DROPLET SENSOR

A droplet sensor includes an optical cover having a curved surface that forms a part of a spheroid, a protective film that covers the curved surface of the optical cover, a light source provided at a first focal point of an ellipse facing the curved surface, and a photodetector provided at a second focal point of the ellipse. The refractive index of the protective film is greater than the refractive index of a liquid to be detected. A sensing region is determined by a range of an incident angle at which a light beam emitted from the light source and incident onto the curved surface is totally reflected at the interface between the protective film and a gas, and is not totally reflected at the interface between the protective film and the liquid to be detected.