{HEAD ~ Prior art devices} wherein {TAIL ~ the thickness} of the layers was not calculated to optimize light efficiency had a product of phase factors ξ1 and ξ2 of less than 80% 
wherein
{ HEAD ~ Prior art devices } wherein { TAIL ~ the thickness } of the layers was not calculated to optimize light efficiency had a product of phase factors ξ1 and ξ2 of less than 80 %
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH