Blood separation systems and methods are provided for controlling the interface between separated blood components. The system includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region therebetween. An optical sensor system emits a scanning light beam along a path toward the light-transmissive portion, which transmits at least a portion of the scanning light beam to the fluid processing region and the light reflector. The light reflector reflects at least a portion of the scanning light beam toward the optical sensor system along a path substantially coaxial to the path of the scanning light beam from the optical sensor system toward the light-transmissive portion of the centrifuge assembly. The scanning light beam may be a white light beam or narrow spectrum beam. The reflected beam may be directed through the optical sensor system via optical fibers.