K9 Optical Glass Prism Dia12/25 Optical Glass Trihedral Retroreflector Silver Coated Corner Cube Prisms Material: K9/N-BK7 Surface flatness: λ/4@632 Diameter: 12(0")/25(1") Comprehensive deflection angle:<15" Height: 9/19mm Clear Aperture>90% Dimension Tolerance: +0/-0 Bevelling: zero degree Surface quality: 60/40 Coating: Three Reflected Surfaces are coated with Silver Exterior with safe black painting