Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: polarizing filters in photography



At 11:48 AM 3/25/01 -0800, Ben Crowell wrote:
Does anyone know of a good reference explaining how
polarizing filters are used in photography? The main
use is supposedly to make the sky darker and more
color-saturated. What seems strange to me is that
they sell both linear and circularly polarizing
filters, and I haven't been able to find any information
about how photographers actually use one as opposed to the
other. It seems to me that a linear polarizer might
indeed have the effects claimed (if you aligned it
properly), but a circular polarizer
should do no more than throw away 50% of the light.
The guy at the camera shop says that the type of filter
you use just depends on the hardware you have on the
front of your lens, as though the two types were
optically equivalent. This seems like nonsense to me,
but maybe I'm missing something.

Good question. See
http://www.geometer.org/beginner/polarizer.html

The point is that what a camera-store clerk calls a "circular polarization
filter" is not what a physicist would call a circular polarization
filter. In particular it is *not* the projection operator

P = 1 0 (equation 1)
0 0

in the [L, R] polarization basis (or any other basis).

In fact it consists of a plain old linear polarizer in front, followed by a
quarter wave plate. That means that
a) one linearly polarized component coming from the scene will be absorbed
b) the other linearly-polarized component will be passed through
c) the passed component will be transformed into circularly polarized
light, to minimize the chance that anything funny will happen to it on its
way through the rest of the optical system.

==============

Note that a physicist would build the aforementioned projection operator
(equation 1) using a piece of linear-polarizing film with a quarter wave
behind it AND ANOTHER IN FRONT OF IT. The camera-store device lacks the
front quarter wave plate.