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