Abstract

A liquid level detector for detecting whether a boundary surface between a liquid and a medium rises beyond a prescribed level comprises a rectangular prismatic refractor body formed of a material which only allows the transmission of a light having particular wavelengths and absorbs light having other wavelengths. The refractor body have first and second inclined flat surfaces and a base. The base of the refractor body is provided with a light-emitting element for projecting a beam of light having the particular wavelength and a photoelectric conversion element for converting the light to an electric signal. Each of these element is equidistantly spaced from the center of the base, and the optical axis of each of these elements defines an angle of 45° with each of the inclined surfaces of the refractor body. Where the first inclined surface contacts an medium, the light beam from the light-emitting element passes through the refractor body, is totally reflected from the first and second inclined surfaces, and finally enters the photoelectric conversion element. Where the first inclined surface contacts a liquid, the light beam from the light-emitting element passes through the refractor body, is refracted on the first inclined surface and enters the liquid, and the photoelectric conversion element receives substantially no light and generates no electric signal.