An optical method of detecting a liquid level is
presented that uses fluorescence radiation generated in an impurity-doped glass
or plastic slab. In operation, the slab is inserted into the liquid and pump
light is coupled into it so that the light is guided by the slab-air interface
above the liquid and escapes into the liquid just below its surface.
Since the fluorescence is generated only in that
section of the slab above the liquid, the fluorescence power will monotonically
decrease with increasing liquid level. Thus, a relationship can be established
between any signal proportional to it and the liquid level.
Because optical
fibers link the pump source and the detector of fluorescence radiation to the
sensor, no electrical connections are needed in or near the liquid. Their
absence vastly decreases the hazard associated with placing a liquid level sensor in a potentially explosive environment.
A laboratory prototype, consisting of a methyl
styrene slab doped with an organic dye, has been built and successfully tested
in water. Its response to liquid level when pumped by a tunable argon-ion laser
at 476, 488, and 496 nm, and by a blue LED, is presented and shown to be
consistent with theory. The fluorescence spectra, optical efficiency,
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