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one-way privacy window

circular-polariser coated windows
  [vote for,

Existing one-way windows and mirrors have two major flaws. They a) require a significant difference in light intensity on either side of the window to be effective, and b) are not really one-way since they transmit the same amount of light in both directions.

My idea is simply a window with a circular-polarising filter.

One surprising thing about circular polarisers is they do not transmit reflected light (see diagram at the bottom of first link). This effect is currently used for things such as testing if a filter is actually a circular polariser, and as an anti- reflective coating on touch screen devices; however as far as I know this effect has not been used as a privacy window.

The idea would require that *all* light entering the private room is circularly-polarised. So all windows and lights would be coated in the circularly-polarising filter material. Obviously, all filters would all circularly polarised in the same chirality (i.e. clockwise or anti-clockwise)

This idea is predicated on my assumption that if an everyday object is illuminated with circularly polarised light then it will simply reflect circularly polarised light (I know there are some things that change the polarisation light as it is reflected, but I am guessing this is exception rather than the rule).

EDIT: I've tested it out and my assumption was wrong! So although this idea doesn't work I'll leave it the idea on HB for entertainment value only.

xaviergisz, Aug 19 2013

circular polariser diagram http://www.apioptic...lar-polarizers.html
[xaviergisz, Aug 19 2013]

You may have to wear one of these. http://www.polyvore...size=l&tid=38263735
Mirror ball suit. [spidermother, Aug 20 2013]


       So, if I understand the reflection business, the point is that light which is circularly polarized one way (say, left-handed) becomes oppositely- handed (to right-handed) on reflection. So, if the lightbulb in your room is covered by a left-handed circularising filter, and the windows are likewise left-handed, then the light shining from the bulb and bouncing off objects in the room will, being right-handed, not be able to get out of the window.   

       Is that right?   

       What about light that undergoes two reflections (eg, a ceiling light that bounces off the ceiling before hitting an object in the room, then bouncing off that and out the window)?
MaxwellBuchanan, Aug 19 2013

       You have explained it better than I have.   

       Yes, the outside observer you would be able to see the lights and secondary reflections clearly, but not primary reflections. If a dark colour scheme was used in the house then secondary reflections could be minimised.
xaviergisz, Aug 19 2013

       Never knew that light could be circularly polarized. Or even circular waves. Mind considerably boggled, even as I look at the diagram.
RayfordSteele, Aug 19 2013

       I agree it is all a bit counter-intuitive. I was just playing around with a camera circular polariser filter and confirmed the mirror effect with my own eyes.
xaviergisz, Aug 19 2013

       My expectation would be that light reflected from a mirror-like surface would not go through the polarizer, but when light reflects from a surface which has texture and depth (partial transparency), that the light would experience multiple reflections on the surface and would therefore be a mix of polarizations. That would work well for reducing glare which is generally light reflected from a mirror-like surface, but would do litle to prevent seeing most objects inside the room.   

       [xaviergisz] Did your experiments with your filter prove otherwise?
scad mientist, Aug 19 2013

       //Never knew that light could be circularly polarized.//   

       Indeed, that's how some modern 3D movie systems (such as RealD) work. One eye is polarized one way and the other eye the opposite way. The advantage to this system over traditional horizontal/vertical polarization is that you can view the screen from any angle or rotation and the 3D effect is retained.
ytk, Aug 19 2013

       I suspect you might be right, scad mientist.   

       Since I only have one circular filter at the moment I haven't been able to properly test my assumption. However, I plan to go and buy another circular filter later today so I can do a proper experiment. I'll put one filter over a torch, look through the other filter, and shine the torch at objects in a dark room. Expect results in roughly 12 hours from now.
xaviergisz, Aug 19 2013

       You may (or, equally, may not) be interested to know that circular polarization is responsible for speciation in the Knapweed Chafer, a type of beetle found in much of Europe.   

       Knapweed Chafers have an iridescent greenish hue which, due to the chirality of chitin (the main component of their cuticles) is circularly polarized. The same polymer is present in the transparent cuticle over their eyes.   

       In the ancestral population, the reflected light was clockwise (right-handed) polarized, and was visible through the correspondingly polarising cuticles over the beetles' eyes. However, very recently (within the last couple of centuries, according to analysis of genetic drift), a mutation arose which inverts the polarization of the cuticle, both over the body and over the eyes. The result is a population of Knapweed Chafers which look green only to others carrying the same mutation.   

       As a consequence, there are now two populations - to all intents and purposes two distinct species - of knapweed chafers. The "lefties" only see the green iridescence of other "lefties", and will mate only with them. The "righties", equally, can only see and mate with other "righties".   

       It might be expected that the minority "lefty" population would have died out quickly for want of mates, and that the "righties" would have won out. However, this is not the case: "lefties" thrive in areas where they occur, until a roughly 50:50 mix is obtained. It seems that when most of the population has the same "handedness", a lot of energy is wasted by fighting off rivals or unwanted suitors. A minority population has fewer choices for mates, and therefore wastes less energy in choosing.
MaxwellBuchanan, Aug 19 2013

       //Knapweed Chafer//   

       Wait a minute… I thought he was the Ninth Duke of Uxmouthshireford—and a distant relative of yours?
ytk, Aug 19 2013

       Whether this pans out or not, I'm bunning it for the original thought and effort.
AusCan531, Aug 20 2013

       I just bought a filter (Hoya 46mm circular polarising filter - left) for $60 (yes, had I been patient I could have bought it online for half that). It is still a few hours until I can perform the experiment.
xaviergisz, Aug 20 2013

       Well, the results are in and... it doesn't work ;(   

       I couldn't even get the (circularly polarised) torch light reflecting off the mirror to be eliminated with my viewing circular polariser, let alone reflection off any other surface to be eliminated.   

       (Although I originally thought I had the experiment partially working this was mistaken - I was using the ciruclar polarisers the wrong way around which made them essentially linear polarisers).   

       In summary, circular polarisation is confusing.
xaviergisz, Aug 20 2013

       That is a shame, [xav], but keep thinking...   

       //Ninth Duke of Uxmouthshireford// No, though you may be thinking of the Sixth Earl of West Norfolk, who is a cousin and great-aunt.
MaxwellBuchanan, Aug 20 2013

       OK folks, an update on the experiment.   

       It turns out that my circular polarisers were actually left and right polarised, whereas I had thought they were both left polarised. Oops.   

       I figured this out by buying some passive 3D glasses and viewing each polariser. Passive 3D glasses work by having one left circular polarised lens and one right circular polarised lens.   

       Lo and behold, light passing through one of my polarisers was obscured by left lens of the 3D glasses but not the right. Light passing through the other of my polarisers was obscured by the right lens of the 3D glasses but not the left.   

       Why didn't I notice this before? mostly because the effect is not as stark in reality as the theory leads one to believe. I can still see some light that passes through both the left circular polariser and then right circular polariser. Basically, I lacked something to compare with and thus made an incorrect inference.   

       I might try the experiment again (this time using two circular polarisers of the same chirality) but the quality of my polarisers is apparently not great.
xaviergisz, Aug 21 2013

       Of course, you already have //two circular polarisers of the same chirality// if you include the lenses in the 3D glasses. Both chiralities, actually.
ytk, Aug 22 2013

       Indeed, ytk.   

       So I performed the experiment again using one lens of the 3D glasses and one of my polarisers.   

       Two polarisers of the same chirality (one covering the torch light,one covering my eye) had the expected effect: The light from the torch was attenuated in the mirror reflection when compared with shining straight into my (circularly filter covered) eye. There was no attenuation in reflection of any other object.   

       The one puzzling thing about the experiment was the colour of mirror-reflected torch light was dependent upon the orientation of the circular filter covering the torch. So as I turned the polariser, the apparent colour of the torch light changed from blue to red to yellow to purple. (Note this same effect happened when a left-circular polariser covering the torch was shon into my right-circular polariser covered eye).
xaviergisz, Aug 22 2013

       Although my original idea did not work, I have a variation that probably will: A one-way mirror with a circular polarising coating.   

       Start with a one-way mirror (i.e. half-silvered) that reflects 70% light and transmits 30%. If a circular polariser coats the 'private' side of the mirror, there is no reflection and only the transmitted (polarised) light can be seen.   

       On the 'public' side of the mirror both transmitted and reflected light can be seen. However, the reflected light will overwhelm the transmitted light, and consequently the private side will remain fairly private.   

       The advantage of this idea over a typical one-way mirror is not needing to keep the private side dark compared with the public side.
xaviergisz, Aug 28 2013


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