Using two polarized filters to block all (?) light is a common classroom demonstration. With the imminent north American total eclipse, the media has been pushing eclipse glasses and emphasizing the inadequacy of sunglasses to protect the eyes. This made me wonder whether perpendicular polarized filters could serve as a sort of adjustable eclipse glasses. The eclipse glasses I got seem awfully dark when looking at the midday sun.


closed as off-topic by Qmechanic Aug 20 '17 at 19:33

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  • $\begingroup$ I'd recommend just going for a pinhole camera to be on the safe side. $\endgroup$ – Sean E. Lake Aug 20 '17 at 18:29
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    $\begingroup$ I'm voting to close this question as off-topic. Phys.SE is not designed to provide authoritative health & safety advices to the general public. $\endgroup$ – Qmechanic Aug 20 '17 at 19:33
  • $\begingroup$ Where else should I ask this question? Getting a non authoritative answer that it won't work (finding the result on Google) is way better than someone finding no answer at all and guessing it will work! However I can understand if it's a liability thing. In that case a "do not try this at home" type of warning would be better than no information. $\endgroup$ – Matt Chambers Aug 21 '17 at 3:06
  • $\begingroup$ Who said anything about Health and Safety? The question is exactly why I came here -- polarized lenses offset from each other does reduce light -- is it enough? Is it too much? At least I wanted people to be able to place non health and safety answers; the health and safety ones can be eliminated by down votes at the answer level! $\endgroup$ – Gerard ONeill Aug 21 '17 at 21:47

Short answer: No - don't do it. When it comes to things like this you need the real deal - there is no point risking your eyesight to see the eclipse.

Long answer: Polaroid filters only work for a specific range of wavelengths this one for example works only for 400-700nm i.e. basically the visible range. It will not protect you from e.g. the invisible uv radiation which you cannot tell how well it is being blocked. As (1) states UV flooding the eye in can cause "solar retinopathy" (aka. Photic retinopathy) which can cause blindness or dark/yellow spots.

Even if you buy polarizers specially designed for the UV range as the images on this page* show even when crossed there is a non-zero transmission of UV radiation.


(1) http://scienceline.ucsb.edu/getkey.php?key=3269

*About 2/3rds of the way down. I will not reproduce them here for copyright reasons.

  • $\begingroup$ Cool, I didn't know about the wavelength limitations. I was reading basic info about polarization and it left that part out. So the next question would be: how much transmission at each range is acceptable. Since I can see this site follows the "don't do this at home" philosophy, I will see if I can read about that in the iso standard for eclipse glasses. :) $\endgroup$ – Matt Chambers Aug 21 '17 at 3:00
  • $\begingroup$ The short answer is off topic. The long answer deals with circular polarizers which are not the same as two linear polarizers at an angle -- the first may have wavelength features, but the latter doesn't seem to have. It would be good to know, and this is pretty much what the OP asked. Stop worrying about biology. $\endgroup$ – Gerard ONeill Aug 21 '17 at 22:00
  • $\begingroup$ @GerardONeill Linear polarizers will also have a wavelength range (see e.g. here edmundoptics.com/resources/application-notes/optics/…). Anyway most of the polarizers you come across are infact circular rather then linear (amazon.co.uk/s/…). In terms of my short answer being off topic - I have no idea who the OP is or how they will use this information. It is best to state the obvious just in case. $\endgroup$ – Quantum spaghettification Aug 22 '17 at 18:01
  • $\begingroup$ Thanks Quantum -- Looks like a combination of extinction ratio and transmission percentage are what we need know and the wavelength range -- so is there no combination of these with a large extinction ratio and poor transmission rate that would reduce the light by 100,000x sunglasses? And if 100% blockage of UV is required, then by adding on a cheap pair of sunglasses? From your link, 2 x 145 for wire grid types might do the trick. $\endgroup$ – Gerard ONeill Aug 22 '17 at 19:24

As far as I know, the main problem is not the visible light, but the invisible light. So your glass pol filters should probably sufficiently block the UV (depending on their thickness and which glass was used) -- that's why people don't get sunburns if they are driving a car (not a cabrio). However, they probably do not block the IR. But both are dangerous to your eye. So don't use your self-made stuff.

  • $\begingroup$ Interesting about IR. Are the retinas actually sensitive to that other than as heat? $\endgroup$ – Matt Chambers Aug 21 '17 at 2:54
  • $\begingroup$ As far as I remember my laser safety lesson, UV lasers mainly damage the lense due to strong absorption and IR lasers mainly damage the retina due to heat. While it is possible to "fix" the lense, it is impossible to repair the retina. $\endgroup$ – Semoi Aug 21 '17 at 18:03
  • $\begingroup$ Ack! Off topic. Unless there is something specific to UV or IR that doesn't apply to other light, it should only be mentioned here as an 'additionally', and not as the primary answer. The primary answer should talk about how two polarizing filters at an angle may or may not do what the OP wants -- which is to reduce light (em) in general. A side note about adding a UV filter (or IR filter) is fine -- but where's the OP's answer? $\endgroup$ – Gerard ONeill Aug 21 '17 at 22:08

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