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(11) | EP 0 129 620 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Colour cathode ray tube with improved phosphor pattern |
| (57) A colour cathode ray rube has an improved phosphor pattern to increase the brightness
ofthe red. By increasing the size of the red phosphor dots the lower efficiency of
the red phosphor can be compensated for. By decreasing the size of the blue and/or
green phosphor dots (preferably the green more than the blue) the display contrast
can be maintained. The overall perceived purity is also improved by changing the relative
sizes of the phosphor dots. The invention is also applicable to colour CRTs using
phosphor stripes. |
[0002] As is well known, colour cathode ray tubes (CRTs) normally have three electron guns producing "red", "green", and "blue" electron beams which are used to stimulate red, green and blue phosphors on the CRT faceplate. By stimulating these three primary-colour phosphors by different amounts, any colour can be displayed on the screen since the human eye is not able to resolve the individual phosphor patterns at normal viewing distances.
[0003] To obtain a pure colour, the various intensities of the three primary colours need to be balanced relative to one another.
[0004] Multi-beam colour cathode ray tubes are of two types, either delta gun where the three guns are placed at the apexes of a triangle or in-line gun where the three guns located along a line normally parallel to the direction of line scan. A shadow mask is employed which consists of a large number of apertures through which the beams are directed onto the phosphors. Each aperture has three phosphor areas associated with it, namely red, blue and green emitting areas. The "red", "blue" and "green" electron beams are directed through the aperture at different angles so that each stimulates the appropriate phosphor. Convergence circuits and assemblies ensure that at any one time the three beams converge at the same aperture. Purity circuits and assemblies ensure that the beams pass through the apertured shadow mask at the correct angle so as to stimulate the correct phosphor.
[0005] The pattern of phosphor associated with each aperture depends on whether the CRT is a delta-gun type or an in-line gun type. The invention is applicable to both types. Indeed although the invention will be described in terms of multi-beam CRTs, it is also applicable to that type of colour CRT known as the Sony Trinitron tube (Sony and Trinitron are trade marks of Sony Corp) in which an electron beam is raster scanned horizontally through vertically extending slots in the shadow mask onto vertically extending phosphor stripes arranged in red, blue and green triplets.
[0006] Whatever the colour CRT technology, the individual areas of phosphor material are surrounded by so-called black matrix material. The purpose of the black matrix (normally formed from colloidal graphite) is to increase the contrast of the CRT display. The relative amounts of phosphor and black matrix materials will affect the amount of ambient light reflected from the screen of the CRT.
[0007] One problem is that the efficiency of the red-emitting phosphor, particularly long persistence red phosphors, is low compared to the green and blue phosphors.
[0008] Much effort has been put into developing higher efficiency red phosphors to obtain a better balance. However in typical colour CRT employing long persistence phosphors the three phosphors will have the following efficiencies (measured at 250 uA beam current):-
[0010] These figures give the photopic efficiency (i.e. including the photopic response of the human eye).
[0011] An object of the present invention is to provide a colour cathode ray tube having an increased brightness of the red whilst maintaining the contrast.
[0012] To this end, a colour cathode ray tube in accordance with the present invention comprises red, blue and green primary phosphors adapted to be stimulated by an electron beam incident thereon through a shadow mask, and is characterised in that the area of each red phosphor element is greater than the area of the blue and green phosphor element thereby to compensate for the lower efficiency of the red phosphor relative to the blue and green phosphors.
[0013] Increasing the size of the red phosphor areas alone would increase the brightness but would decrease the contrast since the area of black matrix would be decreased. It is preferred that the area of phosphor be maintained by correspondingly reducing the area of one or both of the other phosphors. From an efficiency point of view, it is preferred that the green phosphor area be reduced in size, but from considerations of purity it is preferred that both the blue and green be reduced.
[0014] The invention will now be described, by way of example, with reference to the accompanying drawings in which:-
Figure 1 shows the conventional phosphor dot pattern for an in-line CRT;
Figure 2 shows the improved phosphor dot pattern in accordance with the present invention;
Figures 3 and 4 show corresponding patterns for a delta-gun colour CRT; and
Figures 5 to 11 are charts in the form of histograms showning purity performance for various combinations of phosphors and phosphor dot sizes.
[0015] Referring now to Figure 1, red (R), green (G) and blue (B) phosphor dots are arranged in triplets (T) to be stimulated by an electron beam (E) as it scans across the face of the CRT. Black matrix material (M) surrounds the phosphor dots. The size of the electron beam (E) spot corresponds to the size of the apertures in the shadow mask. Those skilled in the art will appreciate that the number of phosphor spots illuminated by the electron beam at any instant will depend upon the beam diameter, the shadow mask aperture diameter and the pitch of these apertures. Of course provided the beam is correctly adjusted for purity, it will only stimulate phosphors of one colour at any one time. Which colour is stimulated depends on the angle at which the beam passes through the shadow mask apertures.
[0016] A problem with the construction shown in Figure 1 is that the efficiencies of the three phosphors vary widely. Thus in a particular CRT employing long persistence phosphors, the following efficiencies were measured with 200 µA beam current:-
[0018] The following efficiencies were measured (also at 200 µA beam current) for a CRT employing short persistence phosphors:-
[0019] In accordance with the present invention, the brightness of the red is increased by increasing the size of the red phosphor dots. This is illustrated in Figure 2. By increasing the red phosphor spot diameter by n%, the red brightness will be increased by n2%. By increasing the size of the red phosphor spots, the area of black matrix material will be correspondingly decreased giving a decrease in the contrast of the CRT display. This can be compensated for by decreasing the size of one or both of the blue and green phosphor dots. As shown in Figure 2, it is preferred that the size of both the green and blue dots is reduced in order to improve purity considerations.
[0020] Approximately, therefore, by increasing the size of the red phosphor spots by 10% and decreasing the size of the green and blue spots by 5% each, the brightness of the red phosphors can be increased by just over 20% and that of the green and blue phosphor decreased by approximately 10% each.
[0021] Figures 3 and 4 are corresponding views to Figures 1 and 2 for the phosphor dot patterns for a delta gun CRT. By increasing the size of the red phosphor dots (R) and decreasing the size of the green and blue phosphor dots (G,B), the lower efficiency of the red phosphor can be compensated for without decreasing the contrast of the display. Those skilled in the art of CRT manufacture will appreciate that the ability to increase the red phosphor spot size and decrease the green-and blue phosphor size does not result in any significant increase in manufacturing cost and is well within current manufacturing capabilities.
[0022] Although the invention has been described with reference to colour CRTs employing phosphor dot patterns, its principles are also applicable to CRTs using phosphor stripes in which the widths will be adjusted to account for the differing efficiencies. It should be noted that in Figures 2 and 4 the green spots are shown as smaller than the blue spots. This is preferred but not essential. Improvements are obtained, as will be seen below, if the blue and green spots are reduced by the same amounts.
[0023] At first sight it might appear that increasing the size of the red phosphor spot, and hence reducing the spacing between it and the shadowed green and blue electron beams might lead to a reduction in the purity margin, that is the possibility of illumination of the wrong colour phosphor dot being increased. Analysis shows, however, that far from giving a problem in this area, increasing the red dot size can improve the overall purity performance.
[0024] The brightness produced by the wrong colour phosphor dot is a function not only of screen geometry, but also of the relative phosphor efficiencies. For example, if a red raster is shifted so that some green is illuminated, the amount of green brightness is quite high (because the red raster beam current is high and green efficiency is high). Conversely, if a green raster is shifted by the same amount, so that some red is illuminated, the amount of red brightness is much lower (because green beam current is lower and red efficiency is lower).
[0025] The chromaticity change produced by this shift is therefore different for the two rasters. The human visual perception of the chromaticity shifts will be different for different primary colours (because the human ability to discriminate chromaticity changes is different for different dominant wavelengths). For example, the human eye is less sensitive to chromaticity changes in the green region of the spectrum than the red.
[0026] Thus a complete analysis of the visual effect of purity loss requires the following factors to be considered:-
Screen geometry.
Primary phosphor chromaticities.
Primary beam currents (determined by white chromaticity).
Human visual chromaticity discrimination in different regions of colour space.
[0027] The steps in the analysis are as follows:-
1. From the basic screen geometry, select a direction and magnitude of beam shift, and calculate the loss of illuminated phosphor area of the primary colour, and the gain in illuminated phosphor area of the second colour.
2. From the known beam currents, and the relative efficiencies of the two phosphors, calculate the percentage brightness loss and gain.
3. Take the two primary chromaticities, and work out the individual (X,Y,Z) tristimulus quantities. Multiply these by the percentage brightness loss/gain, and add the new individual X,Y,Zs. Then recombine to give the new x,y chromaticity.
4. Select a suitable form of the CIE (Commission Internationale de 1'Eclairage/International Commission on Illumination) colour space diagram - which allows chromaticity changes in different colour regions to be easily compared - and plot the chromaticity shifts for all the various primary raster colours and shifts.
5. Measure the chromaticity shift lengths on this version of the CIE diagram, so giving a measure of the human visual perception of each chromaticity shift.
6. Present these lengths in bar chart form, to allow easy comparison of the various chromaticity shifts.
Conventional Long Persistence CRT
Conventional Short Persistence CRT
[0034] 2. Phosphor combinations of different efficiency:- Most of these varying efficiencies were produced by varying the proportions of the components of the phosphor mixes. However, the P39 green phosphor is a new P39 chemistry available under the so-called designation "New improved P39" from the various phosphor vendors. The efficiency comparison is with the standard long persistence phosphors referenced above.
Example 1
Example 2
Example 3
Example 4
Example 5
[0041] Although the basic CIE x,y diagram together with the "MacAdam Ellipses" show the minimum perceptible chromaticity differences that can be detected in various colour regions, the considerable variations in sizes of the ellipses makes this diagram very difficult to use for comparing chromaticity differences in different areas. An alternative is the CIE u,v diagram. Although the MacAdam ellipses show less variation in individual colour regions, there is still a large variation between widely separated regions (e.g. red and green). The Farnsworth diagram, a non-linear transformation of the CIE x,y diagram, is preferred: although the x,y axes are curved, the discrimination ellipses become nearly circular and of constant size over the whole area. Thus this diagram can be used to plot out the analysis results, and the length of each chromaticity shift measured directly from the graph, for final presentation in bar graph form. It should be noted that the absolute magnitude of these measured shifts is not important in this analysis - what we are concerned with is the relative magnitude of one shift compared to another.
[0042] A horizontal beam shift of 0.0675mm was taken in all cases, and results obtained for the various combinations of phosphor efficiencies given above as Examples 1 to 5 and also for phosphor dot sizes of:-
1. Standard. All dots 0.125mm diameter.
2. With red dot 0.136mm, green/blue dots 0.115mm.
[0043] For comparison the conventional short persistence CRT was computed for standard dot sizes. The results are shown in Figure 5. In Figures 5 ,o 12 the letters R, B and G on the abscissa indicates whether the red, blue or green raster is being considered. The letters R, B and G within the bar chart indicates the direction of shift, i.e. towards the red, blue or green respectively. In Figures 6 to 11, the bars on the left hand side show the results for standard-sized (0.125mm) dots whilst the results on the right hand sides are for red phosphor dots increased to 0.136mm and the blue and green phosphor dots reduced to 0.115mm. In Figure 12, although the left hand side shows the results for standard-sized spots, the right hand side shows the results for the red phosphor spot being increased to 0.15mm and the blue and green phosphor dots reduced to 0.11mm and 0.102 respectively.
[0044] It can be seen from Figure 5 that the red raster is much more sensitive to chromaticity changes than the green or blue rasters, even though purity margin is the same for all three colours. This demonstrates clearly that visually perceived chromaticity changes are the result of the interaction of all the parameters previously discussed. This result confirms what can be observed in practice - when varying magnetic fields are applied to a display, almost always purity errors are observed on the red raster before there is any visually detectable change in green or blue.
[0045] Figure 6 shows the conventional phosphor arrangement (long persistence) applied to an in-line CRT. Figures 7 to 11 show the results for Examples 1 to 5 above. These results show how different phosphor mixes can give an improvement in purity performance, as well as their better efficiency: changes in the phosphor dot sizes gives a further improvement in overall purity performance. Figure 9 shows red persistence reduced to 12.5%, and shows that purity performance is maintained.
[0046] Figures 10 and 11 are interesting, because they show that improving phosphor efficiency alone can degrade purity performance. Thus Figure 10 shows the result of blue phosphor reduced to 20% persistence, and Figure 11 the result using "new improved" P39 green phosphor. In each uase the ied raster purity performance (with standard sized dots) is reduced but in both cases the performance is improved by changing the phosphor dot sizes in accordance with the present invention.
[0047] A different combination of phosphor dot sizes was taken for Figure 12 in an attempt to optimise purity performance. Figure 12 shows the result of using 0.15mm red phosphor dots, 0.102mm green phosphor dots and 0.11mm blue phosphor dots. As can be seen, the purity performance is as good as obtained anywhere, and this size dot would give a 44% increase in red brightness over the standard design. It is interesting to note that even with this extreme example of dot size increase, purity performance is better than conventional long or short persistence CRTs with standard phosphor dot sizes.
1. A colour cathode ray tube comprising red, blue and green primary phosphors (R,G,B)
adapted to be stimulated by an electron beam (E) incident thereon through a shadow
mask, characterised in that the area of each red phosphor element (R) is greater than
the area of each of the blue (B) and green (G) phosphor elements thereby to compensate
for the lower efficiency of the red phosphor relative to the blue and green phosphors.
2. A colour cathode ray tube as claimed in claim 1, in which the area of each green
phosphor element (G) is smaller than the area of each blue phosphor element (B).
3. A method of manufacturing a colour cathode ray tube as claimed in either preceding
claim in which dot-shaped phosphors (R,B,G) are found on the face of the cathode ray
tube surrounded by black matrix material (M), characterised in that the relative sizes
of the red, blue and green phosphor dots (R,B,G) and the dot pitch are chosen so as
to increase the brightness of the red phosphor and the perceived colour purity performance
whilst maintaining the contrast due to the black matrix material.