special distortion-correcting means within the amplifier may not be necessary at all. The materials concerned in this connection have heretofore been ignored as material for the screen on account of their low absolute degree of light. The same acquire importance only by reason of the recognitions set forth in this application. A material, for example, disclosing practically no phosphorescence is plaster, which accordingly is admirably suited as a screen material for Braun tubes employed as television transmitters.

As already mentioned, the electro-optical efilciency requires to be considerably larger than has been the case in connection with the tubes hitherto known for carrying out a television method of a useful kind with Braun tubes.

In the case of thefluorescent screens employed heretofore in connection with Braun tubes various disadvantages have been found to exist, which it is intended to avoid by the present invention. In the first place the screens hitherto in use, which in their finest quality consist of calcium tungstate, suffer from a sensitiveness which for numerous purposes is'too small, 1. e., from an insufficient electro-optical efliciency. This lack of sensitiveness has been found extremely disturbing, particularly in the case of Braun tubes intended to be employed for television purposes.

According to the invention, cadmium tungstate is employed for the coating. This cadmium tungstate may, in the manner heretofore known in association with other substances, be mixed with waterglass and painted on to question.

A similar or additional improvement in the electro-optical eiliciency may be accomplished by not mixing the fluorescent material with the attachment substance, but at first evenly distributing the attachment. substance, preferably waterglass, over the surface to be furnished with fluorescent material, for instance painting the same into the tube, and then allowing it to somewhat run until the layer has assumed an even character, the fluorescent material, preferably cadmium tungstate, then being applied in finely divided condition, preferably by means of a nozzle. It is thus accomplished on the one hand that the crystalline structure of the fluorescent material is maintained, and on the other hand that the rays inciting fluorescence meet in direct fashion against the sensitive material, and finally that the layer is substantially more even and homogeneous- As a result of these properties the sensitiveness of the layer is raised to a maximum, so that the sensitiveness is equal at all points, which is an absolute necessity for faulting effect on the luminous surface.

the glass wall in less television images, and finally there is extremely little waste in manufacture. A method of this kind for producing the fluorescent screen is naturally of advantage not only in the case of a screen material consisting of cadmium tungstate, but also in connection with all other materials, inasmuch as the screen materials employed behave, generally spealnng, in practically equivalent manner from a working-technical standpoint, and the sodium waterglass hitherto generally in use for binding purposes results in all cases in appreciable deterioration of the luminosity. Sodium waterglass has, however, the disadvantage that the same causes a darken- According to the invention, this is avoided by the fact that in place of sodium waterglass there is employed as binding agent potassium waterglass. This is natiually not only suitable as a binding agent for cadmium tungstate, but also for the majority of fluorescent materials hitherto in use, such for example as zinc silicate phosphor.

By reason of the high insulating power of glass charges giving rise to disturbances frequently accumulate on the screen in the case of tubes of this kind. These charges cause variation in the speed of the ray, and accordingly intensity distortion; at the same time, however, they also cause spot displacement, even if only a slight one, dependent on the extent of the discharge and its distribution on the surface. To be able to conduct away these charges with sufficient rapidity, it is necessary, assuming the screen does not permit of leaking off per se, to provide a special leak conductance layer which, for example, at the same time also constitutes the binding agent for the fluorescent substance. It is, however, also possible, for instance, to produce an extremely thin layer by cathode atomization or by spraying on graphite or carbon, which ensures a suflicient leak conductance for the charges. This conductive layer, or the screen respectively, assuming the same is sufllcient, is preferably connected with the anode of the Braun tube.

Generally speaking, it is not to be avoided that certain, in particular slow rays disperse from the bundle, which is concentrated in itself, and thus lead to troublesome conjoint lighting effects on the part of other surface portions of the fluorescent coating. To avoid this there is, according to the invention, applied to the coating produced in accordance with the above process a thin, preferably transparent protective layer, which renders the dispersed rays, which as regards their intensity are considerably below that of the main beam, without effect. This preferably transparent protective layer, which may consist, for example, of potassium waterglass, may be employed particularly in the present case, since in accordance with the above method it is possible in itself to raise the sensitiveness to such extent that,

with a certain reduction of this sensitiveness, the

usefulness of the arrangement, from a practical standpoint, is not diminished. By reason of a protective layer of this description the gradation curve of the fluorescent layer is accordingly varied, so that the slow electrons of the space charge do not cause' fluorescence, as they are unable to penetrate the protective coating. This protectivecoating may naturally also consist, for instance, .of a conductive, for example extremely thin aluminium screen, which according to the invention then takes over at the same time the leak conductance of the disturbing charges.

A further arrangement of the invention reage.

lates to a method for colour television. Whereas in the case of the colour television methods hereto-fore known the complete image for transmission was transmitted successively in the different basic colours, the transmission of an image point in the method according to the invention is performed on each occasion in the three basic colours within the completion period of. one im- A method of this kind is accompanied by the appreciable advantage that the formation of colour-seams, which are caused more particularly by chronological parallax, isavoided. Ac-

cording to the invention, the fluorescent surface is produced, preferably in the direction in which line-scanning takes place, line by line from adjacently disposed strips of different fluorescent materials. As fluorescent materials there are preferably employed three different substances which, when met by electrons, each light up in one of the three basic colours red, blue and green. For example, potassium bichromate is suitable for producing a red fluorescent colour, calcium tungstate for producing a blue fluorescent colour, and zinc silicate, cadmium tungstate or zinc sulphide for producing a green colour. In accordance with the desired width of image point, each image strip requires to be divided into three coloured strips, which are scanned and transmitted consecutively. These strips require to possess various densities, or more or less dense distribution of the crystals respectively, dependent on the fluorescent-sensitive nature of. the fluorescent material employed, in order to permit of an even width of line. The transmitter and receiver tubes require to possess fluorescent screens of a similar nature.

As it is done in the plain coloured transmission the cathode ray of the transmitter tube is conducted over the fluorescent area, and now scans successively the strips composed of the different fluorescent materials. If, for example, there is taken the sequence: red, blue, green, the scanning beam, the width of which accordingly amounts to approximately of the desired image-point width, will pass in strip-like fashion in the same consecutive order: red, blue, green, red, blue, green, etc. over the surface composed of strips of the different materials. In order to obtain in effect the same number of lines as in the case of. television without colours, the beam accordingly requires to pass to three times the extent over the surface. By sufliciently fine divisioning the surface of the transmission tube will accordingly light up in white colour. This surface is reproduced through the medium of an optic on the image to be transmitted, and in accordance with the colour of this image a larger or smaller proportion of the particular point of light in the basic colour produced on the transmission tube now passes for example through the diapositive. At the same time the cathode ray of the receiver tube is situated on the same point, whereby naturally the cathode ray of the receiver is required to pass over a fluorescent surface strip of the same fluorescent colour.

In order upon the transmission of. an image point to afford the eye the appearance of a uniform colour, it is essential that the fluorescent materials be so finely divided that their structure does not make itself noticeable. It is accordingly necessary that the receiving means he so constructed that a very small angle of visibility is formed resulting in a subjective mixing of the strip portions of the fluorescent surface lighting in the three basic colours and belonging to one element of the image.

Owing to the different fluorescent-scnsitivities of the various materials concerned as regards the single basic colours, it is necessary for the purpose of obtaining an image in natural colours to so apportion the single fluorescent materials that by reason of a cathode ray of constant duration and constant strength meeting against equalsized superimposed surface particles of an image element the appearance of a certain image point results.

According to the additional subject matter of the invention, it is possible to also employ this surface, composed of the different fluorescent materials, for the transmission of black-and-white images or ordinary diapositivesdor obtaining a white image.

If merely the problem is placed of producing a white fluorescent surface it is obviously unnecessary to dispose the single fluorescent materials in the form of strips. It is quite sufficient to apply the same to the glass wall in any arrangement as desired, attention being merely paid to the fact that the distribution is sufficiently flne, and that the apportioning of the different materials results in a white image upon constant excitation, i. e., constant electric output-that is to say, intensity and speed of the cathode ray.

To obtain a white image it is possible in accordance with the additional subject matter of the invention to dispose the different fluorescent materials in the form of superimposed layers, whereby preferably the layer of greatest fluorescent sensitivity forms the surface directed away from the cathode, whilst the surface with the least fluorescent sensitivity is the one next to the cathode. By suitable allotment and strength it is also in this case readily possible to obtain an admirable white luminous area.

The measures, as represented, may naturally also be employed in conjunction with other television methods, more particularly in the case of those in which the image point scanning operation does not occur in time-linear fashion, but in which the scanning speed is varied dependent on the properties of the particular image point to be transmitted.

The total arrangement is shown in Fig. 4. In the latter I is the Braun tube, 2 the hot cathode thereof, 3 the Wehnelt cylinder, 4 the anode, 5 and 6 the two pairs of deflecting plates, 1 the fluorescent screen and 8 the lens system.

Additional to these, I5 is the photo-electric cell amplifier, and I6 an apparatus for main supply furnishing the heating, anode and cylinder potentials necessary for the operation of the transmission tube. I1 is an image tilting means, for example the frequency 25, and l 8 a line tilting means, for example the frequency 1500. These two tilting devices are connected with both the pairs of deflecting plates 5 and 6 of the transmission tube I as well as the pairs of deflecting plates I9 and 20 of the receiving tube 2|.

As transmitter there was employed a cathode ray tube having a new screen. The advantage of allowing also the transmitter to operate with Braun tubes resides not only in the elimination of mechanically moved parts and in the slight sluggishness of the transmitter, resulting only from lighting and fading .of the fluorescent screen, but also in the fact that in the testing apparatus, by parallel connection of the deflecting plates, to which is connected line and image potential, it is readily possible to produce the requisite synchronism between the location of the point in transmitter and receiver. In the transmitter and the receiver the point scans synchronously an area, which, with correctly selected line and image potentials, appearsin the form of an evenly luminous rectangle on the fluorescent screen. The luminous rectangle in the case of the transmitter tube illustrated in Fig. 1 is reproduced sharply on the diapositive or fllm l2 to be transmitted by means of a strong optic 8. Behind the diapositive there is arranged an opaque sheet l3 and the photo-cell ll. Each momentary position of the fluorescent point corresponds with a lighting current striking the photo-cell. The amount of light depends only,

on the transparency of the diapositive to be transmitted at the point where the flourescent spot is reproduced at the particular moment through the medium of the optic. After sufflcient, non-sluggish amplification of the photo currents there is a potential amounting to a few tens of volts available, which serves for the light control of the receiver tube. This arrangement permits good results, despite its great simplicity, by employing normal measuring tubes, main line apparatus and tilting apparatus, bythe use of normal photo-cells, and by the use of good lowfrequency amplifiers with a degree of amplification amounting between 10,000 and 100,000.

I claim:

1. In a Braun tube'having means for generating and means for deflecting a cathode ray, a luminous screen applied to the end wall of said tube, the luminous material of said screen being cadmium tungstate.

2. In a Braun tube having means for generating and means for deflecting a cathode ray, a luminous screen applied to the end wall of said tube, the luminous material of said screen having a fluorescent sensitiveness greater than the phosphorescence sensitiveness, said material being fixed on said end wall by a binding agent constituting at the same time a leak-conductance layer for leaking oil charges from said screen.

3. In a Braun tube having means for generating and means for deflecting a cathode ray, a luminous screen applied to the end wall of said tube, the luminous material 01' said screen having a fluorescence sensitiveness greater than the phosphorescence sensitiveness, said material being applied to said end wall by a" layer of potassium silicate.

4. In 9. Braun tube having means for generating and means for deflecting a cathode ray, a

luminous screen applied to the end wall of said tube, the luminous material of said screen having a fluorescent sensitiveness greater than the phosphorescence sensitiveness, and a thin transparent protective layer applied onto said fluorescent screen material.

5. In a Braun tube having means for generating and means for deflecting a cathode ray, a luminous screen applied to the end wall of said tube, the luminous material oi said screen having a fluorescent sensitiveness greater than the phosphorescence sensitiveness, and a layer of potassium waterglass applied onto said fluorescent screen material.

6. In a Braun tube having means for generating and means for deflecting a cathode ray, a luminous screen applied to the end wall oi said tube, the luminous material 01 said screen having a fluorescence sensitiveness greater than the phosphorescence sensitiveness, and a layer in the form of an extremely thin aluminium screen applied onto said fluorescent screen material.

7. A fluorescent screen for Braun tubes for television reproduction consisting of materials generating light in the three basic colours upon being excited by the electrons of the cathode ray developed in said Braun tube, said materials being applied to the wall of said tube on a layer of a suitable binding agent such as potassium waterglass, said materials being arranged in the form of superimposed layers, the layer of greatest fluorescent intensity being situated on the side remote from the cathode, the layer of least fluorescent sensitivity being situated on the side directed towards the cathode.

' MANFRED von ARDENNE.