{HEAD ~ The method} also includes detecting at least a portion of the reflected light rays with a photodetector associated with {TAIL ~ the set} of at least four light sources, and generating signals based on the detected light rays for each of the light sources 
{ HEAD ~ The method } also includes detecting at least a portion of the reflected light rays with a photodetector associated with { TAIL ~ the set } of at least four light sources , and generating signals based on the detected light rays for each of the light sources
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