Dichroic Polarisers
Polymer
films can make good polarisers. If
a thin sheet of dichroic polymer is stretched in one direction the long
molecules of the polymer become aligned. This
induces a filtering effect in which oscillations parallel to the long axis of
the molecules are preferentially transmitted.
In
these dichroic polarisers the polymer sheet is sandwiched between two plates
of BK7 or UV Fused Silica. Polarisation
by this means can be quite effective providing an extinction ratio of about
1000:1 over a wide angular field of view. The
polarisers are optimised for use in either the ultra-violet, visible or near
infra-red regions and may also be anti-reflection coated if required.
The
sandwich has a nominal thickness of 3mm. UV
Fused Silica should be selected as protective sheet when used in the
ultra-violet.
Dichroic
polarisers can be supplied either un-mounted or mounted in a metal ring.
Typical diameters of un-mounted polarisers are 12.7 and 25.4 mm.
Clear apertures are 10 and 20 mm when mounted.