The invention provides a secondary processing method and relates to a method of monitoring and controlling the processing of blood and blood samples, particularly the separation of blood into its components. In one aspect, the invention relates to optical methods and devices for measuring two-dimensional distributions of transmitted light intensities, scattered light intensities or both from a separation chamber of a centrifuge. In embodiment, light intensities measured by the methods of the present invention comprise images of a separation chamber or component thereof, such as an optical cell of a separation chamber. In another aspect, the invention relates to multifunctional monitoring and control systems for blood processing. Feedback control systems are provided wherein two-dimensional distributions of light intensities are measured, processed in real time and used as the basis of output signals for controlling blood processing. Optical cells for monitoring and control blood processing are provided.