Abstract: A fluid level and condition detector system including a multimode optical waveguide for insertion into a medium including at least a first fluid whose level is to be monitored, a source of radiation for introducing into one end of the waveguide radiation in a plurality of modes, and means for detecting the difference in radiation flux transmitted by the waveguide with the waveguide covered by the first fluid over its equilibration length, with the waveguide covered by a second fluid of lower index of refraction over its equilibration length, and with the waveguide covered partially by each fluid over its equilibration length and for detecting the difference in radiation flux transmitted by the waveguide due to the combined effects of the change in index of refraction and change in optical absorption coefficient of the first fluid covering at least a portion of the waveguide.