Glan, Thompson prism

A Glan-Thompson prism is a birefringent polarizer made from two pieces of calcite or similar crystalline material cemented together with a refractive index-matched optical cement. Light enters through one face, splits into ordinary and extraordinary rays traveling at different angles through the birefringent crystal, and the ordinary ray is absorbed by a light-absorbing coating on the interior surface where the two halves meet. The extraordinary ray exits as linearly polarized light. This design works reliably across the visible spectrum, typically 400 to 700 nanometers.

The geometry is straightforward: two right-angled prisms are cut from calcite with their optical axes oriented perpendicular to each other, then optically contacted using a transparent cement. The cement layer acts as a weak bond and, critically, matches the refractive index of calcite so that the extraordinary ray (with refractive index around 1.486) crosses the interface with minimal loss to reflection. This overcomes a key limitation of air-gapped designs like the Glan-Foucault, where total internal reflection at an air boundary forces the prism to work only at oblique angles and narrows the field of view.

Glan-Thompson prisms tolerate larger apertures and wider incident angles than air-gap designs, making them useful in spectroscopy, microscopy, and laser polarization control. However, the cement bond is weaker mechanically than the fused or optically contacted surfaces of Glan-Taylor or Glan-Laser variants, so these prisms are more sensitive to thermal shock and mechanical vibration. The cement itself can degrade under UV exposure or at elevated temperatures, causing the bond to fail and the prism to separate into its two halves.

Trade-offs and selection

Choose a Glan-Thompson when you need good extinction ratio (typically 10,000:1 or better) and a reasonable working aperture without the cost and manufacturing complexity of fused designs. They are economical for benchtop optical systems that see gentle use. In harsh environments, thermal cycling, or high-power laser work, the Glan-Laser or Glan-Taylor prism, with their fused or optically contacted surfaces, justify their higher cost. The name honors both the Glan design (a Nicol prism variant with two calcite pieces) and the Thompson contribution to the cemented variant in the early twentieth century.