An object sensor is provided, which has a high object detection rate with respect
to an object such as a raindrop adhering to a detection surface, and which is capable
of detecting the presence/absence of an object with a high sensitivity. A light
source 10, a detection surface 110, a focusing lens 40, and
a plurality of photodetecting elements of a photodetecting unit 50 form
a focusing system of equal magnification that is capable of catching an image in
a size smaller than an object to be detected. Light emitted from the light source
10 is incident on the detection surface 110 via a prism 20 at
a predetermined incident angle. Without a raindrop 120, the total internal
reflection condition is satisfied as shown by a path 140 of a beam, so that
the beam is subjected to the total internal reflection. Then, the beam passes through
a prism 30, and is focused on a photodetecting surface of the photodetecting
unit 50 by the focusing lens 40. On the other hand, with a raindrop
120, the total internal reflection condition is not satisfied as shown by
a trail 130 of a beam, so that the beam passes therethrough and is not focused
on the photodetecting unit 50. A difference in signal level is determined
on the photodetecting element basis, so that an object covering the detection surface
110 is detected.
