The characteristics of image quality of a color CRT (cathode ray tube) monitor are studied by changing the contrast and brightness levels. The resolution is assessed by SQRI (square root integral) measured at 9 different combinations of the contrast and brightness levels. The chromaticity coordinates and luminances of red-green-blue channels as a function of the digital value are measured at these combinations and the relationships among the constant-channel chromaticity, color gamut, maximum luminance of a white point with the channel independence are analyzed. From the results, the optimized combination of levels is obtained.