A centrifuge for continuously separating the various constituents of blood or other biological fluids includes a rotating bowl having high-G and low-G walls. An inwardly directed ramped surface on the high-G wall interacts with an interface region between the separated fluid constituents to provide an optically detectable indication of the position of the interface between the high-G and low-G walls. An optical sensor sensing each passage of the ramped surface past a point develops a changing signal indicative of the interface position. Signal processing circuitry responsive to the signal measures such signal parameters as peak amplitude, signal area and signal shape. By so monitoring these signal parameters, a better indication of actual interface position between the high-G and low-G walls can be obtained. This, in turn, results in better control over centrifuge operation and improved separation of the desired fluid components.