Polarized Lights and Laptops

I was taking apart a laptop's LCD screen and found that sandwiched between the light source and the liquid crystal display were what appeared to be a vertical and horizontal polarized filter. Why is this? I always got the impression that LCD displays had the polarizers attached to the screen itself. Why would there be an extra set of polartization filters behind the screen? Or... am I just plain wrong in thinking the polarization is on the LCD itself? Could these be the only polarization filters? I have searched all over the internet for the answer to these questions, but I think I am just more confused. If these are extra polarization filters, the only thing I can imagine that they are doing is making the screen darker. Any ideas?

westfw8 years ago
There is an excruciatingly detailed tutorial on the components of an LCD display at 3m (link via wikipedia's Backlight page.) What you're looking at is probably either "prism film", which has to do with redirecting light at odd angles into more appropriate directions, or a "reflective polarizer" which has to do with reflecting light that the back polarizer would have absorbed back into the diffuser to try again. But yeah, there a a bunch of thin sheets of material in there with "interesting" optical behavior!
Patrik8 years ago
In the typical LCD, the liquid crystal layer just rotates the polarization of the light passing through it by 90 degrees, or lets it pass straight through when you apply a voltage across it.

Sandwich such a layer in between a horizontal and a vertical polarizing filter, and you get light shining through where there's no voltage, and light completely blocked where there is a voltage. The filters are often thin plastic films glued to the glass substrate of the LCD, so you may not always notice them.

There's other types of LCDs as well, but that's the most common one. See http://en.wikipedia.org/wiki/Liquid_crystal_display
randofo (author)  Patrik8 years ago
Yeah.... that's the problem and exactly why I'm confused.... to clarify, I've attached an image. There appears to be two extra sheets of plastic polarizing the light once more to turn it another 90 degrees. I'm not sure I get why its built this way.
Patrik randofo8 years ago
Wait - that image didn't clarify much! :-D Could you just give a list of the complete optical stack, layer by layer?