Abstract: A chilled mirror hygrometer may comprise a thermoelectric module, a polished mirror surface, two light emitting diodes, and a single photo-transistor or optical detector. A software controller may be employed to rapidly alternate between illuminating the two light emitting diodes. The software controller may allow the single photo-transistor or optical detector to rapidly alternate between observing the light from the first light emitting diode or the second light emitting diode. The chilled mirror hygrometer may further comprise a wand portion and an electrical housing.