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By certification 
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
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
