lexan transmittance
I think it is more like about 90% transmittance for one 0.093" thickness of lexan XL10. I just measured a scrap piece that I have, which is somewhat scratched and dirty: 88%
Still, I think that is much better than the UV filter sheets I have seen. Around 90% seems to be the norm even for the clearest glass you can find. A good dichroic IR filter (hot mirror) will claim 90% transmission of visible light and 10% transmission of IR. That is much better than the plastic film version (Rosco Thermasheild) that only transmits about 81% of the visible light.
The real light sponge is the LCD: Every bit of light goes through a color filter, and through the initial polarizing filter. Every red filter absorbs the green and blue, every blue filter absorbs the red and green, every green filter absorbs the red and blue. So the maximum you get through the color filters is near 33% transmission. Then the initial polarizing filter absorbs all the light that is not in the right polar orientation: near 50% transmission. That is a maximum of 16.5% transmission.
Want some real numbers? I just measured the LUX before my lexan: 31700 LUX. After one piece of lexan, two fresnels, and the LCD with a 100% white image: 4900 LUX
That is 15% transmission. The lexan & fresnels get it down to about 73%, then the LCD takes 80% of the remaining light.