1. Field of the Invention
This invention relates to a shadow mask color cathode ray tube (CRT) with enhanced resolution and/or brightness.
2. Description of Related Art
As is well known, color CRT's normally have three electron guns producing so-called `red`, `green` and `blue` electron beams which are used respectively to stimulate red, green and blue phosphor elements on the CRT faceplate. By stimulating these three primary--color phosphors by different amounts and in different combinations, any color mix can be displayed on the screen. Multi-beam color 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 are 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 across the horizontal dimension of the CRT (i.e. the scan line dimension in the case of a raster scan CRT), either provided as circular holes or elongated slots through which the beams are directed onto the phosphors. Each aperture has three phosphor elements associated with it, namely red, blue and green emitting elements for each scan line. The `red,` `blue` and `green` electron beams are directed through the apertures at different angles so that each stimulates the appropriate phosphor. Convergence circuits and assemblies ensure that at any one time the three beams are coincident at the phosphor screen. 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 element.
A known problem with such color CRTs is that the brightness level of the three different color phosphors is different for the same beam current. Typically, the brightness of the red phosphor is significantly less than that of the blue or green phosphors for the same beam current. In order to achieve an adequate white color point (as defined by a selected point on the CIE chromaticity diagram), it has been common practice to drive the three guns with different value beam currents in order to compensate for the different brightness levels of the phosphors. A consequence and disadvantage of this is that the gun with the largest beam current has a reduced performance in terms of spot size and cathode life and a mismatch of resolutions can occur since spot size is dependent on beam current.
One way of resolving this problem is to vary the size of the phosphor dots or stripes on the CRT faceplate so that the integrated light emission from each phosphor element is constant for the same beam current. Accordingly, the smallest elements are composed of the phosphor exhibiting the highest luminance characteristic and the larger elements are composed of the phosphors exhibiting the lower luminance characteristics. A process for making a CRT screen in which different size phosphor elements are utilized to compensate for different luminance characteristics is described in U.S. Pat. No. 2,687,360. In this example, the relative areas of the different phosphor types are such that the integrated brightness from each element is substantially the same. A further example is to be found in European Pat. No. 0129620 in which the lower brightness efficiency of the red phosphor is compensated for by increasing the size of the red dots or stripes relative to the size of the blue and green dots or stripes.
A disadvantage of this approach is that any increase in the size of a phosphor dot or stripe also necessarily reduces the purity margin. (Purity margin in this context is defined as the distance between the edge of a beam projected through an aperture in the shadow mask onto its associated phosphor dot or stripe and the nearest adjacent phosphor dot or stripe of a different color). Fidelity of color is an important requirement of CRTs in general and particularly for CRTs used in the data and graphic display terminals. Thus, although it is desirable to balance the phosphor emissions in the manner described above it is important to ensure that the performance of the CRT is not degraded in other respects as a consequence.
It is therefore an object of the present invention to balance the phosphor light emission by means of the technique described above without any loss in purity margin. Additionally, as will be shown herein, not only does the modification overcome the problem of reduced purity margin, but CRTs incorporating the invention can be provided with a higher screen resolution and/or brightness for a given level of screen processing cost and technology.