PURPOSE: To reduce the variance of cut-off adjustment and to improve the precision with respect to the cut-off adjusting method for projection television and an illuminance meter.

CONSTITUTION: The illuminance meter is prepared which is provided with a cap 3 covering the whole of a CRT barrel where a sensor 2 to sense the illuminance is provided in the center part, a display device 1 on which the illuminance is displayed, and a cable cord 4 through which data is transmitted between the sensor 2 and the display device 1, and the method consists of the stage where the potential of the cathode electrode is set to the pedestal level, the stage where the CRT barrel is covered with the cap 3, the stage where the value of the illuminance is seen to adjust the screen VR., and the stage where the value of the illuminance at the time of the start of light emission of each CRT tube face of R, G, and B is adjusted. Thus, tracking of the white balance in a considerably low luminance is freely set with less variance.