Abstract: In electron optical system, electron beams emitted by cathode will be focused and deflected by electron optical system, and then it will hit the screen with small size. Because deflection system has deflection aberration, the electron beam that is focused into one point in the centre of screen will not be focused well in the corner of screen, and it will become a circle spot. Because decison of the spot quality on the screen is ac, this paper described the hay research for electron optical syStem quality.The spot size on screen is changed with the changing of screen location, so the' electron optical system quality is receded fuzzily when screen location is changed. According to these fuzzy decisions, a best structure of electron optical system can be founded.

Key words: focus, deflect, spot on the screen, fuzzy, egneral decision of two grades