Near-Infrared Extinction in Rain Measured Using a Single Detector System

The performance and operation of an optical device to accurately measure extinction due to rainfall over a 100 m sample path is described. A collimated beam from an infrared light-emitting diode operating at 0.94 mu m is used as a sensing beam. A PIN diode detector receives reference and sample signals alternately in a switch arrangement using a beam splitter and mirrored chopper wheel. Demultiplexing and phase sensitive detection are used to separate and demodulate the sample and reference signals. The experimental results are in agreement with theoretical predictions and theoretical results obtained for rainfall rates up to 90 mm h(-1). Extinction calculations based on the recent theoretical treatment of Ulbrich and Atlas produced a best fit to the experimental results.