In the book "Light, Science and Magic, by Fil Hunter, Steven Biver, and
Paul Fuqua (About the best book I've ever read on light and lighting for
the photographer) there is a very good section on polarization and
working with polarized light.
Just about any surface or medium will polarize reflected or transmitted
light to some extent. But there is also the direct, or specular,
reflection of the light source which you can not get rid of with a
polarizing filter on the camera. Reflected polarized light looks like an
indistinct, fuzzy halo and it disappears when polarized out with a
filter. The "Clear" (Mirror-like) reflection can't be polarized away.
As bob says, the angle of incidence plays a huge part in how much the
light is polarized, either by the atmospheric layers (The sky) or by the
material that is the reflecting surface.
Of course a polarizing filter is a fantastic tool to assess the degree
of polarization. Just look through it and turn it. If there's no change,
then the light is not polarized. If there's a dramatic change then
there's extreme polarization. :-)
The eye and the brain work so well together!
Herschel
Bob Blakely wrote:
Generally, light might better be thought of as elliptically polarized,
having two components, one collinear with the major axis and the
collinear with the minor axis of the ellipse. If the the two axis are
equal, the light either randomly or circularly polarized and a
polarizing filter will have no effect. If the minor axis becomes
zero, the ellipse degenerates to a line and the light is said to be
linearly polarized and a polarizing filter will have maximum effect.
The more elliptically polarized the light, the greater the effect of
the polarization filter.
1. The (scattered particle) light from certain portions of the sky
becomes (effectively) elliptically polarized by the effects of
varying refraction through varying (with
altitude) atmospheric density. The eccentricity of the polarized light
varies with path angles involved, specifically the angle(s) between
the rays and the normal to the atmospheric"layers". Polarization from
reflection of particles is essentially nil because the the particles
are randomly oriented.
2. Reflected light from a reflective surface becomes elliptically
polarized because the surface attenuates the vertical component of the
reflected light more than the horizontal component. It is also angle
dependent, that is, the lower the angle of incidence the greater and
more polarized the reflection.
Regards
Bob...
---------------------------------------------------------------
"I don't mind if you don't like my manners.
I don't like them myself. They're pretty bad.
I grieve over them long winter evenings."
-- Philip Marlowe (Humphrey Bogart)
----- Original Message -----
*From:* JAMES F LUKENS <mailto:jfl2@xxxxxxx>
In Andy's article on polarized light there is reference to the
natural polarized light of certain parts of the sky. It's my
understanding that such polarization occurs because of light being
reflected from tiny particulate matter suspended in our
atmosphere, and that a polarizing filter over the camera lens
simply eliminates that "glare" the same way it eliminates glare
from water, roadways, etc. Anybody heard of other explanations as
to why you can darken parts of the sky with a polarizer? Just
curious! Lukens
