ized ▁and ▁lost ▁reflect ivity ▁over ▁time ▁in ▁un se aled ▁bin oc ular s . ▁Al umin um ▁mirror ▁coat ings ▁were ▁used ▁in ▁later ▁un se aled ▁designs ▁because ▁they ▁did ▁not ▁t arn ish ▁even ▁though ▁they ▁have ▁a ▁lower ▁reflect ivity ▁than ▁silver . ▁Modern ▁designs ▁use ▁either ▁al umin um ▁or ▁silver . ▁Silver ▁is ▁used ▁in ▁modern ▁high - quality ▁designs ▁which ▁are ▁se aled ▁and ▁filled ▁with ▁a ▁nit ro gen ▁or ▁arg on ▁in ert ▁atmosphere ▁so ▁that ▁the ▁silver ▁mirror ▁co ating ▁does ▁not ▁t arn ish . ▁ ▁Por ro ▁pr ism ▁bin oc ular s ▁and ▁roof ▁pr ism ▁bin oc ular s ▁using ▁the ▁Ab be – K o enig ▁roof ▁pr ism ▁typically ▁do ▁not ▁use ▁mirror ▁coat ings ▁because ▁these ▁pr isms ▁reflect ▁with ▁ 1 0 0 % ▁reflect ivity ▁using ▁total ▁internal ▁reflection ▁in ▁the ▁pr ism . ▁ ▁Die lect ric ▁mirror ▁coat ings ▁▁ ▁Die lect ric ▁coat ings ▁are ▁used ▁in ▁Schmidt – Pe chan ▁roof ▁pr isms ▁to ▁cause ▁the ▁pr ism ▁surfaces ▁to ▁act ▁as ▁a ▁die lect ric ▁mirror . ▁The ▁non - met all ic ▁die lect ric ▁reflect ive ▁co ating ▁is ▁formed ▁from ▁several ▁mult il ay ers ▁of ▁altern ating ▁high ▁and ▁low ▁ref ract ive ▁index ▁materials ▁depos ited ▁on ▁a ▁pr ism ' s ▁reflect ive ▁surfaces . ▁ ▁This ▁multi - mult il ayer ▁co ating ▁increases ▁reflect ivity ▁from ▁the ▁pr ism ▁surfaces