Abstract: A TFT LCD display has a PEL matrix in which the drain lines of the different TFTs are supplied with different drain voltages to achieve a preset number of gray scales. The different drain voltages are set during calibration through use of a test PEL having substantially the same characteristics as the PELs viewable by a user. The characteristics of the test PEL are first measured and the values of drain voltages for achieving the different gray scales are mathematically derived from the measurements. The specific manner in which this is done is by placing a photodiode next to the test PEL and measuring the photodiode outputs corresponding to different drain voltage inputs to construct a unique transmissivity versus drain voltage for the subject monitor.