Glan, Foucault prism

A Glan-Foucault prism is a polarizing optical element made from two blocks of calcite (crystalline calcium carbonate) cemented at their hypotenuse faces with a thin air gap between them, typically 0.1 to 0.5 mm. Light entering the first block splits into ordinary and extraordinary rays according to the crystal's birefringence; the air gap and the angled geometry cause one polarization to reflect away at the interface while the other passes through, producing linearly polarized output from unpolarized input.

The prism works by exploiting total internal reflection. When the ordinary ray strikes the air gap at an angle steeper than the critical angle (around 33 degrees for calcite), it reflects away as waste; the extraordinary ray, traveling at a different velocity, hits the interface at a shallower angle and transmits through. This geometry makes the Glan-Foucault more efficient than simpler designs because the air gap allows a wider acceptance angle and reduces chromatic effects compared to cemented variants.

Variants and practical limits

Related designs include the Glan-Thompson prism (air gap at the perpendicular interface rather than the hypotenuse) and the Glan-polarizer (cemented without an air gap). The Glan-Foucault occupies the middle ground: better extinction ratio and transmission than Glan-polarizers across visible wavelengths, but more sensitive to mechanical stress and temperature change because the air gap can expand or contract. Standard clear apertures run 10 to 25 mm. Extinction ratios typically reach 1:10,000 or better in the visible band.

The prism is named after Augustin-Jean Fresnel's polarization work, later refined by Leon Foucault in the 1850s. It remains common in laboratory spectroscopy, laser work, and optical test equipment where unpolarized light must be converted to a pure linear state without unacceptable loss. High-power laser applications often reject it because the reflected beam can cause optical damage downstream, and thermal lensing in the crystals becomes problematic above a few watts.

Durability depends on keeping the air gap clean and the housing rigid. Dust contamination, humidity cycles, and vibration can degrade performance. The calcite itself is brittle and sensitive to mechanical shock; corners and edges chip easily. Storage in a dry environment and avoidance of thermal shock are standard practice. Cost is moderate relative to other precision polarizers, making the design common in teaching laboratories and standard-duty optical benches.