Polarization Questions...

[Pete Hanlin]

Howdy,
If I look at the LCD display on my laptop while wearing sunglasses (don't ask :-), I notice that the screen is polarized at 45 degrees (like a car radio display). I knew crystal displays were polarized, but it just made me wonder... "why?" I imagine it must have something to do with the properties of the crystal (oooh, there's a brilliant assumption, huh?)...

Second dumb question: If a lens was made with four or five polarizing sheets that were each 2 degrees apart in orientation, wouldn't the result be a lens that "maintained" it's polarization as you tilt your head slightly (within a few degrees, anyway)? How thick is the polyvinyl alcohol sheet that is inserted in the lens, anyway?

Finally, is there a good reference out there that shows why stretching the cells polarizes light to begin with. I understand the concept behind polarizing filters and all, but I'm still sketchy on the overall mechanics.

Pete "Would still like to see a clear polarizing lens" Hanlin

[Blake]

Pete,
LCD's are made with polarizing filters on the glass plates containing the liquid crystal substance. Normally, the polarizers are crossed, but the polarization is modified by applying an electric field to the liquid crystal. Because of circular polarization within the LCD, linearly polarized light leaving the LCD is polarized at a different angle, hence the polarization along 45 degrees.
I don't see why you're idea of multiple films wouldn't work, except that in the overlapping areas the transmission percentages might be affected, causing some dark spots... but I'm merely speculating!
Hope this helps...

Blake
*note: the info in the first paragraph came from an MIT web page(someone's research paper).*

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where you can get a complete pair of glasses starting at $99.

[This message has been edited by Blake (edited 06-10-2000).]

[Darryl Meister]

Originally posted by Pete Hanlin:
Second dumb question: If a lens was made with four or five polarizing sheets that were each 2 degrees apart in orientation, wouldn't the result be a lens that "maintained" it's polarization as you tilt your head slightly (within a few degrees, anyway)? How thick is the polyvinyl alcohol sheet that is inserted in the lens, anyway?...

Finally, is there a good reference out there that shows why stretching the cells polarizes light to begin with...

Hi Pete,

Combining multiple sheets of Polaroids together will create a single net effect, representing the sum of all the Polaroids, not a varying effect. Remember that as the light passes through each subsequent Polaroid a portion of it (at some orientation) has already been absorbed by the preceding Polaroid. You can determine this for two Polaroids crossed at a given angle (Angle) using Malus'law:

New Intensity = Old Intensity * cos^2 (Angle)

With 4 sheets, each crossed at 2 degrees, the first sheet absorbs 50% of all light. The second, third, and fourth sheets will absorb:

New Intensity = 50% * [cos^2 (2)]^3 = 50% * cos^6 (2)
New Intensity = 49.8%

The orientation of the final light waves will be equal to the last Polaroid. However, you could implement your suggestion by using a single Polaroid oriented at say 2 or 3 degrees, instead of 180 degrees. Keep in mind, though, that this would only help if you tipped your head in one direction (not the other).

Polaroid films work on the action of molecules, and the sheets are quite thin. (I don't have a value for you, though.)

I'm sure that there are several good references on Polaroid films. Most ophthalmic optics text, however, will only give you a general explanation. To make a long story short... The film has molecules that are all aligned in the same direction. This is often done by stretching a thin sheet of polyvinyl alcohol, for instance (and then soaking it in iodine). These molecules have electron bonds that are stronger in one orientation and weaker in another (and all of them are now aligned). When light passes through the film, the electric field component of the light is able to "do more work" on the electrons with the weaker bonds. (Remember that light has energy.) As the light does work, its energy is converted into heat. Consequently, the light is effectively absorbed. This only occurs though, for light waves vibrating at the same orientation as the aligned electrons.

Anyway, that's my understanding of it -- and I hope that makes sense.

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 06-11-2000).]

[Darryl Meister]

*bump*

[yzf-r1]

Darryl Meister said:
*bump*

Oh, look everybody. Darryl just bumped into us!!!

[harry a saake]

Darryl, physically speaking i suppose, how is the polyvinyl sheet actually strechted and by what means and how do they know when it s right.

[Darryl Meister]

Hi Harry,

I believe that the films are heated and stretched to about four or five times their original length before exposure to an iodine solution. We don't manufacture the films at SOLA, so I would not know what kind of apparatus they use to do this without making some phone calls. ;)

Best regards,
Darryl