The focus information of {HEAD ~ each solder ball} is used to determine {TAIL ~ the distance} to the tips of each solder ball 
The focus information of { HEAD ~ each solder ball } is used to determine { TAIL ~ the distance } to the tips of each solder ball
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH