Droplet sensor

A droplet sensor comprising:

• an optical cover having an ellipsoid surface that is a portion of a spheroid;
• a light source disposed at or in proximity to a first focal point of the ellipsoid surface;
• and
• a light detector disposed at or in proximity to a second focal point of the ellipsoid surface,
• wherein the ellipsoid surface is an effective detection area configured to reflect light emitted from the light source toward the light detector, and an amount of light reflected by the effective detection area changes in accordance with adhesion of droplets on the ellipsoid surface,
• wherein the optical cover has a curved surface that is connected to an outside of the effective detection area and having a curvature greater than a curvature of the ellipsoid surface,
• wherein a tangent to an imaginary curved surface at a boundary and a tangent to an imaginary ellipsoid surface at the boundary are the same, the imaginary curved surface being formed by extending the curved surface beyond the boundary, the imaginary ellipsoid being formed by extending the ellipsoid surface beyond the boundary, the boundary being a boundary between the curved surface and the ellipsoid surface,
• wherein in the optical cover, a first space having a sphere surface centered on the first focal point and a second space having a sphere surface centered on the second focal point are formed, and
• wherein the second space includes, at an interface with the optical cover, an optical path change portion that changes an optical path of extraneous light entering the sphere surface from outside the optical cover and traveling toward the second focal point.