I guess not quite all kinds of LCDs would be blocked by just one-direction polarization. Would they?
Actually polarized sunglasses have been existing for a long time. I happen to have a Ray-Ban pair. They are great for color preservation.
Anyway, with those on, only *some* types of LCD screens become invisible, but not all. (For the record, they tend to render TN-LCDs invisible, such as the ones used in gas stations - which bites btw when you're filling your tank, it's sunny and you can't see the screen 
- but active matrix ones tend to remain fully visible, particularly IPS panels.)
What would make their project different and what's the added technology?
The trick is to use multiple polarisation layers in different directions. You'd be able to block almost any polarised screen with two or three layers. Conversely, sunglasses are typically designed with the polarisation layer in a direction least likely to cause issues, which I think might be diagonal.
I can explain how this works.
Yes sunglasses are vertically polarized because when light bounces from a shallow angle off a level surface it preferably bounces off horizontally polarized. Since the polarization of the light is 90 degrees to what the polarizer on sunglasses wants means that all of that light is blocked while the rest makes it trough. This means that the glasses attenuate glare significantly more than randomly polarized light.
But a lot of LCD displays are also horizontally polarized (likely to provide a better viewing angle in the horizontal plane), so this sunglasses trick stops working once you tilt the LCD in portrait mode. So what if you use both horizontal and vertical polarizes in the sunglasses to block both? That would indeed block the screen, but it would block everything else too, making the glasses completely black. The reason for that is that the first polarizer forces even randomly polarized light to align so when you put a extra 90 degree offset polarizer behind it then it blocks 100% of the light because all the light coming trough the first polarizer is the exact polarization that it blocks. Interestingly if you add a third polarizer at 45 degrees between the two you once again have light coming trough (This is a nice demonstration of wave behavior of light, you can see it demonstrated on youtube and explained why it works)
But as some have noticed this doesn't work perfectly on all displays. This is because some materials can scramble the polarization of light. If you place objects made of certain kinds of transparent plastic between the display and your sunglasses you can see the picture on the screen again. Some displays are covered by such materials that scramble polarized light coming off the LCD panel, not sure if this is intentional or just a coincidence of the materials they happen to use. But it still doesn't result in a perfect image, it usually makes it a dark image with weird rainbow patterns in it.
This trick is actually used for inspecting such polarization scrambling materials. Because this effect has to do with the tangled long polymer chains in plastics means that if you look carefully you can see how the plastic has flowed into the mold and can be used to see stress concentrations in the object.
Home
Help
Search
Login
Register
