Patent Application
20070007873
The present invention relates to a colour cathode ray tube with a noticeably flat screen, and more specifically a colour selection frame/mask assembly equipping such a tube.
The invention finds its application in any type of tube comprising a colour selection mask and is more particularly adapted for tubes with a mask that is realised by stamping and is held in place within the tube by a rigid frame to which it is joined.
A conventional cathode ray tube comprises a glass envelope under vacuum. Within the envelope, the tube comprises a colour selection mask located at a precise distance from the glass front face of the tube, front face on which red, green and blue luminophore networks are laid to form a screen. An electron gun is arranged within the rear part of the tube according to the direction of the longitudinal axis of the tube, the said axis passing through the centre of the front face, the said gun generating three electronic beams in the direction of the said front face. An electromagnetic deflection device, generally located outside the tube and close to the electron gun has the function of deviating the electronic beams so as to sweep them over the surface of the panel on which the luminophore networks are arranged. Under the influence of three electronic beams each corresponding to a determined primary colour, the luminophore networks enable colour pictures to be reproduced on the screen, the mask enabling each determined beam to illuminate only the luminophore of the corresponding colour.
The colour selection mask must be arranged and maintained in a specific position within the tube during the operation of the tube. The mask support functions are realised owing to a generally very rigid rectangular metal frame on which the mask is conventionally welded.
The frame/mask assembly is mounted in the front face using suspension means welded on the frame and co-operating with lugs inserted into the glass constituting the front face of the tube.
The colour selection mask is realised from a metal sheet of a very low thickness and comprises a surface called the effective surface, pierced with openings, the said openings being realised by chemical etching and generally arranged in vertical columns; the effective surface is surrounded by a peripheral non-pierced border; a peripheral flange, generally realised by stamping, borders the assembly by extending in a direction noticeably perpendicular to the effective surface. The mask is integrated with the frame by spot welding at the level of the peripheral flange.
The generally rectangular frame has a pair of long sides and a pair of short sides, with a cross-section that is generally L-shaped. The frame must have excellent rigidity, allowing the entire frame/mask to be able, without any modification of shape, to undergo the many manipulations that occur during the manufacturing stages of a cathode ray tube, as explained in the patent U.S. Pat. No. 4,639,230.
To obtain this level of rigidity, the frames of the prior art have a thick cross-section and large weight, which has two disadvantages:
Moreover, it has been observed that a mask formed by stamping is extremely sensitive to heating, caused by the fact that the mask intercepts approximately 70% of the electrons emitted by the gun. The cause of this sensitivity lies in the significant difference in mass between the frame and the mask, meaning that the thermal behaviour of two parts are very different for the assemblies according to the prior art.
Among other advantages, the frame according to the invention reduces the weight of the materials used to manufacture the frame/mask assembly, while improving the thermal behaviour of the said assembly.
To do this, the tube according to the invention comprises:
the said frame being characterized in that the said L-shaped cross-section sides are the short sides of the frame and in that the width of the flange of each short side is smaller in the middle of the side than at its extremities so that the inner edge of the flange defines a curve whose concavity is oriented towards the longitudinal axis of the tube.
The invention and its different advantages will be better understood from the following description and drawings, wherein:
The
The tube comprises a rear part in the form of a funnel 30 terminating in a cylindrical collar 31 within which there is an electron gun 40 generating three electronic beams 41 in the direction of the colour selection mask 21. The electronic beams sweep the surface of the screen 11 under the influence of the magnetic deflection device 50.
When the tube operates, approximately 20 to 30% of the electrons emitted by the gun 40 can pass through the mask by the openings arranged on the surface facing the screen. A large quantity of the electrons are therefore intercepted by the colour selection mask 21 and also collide with the support frame 22. The result is that the frame and the mask undergo thermal dilation, and when the frame and the mask are realised in the same material, for example steel, the dilation of the mask will be more rapid than that of the frame owing to the large difference between the masses of the two elements. The mask, maintained on its periphery by the frame, will then become warped (phenomenon known as “doming”) and the distance from its surface to the luminophore screen will change until the assembly reaches a thermal equilibrium. The modifications of the mask 21/screen 11 distance cause discolorations of the picture on the said screen, the electronic beams no longer correctly illuminating their corresponding luminophores.
It has revealed that two factors affect the appearance of this phenomenon:
The temperature at the periphery of the mask is governed by conditions at the limits: contacts with the frame, thermal inertia of the frame, shape of the edges of the frame enabling the electronic beams to strike the folded peripheral flange 23 of the mask.
The short sides have a flange whose width varies from the centre to the ends such that the width 150 of the said flange in its middle is less than the width 151 at its extremities. In this manner, the inner edges 130 show concavity oriented inwards so as to allow the electronic beams to intercept a larger part of the surface of the peripheral flange of the mask which is welded to the frame.
The electronic beams reach the middle of the short sides at higher angles of incidence in the middle of the sides of the frame than at their extremities. Hence, when the flange has a constant width (dotted line), the peripheral flange of the mask is less covered by the electronic beams in the middle than at its extremities, whereas if the width of its extremities reduces towards its centre (solid line) the electronic beams can reach a noticeably constant width of the peripheral flange along the entire length of the part of this flange welded on the short side of the frame. The result is that the mask heats up more evenly with a low temperature gradient between the different parts of the mask. The inner edge 130 of the flange thus has a concavity oriented inwards, with an average radius of curvature lower than the average radius of curvature of its outer edge 131. In an embodiment according to the invention designed for a tube with a screen diagonal in the order of 60 cm, illustrated by the
The arched form of the flange gives the frame a great mechanical rigidity allowing a reduction in the quantity of metal used to produce the said frame. The rigidity of the frame/mask assembly can be improved by coupling the arched form of the flanges of the short sides with the flanges of the long sides presenting on the contrary a width 160 at its middle that is greater than its width 161 at its extremities. In the embodiment according to
Although the type of frame structure according to the invention can be adapted to all types of suspension means in the tube of the frame/mask assembly, the rigidity of the said assembly, once inserted in the tube is improved by suspension means arranged at the corners. Hence, suspension springs can be arranged in the corners 140 of the frame to co-operate with the lugs inserted in the glass peripheral flange of the front part 1 of the tube. In this manner, it is possible both to reduce the quantity of material used to produce the frame and to use a steel frame/mask assembly, a less expensive material than invar.
In the embodiment according to
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2003 A 001473 | Jul 2003 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR04/50323 | 7/9/2004 | WO | 8/9/2006 |
