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I have bought two cheap polarizing lenses for camera: New View Filtro Polarizzatore Circolare CPL Filter 58mm per Camera (Amazon Italia)

I have done the following observations:

  1. If I put them one after the other in front of a common lamp and rotate them in any possible relative orientation, I NEVER get complete darkness,but I only get a mild loss of intensity and change of light color. I used to think that when polarization directions are orthogonal I should get complete darkness, why it is not so?

  2. On the other hand, if I put one of them in front of a laptop display I easily find an orientation giving complete blackness.

I am in trouble in conciling the two "experiments": I can assume that the filters are no 100% efficient, but then why do I get complete blackness when filtering the polarized light coming from laptop display?

I suspect that those filters do not simply polarize at a given direction, but even thinking about circular polarization I cannot explain the dircrepancy.

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  • $\begingroup$ Related: physics.stackexchange.com/questions/70384/…. That's the behaviour you'd expect for the simplest possible circular-polarization analysers (though if they've been designed to keepthe light circularly polarized instead of linear, by the addition of a second QWP at the end, the behaviour will change). $\endgroup$
    – Emilio Pisanty
    Oct 8, 2017 at 16:23

3 Answers 3

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They are circular polarising filters. They only block light which is already polarised, which is why you use them with a camera - to block reflections from windows or water (which are polarised).

Putting two of them together doesn't block any more light. If you had 2 linear polarisers then the first would only let through light of one polarisation and the other would block that — unless they were exactly aligned — this is probably the experiment with rotating polarisers you have seen.

The LCD screen uses polarisers, so the light coming from it is already polarised, and the circular polariser will block that if rotated in the correct direction.

See All about Polarizers - Linear and Circular

P.S. Cameras generally used circular polarisers because light was reflected with mirrors inside the camera before reaching some sensors - this would block linearly polarised light and so the reading would be wrong. This isn't really true on modern mirror-less digital cameras.

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  • $\begingroup$ Just to clarify your postscript, some modern cameras have mirrors (SLRs) and some do not (everything else AFAIK). $\endgroup$
    – nasch
    Oct 8, 2017 at 21:32
  • $\begingroup$ I was under the impression that the problem with linear polarisers was with certain phase detect autofocus (PDAF) systems, which means that even mirrorless cameras with PDAF may be susceptible. In any case, circular polarisers solve this issue quite well. $\endgroup$
    – thomasrutter
    Oct 9, 2017 at 1:07
  • $\begingroup$ @thomasrutter - phase detection generally has another mirror to reflect the light into the phase detector so suffers from the same problem, even if it isn't otherwise an SLR (actually pd-af is only used on high DSLRs anyway). Contrast AF can be done on the main sensor with no other optics in the way $\endgroup$
    – Martin Beckett
    Oct 9, 2017 at 3:06
  • $\begingroup$ If they're circular polarizers you should be able to get blackness by combining them if you flip one over, no? $\endgroup$
    – user253751
    Oct 9, 2017 at 3:20
  • $\begingroup$ @immibis - yes, although the screw threads mean you can't use it like this normally. In practice the extinction factor (amount of blocking) for a film polariser isn't that great so you might still get a % of light through. $\endgroup$
    – Martin Beckett
    Oct 9, 2017 at 3:26
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To add to the answer of Martin Beskett, camera filters are directional, because they have two layers. The external layer is a linear polarizer; the internal layer changes the polarization from linear to circular. The filter acts as a circular polarizer in the normal direction, but as a linear polarizer in the opposite direction. To achieve a complete darkness by rotating two filters, you must reverse the direction of one of them (the external sides must face each other). You would have to hold it with your hand, because the threads would be pointing outside. As Martin said, make sure the filter closest to the camera is not reversed, because the camera needs a circularly polarized light.

This is a photo of the sun spots taken with two polarizers (external reversed) on a hot afternoon:

enter image description here

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    $\begingroup$ Wonderful, tried in real time and it works! THX everybody! $\endgroup$
    – Arnaldo Maccarone
    Oct 8, 2017 at 16:46
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Circular polarization is a thing that to my conclusion dont exist. These kind of filters exist of 2 layers, 1 polarization filter and behind that a layer that splits the polarization into 2 different angles, these materials exist.

So experiment further, and put the front sides of lenzes to each other, so one lens backward . If that still wont give total darkness, do the same with backsides together.

I think the first will work.

Sometimes in 3D glasses also these kind of filters are used, and also in LCDisplays like calculators .

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