Patent ID: 7973916

Claim:
Apparatus comprising: a. a vessel containing liquid having a reflective surface and an index of refraction which changes with temperature, b. a lens situated in an optical path between the reflective surface and a focal plane of the lens, c. a point source to emit light through the lens toward the liquid surface, d. an array of detector elements located such that light reflected from the reflective surface passes through the lens onto the detector elements, each detector element producing a value corresponding to amplitude of incident light, e. a processor responsive to the values produced by the detector elements to calculate a center of gravity of the image formed on the detector elements, wherein the calculated center of gravity is dependent on inclination of the vessel, and f. a sensor producing a signal dependent on ambient temperature, wherein the processor is responsive to the signal for applying a temperature correction which compensates for changes in the index of refraction of the liquid when calculating the center of gravity of the image formed on the detector elements.