know that a polarizing filter changes the appearance
of reflected light. Mostly, it is used to darken
a clear, blue sky or reduce the reflection from
a water surface or wet foliage. It's another way
for a photographer to control light and contrast
adjusting the elements in a scene and producing
a more pleasing composition.
reflecting from some materials becomes polarized.
Polaroid material (such as a polarizing filter)
also causes light passing through it to become polarized.
Parts of the scene will darken as we rotate the
polarizing filter depending on how the light is
we have used a polarizing filter on a digital camera
to demonstrate how we can construct a multi-color
image from black and white images whose light is
passing through different polarization angles. The
picture at left is a "straight" color
photo using no polarization filters. The building
is a section of the Space Telescope Science Institute
in Baltimore, Maryland (and home of the Hubble Heritage
we have three black and white photos of the same
scene taken through a polarizing filter at three
different angles (rotated by 60° from each other):
The middle image in each set has hints on where
to look to see differences in the images!
close observation of the black and white images
shows that there are subtle differences in the sky
and the reflections in the windows in the three
images, but not much else changes. Now we have "colorized"
the three black and white photos by applying a separate
hue (color) to each photo.
we have combined the three colorized images together
into a single full color image (the individual "primaries"
add together to produce all possible colors):
The color here is quite different from the original
"natural" color image at the top. For
example, the sky is an unnatural yellowish-green.
Here, the color doesn't really have anything to
do with the colors we perceive with our eyes. Rather,
the color represents the physical phenomenon of
polarization, with the actual colors representing
(in some quite complicated way) the various angles
of reflection and polarization of the light interacting
with various materials: air, glass, paint, etc.
you compare the individual black and white images
with the final color composite, you can see roughly
where each color comes from: the windows at top
center are purplish because most of the light is
coming from the second (blue) and third (red) image
above; blue and red combine to form purple. Similarly,
the sky is slightly brighter in the first (green)
and third (red) images; some red added to the green
combine to form yellow-green.
we could analyze the individual exposures in some
detail (measuring the brightness of each part of
the image in each polarization angle, say, along
with some understanding of the light being reflected)
we could, in principle determine some physical property
of each material. This is how astronomers deduce
properties of the objects they can't touch but can
only observe using telescopes and instruments with
radiation (light) consists of waves of electric
and magnetic fields, each individual wave
vibrating at an arbitrary angle perpendicular
to the direction of motion. If the angle of
vibration of the individual waves in a beam
is mostly in a particular direction, the light
is said to be polarized.
passing through or reflected by certain materials
causes light to be polarized. For example,
the Sun's light is scattered and polarized
by the air in the Earth's atmosphere; the
strongest effect being perpendicular to the
line of sight to the sun. Also, light reflecting
from water, glass, plastic and many other
non-metallic materials becomes polarized to
can deduce the angle of polarization of the
light by passing it through another polarizing
material whose angle of polarization is known.
If the angles of the polarized light and the
polarizing filter are perpendicular, the light
will be blocked by the polarizing filter.
Conversely, if the angles are parallel, the
light will pass.