system
Given the following post, respond as if you're commenting on Reddit:
human
Why do polarised sunglasses make some screens difficult to read?

I'm a crane driver and find it hard to look at my instrument screens when wearing my polarised sunnies. It's difficult having to choose between reading my vitals and avoiding sun strike.
gpt
It's because those screens also work through polarization.

Polarized sunglasses work by filtering horizontally polarized light, which is common in reflections of the sun (like off the ocean or snow). 

TN-style LCDs work by having a backlight, a polarizing filter, a thin film that can twist the light, and another polarizing filter. The thin film can switch from "twist light" to "don't twist light" for each pixel on the screen, so when it twists it can make the light get filtered and when it's not twisted, it doesn't get filtered. 

The end result is that all the light coming off your screen has the same polarization (if it didn't, the second filter would have already filtered it for you).