WO2006090077A2

WO2006090077A2 - Colour cathode ray tube mask

Info

Publication number: WO2006090077A2
Application number: PCT/FR2006/050113
Authority: WO
Inventors: Fabrizio Berton; Stefano Necci; Paolo Ginesti
Original assignee: Thomson Licensing
Priority date: 2005-02-25
Filing date: 2006-02-09
Publication date: 2006-08-31
Also published as: ITMI20050300A1; WO2006090077A3

Abstract

The invention relates to a cathode ray tube having an essentially-flat front face, whereof the colour-selecting mask is produced by swaging. The mask is defined by two axes of symmetry, the major horizontal axis X and the minor vertical axis Y intersecting at the centre O of an active surface (19) which is pierced with holes and surrounded by a solid peripheral border. In order to minimise the effects of microphonicity, the active surface (19) of the mask (9) is characterised in that: along any line parallel to the major axis, the value of the radius of curvature of the mask Rx in the horizontal direction is maximum in the zone located close to the intersection with the minor axis; and the radius of curvature Rx along the minor axis varies from the centre O towards the end of the axis such that the maximum value thereof is located close to the end of said axis.

Description

MASK FOR TUBE WITH COLORED CATHODE RAYS

The present invention relates to a color cathode ray tube having a substantially planar screen, and more specifically to the color selection mask equipping such a tube.

The invention finds its application in any type of tube comprising a color selection mask and is particularly suitable for tubes whose mask is formed by stamping and held by a rigid frame on which it is secured.

A conventional color cathode ray tube is composed of a vacuum glass envelope. The tube comprises inside the envelope, a color selection mask located at a precise distance from the glass front face of the tube, the front face on which are deposited red, green and blue phosphor arrays to form a screen. An electron gun arranged inside the tube, in its rear part, generates three electron beams in the direction of the front face. An electromagnetic deflection device, generally disposed outside the tube and close to the electron gun has the function of deflecting the electron beams to scan the surface of the panel on which the phosphor arrays are arranged. Under the influence of three electron beams each corresponding to a predetermined primary color, the phosphor arrays allow the reproduction of colored images on the screen, the mask allowing each determined beam to illuminate only the phosphor of the corresponding color. .

The color selection mask must be arranged and maintained during operation of the tube in a precise position within the tube. The functions of maintaining the mask are achieved through a generally very rigid rectangular metal frame on which the mask is conventionally welded. The frame / mask assembly is mounted in the front face of the tube by means of suspension welded to the frame and cooperating with pins inserted in the glass constituting the front face of the tube.

The color selection mask is made from a metal sheet of very small thickness and comprises a so-called active surface pierced with openings, made by chemical etching and generally arranged in vertical columns; the active surface is surrounded by a non-perforated peripheral edge; a skirt generally made by stamping, borders the assembly extending in a direction substantially perpendicular to the active surface. The mask is secured to the frame by welding at the skirt.

Most of the tubes today have a substantially flat front face and the current trend tends to shorten the depth of the tube by using for example at maximum deflection angles electron beams greater than 110 °.

In general, the surface of the mask must follow the shape of the inner part of the front face of the tube, so that their curvature is substantially identical. The color selection mask of a conventional tube has a surface defined by horizontal and vertical profiles whose radii of curvature are small, of the order of one or two meters in the central zone; this curved surface can be represented by a complex polynomial expression and the low value of the radii of curvature ensures the mechanical rigidity of the surface of the mask.

In the case of tube whose appearance of the screen is substantially flat, the radii of curvature defining the surface of the mask are large values. In this case, the surface of the mask facing the screen of the tube is substantially flat and no longer has sufficient mechanical rigidity to maintain the entire surface at a predetermined distance from said screen. Moreover, the mask becomes very sensitive to external vibrations; under the influence of shock or external mechanical vibrations, for example acoustic vibrations due to the speakers of the television set into which the tube is inserted, the mask may then enter into vibration according to its natural frequency of resonance. The vibrations of the mask have the effect of modifying the landing zone of the electron beams on the screen of the tube, the points of impact of each beam then being offset relative to the network of phosphors associated, thus creating a discoloration of the image reproduced on the screen.

This so-called microphonicity phenomenon is also more important for the tubes of reduced depth because this reduction also passes through the enlargement of the radius of curvature of the mask. The present invention relates to a color cathode ray tube whose mask, formed for example by stamping, has among other advantages, to have sufficient mechanical rigidity to avoid the disadvantages associated with a substantially flat surface.

For this, the color cathode ray tube according to the invention comprises:

a rectangular front face, supporting on its inner surface a luminescent screen,

a rectangular color selection mask defined by two axes of symmetry, the horizontal major axis X and the vertical minor axis Y crossing at the center O of an active surface pierced with orifices,

an electron gun for generating electron beams for scanning the luminescent screen at a maximum deflection angle Θ _m ,

a substantially rectangular frame to which the mask is secured, the active surface of the mask being characterized in that:

- along the major axis parallel to the major axis, the value of the radius of curvature Rx in the horizontal direction is maximum in the area near the intersection with the minor axis, - the radius of curvature Rx along the minor axis varies from center

O towards the end of said axis so as to have a maximum value located near the end of said axis. The invention as well as its various advantages will be better understood by means of the description below and the drawings among which:

FIG. 1 shows in section a cathode ray tube according to the invention with its various operating members

FIG. 2 shows in a cavalier perspective a color selection mask for a cathode ray tube according to the invention;

FIG. 3 illustrates the variations of the radius of curvature Rx of the mask in the horizontal direction along different parallels to the major axis in one embodiment of the invention.

FIG. 4 illustrates the variations along the minor axis of the radius of curvature Rx of the mask in the horizontal direction in the zone corresponding to the active surface in one embodiment of the invention.

FIG. 5 shows the variations of the derivative of the radius of curvature Rx in the horizontal direction in one embodiment of the invention.

Figure 1 depicts a color cathode ray tube according to the state of the art. The tube comprises a glass envelope in which a high vacuum prevails, the envelope being composed of a front face 2 and a funnel-shaped rear portion 4. A side skirt 1 surrounds the front face 2 which has an outer face 10 substantially flat and rectangular. The major axis of the front face is a horizontal axis X, the minor axis is a vertical axis Y and the two axes X and Y intersect the main axis Z of the right angle tube. The funnel-shaped rear portion 4 ends in a cylindrical portion 3 inside which an electron gun 12 is disposed. A luminescent screen 5 constituted by a network of bands of green, red and blue luminescent materials is deposited on the inner surface of the front face. A color selection mask 9 is disposed inside the glass envelope and is secured on its periphery to a rigid frame 8 intended to hold it in place with respect to the screen 5. FIG. 2 illustrates an embodiment of a mask according to the invention in which the mask comprises an active surface 19, substantially rectangular, pierced with a multitude of openings arranged at regular intervals, the active surface being surrounded by a full peripheral edge 18; during the forming of the mask generally carried out by stamping a flat metal sheet, a skirt 17 is produced, extending in the direction of the main axis Z, substantially perpendicular to the edge 18.

The electron gun 12 emits three electron beams 11 in the direction of the screen 5. The three beams are deflected by a magnetic deflection device 13 also called deflector. The colored images are displayed on the screen 5 by the horizontal and vertical sweeps of said screen by the electronic beams 11 passing through the openings 6 of the mask.

The electron beams scan a maximum angle Θ _m corresponding to the two extreme positions of the beams when they reach two opposite corners by a diagonal of the screen 5.

The tube according to the invention has an outer surface 10 of the substantially flat front face, whose average radius along the horizontal axis is greater than 20m. To avoid distortions of images formed on the screen of the tube and which are annoying for the viewer, such as differences in brightness rendering on different parts of the screen, differences due to varying thicknesses of the front face, the current trend is to make the inner surface of said front face as flat as possible so as to minimize the variation of glass thickness. The designer of the tube is then confronted with the choice to use a mask with a large radius of curvature, or to use a mask whose curvature follows the curvature of the internal surface of the front face.

The first solution has the advantage of offering significant mechanical rigidity and also the advantage that during the forming step of the mask, usually made by stamping, the mechanical stresses generated by the shape of the mask guarantee the maintenance of this shape. However, the variations in distances between the active surface of the mask and the screen for different zones of this active surface induce deteriorations in the quality of the image formed on the screen, in particular on the peripheral zones, the impacts of the electron beams on the screen being then widened and deformed, with respect to the center.

The second solution makes it possible to minimize the variations in distance between the active surface of the mask and the screen; however, the mask will then have a substantially flat surface with few mechanical stresses induced by this shape. As a result, once secured to the frame, the mask has areas of weakness making it very sensitive to the mechanical vibrations of its environment.

The analysis of the behavior of a tube mask according to the state of the art having a substantially flat front face has shown that the amplitude of the vibrations of the surface of said mask is maximum near the vertical sides as well as in the corners.

Moreover, the analysis of the consequences of the mask vibrations on the landing errors of the electron beams on the luminescent screen shows that the visibility of the landing errors is greater near the corners and the vertical sides, whereas this visibility is weaker along the minor axis.

One of the principles of the invention is to move the regions of maximum amplitude of vibration to areas of the active surface of the mask where the consequences will be less visible to the viewer, that is to say by ensuring that the the amplitude of the vibrations on the natural frequency of the mask is greater in the zone situated along the minor axis.

Moreover, the investigations conducted to minimize the effects of microphonicity have shown that these effects are very sensitive to the radius of curvature Rx, which is the radius of curvature of the mask in the direction parallel to the horizontal major axis. In one embodiment of the invention, a mask for a diagonal tube equal to 68 cm and a maximum deflection angle Θ _m has been designed.

FIG. 3 illustrates the variations of the radius of curvature Rx of its mask in the horizontal direction along the major axis and along two parallels to the major axis situated respectively at 140 mm and 195 mm from said axis.

Figure 4 shows the evolution of its horizontal radius of curvature Rx along the minor axis. Thus, along parallel to the major axis, the value of the radius of curvature Rx in the horizontal direction is maximum in the area near the intersection with the minor axis to gradually decrease towards the vertical edges of the active surface . Near the center O the radius of curvature Rx has a value of approximately 4 meters and at the end of the minor axis, this value increases to more than 10 meters, whereas towards the vertical edges of the active surface of the mask it is about 0.5 meters.

This characteristic can be checked preferably for any parallel between the major axis and the horizontal edges of the active surface of the mask. Experience has shown that it must be at least for the areas on the minor axis at a distance from the center of the active surface greater than two-thirds of the length separating said center from said edges of the active surface.

In order to better focus the vibrations in the area near the minor axis, experience has shown that it is preferable to have the radius of curvature Rx along the minor axis vary from the center O towards the end of said axis so as to present a maximum value located near the end of said axis; preferably, increasing the value of Rx in the area around said axis accelerates from the center to the edge of the image. Thus, Rx increases, along the minor axis, by about 40% between the center O and the point situated at 150 mm from this center (respectively from 4200mm to 5500mm) and from 200% between 150mm and 195mm ( respectively from 5500mm to 11000mm). In another embodiment of the present invention, illustrated in FIG. 5, the behavior of the mask with microphonicity is compared according to the variations of curvature of said mask along the major axis. It was noted that said behavior was less sensitive when the derivative of the radius of curvature Rx in the horizontal direction c) Rx along this axis R'x =, kept the same sign from the center to the edge of the dx active surface (case 1 in Figure 5) and this behavior was deteriorating

when the derivative changed sign between the center and the edge of the dx active surface (case 2 of Figure 5).

This evolution of Rx can be used in isolation or in conjunction with the features illustrated in Figures 3 and 4.

Although perfectly adapted to tubes having a flat front face, and / or having maximum deflection angles greater than 110 ° invention is not limited to these types of tubes. Microphonicity problems occur for any type of tube, with more or less visibility, and the invention has the advantage of reducing the visibility of the vibration effects of the mask to a level acceptable by the viewer.

Claims

1 / cathode ray tube in color, comprising: - a rectangular front face (2), supporting on its inner surface a luminescent screen (5),

a rectangular color selection mask (9) defined by two axes of symmetry, the horizontal major axis X and the vertical minor axis Y crossing at the center O of an active surface (19) pierced with orifices ,

an electron gun (12) for generating electron beams for scanning the luminescent screen at a maximum deflection angle Θ _m ,

- A substantially rectangular frame (8) to which the mask is secured, the active surface of the mask being characterized in that:

O towards the end of said axis so as to have a maximum value located near the end of said axis.

2 / color cathode ray tube according to claim 1, characterized in that the radius of curvature Rx evolves monotonically along the major axis.

3 / color cathode ray tube according to claim 1, characterized in that the maximum value of the radius of curvature Rx along the minor axis is at least two times greater than the value of Rx in the center of the active surface. 4 / color cathode ray tube according to claim 1, characterized in that the outer surface (10) of the front face is substantially flat.

5 / cathode ray tube according to claim 1, characterized in that the deflection angle Θ _m is greater than 115 °.

6 / cathode ray tube according to claim 1, characterized in that along the minor axis, the ratio between the maximum value of Rx and its value at the center O is greater than 2.

Cathode ray tube according to claim 4, characterized in that along the minor axis, the maximum value of Rx is greater than 8 meters.

8 / Cathode ray tube according to claim 1, characterized in that along the minor axis the increase of the radius of curvature Rx is faster near the edge of the active surface than near its center O.