Glan Thompson Polarisers
Glan
Thompson polarisers consist of two prisms of calcite cemented together in such a
way as to permit only the p-polarisation to be transmitted.
They produce a high degree of polarisation in the transmitted beam with
an extinction ratio of 1x10-6. Glan
Thompson polarisers have a wide angular field of view, which increases with a
longer aspect ratio. Two aspect
ratios are offered, 2.5X which provides a 14° field of view and 3X which
provides a 22° field of view. They have a transmittance of at least 72%.
Calcite
is available in several different grades. For
low power laser use, Glan Thompson polarisers must be made of laser grade
calcite, which is of high homogeneity, has few inclusions and shows no
scattering when illuminated with a helium neon laser beam.
However, these polarisers should not be used for high power laser
applications since the cement layer will cause scattering and could induce laser
damage. For most general
applications it is acceptable to use optical grade calcite, which shows no
scattering when viewed under a 40 watt incandescent lamp.
Standard grade calcite, which shows some scattering, may be used for less
demanding applications.
Glan
Thompson polarisers are normally supplied as square cross section prisms with an
aperture of 8, 10, 12, 15 or 20 mm. Since
they are cemented, they may be supplied un-mounted.
Alternatively, they may be mounted in an absorbing black potting compound
within a metal cell.
Single
layer MgF2 anti-reflection coatings may be applied if required. These coatings provide a reflectance of less than 2% over a
range of 220-400 nm, 400-700 nm or 700-1100 nm.