Abstract: An optical sensor (40) that provides a monotonic level-dependent output signal that varies smoothly and at an easily detectable rate over the entire length of the sensor includes first and second spaced waveguides (45, 50) extending along an axis, with each extending between its respective first and second ends. The first waveguide does not have surfaces contacting the liquid along the first waveguide's length. The second waveguide has at least a longitudinally extending portion of an outer surface in optical contact with the liquid. One or more web portions (50a, 50b, 50c) extend along and between both waveguides so that some of the light traveling along the first waveguide is coupled through the web portion(s) into the second waveguide. Light is injected into an end of the first waveguide (52), allowed to travel along the first waveguide, with a portion leaking into the second waveguide. Light is detected (55), with the amount representing the level of liquid.

Abstract: Ice in particular and also other substances such as water and glycol-containing anti-icing fluids can be detected and distinguished by means of differences in their optical index of refraction. A plurality of beams at different angles of incidence are multiplexed onto the reverse side of an optical surface which is embedded in a surface to be monitored or otherwise positioned in a sampling environment. The reflected beams at the different angles are measured and refractive indexes calulated form the measured reflected beams. If the calculated refractive indexes are the same at all angles, then the substance is positively identified. If the values do not agree, then there is partial coverage, and the substance may be identified by comparison with predetermined values. The system is applicable to aircraft, where in-flight and pre-flight monitoring of icing conditions and surface contamination is needed.