This industrial-grade optical rainfall measuring device is intended to be used for wiper control, drop detection, condensation sensing and irrigation control.
It uses beams of infrared light within a plastic lens about the size of a tennis ball. It is maintenance-free: with the round surface of the lens, there are no collections of debris, no moving parts to stick, or water-pathways to clog.
This is a model comparison.
More information is available on the website of Hydreon.