Post-deflection focusing type colour cathode ray tube

Claim 1

In a post-deflection focussing type colour cathode ray tube of the type comprising an envelope including a neck, a funnel and a face plate, an electron gun contained in the neck for emitting an electron beam, means for controlling the travel of said electron beam, a shadow mask electrode upon which said electron beam impinges having a plurality of perforations and supported by a supporting frame adjacent the face plate between said electron gun assembly and said face plate, a tri-color phosphor film provided on the inner surface of said face plate to luminesce in color when impinged upon by said electron beam which has passed through the perforations of the shadow mask, said luminescence subject to halation from secondary electrons, a metal back electroDe formed on said phosphor film for creating a high intensity electric field between said shadow mask electrode and said metal back electrode, and an electrode film formed on the inner surface of the funnel, the improvement which comprises a source of potential, and means connecting said source between said shadow mask electrode and said electrode film for applying a potential to said electrode film about 30 to 800 volts higher than the potential of said shadow mask electrode to absorb secondary electrons at said electrode film without unacceptable degradation of color purity due to drift of the electron beam impinging on said phosphor film.