US2211843A - Cathode ray tube - Google Patents

Description

Aug. 20, 1940.

A. BOUWERS CATHODE RAY TUBE Filed Feb. 5, 1938 INVENTOR ALBERT BOUWERS BY H w m ATTORNEY Patented Aug. 20, 1940 UNiTED STATES CATHODE RAY TUBE Albert Bouwers, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America,

New York, N. Y., a corporation of Delaware Application February 5, 1938, Serial No. 188,876

In Germany October 23, 1936 5 Claims.

Cathode ray tubes for the transmission of pictures are known wherein a thin metallic screen is brought to luminescence owing to its being heated by electron bombardment. Such a screen will hereinafter be referred to as an incandescent screen While a cathode ray tube comprising such a screen will be designated as an incandescentscreen tube.

The operation of these tubes is for the rest similar to that of cathode ray tubes having a fluorescent screen. Like in the latter tubes a narrow modulated electron beam may be moved within the tube by suitable electrostatic or magnetic deflecting means so as to scan the whole of the area of the picture and to produce, in accordance with the modulation of the electron current, different degrees of brilliancy on the incandescent screen. With respect to cathode ray tubes having a fluorescent screen, incandescent screen tubes afford the advantage that the picture on the incandescent screen may be of sufficient brightness to be projected enlarged on a viewing screen by means of a lens system.

In the short period of time'during which the screen portions are struck by the rays in one movement of the cathode ray beam over the incandescent screen, the material must be heated to the required temperature of incandescence. The cooling, which is due to thermal radiation, must take place in a fraction of one second, that is to say in the lapse of time between two complete scannings of the picture.

In order to fulfill these conditions, it is not absolutely necessary that the incandescent screen should be thinner than 20 microns. The thinner screens are chosen the more quickly the heat is radiated. and the higher the temperature has to be raised to obtain a sufficint mean brilliancy. Since tungsten is capable of withstanding higher temperatures than other refractory metals such as molybdenum and tantalum, this metal has already been employed as the material of an incandescent screen of 6 microns.

It has been found, however, that a disadvantage inherent to the screens employed upto the present consists in that it is not possible to obtain pictures which are as sharply defined as would allow the width of the scanning lines.

The invention is based on the recognition that this phenomenon has to be ascribed to an effacement of the diiferences in brilliancy produced on the incandescent screen (referred to hereinafter as broadening of the lines) which is harmful broadening of the lines is avoided by reducing the thermal conduction in the incandescent screen. i

The reduction of the sharpness of definition of the pictures due to broadening of the lines is immaterial when it is not larger than the breadth of the scanning lines which is in practice at the utmost-about 0.4 mm.

For reducing the thermal conduction in the incandescent screen to such an extent that the broadening of the lines does not surpass the breadth of the scanning lines, use may be made of an incandescent screen whose thickness is not greater than 1.2 microns. However, other metals than tungsten hardly enter into account for a screen of such thinness because with the great increase of temperature required in this case they would have a length of life which is too short for practical use. An addition of thoria (as is also usual for filaments of electric incandescent lamps) of the order of 1% is advantageous for the durability of these thin incandescent screens.

It is nevertheless possible to remove the above mentioned drawback also with incandescent screens of larger thickness, that is to say of a thickness up to 6 microns, if, in accordance with the invention, they are divided by a perforation into elements. These screen elements should preferably have approximately the same width as, the scanning lines. They preferably have the shape of quadrangles.

The invention will be explained more fully with reference to the accompanying drawing wherein:

Fig. 1 represents diagrammatically, by way of example, one embodiment of an incandescent screen tube, while Figs. 2 and 3 represent on a greatly enlarged scale parts of an incandescent screen as they may be formed, by way of example, in accordance with the invention.

In Fig. l, l denotes a glass bulb which has the shape of a cylinder with a funnel-like widened portion as is usual for Braun tubes. The cylindrical bulb portion contains the electrodes for the production of a narrow directed cathode ray beam. The tube contains an indirectly heated incandescent cathode 2, a control electrode 3 and an electrostatic lens system consisting of a perforated disc 4 and of two cylinders 5 and 6.

By means of a magnet system (not shown in the drawing because of simplification in illustration), the beam of rays may be deflected in a direction perpendicular to the plane of the drawing. Defiecting plates 1 and 8 serve for the deflection in a direction parallel to the plane of the drawing. 55

The wall of the tube is provided with the usual silver mirror 9 which is connected at to a current supply conductor.

In the funnel-shaped tube portion is arranged a metallic frame II to which an incandescent screen I2 is secured at its edges. A current supply conductor I3 is taken out of the glass bulb through an extension M. The current supply conductors of the cathode 2 and of the electrodes 3 and 4 are hermetically sealed. into a pinch 15.

In known manner the narrow cathode ray beam produced and focussed bythe electrode system may be moved within the tube by the deflecting means in such manner that the point of impact of the beam on the screen l2 moves in parallel lines, the scanning lines, over the screen, sweeps across the Whole of the screen in a lapse of time, for example, of 0.04 second and then starts anew on its Way of the screen so that the latter is scanned 25 to 30 times in one second, for example. According as the current intensity of the beam is higher or lower each screen portion struck by the beam is raised by electron bombardment to higher or lower incandescence so that owing to a suitable modulation of the beam by means of the electrode 3 the brilliancy varies and a luminous picture may be produced on the screen.

The number of lines on which the whole of the picture is composed may not be chosen arbitrarily. The transmission possibilities set a limit thereto. It is as a rule possible to go up to 200 lines, satisfactory sharpness of definition of the picture being ensured in this case. With a breadth of the lines of 0.4 mm., the screen consequently acquires a height of 8 cms. With a screen of tungsten having a thickness of 1.2 microns an energy of about 10 Watts per cm. is needed for heating the screen to the required temperature of incandesence. The total energy consequently amounts to about 700 to 800 watts, in which event, for example 16 ma. at 50 lav. may be utilized. A smaller line breadth for example 0.3 mm. may be used.

Fig. 2 shows, by way of example, a model of a perforation which may be utilized in accordance with the invention for the reduction of the broadening of the lines if it is desired to employ screens thicker than 1.2 microns or metals other than tungsten. Owing to the perforation similar to that of stamps the picture is decomposed into elements. These picture elements have slight coherence with the result that there is also slight conduction of heat from one picture. element to the other. The mid-points of the rows of holes should preferably be spaced from one another as far as the midpoints of the picture lines, for example 0.3 mm. in the case of a line breadth of 0.3 mm., Whereas the diameter of the holes is, for example, 50 microns with a distance between their centres of '75 microns.

Another example is shown in Fig. 3 wherein only at the angles of the picture elements there is a connection with the neighboring elements. The holes have an elongated shape and are, for example, 0.2 mm. long and 0.03 mm. wide while the midpoints of successive rows are spaced apart by 0.3 mm.

Other shapes are also possible; thus, for example, the holes may be cross-shaped so that the picture elements are connected to one another in the middle of their sides or the picture elements may have a shape different from that of a rectangle.

Incandescent screens which are perforated according to Figs. 2 and 3 or in any other suitable manner may have a thickness up to 6 microns without harmful broadening of the lines occurring. With this thickness a lower temperature, for example 2200 K., is sufiicient. They may consequently consist of tantalum or of any other metal or alloy which is capable of withstanding this temperature.

It is advisable to utilize a thickness less than 2.5 microns with tungsten alone for in this case the temperature has to be raised to a higher value, for example to 2500 or even to 2700 K. because the heat is lost more rapidly. With other metals the life becomes in this case too short. For this reason, only tungsten enters into account for unperforated screens which should preferably not be thicker than 1.2 microns to avoid harmful broadening of the lines. An incandescent screen thinner than 0.3 micron loses its heat so rapidly that even with the highest admissible increase of temperature the mean brilliancy is under every condition no longer sufficient.

What I claim is:

'1. A cathode ray tube comprising an envelope containing means for generating a beam of electrons, means for deflecting said beam and a luminescent screen adapted to incandesce in re sponse to the impingement of the beam of electrons thereon, said screen consisting of a solid plane sheet of material having a plurality of open perforations therein, and supported solely at its periphery.

2. Apparatus in accordance with claim 1, wherein said solid plane sheet of material comprises tungsten.

3. Apparatus in accordance with claim 1, wherein the open perforations in said solid plane sheet of material are formed so as to divide said material into substantially square sections.

4. Apparatus in accordance with claim 1, wherein said solid plane sheet of material comprises tungsten, having open' perforations thereon in such configuration as to divide said material into substantially square sections.

5. Apparatus in accordance with claim 1, wherein said solid plane sheet of material having open perforations thereon comprises tungsten and thoria.

ALBERT BOUWERS.

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