To provide a method of measuring the phosphor stripes of a color cathode ray tube(CRT) that properly compensates for lack of illuminance at low cost by employing conventional lenses and ring light guide in order to improve the measurement preciseness.

A lens with a focal length of 90 mm is used and the ring light guide is arranged to make a ring light obliquely auxiliary-illuminate. The lens is offset from the ring light guide toward the color CRT side by about 10 mm, and an angle of the light directing toward the focus of the lens from the ring light with an optical axis is set to about 10°. Under the conditions described above, the measurement is carried out by photographing the phosphor stripes with a camera.