Abstract: An apparatus and a method are provided to monitor a generally transparent blood tube at a location that is downstream from a blood separator in order to detect the presence of red blood cells in a tube that is intended to carry a different blood component, for example platelets. A red light source and a green light source are located in relation to the tube so that red light emission and green light emission are directed toward the tube and toward the flowing blood component(s) that is carried within the tube. The red light source produces a red reflection intensity from red blood cells whose magnitude increases as a function of an increase in the concentration of red blood cells. The green light source produces a green reflection intensity from the red blood cells whose magnitude decreases as a function of an increase in the concentration of the red blood cells.