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