In its simplest operation, a machine will simply see how much light is reflected off the object using a simple photodetector (such as a photoresistor) and accept or reject the item depending on how reflective it is (light or dark). More sophisticated systems use image processing to discriminate the colors of the object, often via a controlled spectrum of light, even beyond the visible spectrum into the IR and UV range. Shape detection is an evolving ability. The common method of removal is jets of compressed air, but others exist.
The term optical sorting also includes manual seeing and manipulating processes.
See also 
Further reading 
- http://www.woodheadpublishing.com/en/book.aspx?bookID=614&printer=yes Detecting foreign bodies in food
- "Image Processing for the Food Industry", E. R. Davis; World Scientific Publishing Co. Pte. Ltd.; London, 2000
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