Replacement cover slide or debris shield Diameter 55mm x 1.5mm thick. TWFD: < 1/2 waves @ 632.8nmSurface quality: 40/20Parallelism: < 45 arcminutesChamfer: protectiveCoating: R< .25% @ 1064nm & < 10% @ 632.8nm @ 0º AOIDownload our free e-book for protecting your laser today.http://info.unitedlens.com/debris-shields-manufacturing-and-procurement