Workpieces (10; FIG. 7), differing from each other only in their color, are optically sorted by illuminating the workpieces with a light beam (13) of stable color temperature. The diffuse reflection from the workpieces is analyzed by three photo-detectors, each of which is filtered to respond to a different...http://www.google.de/patents/US4278538?utm_source=gb-gplus-sharePatent US4278538 - Methods and apparatus for sorting workpieces according to their color signature
