A METHOD OF REDUCING DOMING IN A COLOUR DISPLAY TUBE AND A COLOUR DISPLAY TUBE MADE IN ACCORDANCE WITH THE METHOD

Abstract: Anti-doming measures taken in colour display tubes normally comprise applying radiation absorptive layers to the back of the screen. In contrast, in the present invention the thermal radiation reflectivity between the upright edge of the faceplate and at least the edge portion of the shadow mask is adjusted to obtain a desired temperature stabilisation level which avoids spot misalignment. This may be achieved by having selected areas of the upright edge non-­ aluminised whilst the remainder of the upright edge together with the back of the screen have a layer of aluminium thereon. The size and/or shape and/or disposition of the selected areas is/are chosen to obtain an optimum ratio of aluminised and non-aluminised glass surface which will provide a desired radiation coefficient. Typically at least 35% of the upright edge is aluminised. In certain situations it may be necessary to apply a material having a high radiation coefficient, such as a low melting point glass with a high lead content, to a peripheral portion of the shadow mask and the adjoining mounting frame. Measures may also be taken to counter local doming.

Description: The present invention relates to a method of reducing doming in a colour display tube and to a colour display tube made in accordance with the method. Colour display tubes consist of an envelope comprising a faceplate, a cone and a neck. An electron gun system is arranged in the neck. A cathodoluminescent multicolour screen is provided on the interior of the faceplate and an apertured shadow mask is mounted at a short distance from the screen. In operation three electron beams produced by the electron gun system are scanned across the shadow mask by deflection coils mounted at the outside of the neck-cone transition of the envelope. A high percentage of the electrons, up to 80%, impinge upon the shadow mask causing it to heat-up. In the initial phase after switch-on the apertured portion of the shadow mask expands and is deflected or domes towards the screen. This causes mislanding of the electron beams leading to colour distortion. Subsequently heat is lost from the apertured portion of the shadow mask by radiation to the faceplate and by conduction to the frame of the shadow mask which expands and in so doing tightens the shadow mask sheet causing it to be deformed oppositely. This tightening of the shadow mask sheet together with a suitable temperature compensating shadow mask suspension system such as that disclosed in British patent Specification No. 1192725 or British Patent Specification No. 2097996A ideally causes the shadow mask to be restored to its undeflected state. U.S. Patent Specification 3 392 297 (Schwartz) discloses applying a layer of a heat absorptive material to the aluminium layer normally covering the phosphors of the cathodoluminescent screen. The patentees remark that the screen/faceplate absorbing radiated heat from the part-spherical shaped shadow mask a temperature equalisation state is achieved and consequently doming is compensated for. U.S. Patent Specification 3 878 428 (Kuzminski et al) discloses applying one of a variety of heat absorbing layers to the centre portion of the screen and a heat reflective material to a peripheral portion of the forward facing surface of the shadow mask; the purpose of this mixture of layers again being to equalise more easily the temperature difference between the part spherical shadow mask and the screen. In spite of these known methods to counter doming and the consequent misregistration of colours, there still exists a doming problem which is more pronounced in flatter, squarer display tubes currently entering production. An object of the present invention is to improve the antidoming characteristics of a colour display tube. According to one aspect of the present invention there is provided a method of reducing the effects of doming in a colour display tube comprising a glass faceplate having an upright edge and a cathodoluminescent screen on the inside surface thereof and a shadow mask comprising an apertured sheet having an edge portion which is connected to a mounting frame, characterised in that the thermal radiation reflectivity between the upright edge and at least the edge portion of the apertured sheet is adjusted to obtain the desired temperature stabilisation level. This adjustment may be achieved by applying the aluminium film which normally covers the luminescent screen layer on the faceplate so that it extends over the upright edge leaving selected areas of the glass of the upright edge non-aluminised. Typically 35% of the upright edge is covered with aluminium. The size and/or shape and/or disposition of the selected areas is/are chosen to obtain the optimum ratio of aluminised and non-aluminised glass surface which will provide a desired radiation coefficient. Additionally selected areas of the edge portion of the apertured sheet and the mounting frame facing the upright edge of the faceplate may be made extra radiation absorptive by applying a material, such as a low melting point glass with a high lead content, having a high value radiation coefficient thereto. In order to reduce local doming as may occur more likely in flat square display tubes and higher resolution display tubes, the gun facing side of the shadow mask may be treated so that it has a high electron reflection coefficient and a high thermal radiation coefficient. According to another aspect of the present invention there is provided a colour display tube comprising an envelope including a faceplate having an upright edge, a cone connected to the upright edge and a neck, a cathodoluminescent screen applied to the inside surface of the faceplate, and a shadow mask formed by an apertured sheet having an edge portion to which a mounting frame is connected, characterised in that the surface of at least the upright edge of the faceplate has been treated to adjust the thermal radiation reflectivity between the upright edge and at least the edge portion of the apertured sheet in order to obtain a predetermined temperature stabilisation level in operation of the display tube. The present invention will now be described, by way of example, with reference to the drawings, wherein: Figure 1 is a perspective view of a colour display tube with a portion of the envelope broken away, Figure 2 is a diagrammatic cross-sectional view through a portion of faceplate and a shadow mask, Figure 3 is a graph illustrating displacement (D) of the spot on the phosphor line in micrometres (µm) against time (T) in minutes, the ordinate being a logarithmic scale, of a mild steel shadow mask, Figure 4 and 5 are perspective views through a portion of an upright edge of a faceplace in which portions of the glass are selectively covered by an aluminium layer, and Figure 6 is a diagrammatic cross-section through a portion of a faceplate and a shadow mask. In the drawings corresponding reference numerals have been used to refer to the same features. The colour display tube shown diagrammatically in Figure 1 comprises a glass envelope 10 in which three (diagrammatically shown) electron guns 12, 13 and 14 are present to generate three electron beams 15, 16 and 17. A display screen 18 is built up on a faceplate 11 from a recurring pattern of phosphor stripes 19, 20 and 21 luminescing in blue, green and red and which are associated with each of the electron beams 15, 16 and 17 in such a manner that each electron beam impinges only on phosphor stripes of one colour. This is realized in known manner by means of a shadow mask 22 which is suspended at a short distance before the display screen 18 and has rows of apertures 23 which pass a portion of the electron beams 15, 16 and 17. Only approximately 20% of the electrons pass through the apertures 23 on their way to the display screen 18. The remainder of the electrons are intercepted by the shadow mask 22, in which their kinetic energy is converted into thermal energy. In normal operating conditions of a colour display tube, the temperature of a mild steel shadow mask 22 may increase to approximately 75 to 80°C depending on the beam current. Although in the interests of clarity the means for suspending the shadow mask have not been illustrated, a temperature compensating shadow mask suspension system is used. Two alternative types of suitable mask suspension systems are disclosed in British Patent Specification 1192725. Referring to Figure 2, the faceplate 11 comprises an upright edge 24 and the shadow mask 22 comprises a central portion having the apertures 23 and a peripheral portion 25 with an upright edge which is connected, for example by laser welding, to a lightweight mild steel frame 26. The shadow mask 22 and its frame 26 are thermally blackened as indicated by a layer 33. Generally as film 27 of evaporated aluminium covers the screen 18 and the upright edge. This aluminium layer has a low infra-red radiation coefficient which in turn affects in an adverse way the overall and local doming behaviour of the shadow mask. The problem of doming of the shadow mask 22 is generally known and concerns the warming-up phase of a colour display tube, More particularly at switch-on the faceplate is at ambient temperature and the perforated area of the shadow mask 22 becomes heated in response to electron beam impingement. This heating causes the perforated area of the shadow mask 22 to move towards the screen 18, as shown in broken lines. This effect can lead to some colour distortion resulting from mislanding of the electron beams passing through the apertures 23 in the shadow mask. As the display tube continues to warm-up the peripheral portion 25 of the shadow mask and the frame 26 become heated by way of thermal conduction and radiation, and in consequence expands causing tensioning of the perforated central portion which ideally restores it to its original shape. However, the mask is nevertheless at a higher temperature level so that each mask hole is shifted outwards. This needs to be adjusted by an appropriate temperature compensating shadow mask suspension system which moves the mask towards the screen, so that, ideally the original position of the mask hole, looked at from the deflection point is restored. Figure 3 illustrates a hypothetical situation where, very shortly after switch-on, there is a pronounced displacement of the spot on the phosphor line due to doming but after a time, say 15 minutes, thermal stabilisation has been achieved and the displacement of the spot has stabilised ideally at zero. If, however, by applying certain doming, reducing measures there is more a radiation loss from the perforated portion, then the resultant thermally stabilised position of the mask is one where there is doming or overtensioning of the shadow mask in the opposite direction and the movement of the spot in this situation is as shown by the dotted line in Figure 3. Conversely if less heat is lost then the shadow mask will remain slightly domed in the original direction and the movement of the spot is as indicated by the dash-dot line. In accordance with the method of the present invention the anti-doming technique is concerned with adjusting the radiation between the upright edge 24 of the faceplate and shadow mask. The radiation coefficient α of glass is of the order of 0.95 and that of aluminium, that is, the film 27, is of the order of 0.10. In order to obtain a constant temperature profile so that a substantially constant stabilisation level is achieved leading to elimination of doming then the overall radiation coefficient can be adjusted empirically between these values. For example α = 0.65 is obtained by covering the surface of the upright edge for 35% with aluminium. An accurate and consistent way from the point of view of manufacture to obtain an acceptable overall coefficient of radiation especially in the case of a colour display tube with a mild steel shadow mask, is to have predetermined areas of glass at the upright edge 24 non-aluminised whilst concurrently ensuring that there is sufficient of the aluminium film 27 to avoid flashing phenomena on the screen the few seconds after switching-on the set. This phenomena is caused by cold emission sources, such as residues from the screen making process, on the non-aluminised glass surface. One embodiment is illustrated in Figure 4 where the aluminium film 27 comprises finger-like extensions 30 of the main film 27 covering the screen 18. The extensions 30 which are for example 30 mm long stop short of the end of upright edge 24 by approximately 10 mm. In the embodiment of Figure 5, the selected areas of exposed glass comprise suitably shaped windows 31 in the aluminium film 27. In implementing the embodiments of Figures 4 and 5, the required area of glass to be exposed has to be determined empirically for the particular model of the display tube and then either by selective masking the required pattern of aluminium is evaporated onto the screen 18 and exposed areas of the upright edge 24 or by selective etching, predetermined areas of an aluminium film 27 covering the screen 18 and upright edge 24 are removed from the upright edge. In an extreme case no aluminium is present on the upright edge. Consequently the flashing phenomena can either be accepted or special measures taken, such as careful cleaning, to remove sources giving rise to this phenomena. Figure 6 shows an embodiment in which the measures already described are inadequate in the sense that insufficient heat is absorbed by the exposed glass of the upright edge 24 which means that α is too low. Consequently additional measures have to be taken and in Figure 6 these are applied to the peripheral portion 25 and the frame 26. Selective areas of the thermally blackened layer 33, which has an α of the order of 0.7, on the peripheral portion 25 and the frame 26 have a coating 35 of an extra heat absorptive material applied thereto. Such a material will have an α of the order of 0.95 and may typically comprise a low melting point glass with a high lead content. The thickness of the coating 35 is in the range 1 to 10 µm. The selective areas comprise patterns which enable the overall α of the peripheral portion 25 and the frame 26 to be between 0.7 and 0.95 thereby influencing the stabilisation level of the curve shown in Figure 3. In the case of flat square tubes or tubes with a higher resolution whose performance is more susceptible to the effects of doming, additional measures may have to be taken to reduce or eliminate the effects of local doming. Generally speaking the side of the shadow mask facing the electron gun should have a high electron reflection coefficient and a high thermal radiation coefficient. Amongst the options available is to apply a porous layer of a heavy metal or compounds, alloys or mixtures of heavy metals having an atomic number exceeding 70 to the surface of the shadow mask of mild steel or alloys, facing the electron gun. Such a method is disclosed in British Patent No. 2080612. Other options include applying lead glass to said surface but the benefits are reduced if the lead glass is covered by barium from the getter since it decreases the infra-red emissivity of the mask.Thermally blackening said surface is another possibility.

Claims: A method of reducing the effects of doming in a colour display tube comprising a glass faceplate having an upright edge and a cathodoluminescent screen on the inside surface thereof and a shadow mask comprising an apertured sheet having an edge portion which is connected to a mounting frame, characterised in that the thermal radiation reflectivity between the upright edge and at least the edge portion of the apertured sheet is adjusted to obtain the desired temperature stabilisation level. A method as claimed in claim 1, characterised in that an aluminium film is applied over the luminescent screen layer on the faceplate and over the upright edge leaving selected areas of the glass of the upright edge non-aluminised, the size and/or shape and/or disposition of said selected areas being chosen so that a desired radiation coefficient is obtained. A method as claimed in claim 2, characterised in that substantially 35% of the upright edge is covered with aluminium. A method as claimed in claim 1, 2 or 3, characterised in that selected areas of the edge portion of the apertured sheet and the mounting frame facing the upright edge of the faceplate are made extra radiation absorptive by applying a material having a high value of radiation coefficient thereto. A method as claimed in claim 4, characterised in that the material comprises a low melting point glass with a high lead content. A method as claimed in anyone of claims 1 to 5, characterised in that the side of the apertured sheet facing away from the faceplate is coated with a heavy metal or a compound of it having an atomic number exceeding 70 and a high electron reflection coefficient. A method as claimed in claim 6, characterised in that the heavy metal is bismuth oxide. A method as claimed in any one of claims 1 to 5, characterised in that the central apertured portion of the side of the apertured sheet facing away from the faceplate is thermally blackened. A method as claimed in claim 8, characterised in that a radiation reflective coating is applied to the peripheral area of the apertured sheet and the mounting frame on the side facing away from the faceplate. A colour display tube comprising an envelope including a faceplate having an upright edge, a cone connected to the upright edge and a neck, a cathodoluminescent screen applied to the inside surface of the faceplate, and a shadow mask formed by an apertured sheet having an edge portion to which a mounting frame is connected, characterised in that the surface of at least the upright edge of the faceplate has been treated to adjust the thermal radiation reflectivity between the upright edge and at least the edge portion of the apertured sheet in order to obtain a predetermined temperature stabilisation level in operation of the display tube. A colour display tube as claimed in claim 19, characterised in that an aluminium layer is provided on the screen and the upright edge, selected areas of the upright edge being non-aluminised. A colour display tube as claimed in claim 11, characterised in that substantially 35% of the upright edge is covered with aluminium. A colour display tube as claimed in claim 10, 11 or 12, characterised in that selected areas of the edge portion of the apertured sheet and the mounting frame facing the upright edge have thereon a material having a high value of radiation coefficient. A colour display tube as claimed in claim 13, characterised in that the material comprises a low melting point glass with a high lead content. A colour display tube as claimed in any one of claims 10 to 14, characterised in that the surface of the apertured sheet remote from the faceplate has thereon a layer of a heavy metal or a compound of it having an atomic number exceeding 70 and a high electron reflection coefficient. A colour display tube as claimed in claim 15, characterised in that the layer comprises bismuth oxide. A colour display tube as claimed in any one of claims 10 to 14, characterised in that the surface of the apertured portion of the apertured sheet remote from the faceplate is thermally blackened.

Inventors: COMPEN, JOHANNES MARIA AZALINA ANTONIUS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01J29
IPC codes: H01J29, G03B27