A centrifuge blood processing system for separating fluid components, said system comprising: a separation chamber arranged to rotate about a central rotation axis, said separation chamber having an outflow passage, at least one pump (158, 160, 162) regulating fluid flow in said separation chamber; a light source in optical communication with said centrifuge blood processing system, said light source providing an incident light beam for illuminating an observation region on said centrifuge blood processing system and a viewing region on said outflow passage; a first detector in optical communication with said separation chamber to receive and detect said light from said observation region; a second detector in optical communication with said outflow passage; a computational apparatus distinguishing one or more phase boundaries in said observation region and further distinguishing fluid composition in said viewing region as a function of light intensity received from said viewing region; a controller regulating speed of said at least one pump (158, 160, 162) or of said separation chamber in response to signals from said computational apparatus; characterised in that said system is arranged to calculate: (a) a short-period arithmetic average A of intensity in said outflow passage; (b) a longer-period median average B of intensity in said outflow passage; and (c) a weighted combination of arithmetic average A and median average B; wherein said system is arranged to bias control in favour of said median average B when a relatively steady state has been achieved. a separation chamber arranged to rotate about a central rotation axis, said separation chamber having an outflow passage, at least one pump (158, 160, 162) regulating fluid flow in said separation chamber; a light source in optical communication with said centrifuge blood processing system, said light source providing an incident light beam for illuminating an observation region on said centrifuge blood processing system and a viewing region on said outflow passage; a first detector in optical communication with said separation chamber to receive and detect said light from said observation region; a second detector in optical communication with said outflow passage; a computational apparatus distinguishing one or more phase boundaries in said observation region and further distinguishing fluid composition in said viewing region as a function of light intensity received from said viewing region; a controller regulating speed of said at least one pump (158, 160, 162) or of said separation chamber in response to signals from said computational apparatus; characterised in that said system is arranged to calculate: (a) a short-period arithmetic average A of intensity in said outflow passage; (b) a longer-period median average B of intensity in said outflow passage; and (c) a weighted combination of arithmetic average A and median average B; (a) a short-period arithmetic average A of intensity in said outflow passage; (b) a longer-period median average B of intensity in said outflow passage; and (c) a weighted combination of arithmetic average A and median average B; wherein said system is arranged to bias control in favour of said median average B when a relatively steady state has been achieved.