Polarizers have to block out 50% of unpolarized light, so how can LCD screens (especially the black and white ones found on calculators and watches) be so white? They’re clearly blocking out less than 50% of the light.
If the polarizer blocks all wavelengths equally, then the output will be white (assuming the starting light was white as well). I think you’re really asking how they can be so bright. I suspect that if you took off the polarizer you’d find that the background is brighter than you expected, precisely to make up for that 50% loss.
Watch this five minute video. Will answer this question very well for you (well done video).
White light dimmed by 50% is still white. Unless you compare it to the original lamp, in which case it would look light gray.
With a backlit LCD, you don’t get to see the unfiltered backlight at all. If you could, it would look twice as bright as the “white”.
With a reflective LCD (like wristwatches and calculators), the background is not white. Place a white piece of paper next to the LCD and you’ll see that the background is a fairly dark gray.
At 1:30 in the video I linked above you see him put a polarizer on the display. You can see it is darker than the surrounding area that is unpolarized but it is still pretty bright.
As I tell my students all the time, the human eye is fine for determining qualitative brightness differences, but terrible for quantitative differences. That is to say, you can generally say “this is brighter than that”, but if you try to say how much brighter (“this is twice as bright as that”, or the like), you’ll almost certainly get it wrong.
Indeed. In the video I would never guess the light coming through the polarizer is 50% less than the surrounding area. It looks darker but not half as dark.
I have always been amazed that my sunglasses, which are polarized, don’t seem darker than they are.
I assume this is, at least in part, due to our irises opening to allow more light in.
Here’s an example.
That one always gets me. Even with the color bar showing the reality I cannot rejigger my brain to stop think the squares are different.
Oops you’re right everyone, I was asking how they could be so BRIGHT, not WHITE. Thanks for guessing what I meant.
I suspected they really are 50% darker, but don’t appear that way because of the perception of light (it probably scales logarithmically, like sound). My sunglasses pass 10% of light, but they don’t feel that dark.
If this is the case, why are polarizing sheets so much darker than reflective LCDs? They should both be 50%.