Color projection tv systems

Abstract

A projection television system having three different primary color pitcure tubes in an in-line configuration, one centrally located on the projection screen axis and the other two on opposite sides of the axis. In order to compensate for the defocussing which these off-axis locations produce at the edges of the projected image, the off-axis picture tubes are rotated relative to the optical axes of their respective focussing lenses, thus rotating the corresponding projected images into coincidence with the projection screen. The off-axis positioning also produces color shading by making the square law fall-off in illumination vary across the projection screen, and also by producing a trapezoidal distortion which is compensated by pre-distortion which introduces additional color shading effects. The first effect is reduced by making the lenses tangent or nearly tangent to each other, thus minimizing the angular deviation of the projection axes. The second effect results from the use of slanted CRT electron guns to achieve the compensatory trapezoidal pre-distortion of the off-axis object images, and is corrected by laterally offsetting these object images (CRT rasters) from the optical axes of their focussing lenses. This off-set can be achieved by laterally displacing the entire picture tube, or by laterally displacing the electron gun relative to the tube faceplate, or by magnetically deflecting the electron beam. The degree of offset is preferably such as to equalize the distances of all object image corner points from the lens axis. The offset then must itself be compensated by rotating the off-axis lenses so that their optical axes are underconverged.