Glan-thompson polarizer is made of two calcite prisms cemented together. This is the long form and length to aperture ratios are 3.0 : 1 respectively. Glan-thompson tends to have higher extinction ratio than air spaced polarizers. In the ultra violet spectrum, their transmission is limited by absorption in calcite as well as the cement layer. This polarizer can be used from about 350 to 2300 nm. The polarization axis is marked by two white lines into th surface of its metal case.