The two flat mirrors are rotated in {HEAD ~ synchronism} for scanning {TAIL ~ the light beams} longitudinally along the document 
for scanning
The two flat mirrors are rotated in { HEAD ~ synchronism } for scanning { TAIL ~ the light beams } longitudinally along the document
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH