A centrifugal blood separation system comprising a rotor, a light source, an optical sensor, a control system, a separation vessel, and an optical cell on the separation vessel. The optical cell has a first extraction port extending radially outwardly into the optical cell, a red blood cell extraction port downstream from the first extraction port and extending into the optical cell beyond the first extraction port； and a dam between said first extraction port and said red blood cell extraction port, having an upper edge and a lower edge, wherein the first extraction port and the red cell extraction port are radially between the upper edge and the lower edge of the dam. Also, a first extraction port having a bore having a first diameter, a lumen having a second diameter smaller than the first diameter, and a frustro-conical passageway coupling the bore to the lumen.
