From AMS Glossary
The onset of condensation is measured by bouncing a light beam off the mirror and recording when the reflection changes from specular to diffuse. The mirror can be cooled and heated by a variety of methods, one of which is thermoelectrically. Electronic circuits continuously monitor the light reflection and adjust the mirror temperature to maintain it at the dewpoint, even while humidity is varying. These devices often have high absolute accuracy and precision, but somewhat slow response.