Abstract: In contrast to the non-linear systems used in certain past infrared thermometers, the present invention uses an empirical data set to determine patient temperature. The empirical data set provided by the present invention represents actually measured thermometer sensor outputs over a substantial number of target and ambient temperature points within the thermometer's operating range. The empirical data set is collected during a testing process, and is stored in a non-volatile memory within the thermometer. At temperature measuring time, the thermometer accesses the appropriate cell in the non-volatile memory to determine temperature. The substantial size of the empirical data set eliminates guesswork and estimation--since the most accurate indication of how a thermometer will perform under certain conditions is a record of how it previously performed under those same conditions.