{TAIL ~ The first and second diffractive structures} may be on {HEAD ~ a same surface} or may be on opposite surface of a same substrate 
{ TAIL ~ The first and second diffractive structures } may be on { HEAD ~ a same surface } or may be on opposite surface of a same substrate
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH