A low-cost, high-speed, near-infrared hygrometer

We describe a simple near-infrared hygrometer based on a broadband light-emitting diode (LED) centered at 1320 nm. Light from the LED is collimated and passes through a rotating optical-filter assembly and a sample cell. The water vapor concentration is measured using a synchronous detection scheme. This compact and robust instrument has a response time of 0.15 s and can reliably detect short-term absolute humidity changes as small as 0.6 mg/L (which equate to 0.02% absorption); the long-term stability of the absolute humidity measurement over a period of 4 days is approximately 1 mg/L. These performance characteristics are particularly well suited to measurements associated with human respiration, and we present results in this humidity regime.