Beam focusing means in a unitized tri-potential CRT electron gun assembly

Claim 1

A beam focusing improvement in a CRT plural beam tri-potential in-line electron gun assembly embodying a center and two side-related gun structures for use in a color tube having a forwardly positioned cathodoluminescent screen, each of said guns having individual coordinate beam path axes therethrough with the "x" axes thereof oriented in a common horizontal plane, said gun assembly being a construction of unitized in-line apertured electrode members sequentially positioned forward of individual electron producing cathode elements to provide for each gun an initial beam forming electrode (G1) embodying a discrete substantially elongated aperture-related beam shaping configuration therein whereof the major axis of said configuration is substantially coincident with the "y" axis of said gun structure, an initial beam accelerator electrode (G2), a first high focusing electrode (G3), a low focusing electrode (G4) having a longitudinal dimension defined between rear and forward apertured portions, a second high focusing electrode (G5), and a final accelerator electrode (G6), the "G3-G6" region forming a distributed focusing lens for said beam; said improvement relating to means for particularly modifying that portion of the beam lensing field associated with the forward end of said low focusing electrode (G4) of at least one of said gun structures in said assembly wherein the inherent substantially ovate cross-sectional shaping of the beam, having a major dimension substantially coincident with the "x" axis of said gun structure, is modified to provide a focused substantially circular beam spot at the center of said screen, said improvement comprising: a pair of substantially planar metallic sideboard elements oriented in standing parallel positions in a manner to project inwardly within said low focusing electrode (G4) and being substantially perpendicular to the interior surface of said forward end thereof, one of said sideboard elements being oriented on either side of the center forward aperture in planes substantially parallel with the "x" axis of said gun structure, said orientation effecting positional adjustment of the equipotential lines in the most effective "G4-G6" area of the distributed focusing lens formed spatially within the "G3-G6" region to provide substantially symmetrical lensing for focusing said respective electron beam.