CRT WITH IMPROVED SLOTTED MASK

Abstract: The CRT has a tensioned shadow mask with between 160-180 or between 210 and 240 slits per row and display devices having such CRT's. The first embodiment is in particular suited for CRT"s having a screen size larger than 21" and for TVT applications, giving a substantially Moire free image for an NTSC, PAL and VGA signal, the second embodiment is in particular suited for CRT's having a screen size of 23" or smaller and for CMT applications, giving a substantially Moire free image for a SVGA and XGA signal.

Description: CRT with improved slotted mask The invention relates to a CRT (cathode ray tube) with a colour selection electrode having rows of elongated openings The colour selection electrode is also called a 'mask'. More in particular it relates to a CRT with a tensioned colour selection electrode in which tension is applied in a direction and the colour selection electrode comprises openings elongated in said direction, the openings being separated from each other in said direction by bπdges. In US 4,942,332 a slit-type flat foil tension mask is descπbed having slits with large (in compaπson to conventional masks) longitudinal dimensions. Typically the length of the slits is of the order of 1 inch (2 5 cm). The distance between bπdges between slits is sometimes also called the vertical pitch or in short a v The bπdges in between the slits provide mechanical strength to the tensioned colour selection electrode, without substantially reducing the image bπghtness or deforming the mask when the mask is put under tension. However, they also pose a problem in that the bπdges may be visible as two straight hoπzontal lines, reducing the image quality. In US 4,942,332 a solution for this problem is descπbed, being constituted by randomizing the length (pitch) of the slits It is proposed in 'Stretched tension mask with large vertical pitch for CMTs' IDW 99, page 573-576 to use, in order to remove or at least reduce, the visibility of the bπdges a vertical pitch from 2 4 mm to 6 mm for a 21 " CRT Smaller vertical pitches (between 1 and 2.4 mm) provide large Moire patterns and are thus excluded. Both of these solutions, however, have shortcomings, in both the visibility of the bπdges is still appreciable. The inventors have realized that randomization of the position of the bπdges as descπbed in US 4,942,332 leads to unexpected problems At some parts of the image the bπdges become visible, namely there where a number of bπdges happen to be at the same hoπzontal position, or where adjacent bπdges happen to be spaced at such distances that Moire effects occur, where at ot er parts they are not The randomization in fact does not change the distance between the bπdges seen in a hoπzontal dimension by a large amount The randomization as descπbed in US 4,942,332 is for example 0.02', which means that on average the distance between adjacent bπdges is 0.01 ' or 0.25 mm, and because of the randomization the distance is sometimes much less. This still leads to lines being visible in the image, to which lines the human eye is very sensitive. Randomizing invaπantly also leads to clusteπng. resulting being visible in parts of the image. The human eye is very sensitive to such liregulaπties in the image The image is perceived as 'patchy' because of this effect In some sense this problem is a greater pioblem than a straight line being visible. The straight line is always there and it is a problem that the viewer understands and will most likely be at least to some degree be visible when the device is bought and will effect all modes of image reproduction in more or less the same degree. The 'patchy' image due to randomization is something that is dependent of the image that is displayed, and also on the particular mode (VGA, UGA. XVGA etc; with which the image is displayed. These problems become usually visible at higher solution of the image, i.e the higher the image quality. Such problems manifest themselves more often than not after sale and effect in particular the high quality image modes, leading to the clearly unwanted effect that the 'lower image quality' in fact gives a higher quality image than the 'high quality image'. A vertical pitch between 2 4 and 6 mm (as in the above cited article) does reduce the visibility of the bπdges, but they are nevertheless visible, especially in a CMT (Colour Monitor Tube, that is a CRT for a computer monitor) The present invention has as an object to provide a CRT in which some or most of the above cited problems are resolved or reduced, more in particular to provide a CRT in which the presence of the bπdges is less noticeable To this end the CRT in accordance with the invention is characteπzed in that the number of slits per row lies between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21' ' or between 210 and 240 for a diagonal dimension smaller than 23". The inventors have realized that, although the resulting vertical pitch or pitches is for a 21" CRT well within the range for which in the cited article Moire patterns are predicted, the indicated range surpπsingly enough provide for a CRT which, when used for either TVT or CMT application shows little or no Moire patterns, while yet offeπng increased intensity of the image, and strong masks, but no visible effects of individual bπdges The first indicated embodiment (the number of slits per row lying between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21") provides a CRT which shows little or no Moire foi an NTCS or PAL or VGA signal and onh \ small Moire for an SVGA signal as will be explained below NTSC, PAL and to a lesser degree VGA signal are typically signals for TVT applications The size of the CRT (larger than 21") makes for a comfortable viewing distance (roughly three to seven times the height of the image) at which viewing distance the resulting distance between the bπdges is such that individual bπdges are no longer or less visible The second embodiment provides for a CRT which shows little or no Moire for an SVGA or XGA signal which are signals typical for CMT applications For such applications the viewing distances are typically smaller, but because the number of slits (as compared to the first embodiment) is also increased, visibility of individual bπdges is still small The invention is amongst others based on the insight that, although in general Moire patterns do indeed cause severe Moire patterns in the range (1 to 2.4 mm) indicated m the cited article, the ranges of the embodiments of the present inventions do not show appreciable Moire patterns Namely, in the range from 1 to 2 4 mm there is a substantial vaπation in the intensity and wavelengths of the Moire patterns. In the range of the embodiments of the present invention the Moire intensities are lowest and the Moire is least visible Preferably the height of the bπdges is for the first embodiment less than 100 micrometer, for the second embodiment less than 70 micrometer. The inventors have released that, for tension masks with relatively large slits as in the present invention, the height of the bπdges may be considerably less than the Height of the bπdges in conventional shadow masks, which e around 140 micrometer This reduction m height reduces the negative influences of the bπdges on the image intensity and Moire patterns, thus increasing the image The invention furthermore relates to a cathode ray tube for TVT applications with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode compπses elongated openings in said direction, the openings being separated from each other m said direction by bπdges. characteπzed in that the number of slits per row lies between 160 and 180 slits per row and to a cathode ra\ tube for CMT applications with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction by bπdges, characteπzed in that the number of slits per row lies between 210 and 240 slits per row It is remarked that 'for TVT applications' and 'for CMT applications' in the art of CRT's is more than an indication of possible use CRT's are specifically made for such applications, in fact the international code for CRT's indicates (by means of an M) which CRT's are for CMT applications Furthermore CRT's for CMT applications have to comply with certain radiation requirements (due to the relatively small viewing distance) and safety requirements whereas CRT's for TVT applications have less stπngent requirements Handbooks and brochures also specifically indicate which CRT's are for which application The invention also relates to a TVT compπsmg a cathode ray tube with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction by bπdges, characteπzed in that the number of slits per row lies between 160 and 180 slits per row The invention also relates to a CMT compπsing a cathode ray tube with a tensioned colour selection electrode in which electrode tension is applied in a direction and the colour selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction by bπdges, characteπzed in that the number of slits per row lies between 210 and 240 slits per row 'Between x and y' includes within the framework of the invention a number of y (or y) slits Within the framework of the invention some of the outermost rows may have a smaller number of slits These and other aspects of the invention are apparent from and will be elucidated in greater detail by way of example and with reference to the accompanying drawings in which Fig 1 schematically shows a CRT Fig 2 shows schematically a tensioned colour selection electrode Fig 3 shows schematically a part of a shadow mask, showing the elongated slits and the bπdges Fig 4 shows as a function of diagonal size D for a 16 9 tube the vertical pitch a for CRTs in accordance with the invention. Fig. 5 shows as a function of diagonal size D for a 4:3 tube the vertical pitch a v for CRTs in accordance with the invention. The Figures are not drawn to scale. In general, like reference numerals refer to like parts. A color display device 1 (Fig. 1) includes an evacuated envelope 2 compπsing a display window 3, a cone portion 4 and a neck 5. In said neck 5 there is provided an electron gun 6 for generating three electron beams 7, 8 and 9. A display screen 10 is present on the inside of the display window. Said display screen 10 compπses a phosphor pattern of phosphor elements luminescing m red, green and blue. On their way to the display screen the electron beams 7, 8 and 9 are deflected across the display screen 10 by means of a deflection unit 11 and pass through a shadow mask 12 which is arranged in front of the display window 3 and which compπses a thin plate having apertures 13. The shadow mask is suspended in the display window by means of suspension means 14. The three electron beams converge and pass through the apertures of the shadow mask at a small angle with respect to each other and, consequently, each electron beam impinges on phosphor elements of only one color. In figure 1 the axis (z-axis) of the envelope is also indicated Figure 2 shows schematically a tensioned color selection electrode. The color selection electrode compπses a frame 14, having attachment sides 14A and 14B and sides 14C and 14D perpendicular to the attachment sides 14A and 14B. Before the mask is provided the sides 14A and 14B are pushed inwards. When the mask is attached and the pushing force is released, the mask is put under tension. The direction of tension T is indicated in Fig.2. Fig. 3 shows schematically slits 13 in shadow mask 12 The slits are elongated in direction T. The vertical pitch a is schematically shown. The slits 13 are separated from each other (in the direction T) by bπdges 15. The bπdge height d v is also indicated in Fig. 3, as well as the slit length p. The following relation holds: a % =d v +p. Adjacent rows are offset by a distance Δy. As can be seen in Fig. 3 all bπdges e in this example at a set of parallel lines. When a becomes large these lines of bπdges become visible to the naked eye Randomization of Δy reduces the visibility of the lines as such, but introduces patchiness of the image The CRT in accordance with the invention is characteπzed in that the number of slits per row lies between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21" or between 210 and 240 for a diagonal dimension smaller than 23" Moire patterns are due to an interference effect between the scan lines (with a scan pitch s) and the pattern ol openings (with a vertical pitch a ) The inventors have realized that there is a relatively small range of s/a values between 4/8 and 2/8, namely between 3 2/8 and 2 4/8 within which the Moire patterns are small to negligible. It is remarked that the centre of this range is not 3.0/8 but more towards 2/8 than towards 4/8 For first range of number of apertures (160-180) the s/a v ratio for a NTSC, a PAL and a VGA signal are all within 2.4/8 and 3.2/8 This first embodiment provides a CRT which on the one hand has a considerable larger image intensity (because the number of bπdges is strongly reduced from the usual number which lies in the order of 500), with a typical gam in intensity of 10-20%, while yet providing an image substantially free from moire for NTCS, PAL and VGA signals, which are signals typically used in TVT applications The number of lines is relatively small, but sufficient since for TVT applications the viewing distance is relatively large (roughly 2 meter or more) as are the CRT sizes (typically larger than 21"). For second range of number of apertures (210-240) the s/a v ratio for a SVGA and XGA signal are within 2 4/8 and 3.2/8. This second embodiment provides a CRT which on the one hand has a considerable larger image intensity (because the number of bπdges is strongly reduced from the usual number which lies in the order of 500), with a typical gain in intensity of 10-20%, while yet providing an image substantially free from moire for SVGA and XGA signals, which are signals typically used in CMT applications The number of lines is relatively large, since for CMT applications the viewing distance is relatively small (0.5 to slightly more than 1 meter) as are the CRT sizes (equal or less than 23") Fig 4 shows graphically and approximately a v as a function of the diagonal size of the CRT for a 16:9 tube The indicated ranges are although giving a good indication, indicative firstly because not all CRT which have the same nominal size actually have the same size and secondly because the distance between the mask and the phosphor screen and the angle of deflection of the electron beams have some influence on the relation between screen size and a value Fig 5 shows graphically and a v as a function of the diagonal size of the CRT for a 4 3 tube Again, as in figure 4, the indicated ranges form an indication Also given is the range lor a 21" CMT which according to the article 'Stretched tension mask with large vertical pitch for CMT's' IDW 99, page 573-576' provide for large Moire problems The inventors ha\ e released that, although in general the indicated range indeed provide foi large Moire problems, there is nevertheless, and this is formed by the ranges of the l ention, some particular sub-ranges for which Moire pioblems do not occur The cross in figure 5 indicate the value for d mentioned in the cited article as optimal foi NTCS and PAL It will be clear that within the framework of the invention many vaπations are possible For instance, the height of the bπdges is preferably reduced substantially in respect to con\ entional values which e around 140 micrometer Preferably the height of the bπdges is for the first embodiment (160-180 bπdges) less than 100 micrometer, for the second embodiment (210-240 bπdges) less than 70 micrometer For tensioned masks the height of the bπdges can be reduced substantially in respect of the height for con\ entional (non-tensioned) masks This reduction reduces the visibility of the bπdges, increases the image intensity, and has some positive effect on any residual Moire effects Also, for instance, not all of the bπdges need to be full bπdges, I e attached to the mask at both sides In embodiments it is possible that some (for instance 2 of every three) of the bπdges span a substantial part of the slit (for instance 50-75%) bot not the whole slit Although the strength of the mask increased as the number of bπdges increases (which thus would favour all bπdges forming mechanical connections), the effect of the invention (reduction of visibility of the bπdges) is also apparent when some of the bπdges are false bπdges as long as the total number of true and false bπdges lies within the indicated numbers The advantage of using false bπdges lies in a further inci eased image intensi In short the invention can be descπbed as follows The CRT in accordance with the invention has a tensioned shadow mask with between 160-180 or between 210 and 240 slits per row and display devices having such CRT's The first embodiment is in particular suited for CRT's having a screen size larger an 21' and for TVT applications, giving a substantially Moire free image for an NTSC, PAL and VGA signal, the second embodiment is in particular suited for CRT's having a screen size of 23" or smaller and for CMT applications, giving a substantially Moire free image for a SVGA and XGA signal

Claims: CLAIMS 1 Cathode ray tube with a tensioned color selection electrode in which electrode tension is applied in a direction and the color selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction b> bπdges, characteπzed in that the number of slits per row lies either between 160 and 180 slits per row for a diagonal dimension of the CRT equal to or larger than 21" or between 210 and 240 for a diagonal dimension smaller than 23" 2. Cathode ray tube for TVT applications with a tensioned color selection electrode in which electrode tension is applied in a direction and the color selection electrode compπses elongated openings in said direction, the openings being separated from each othei in said direction by bπdges, characteπzed in that the number of slits per row lies between 160 and 180 slits per row. 3. Cathode ray tube for CMT applications with a tensioned color selection electrode in which electrode tension is applied in a direction and the color selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction by bπdges, characteπzed in that the number of slits per row lies between 210 and 240 slits per row. 4 Cathode ray tube as claimed in claim 1, 2 or 3, characteπzed in that the height of the bπdges is less than 100 micrometer. 5 Cathode ray tube as claimed in claim 4, characteπzed in that the height of the bπdges is less than 70 micrometer 6. TVT apparatus compπsing a cathode ray tube with a tensioned color selection electrode in which electrode tension is applied in a direction and the color selection electrode compπses elongated openings in said direction, the openings being separated from each othei in said direction by bπdges, characteπzed in that the number of slits per row lies between 160 and 180 slits per row 7 CMT apparatus compπsing a cathode ray tube with a tensioned color selection electrode in which electrode tension is applied in a direction and the color selection electrode compπses elongated openings in said direction, the openings being separated from each other in said direction by bπdges, characteπzed in that the number of slits per row lies between 210 and 240 slits per row

Inventors: VRIENS LEENDERT, VRIENS, LEENDERT
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H01J29
IPC codes: H01J29