Can a filter block one handedness of circularly polarized light, and would it be useful in photography?
Asked 2/26/2012
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I learned that a standard photographic circular polarizer is really a linear polarizer followed by a quarter-wave plate, so it first filters linear polarization and then converts the transmitted light to circular polarization.
Does a filter exist that does the opposite: selectively blocks left- or right-handed circularly polarized light? If so, is it used in photography, and what visual effect would it have?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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As the photographic circular polarization filter is a polarization filter combined with a λ/4-plate, which converts the linearly polarized light to circularly polarized light, you can simply turn the filter around and you'll get a filter that filters for one direction of circularly polarized light and makes the result linearly polarized.
However, as the light that is reflected from an appropriate surface (like glass or water) is linearly polarized, there wouldn't be much use for such a filter, except maybe to take pictures of a 3D movie in a cinema (which uses circular polarization), of which the cinema guys probably wouldn't approve.
In addition, you'd have to put another filter (in the "normal" direction) behind it to convert it back to cirularly polarized light because the linearly polarized light confuses the autofocus sensors of some cameras.
Originally by user6879. Source · Licensed CC BY-SA 4.0
user6879
14y ago
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Yes. In principle, a standard photographic circular polarizer can be used in reverse: the quarter-wave plate first converts circular polarization into linear polarization, and the linear polarizer then blocks one handedness while passing the other. The output would be linearly polarized light.
In normal photography, though, this is rarely useful. Reflections from water, glass, and many other common photographic subjects are typically linearly polarized, which is why ordinary polarizers are effective. A handedness-selective circular-polarization filter would usually have little visible effect on everyday scenes.
One place it can matter is systems that intentionally use circular polarization, such as some 3D cinema projection methods. In that kind of situation, such a filter could isolate one circular polarization state.
So: yes, the concept exists, but it has little practical value for most photographic use compared with a normal linear or circular polarizer.
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