I’ve mentioned in another thread (
http://www.thephotoforum.com/forum/showthread.php?p=1088303) that the materials used to make photographic polarizers are never perfect. This means that polarizers will never behave exactly as theory would suggest. For example, circular polarizers may not produce perfectly circular polarized light – in fact it is almost impossible to do it for all wavelengths of light. There will be some linear component. This doesn’t matter much in normal use as a single polarizer, but it could affect their performance when used in crossed polarizers when they will behave like imperfect linear polarizers.
Here is a list of the
ideal properties of various elements. ‘Unpolarized light’ refers to light with no dominant direction of polarization.
Linear polarizer
Unpolarized light in: linear polarized light out, with one stop reduction.
Linear polarized light in: linear polarized light out, with the degree of reduction varying from none to all depending on the angle between the planes of polarization of the incoming light and the linear polarizer.
Circular polarized light in: linear polarized light out, with one stop reduction.
Quarter-wave delay plate
Unpolarized light in: unpolarized light out, no reduction.
Linear polarized light in: circular polarized light out, no reduction.
Circular polarized light in: linear polarized light out, no reduction.
Circular polarizer, used forwards (ie linear polarizer followed by a quarter-wave delay plate at 45° to the linear polarizer)
Unpolarized light in: circular polarized light out, one stop reduction.
Linear polarized light in: circular polarized light out, with the degree of reduction varying from none to all depending on the angle between the planes of polarization of the incoming light and the linear element.
Circular polarized light in: circular polarized light out, one stop reduction.
Circular polarizer, used backwards (ie quarter-wave delay plate followed by a linear polarizer at 45° to the delay plate)
Unpolarized light in: linear polarized light out, one stop reduction.
Linear polarized light in: linear polarized light out, one stop reduction for all angles of the polarizer, the plane of polarization may be rotated.
Circular polarized light in: linear polarized light out, either no reduction or total reduction, depending on the sense of rotation (ie left-rotating light may pass, right-rotating light may be blocked, or vice-versa depending on how the filter was assembled).
Best,
Helen