▁simpler , ▁easier ▁to ▁apply , ▁and ▁less ▁cost ly . ▁ ▁However , ▁the ▁reflect ivity ▁is ▁lower ▁than ▁the ▁near - 1 0 0 % ▁reflect ivity ▁of ▁a ▁phase - correct ing ▁co ating , ▁so ▁a ▁p - co ating ▁is ▁des irable ▁for ▁low - light ▁applications . ▁▁ ▁Bin oc ular s ▁using ▁either ▁a ▁Schmidt – Pe chan ▁roof ▁pr ism ▁or ▁an ▁Ab be – K o enig ▁roof ▁pr ism ▁benefit ▁from ▁phase ▁coat ings . ▁Por ro ▁pr ism ▁bin oc ular s ▁do ▁not ▁split ▁be ams ▁and ▁therefore ▁they ▁do ▁not ▁require ▁any ▁phase ▁coat ings . ▁ ▁Met all ic ▁mirror ▁coat ings ▁▁ ▁In ▁bin oc ular s ▁with ▁Schmidt – Pe chan ▁roof ▁pr isms , ▁mirror ▁coat ings ▁are ▁added ▁to ▁some ▁surfaces ▁of ▁the ▁roof ▁pr ism ▁because ▁the ▁light ▁is ▁incident ▁at ▁one ▁of ▁the ▁pr ism ' s ▁glass - air ▁boundaries ▁at ▁an ▁angle ▁less ▁than ▁the ▁critical ▁angle ▁so ▁total ▁internal ▁reflection ▁does ▁not ▁occur . ▁Without ▁a ▁mirror ▁co ating ▁most ▁of ▁that ▁light ▁would ▁be ▁lost . ▁Schmidt – Pe chan ▁roof ▁pr ism ▁al umin um ▁mirror ▁co ating ▁( reflect ivity ▁of ▁ 8 7 % ▁to ▁ 9 3 %) ▁or ▁silver ▁mirror ▁co ating ▁( reflect ivity ▁of ▁ 9 5 % ▁to ▁ 9 8 %) ▁is ▁used . ▁ ▁In ▁older ▁designs ▁silver ▁mirror ▁coat ings ▁were ▁used ▁but ▁these ▁coat ings ▁ox id