These instruments feature a sensor which thermo-electrically chills a mirror surface until dew or frost begins to form, providing the most fundamental measurement of dew or frostpoint. Formation of dew or frost is precisely monitored by an electro-optical system; measurements are traceable to national and international standards. To account for the buildup of contaminants on the mirror surface, the instruments incorporate Dynamic Contamination Correction (DCC). Periodically, the instrument automatically heats the mirror to vaporize the accumulated dew or frost, recalibrates itself for altered reflectivity due to contaminants, and then returns to normal measuring. Instruments capable of measuring dewpoints from -148F to +266F with accuracies to +/-0.18F are offered.