Abstract: Two types of optical liquid level sensor are disclosed. In the first refractive-type (20,65), light is selectively refracted by the particular fluid(s) in a plug recess between primary and secondary detectors (D.sub.1,D.sub.2). The output signal from either detector indicates whether the sensor is above or below the liquid level. The signals of the respective detectors are cross-compared and used to corroborate one another. In the second absorptive-type (100), light at a particular wavelength is passed through a body chamber (86'), which is exposed to the liquid whose level is to be sensed. Such light is selectively absorbed by the liquid. A detector (D) is arranged to measure the intensity of such light as has passed through the chamber, and to indicate the level of liquid in said chamber.