Color CRT electron gun with progressively reduced electron beam passing aperture size

An electron gun for use in a cathode ray tube (CRT) for producing a video image on a display screen, said electron gun comprising:

• a cathode for providing energetic electrons;
• a beam forming region (BFR) aligned with said cathode and disposed intermediate said cathode and the display screen for receiving and forming said energetic electrons into a narrow beam, said BFR including plural spaced first charged grids each having one or ore first aligned apertures, wherein said electrons are directed through said first aligned apertures and said electron beam increases in cross section in proceeding from said BFR toward the display screen;
• and
• an electrostatic lens disposed intermediate said BFR and the display screen and including plural spaced second grids charged by a respective focus voltage, each of said second grids having one or more second aligned apertures through which said electron beam is directed for focusing said electron beam on the display screen, said electrostatic lens including first and second dynamic quadrupoles each having a respective third grid and a respective fourth grid, wherein each of said third grids includes plural spaced apertures for passing a respective electron beam and each of said fourth grids includes a single common aperture having plural spaced aligned portions each adapted for passing a respective electron beam, and wherein each spaced aperture in each of said third grids is larger than an aligned enlarged portion of the single common aperture in an associated fourth grid when said fourth grid is disposed intermediate said cathode and its associated third grid, and is smaller than an aligned enlarged portion of the single common aperture in an associated fourth grid when said third grid is disposed intermediate said cathode and its associated fourth grid.