gun and a phosphor screen reference numeral 5 designates a focus electrode 6 an anode 10 an electron beam 13 a phosphor screen 14 a panel portion and 38 a main lens a space between the anode 6 and the phosphor screen 13 is field free and called a drift space the electron beam 10 subjected to focusing action by the main lens 38 moves toward the phosphor screen 10 and continues to be focused further during this travel of the electrons a diverging force is also exerted on the electrons by charges of the electrons themselves within the electron beam that is space charge repulsion the electron beam is focused into a beam of the minimum diameter d 4 on the way to the phosphor screen is spread again by the diverging force of the space charge repulsion prevailing over the focusing action by the main lens and forms a beam spot of a diameter d 1 larger than d 4 on the phosphor screen fig1 is an explanatory view of the relationship of an electron beam spot diameter versus the distance between the main lens and the phosphor screen the phenomenon explained in connection with fig1 becomes pronounced as a distance l 2 between the main lens and the phosphor screen increased as shown in fig1 and deteriorates resolution further as the distance l 2 increases the imaging magnification for projecting a virtual object in the vicinity of the cathode of the electron gun onto the phosphor screen increases the diameter of the spot formed on the.