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How polarized test image cards work

  1. Jan 27, 2016 #1
    in optical stores.. you can see polarized sunglasses test card with image (like boat or trees). when you wear the polarized sunglasses, you can see the image that you wont see with normal eyes or normal glasses. i know polarized sunglasses work by filtering and seeing only the vertical light. so when wearing them, how can the image appear when without polarizer, you can see both vertical and horizontal.. meaning more... with polarizer where you can see only vertical..you can see the image.. its quite counterintuitive.. i cant find the answer goggling.. any one got idea?
     
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  3. Jan 27, 2016 #2

    A.T.

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    And which version of the test image reflects more light?
     
  4. Jan 27, 2016 #3
    It is quite simple. The "secret" image is drawn with special paint, that reflects only polarized light, while all other painting is drawn with ordinary paint. When you look at the painting it is hard to distinguish secret image as your eyes are not sensitive to polarization state (a lot of light is reflected from the secret painting and it appears white/shinny and disappears in the background)..When you put on polarizing glasses, these glasses block polarized light from the secret image, and it appears darker than background.
     
  5. Jan 27, 2016 #4
    Thanks. I couldn't have figured it out by myself. Do you know of other ways to read secret messages where only the wearer particular glasses can read it (like it's encrypted in some unique angle that matches the paper's).. maybe there are bosons (higgs like) where you can have different directions and angles of polarizations? Why do photons only have vertical and horizontal polarizations?
     
  6. Jan 27, 2016 #5
    I guess there are many tricks to hide a message and then "decrypt" it with glasses. Most obvious are using luminous marking - a dye that slightly reduces reflection of some particular color, when you wear glasses with filters the marking become obvious (a technique used by card cheaters); also there are fluorescent dye (like these found on paper money that shine under uV light) - with right chemical composition these dye can fluorescence in daylight and with good filters you can clearly identify marking. I bet there are many more techniques.

    This has nothing to do with photons/bosons and etc. It is classical physics and it is better to use wave analogue for light in this case, otherwise you will get confused soon :) Photons do not have nor vertical, nor horizontal polarization - they are right-handed and left-handed :)
     
  7. Feb 7, 2016 #6
    Domullus or others...

    I bought many light amber or yellow lens that is supposed to be polarized... but all of them are only mildly polarized (one can see light yellow letter instead of black). However their dark color lens are really polarized such as in the following set of black and yellow I bought.

    https://www.amazon.com/gp/product/B015BYT3G6?redirect=true&ref_=cm_cr_ryp_prd_ttl_sol_0

    For full polarized lens to work.. should the color be of certain darkness? because in the above.. only the dark lens are truly polarized.. the yellow one and other brands I bought are not (only maybe 10% as only light yellow lettering can be made out in polarizing test card instead of black letterings). Any idea why?
     
    Last edited by a moderator: May 7, 2017
  8. Feb 7, 2016 #7

    sophiecentaur

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    Only the most expensive forms of polariser work well over the whole of the visible spectrum - it often doesn't matter very much, actually. Anti reflection sun glasses only work as well as 'necessary' and a crossed pair will still tend to let through some visible level of light (often dark blue if I remember right) where a certain amount of blue cross polar light gets through each.
    I would suspect that the paint effect doesn't involve really good polarisation discrimination and that it's the paint, rather than the glasses film that is failing. Try a good quality photographic polarising filter. Many of them are circular polarised, for the benefit of the internal focussing optics so make sure you use it the right way round of you will not get the effect.
     
  9. Feb 7, 2016 #8
    I have a camera linear polarizing. I have tested it on surfaces.. it can suppress some reflections that the yellow/amber lens can't do (that the black lens can). So I'm wondering what is the lightest color you can make a full polarized lens? For example. Can you make a totally clear polarizer lens? Or what is the least darkening effect for it to work?
     
  10. Feb 7, 2016 #9

    sophiecentaur

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    I don't know the numerical answers to those questions but if you have a digital camera, you could get a good idea. Take a picture of an evenly lit white card with a single filter in front of it (make sure it casts no shadow on the paper), With your photo processing package, look at the RGB values of the paper and the filter,making sure that the paper is not limiting at 255,255,255. The ratio of the values for the paper and the paper through the filter will show you the loss through the filter. Likewise for two filters (not crossed). You could try the same with crossed polarisers but you may get R,G or B of zero - meaning that the exclusion is better than 1/white level i.e better than you can measure easily. If you use manual settings for exposure, you can use a range of pictures with different stops and compare pictures. That can give you a bigger range of measurement than 1:255 if you get your sums right.
    Digital cameras are amazing measuring instruments if you use them inventively.
     
  11. Feb 7, 2016 #10
    Ok will have them in mind. But some more details.

    http://www.hoyafilter.com/hoya/products/generalfilters/plplcir/

    I used the Hoya PL (Linear polarizing). On glacing surfaces with horizontal lighting components. It can block the glare.. turning it 90 degrees would have all the glare visible. In the https://www.amazon.com/gp/product/B..._=cm_cr_ryp_prd_ttl_sol_0&tag=viglink20267-20

    the black lens work like the hoya PL too.. but the amber one doesn't work (they are from same company and package).. there is at most only 5% effect. I tried many other light amber lens brands. They are only at most 5% polarized versus the black which are almost 95%. So is the reason because the amber lens are made in cheap labs. Can an expensive lab or company make light amber polarized lens at all or is it theoretically impossible because a real polarized lens would have a minimum darkened shading effect that can't be done on the light amber lens?
     
    Last edited by a moderator: May 7, 2017
  12. Feb 8, 2016 #11

    sophiecentaur

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    You have some 'good' polarisers, which you can check against each other. You have a 'fashion' polariser, which doesn't appear to work very well, when checked against the good ones. It strikes me that the orange filter is just not a good one.
    It is possible, of course, to make a filter that is a combination of polariser and a layer of orange tinted filter. Does what you see with the orange filter fit in with that possibility? (Let's face it, the sunglasses are not likely to be fantastic scientific equipment. :smile:)
     
  13. Feb 8, 2016 #12
    It's yellow filter. Did you see it as orange??

    It's not more of 'fashion' but as driving aid at daytime. It can block blue light or at least a part of it so there is less glare. Btw.. I also need to know why when you use a yellow lens.. the grey colors in the images viewed have better contrast.. this is true for brown lens too.. any explanation?
     
  14. Feb 9, 2016 #13
    By definition, a 100% effective polarizer will filter 50% of the light. This is not about tint (colour) - any tint will additionally filter out certain ranges of wavelength.
     
  15. Feb 9, 2016 #14
    So is it possible to make a clear polarizer? Can anyone point me to such product?
     
  16. Feb 9, 2016 #15
    I don't think you understood what I wrote. Again, by definition, if a "filter" is clear it passes 100% of light of all polarisations.
     
  17. Feb 9, 2016 #16
    Ok got it.. for a lens to filter half of light.. how dark must it become and what color would it become (without any tint present). Any application to show a screen in cellphone or lcd screen with half dark? Is it the same as seeing thru a half darkened lens?
     
  18. Feb 9, 2016 #17

    sophiecentaur

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    Are you really aware of what a polariser does? Some of what you wrote here makes me wonder. A linear polariser selects the component the E field of a wave that's going through it. That means it will select exactly half of the power going through it when the light is unpolarised (i.e. the source contains a random selection of different waves with random polarisation). A linear polariser will also let through exactly half of the energy of a circularly polarised beam. A polariser is not just a set of slots that only let through waves that are perfectly aligned with it; that would only let through an infinitely small fraction of the light.
    Polarisers tend to have a limited bandwidth over which they act well. The more you pay, the better they will be. If they do not polarise (for instance) red light very well then two such polarisers will always let through some red light, whatever orientation. Polaroid material consists of many small dipole elements, embedded in the plastic which are aligned in one particular direction. They will absorb the light with one polarisation by absorbing the component of the E field in the preferred direction. It's a very smart but crude mechanism which tends only to work well over a limited wavelength range. It's so much cheaper than any other form of polariser that pretty well everyone uses it.
    You can expect any layer of transparent film to be slightly selective in how it passes light; there will be a 'tint', which may not be discernible but will be measurable. How much do you want to pay?
    A 50% neutral filter will achieve this. Play with your digital camera, looking at pictures of an LCD screen, as I suggested earlier. You will be surprised at just how far adrift things can be from 'ideal', yet still look OK. There as many 'greys' as you have had hot dinners.
     
  19. Feb 9, 2016 #18
    So I guess the light amber/yellow shades would no longer be light yellow if it can polarize half of light or the horizontal components... so for it retain light yellow.. it shouldn't be polarized. I bought a number of it just to test if it works. Manufacturers claimed they were polarized when they really weren't (except the black, brown or dark color shades).
     
  20. Feb 10, 2016 #19

    sophiecentaur

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    There are two issues here, I think. You can have a tinted film with an additional polaroid layer. That can produce a whole set of effects depending on the proportion of each and how 'good' the polaroid is.
    But what exactly do you want this stuff for?
     
  21. Feb 10, 2016 #20
    I already wasted money on two sets of light yellow shades which are not really polarized (even if they are advertised as polarized). I'm looking for a third pair of light yellow lens that is really polarized.. wonder if you know where to find such product..
     
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