To attain high image resolutions over the entire screen, it is necessary to obtain a small beam spot diameter in a peripheral area as well as a center area of the screen. If a focus voltage is a constant value and adjusted so that the smallest beam spot diameter can be obtained in the center portion, overfocussing may occur in the peripheral portion of the screen, and the beam spot diameter may grow in the peripheral portion.
"Dynamic focussing", which changes the focus voltage in synchronization with the deflection of the electron beam, is a conventional method with which an optimal focus can be attained over the entire screen (see Tokukaisho 61-99249, for example). In this conventional method, first and second focussing electrodes are provided, and a voltage applied to the second focussing electrode is raised along with an increasing deflection angle of the electron beam so that a main lens formed between the second focussing electrode and a final accelerating electrode is weakened. Thus, overfocussing is compensated in the peripheral portion of the screen.
Additionally, in the above mentioned prior art disclosed in Tokukaisho 61-99249, a so-called "four-pole lens" is formed between the first and second focussing electrodes to compensate a non-axisymmetric beam spot distortion in the peripheral portion of the screen. This four-pole lens is formed by providing vertical oblong through holes in the first focussing electrode and horizontal oblong through holes in the second focussing electrode for passing electron beams, for example.
Another prior art disclosed in Japanese laid open patent application (Tokukaihei) 8-22780 is a method for increasing the beam spot diameter along with raising the current density of the electron beam, and compensating a deterioration of image resolution in the peripheral portion of the screen that is caused by a non-axisymmetric distortion of the beam spot due to a spherical aberration of the main lens. In this prior art, a tube-like intermediate auxiliary electrode is provided between the focussing electrode and the final accelerating electrode, and the intermediate auxiliary electrode is supplied with a voltage between the focus voltage and an anode voltage (voltage applied to the final accelerating electrode). Thus, a potential gradient in the axial direction of the main lens becomes gentle, so that the spherical aberration of the main lens can be reduced.
It is a first object of the present invention to raise the resolution over the entire screen by combining two such prior art methods as described above. It is a further object of the present invention to solve the problems occurring when these two prior art method are combined, that is, the shifting of the beam spot, and a difference of focussing ability between horizontal and vertical directions.