Compact moisture sensor with efficient, high obliquity optics

Compact moisture sensor (10) mounted on inner surface (30) of windshield (18) to detect moisture on outer surface (32) and control windshield wipers (20). Includes coupler (24) having collimator (37) and focuser (42) and housing (28) detachable covering coupler. Planar circuit board (26) disposed within housing (28) includes emitters (56) and detectors (58) mounted such that the axes of emission and detection are perpendicular to windshield (18) when housing (28) is secured to coupler (24). Collimator (37) and focuser (42) are disposed adjacent emitter (56) and detector (58) such that the optical axes of collimator (37) and focuser (42) form oblique angles with respect to emission and detection axes. Two emitters and two detectors are used to form four optical paths of equal length and optical efficiency. In operation, a light beam from emitter (56) enters windshield (18) at a forty-five,degree angle and is reflected back from outer surface (32) of windshield (18) to detector (59), which generates a control signal based on the amount of light reflected from the outer surface.