Since the overlapped area of the focused target track S11 and the pinhole SS12 is identical to that of the focused target track S11 and the pinhole SS13, the amount of a second portion of the focused laser beam passing through the pinhole SS12 is the same as that of a third portion of {HEAD ~ the focused laser beam} passing through {TAIL ~ the pinhole SS13} 
Since the overlapped area of the focused target track S11 and the pinhole SS12 is identical to that of the focused target track S11 and the pinhole SS13 , the amount of a second portion of the focused laser beam passing through the pinhole SS12 is the same as that of a third portion of { HEAD ~ the focused laser beam } passing through { TAIL ~ the pinhole SS13 }
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 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 HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL