The measurement system incorporates an LED (21) sending light through a first optical fiber (22) to a Y-junction (23). Light travels through a second optical fiber (24) to a measurement head (25) containing an elongated cylindrical graduated refractive index lens (26). Light spreads out from the end of the fiber and is brought to a focus (29) just in front of a flexible membrane (8). Light reflected from the membrane (30) passes back through the lens. Some light passes back through the second optical fiber to the Y-junction and to a photodetector (33) connected to a signal interface (34) and a signal processing circuit (35).
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