In CRT monitor characterization accuracy is largely affected by the brightness setting on the monitor. At the optimum brightness level, which adjusts the offsets of RGB channels to nearly zero, CRT monitors have a simple relationship between DAC count and luminance so characterization can be achieved accurately. However, since the optimum brightness levels of CRT monitors are different from one another monitor users, have to make adjustments to the monitor themselves. In this study, a simple device named, brightness optimizer, is fabricated to determine the optimum brightness level objectively. This method uses change of curvature in tone reproduction curves plotted in log-log scale, according to the changes of brightness level. In this article, the configuration of the brightness optimizer and the measurement processes are described. The repeatability and the accuracy in the measurement of the optimum brightness level for three different CRT monitors are also evaluated. (C) 2003 Wiley Periodicals, Inc.