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Polarized Photographic Images
by Zolt Levay (STScI)

Photographers 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.

Light 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 polarized.

Here 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 Project).

Next 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!

A 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.

Finally 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.

If 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.

If 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 polarizing filters.

Polarization

Electromagnetic 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.

Light 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 varying degrees.

We 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.