Can salt on an optical rain gauge lens affect performance?

The optical rain gauge (ORG) by ScTI is designed to be tolerant to reduction of infrared beam intensity due to a dirty lens. Recently there is interest in long term use of optical gauges onboard buoys at sea. Because of logistics, these systems are serviced infrequently, i.e., every several months. Due to the proximity of the gauges to the sea surface, salt can be expected to be deposited on the lens. To obtain an indication of the potential for dirty lens to affect the ORG calibration, two simple experiments were conducted. In the control experiment, a sample ORG was compared to three other ORG's during natural rain events. Next a translucent mask was placed on the transmitter lens of the sample ORG and again data were collected under natural rain conditions. The mask reduced the gain of the perturbed ORG by about 30%. The perturbed ORG operated rather well in that the mask only causes a change in the gain and does not cause data drop out at low rain rates. However, the reduced gain would seriously impact an assessment of rain statistics.