An optical detection system for detecting rain or other water droplets on the outer surface of a window and fog on the inner surface with a single photo-detector. The invention measures the accumulation of water droplets on the window by light refraction of a first light beam with droplets to redirect...http://www.google.co.uk/patents/USRE35762?utm_source=gb-gplus-sharePatent USRE35762 - Optical detection of water droplets using light refraction with a mask to prevent detection of unrefracted light 
