A centrifuge blood processing system for separating fluid components comprising: a separation chamber arranged to rotate about a central rotation axis, said separation chamber having an optical reference mounted thereon, a first detector in optical communication with said separation chamber to detect a plurality of observations from an observation region (202, 204) on said separation chamber from rotation to rotation; a computational apparatus arranged to distinguish said optical reference and to establish reference co-ordinates for gathering data from said observations of said observation region (202, 204) as detected by said first detector, wherein said optical reference comprises at least two non-parallel sides (232, 234) and a known angle between those sides; and characterised in that said computational apparatus is arranged to: determine an observed first edge and an observed second edge by collecting a set of data points measured by the first detector for each edge, and computing a line for each edge through the points; compute an error measurement for the line of said first and second edge; select the line with the least error as the referent line; recognize a first side represented by the referent line and calculate a dependent line for a second side according to said known angle between said first side and said second side, wherein said dependent line is a mathematical construct created at the known angle between the first and second sides; calculate an error measurement for the dependent line; and reject an observation of said separation chamber if the error measurement exceeds a selected maximum. a separation chamber arranged to rotate about a central rotation axis, said separation chamber having an optical reference mounted thereon, a first detector in optical communication with said separation chamber to detect a plurality of observations from an observation region (202, 204) on said separation chamber from rotation to rotation; a computational apparatus arranged to distinguish said optical reference and to establish reference co-ordinates for gathering data from said observations of said observation region (202, 204) as detected by said first detector, wherein said optical reference comprises at least two non-parallel sides (232, 234) and a known angle between those sides; and characterised in that said computational apparatus is arranged to: determine an observed first edge and an observed second edge by collecting a set of data points measured by the first detector for each edge, and computing a line for each edge through the points; compute an error measurement for the line of said first and second edge; select the line with the least error as the referent line; recognize a first side represented by the referent line and calculate a dependent line for a second side according to said known angle between said first side and said second side, wherein said dependent line is a mathematical construct created at the known angle between the first and second sides; calculate an error measurement for the dependent line; and reject an observation of said separation chamber if the error measurement exceeds a selected maximum. determine an observed first edge and an observed second edge by collecting a set of data points measured by the first detector for each edge, and computing a line for each edge through the points; compute an error measurement for the line of said first and second edge; select the line with the least error as the referent line; recognize a first side represented by the referent line and calculate a dependent line for a second side according to said known angle between said first side and said second side, wherein said dependent line is a mathematical construct created at the known angle between the first and second sides; calculate an error measurement for the dependent line; and reject an observation of said separation chamber if the error measurement exceeds a selected maximum.