The distance between two sensing chip 31, 32 is represented by L2, and {HEAD ~ the distance} between {TAIL ~ two light focusing} means 16, 16 is represented by L1 
The distance between two sensing chip 31 , 32 is represented by L2 , and { HEAD ~ the distance } between { TAIL ~ two light focusing } means 16 , 16 is represented by L1
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH