An optical element 13, wherein the first reflectivity is {HEAD ~ the substantially maximum reflectivity} for {TAIL ~ the material} 
An optical element 13 , wherein the first reflectivity is { HEAD ~ the substantially maximum reflectivity } for { TAIL ~ the material }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL