The optical principle on which the rain sensor functions
remains unchanged. An LED emits light in such a way that
when the windshield is dry almost the entire amount of light
is reflected onto a light sensor. When the windshield is wet,
the reflective behavior changes: the more water there is on
the surface, the less light is reflected. In the new rain
sensor, infrared light is used instead of conventional
visible light. This means that the sensor can be mounted
in the black area at the edge of the windshield and cannot
be seen from outside.
(http://www.autospeed.com/cms/article.html?&A=1168)