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