There are several different faceplate contours presently used in CRT's having 4.times.3 aspect ratios. The two most common contours are spherical and cylindrical. Other contours in use include biradial and more complex variations of biradial contours. Recently, development of tubes having aspect ratios of 16.times.9 has begun. Presently, there is a need for a design of a faceplate contour for CRT's having 16.times.9 aspect ratios that will meet certain requirements, such as those needed for high definition television (HDTV).
A cathode-ray tube, such as a color picture tube, must have several features if it is to be useful for HDTV. First, the faceplate contour of such tube should be as flat as practicable. The tube must have sufficient resolution to meet any future HDTV standard. The tube also must have good color purity and white uniformity at high electron beam current density. It is desirable that the tube have an optimized raster geometry to eliminate the need for extra circuitry to correct for raster distortion. The tube should have good implosion protection, while using glass having minimum thickness to reduce cost and tube weight. Finally, the tube should be usable for both line and dot screens.
The above features are somewhat related and have an effect on faceplate contour and on faceplate panel design. (A faceplate panel includes a faceplate as well as a peripheral sidewall that extends from the faceplate.) Some of the desired features are inconsistent with other features in that, in providing for one feature, another feature is adversely affected. The present invention provides a faceplate contour that is a compromise to ensure that all of the above features are attainable to some extent, although any particular feature may not be optimized.
In the present specification and claims, the term "equivalent radius" is used. Use of this term is not meant to imply that the contour curvature of any cross-section of a faceplate is circular. Such contours are more complex and can only be defined by the equations presented herein As used, the term "equivalent radius" indicates a circle that touches the center of a faceplate and the extremes of the faceplate at the border of the viewing screen.