{HEAD ~ The optical touch device} can avoid a blind zone and can be used as {TAIL ~ a dual touch device} or a multi touch device 
used as
{ HEAD ~ The optical touch device } can avoid a blind zone and can be used as { TAIL ~ a dual touch device } or a multi touch device
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH