A LOW-COST, HIGH-SPEED, NEAR-INFRARED HYGROMETER

We describe a simple near-infrared hygrometer based on a broadband lig ht-emitting diode (LED) centered at 1320 nm. Light from the LED is col limated and passes through a rotating optical-filter assembly and a sa mple cell. The water vapor concentration is measured using a synchrono us detection scheme. This compact and robust instrument has a response time of 0.15 s and can reliably detect short-term absolute humidity c hanges as small as 0.6 mg/L (which equate to 0.02% absorption); the lo ng-term stability of the absolute humidity measurement over a period o f 4 days is approximately 1 mg/L. These performance characteristics ar e particularly well suited to measurements associated with human respi ration, and we present results in this humidity regime. (C) 1995 Ameri can Institute of Physics.