A high-quality polarizer made of two calcite prisms cemented together. The cement's refractive index is chosen so that the ordinary ray undergoes total internal reflection at the interface while the extraordinary ray transmits. It offers a very wide acceptance angle but cannot handle high-power lasers because the cement absorbs energy and degrades.
Heard on the job
Swap out the Glan-Thompson for something with an air gap before you turn up the pump power, or you'll fry the cement.