These are air-spaced Glan Laser polarizers. They use the strong bi-refringence of natural crystal calcite to produce a high degree of p-polarization in the transmitted beam. They provide an extinction ratio of 500,000: 1. In order to permit the use of high power laser beams these cubes have polished side exit faces and a cut out cell to permit the exit of the rejected s-polarized beams. Since Calcite has a good transmittance from the visible to 2300nm and they are useful with most visible and near IR lasers.