A centrifuge for separating blood having a camera observing fluid flow, and a controller controlling the flow. The location of an interface is detected by image processing steps, which may comprise the steps of “spoiling” the image, “diffusing” the image, “edge detection”, “edge linking”, “region-based confirmation”, and “interface calculation”. “Spoiling” reduces the number of pixels to be examined preferentially on orthogonal axis oriented with respect to the expected location of the interface or phase boundary. “Diffusing” smoothes out small oscillations in the interface boundary, making the location of the interface more distinct. “Edge detection” computes the rate of change in pixel intensity. “Edge linking” connects adjacent maxima. “Region-based confirmation” creates a pseudo image of the regions that qualify as distinct. “Final edge calculation” uses the points where the shade changes in the pseudo image, averages the radial displacement of these points for the interface position.