The present invention is a system that uses a single channel light meter to measure the actual red, green and blue light output response curves of a CRT monitor to the range of possible input monitor code values for each color. A computer is used to transform a desired corresponding tone scale through the corresponding response curve to obtain a look-up table or video shaper that calibrates the monitor to the desired corresponding tone scale. The transformation includes obtaining a tone scale intensity value for each possible input value and searching the actual response values for a closest match. The input monitor code value used to obtain the actual response matching intensity for the possible input value is the calibrated value displayed by the monitor when an image includes the possible input value. The system is used throughout the life of the monitor to bring the monitor back to calibration as it changes.