display tube

Abstract: A method of manufacturing a display tube (1) comprising an envelope having an envelope portion (2) and a display window. The quantity of X-rays released from the envelope portion (2) is determined, after which discrete X-ray absorbing means (7) are provided in those places of the envelope (2) where the quantity of X-rays released exceeds a limiting value. In a display tube (1) manufactured according to such a method and comprising, in particular, a substantially rectangular display screen and a cylindrical neck portion having an electron gun the diagonals of the envelope of which are provided with discrete X-ray absorbing means (7), the desirable quantity of X-rays is absorbed in a simple and cost-­ effective manner.

Description: The invention relates to a display tube comprising an envelope containing a conical envelope portion provided with X-ray absorbing means between a display window and a neck portion having an electron-generating system. Such a display tube can be used in black-and-white, colour and projection television, apparatuses for displaying digits and letters (Data Graphic Display) and in other apparatuses in which a display tube is used. In conventional display tubes an image is produced by deflecting an electron beam generated in the envelope across a luminescing display screen provided on the inside of the display window. When an image is displayed, X-rays are generated in the display tube. Safety requirements determine the permissible maximum X-ray release. This maximum quantity will hereinafter be termed the limiting value. United States Patent Specification US-A-3,562,518 discloses a method of absorbing X-rays released from a display tube by surrounding the neck portion of the envelope portion with coating means impregnated with bismuth and the remaining portion of the envelope portion almost completely with a coating containing bismuth trioxide. The absorption must be such that the quantity of X-ray release remains below the limiting value. The display tubes which are developed and manufactured at present increasingly exhibit a rise in the quantity of X-rays produced in operation. Moreover, safety requirements are becoming tighter, in general, and are different from country to country. If the tighter safety requirements are to be met, the absorption of X-rays must be increased. In the case of a display tube as described in US-A-3,652,518, said tighter safety requirements can be met by increasing the quantity of bismuth and bismuth trioxide. However, this leads to an increased use of bismuth and to a higher weight of the display tube. An object of the invention is to provide a method of manufacturing a display tube, in which the desired quantity of X-ray can be absorbed by using a minimum of X-rays absorbing means. For this purpose, a display tube of the type described in the opening paragraph is characterized in that the conical envelope portion is provided with localized X-ray absorbing means on parts where more X-rays are released during operation of the tube compared with other parts. The X-ray absorbing means may be formed by, for example, one or more localized layers containing a heavy metal. Preferably, a heavy metal-containing tape is used which can be easily adhered to the enyelope portion in the desired places. If the tape is self-adhesive, a separate adhesive can be omitted, so that the tape can be easily provided. In practice it has been found that a tape impregnated with a heavy metal is very suitable. The invention is based on the insight that the quantity of X-rays released from the display tube during the display of an image is not uniformly distributed over the envelope portion. This non-uniform distribution of X-rays may have various causes. For example, the envelope may be manufactured with a non-uniform wall thickness, so that the X-rays are not uniformly absorbed, or it is alternatively possible that the parts in the envelope on which the electron beams impinge emit X-rays in a non-uniform manner. According to the invention, the possibility of adapting the number of means of the quantity of X-ray absorption of every single means to the safety requirements and the limiting value is obtained by providing the conical envelope portion with discrete X-ray absorbing means in places where the X-ray release exceeds the limiting value. The discrete X-ray absorbing means, for example, only have to be provided on those parts of the conical envelope portion where they are required to obtain a desired X-ray absorption. In this manner, a saving in X-ray absorbing means is obtained, so that a display tube manufactured by using the inventive method is more economical than the known display tubes in which the envelope portion is exclusively provided with a uniform quantity of X-ray absorbing means, also in those places where a smaller quantity would be sufficient. With certain types of display tubes, the distribution of the X-rays released and, consequently, the places on the walls of the conical envelope portion where X-ray absorption is desirable can be determined by measuring the radiation of each individual display tube or of a test tube during operation. When it is not necessary to measure the tube itself during operation, the discrete X-ray absorbing means can be advantageously provided on the inside of the conical envelope portion, so that more space is available on the outside of the envelope for, for example, providing other parts of the tube. A preferred embodiment of a display tube according to the invention, is characterized in that the display tube comprises a display window bearing a display screen having a substantially rectangular shape and a cylindrical neck portion, the localized X-ray absorbing means being provided at locations of the conical envelope portion where the planes passing through the diagonals of the display screen and through the axis of the display tube intersect the conical envelope portion. In practice it has been found that in such a display tube the X-rays are released mainly at said locations of the conical envelope portion. This can be attributed to the fact that the envelope portion is manufactured so that it is thinner at said locations than in other places. Consequently, in such a display tube a desired X-ray absorption can be attained in a cost-effective manner without previously measuring of the quantity of X-rays released. A further preferred embodiment of a display tube according to the invention is characterized in that the envelope portion is further provided with a heavy metal-containing layer which substantially covers said envelope portion. If the glass wall of the envelope portion insufficiently absorbs the X-rays, it is to be preferred to first provide a layer containing a heavy metal on the envelope portion so as to substantially cover it. In this manner, the quantity of X-rays released can be kept below the limiting value over a large part of the envelope portion. Localised parts of the envelope portion where the X-ray release is still too high are then provided with discrete means. These and other aspects of the invention will be described and explained by means of examples and with reference to the accompanying drawing. By way of example, the invention will be explained in more detail with reference to the accompanying drawing, in which Fig. 1 is a diagrammatic, perspective, partly cut-away elevational view of a display tube, Fig. 2 is a diagrammatic elevational view of an embodiment of a display tube according to the invention, and Fig. 3 is a diagrammatic sectional view of the display tube shown in Fig. 1, taken on the line III-III, provided with X-ray absorbing means on the diagonals of the envelope portion. Fig. 1 is an elevational view of a display tube comprising a glass envelope having a display window 1 secured to a conical enveloping portion 2 having a cylindrical neck portion 3. In said neck portion 3 there is provided an electron-generating system 4 for generating an electron beam 10 which is focused on a luminescing display screen 8 provided on the inside of the display window 1. Said display screen 8, comprises for example, large number of phosphor elements 9 luminescing in red, green and blue, said phosphor elements being strip-shaped in this case. On its way to the display screen 8 the electron beam 10 is deflected across the display screen 8, by means of a number of deflection coils (not shown) which are coaxially arranged about the tube axis 5. When an image is displayed, X-rays are generated in the display tube, for example, because the electrons in the electron beam 10 impinge on the display screen 8 or, in the case of a colour display tube, are incident on the colour selection system 6. To reduce the quantity of X-rays released to a permissible level, the envelope portion is provided with X-ray absorbing means (not shown in Fig. 1) in a process step in the manufacture of the display tube. According to the invention, discrete X-ray absorbing means are provided after the quantity of X-rays released from the envelope portion during operation of the display tube is determined, for example, by measuring the quantity of X-rays by means of X-ray detecting equipment. In practice it has been found, that the quantity of X-rays released is not uniformly distributed over the envelope portion. Certain parts of the envelope portion release more X-rays than others. The quantity of X-ray absorbing means can be minimised, while maintaining the desired X-ray absorption by providing, in accordance with the invention, discrete X-ray absorbing means in those parts of the envelope portion where the quantity of X-rays released exceeds a limiting value. Thus, according to the invention, a permissible X-ray release, below the limiting value dictated by safety requirements, can be attained by attuning the quantity of X-ray absorbing means to the quantity of X-rays released. This can be realised, for example, by increasing the number of means or the quantity of X-rays absorbed by every single means until a sufficient X-ray absorption is attained. Fig. 2 is a diagrammatic elevational view of an embodiment of a display tube according to the invention, viewed in the direction of the tube axis 5. The display window 1 and the display screen (not shown) are substantially rectangular, permitting the diagonals A and B to be defined. For clarity, the deflection coils are not shown. In such a display tube, the shape of the section of the display tube extends transversely to the tube axis 5 in the direction of said tube axis 5. The section near the neck portion 3 exhibits a substantially circular inner wall of the envelope portion 2, and the section near the end of the envelope portion 2 to which the display window 1 is secured exhibits a substantially rectangular inner wall. This shape of the envelope portion 2 influences the deflection of the electron beams, as will be explained below. During operation of the display tube, electron beams are deflected across the screen by the deflection coils. When the display screen is rectangular, the angle of deflection of the electron beams being deflected to the corners of the display screen is largest. To preclude that these electron beams are incident on the inner wall of the envelope portion 2 and are reflected there in an undesirable manner, said envelope portion 2 is constructed such that it is thinner at the location of the diagonals. This is realised, for example, by constructing the tools used in the manufacture of said envelope portion in such a manner that the wall thickness of the envelope portion is less at the diagonals than elsewhere. Measurements have shown that in this type of display tube more X-rays are released at the location of the diagonals of the envelope portion than in other parts of the envelope portion. It has been found that in this case it is not necessary to measure each display tube separately. Measuring the quantity of X-rays released from a test tube which is representative of this type of display tube is sufficient. The shape of the envelope portion 2 is diagrammatically shown in Fig. 3 which is a sectional view of a display tube as shown in Fig. 1, taken on the line III-III. In this type of display tube, the discrete X-ray absorbing means 7 can be provided on the outside of the envelope portion 2, on the diagonals, to obtain a sufficient absorption of the quantity of X-rays released. The envelope portion 2 need only be provided with discrete X-ray absorbing means on the diagonals, when the glass wall of the envelope portion sufficiently absorbs the X-rays everywhere else. However, when more X-ray absorption is required in other part of the envelope portion, this can be realised, for example, by providing further discrete means. Besides, it is alternatively possible to use a heavy metal-containing layer which is uniformly provided on the envelope portion. Since the discrete means 7 absorb the X-rays released in places where the wall thickness of the envelope portion 2 is small (in this case the diagonals), the wall thickness to be used only has to be sufficient to withstand the vacuum pressure and need not be geared to the X-ray absorption. As a result hereof, a substantial reduction in the quantity of glass necessary for the manufacture of the envelope portion can be attained. Moreover, a thinner wall of the envelope portion also permits to reduce the distance between the deflection coils and the tube axis 5, so that the electron beams can be deflected with less energy. In practice it has been found that the discrete X-ray absorbing means do not influence the deflection of the electron beams. To facilitate the provision of other parts of the tube on the outside of the envelope portion, the discrete X-ray absorbing means 7 are preferably arranged on the inside of the envelope portion. The discrete X-ray absorbing means are formed, for example, by a suspension containing Ba, Zr, Sr or Pb, which is provided on the envelope portion in the form of a layer, for example, by painting. Said means may alternatively be formed by a heavy metal which is provided in the glass of the envelope portion during the manufacture of said envelope portion. Preferably, the discrete X-ray absorbing means 7 consist of a tape containing a heavy metal, for example Pb, which tape can be provided on the envelope portion in a simple and accurate manner by, for example, adhering. Self-adhesive tapes provided with a layer containing a heavy metal are particularly easy to use. Such self-adhesive tapes containing heavy metal in various layer thicknesses are commercially available. The type of tape can be determined in accordance with the required X-ray absorption. Taking the current safety requirements into account, a required X-ray absorption can mostly be obtained by means of a self-adhesive tape having a 50 µm thick layer of Pb. Apart from a tape having a layer containing a heavy metal, for example, a heavy metal-impregnated tape can also be used. The shape of the tape depends on the shape of the portion of the envelope where X-ray absorption is required.

Claims: A display tube comprising an envelope containing a conical envelope portion (2) provided with X-ray absorbing means between a display window (1) and a neck portion (3) having an electron-generating system (4), characterized in that the conical envelope portion (2) is provided with localized X-ray absorbing means (7) on parts where more X-rays are released during operation of the tube compared with other parts. A display tube according to Claim 1, characterized in that the localized X-ray absorbing means (7) are provided on parts where, during operation of the tube, more X-rays are released than a limiting value given by the safety requirements. A display tube according to Claim 1 or 2, characterized in that the display tube comprises a display window (1) bearing a display screen having a substantially rectangular shape and a cylindrical neck portion (3), the localized X-ray absorbing means (7) being provided at locations of the conical envelope portion (2) where the planes passing through the diagonals of the display screen and through the axis of the display tube intersect the conical envelope portion (2). A display tube as claimed in Claim 1, 2 or 3, characterized in that the localized X-ray absorbing means (7) are provided in the inside of the conical envelope portion (2). A display tube as claimed in Claim 1, 2, 3 or 4, characterized in that the localized X-ray absorbing means comprise a heavy metal containing tape. A display tube as claimed in Claim 5, characterized in that the tape is a self-adhesive tape. A display tube as claimed in Claim 5 or 6, characterized in that the tape is impregnated with said heavy metal.

Inventors: BAKKER GIJSBERTUS, REUKERS WOUTER MARINUS, BAKKER, GIJSBERTUS, REUKERS, WOUTER MARINUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H01J29
IPC codes: H01J9, H01J29