"Circular" polarizing filters allow the partial removing of non specular reflections like glass, water or snow reflections.

But what does circular mean? Does it mean that only the circular polarized light goes through or at the opposite, only the circular polarized light is stopped?

Last, how is the coating made?

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    Possible duplicate of What is the difference between a linear and a circular polarizer? – Olivier Sep 27 '16 at 17:17
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    The question isn't the same... – Patrick Augeau Sep 27 '16 at 17:20
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    these are two separate question. also, did you try googling first? What you'd like to clarify then? did you read wikipedia article? – aaaaa says reinstate Monica Sep 27 '16 at 17:27
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    If we closed every question about things that use physics and material science to turn light into a photograph there wouldn't be much at all to talk about here. Lenses? Nope, that's optics. Sensors? Nope, that's semiconductors. Depth of Field? Nope, that's mathematics... – Michael C Sep 28 '16 at 19:47
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    And if I ask the question on the physics forum, I probably will have a guy writing "I vote to close this because it's a question about photography" :-p – Patrick Augeau Oct 3 '16 at 6:56

You have seen, water waves ungulate in an up and down direction. Light waves are not limited as to the direction vibration. Light waves can and do vibrate like water waves but their direction of vibration can be in any orientation. We are taking up – down – left – right and angled (not parallel to the horizon) ect. Scientists that first discovered that light moved with a wave motion compared the up and down trails to the poles of a magnet. They reasoned that light must a north and South Pole. Investigation showed that when light transversed some transparent material or when light was reflected off some surfaces, the direction of vibration was changed, they said the light ray had been polarized. The north pole south pole idea failed to materialize the jargon remained thus “polarization”.

A transparent crystalline mineral called Islandic Spar was observed to split a light beam into two different paths. Other minerals acted as if they were ruled with closely parallel lines. Edwin Land mixed these into a plastic and made lenses for sunglasses. His sunglasses only allowed light to transverse if the direction of vibration aligned with the crystals. Now light changes its direction of vibration upon reflection from some objects. Same thing happens as light transverses air that contains water vapor and or dust. The polarizing sunglasses thus stop some light rays and pass some light rays. The result, a great deal of glare is mitigated.

In photography we use the polarizing filter to darken blue sky making the white clouds stand out. The polarizer adds saturation to color images without changing the basic color of the vista. The polarizing filter mitigates reflections from water and glass and many other surfaces. The polarizing filter cuts through haze in the air clarifying the images of distance landscapes and views from high flying airplanes. The polarizing filter is likely the most useful of all optical filters for the camera.

Modern digital cameras have auto focus and auto exposure mechanisms. Many times a polarizing filter is in the light path used by these devices. For this reason, mounting a polarizing filter can defeat this automation. Circular Polarizer to the rescue: Two filters are sandwiched together. The first is a regular “linear” polarizer. This filter does the intended job. Only the select rays vibrating in the correct ordination transvers the filter, a second filter called are “retarder” is next in line. This filter scrambles the light, now it vibrates in all planes. This combination will not harm the sensors, they auto focus and measure exposure as if nothing had interfered.

  • This is inaccurate -- a proper circulizing polarizer will retard one axis by 45 degrees compared with the other. Also, your description of the effect a linear polarizer has on sky/clouds is weak: it doesn't explain that the choice of orientation strongly affects the relative transmission from various objects. – Carl Witthoft Sep 28 '16 at 11:51
  • The polarizing screen filter is used to contribute a Dark-Sky Effect. Sunlight is unpolarized before it strikes water and dust particles in the air. A high percentage of light rays are polarized due scattering after collisions. With a polarizing screen mounted, you rotate the filter for maximum effect. The effect is best if the camera is at a right angle (90⁰) to the sun. Rotating the filter produces various effects. We use this filter to reduce reflections from shiny surfaces. The reflection mitigation is maximized if the surface is a non-conductor. – Alan Marcus Sep 28 '16 at 14:44

Does it mean that only the circular polarized light goes through or at the opposite, only the circular polarized light is stopped?

Neither. It means only light polarized in one direction is allowed to pass. Then the quarter-wave plate underneath the polarizing filter repolarizes it in a kind of circular way.

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