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