Here's a conversation BlackShark and I were having over at AVS. Neither of us completely get it, so I thought I'd appeal to you geniuses here:

xhonzi:
Why do (most) LCD projectors have R&B at 0 degrees and G at 90? Is that an intentional design decision? Is it the LCD panel itself that polarizes the light, or is there a separate filter in the light path? Assuming it's a separate filter- can the green filter be rotated to 0 degrees?

RE: Green at 90 degrees

I've had a thought:
The dichroic mirrors used to break up the bulb light initially are essential a "high band" filter and a "low band" filter. The "high" filter reflects the "higher" wavelengths of blue and green, while passing the lower of red. Then a "low" filter is used to reflect the relatively low wavelenghts of the green while passing the blue.

If the filters in the x-cube prism are working off of the same wavelength bands, that would explain why green has to go through at a different polarity. Both the high and low filters reflect green. So if a low filter is used to reflect the red wavelengths into the lens and a high filter is used to reflect the blue wavelengths likewise... both filters would reflect green- ultimately reflecting it right back to the lcd panel.

The dichroic surfaces used in the x-cube prism must only filter high and low wavelengths at a certain polarity. Meaning reflect high (blue) at P and reflect low (red) at P, but pass everything at S (green).

This could be solved (perhaps the way D-ILA/LCoS works) by using narrower band dichroic filters. If a "higher" filter were used that reflected only blue wavelengths and passed green (and red), and a "lower" filter were used that reflected only red wavelengths and passed green (and blue), then I think that all panels could be transmitted at polarity P.

Am I full of crap?