The deviation angle of 90° is thus independent of the orientation of the prism, making it especially important in applications in which the prism orientation cannot be precisely controlled. Due to geometry that total internal reflection cannot be used, the reflecting surfaces must be coated with a metallic or dielectric coating. The standard Penta Prism reflecting surfaces are coated with aluminium or enhanced aluminium. Sometimes, the coated surfaces are black painted for protection.

Specifications

Material: BK7 Grade A optical glass
Dimension Tolerance: ±0.25mm
90° Deviation Tolerance:
Standard series: < 30 arc seconds
Precision series: up to 10 arc seconds
Flatness:
Standard series: l/2 at 632.8 nm
Precision series: up to l/4 at 632.8 nm
Reflectivity: R > 95% per face from 630 to 680 nm
Surface Quality: 60-40 scratch and dig