<https://www.lambda.cc/product/plane-mirror-substrates-pm/#product>
<http://schema.org/description>
"General purpose plane mirror blanks\r\n \tFor first surface use. Second surface is commercial polish\r\n \tApply any reflecting coating\r\n\r\n&amp;nbsp;\r\n\r\n&amp;nbsp;\r\n\r\nThese general-purpose mirror blanks are polished to &amp;lt; λ /10 flatness except where indicated otherwise. They are intended for front surface use and are parallel to within 3 arc min. These blanks are not recommended for use as transmitting optics as the rear surface is only commercially polished.\r\n\r\n\r\n\r\nSurface Quality:\r\n&amp;lt; 10/5\r\n&amp;lt; 40/20 for Sapphire\r\n\r\n\r\nSurface Flatness:\r\n&amp;lt; λ / 10 @ 633 nm\r\n&amp;lt; λ / 2 @ 633nm for Sapphire\r\n\r\n\r\nDiameter Tolerance:\r\n&amp;lt;+ 0.00 / - 0.20 mm\r\n\r\n\r\nThickness Tolerance:\r\n&amp;lt; ± 0.25 mm\r\n\r\n\r\nClear Aperture:\r\n&amp;gt; 85% of diameter\r\n\r\n\r\nParallelism:\r\n&amp;lt; 3 arc min\r\n\r\n\r\nBevel:\r\n0.3 mm (typical) x 45°\r\n\r\n\r\n\r\nPlane Mirror Substrates (PM)"
<https://www.lambda.cc/product/plane-mirror-substrates-pm/>