Polarisation & LCDs: How Does Darkening On Turn On Work?

[vickyc]

I think I understand how polarised light is used to light up an LCD screen. The polarised light twists as it passes through the LCD, then goes through another polaroid filter which allows twisted light through to light up the display - unless there's a current in the electrodes, which means that the crystals don't twist the light. Is that right?

My question is: what about displays which darken when you turn them on? I have a calculator which is a kind of pale greeny non-colour turned off, but when I turn it on and start typing numbers in, the numbers show up as dark. How does that work?

 

[mgb_phys]

My question is: what about displays which darken when you turn them on? I have a calculator which is a kind of pale greeny non-colour turned off, but when I turn it on and start typing numbers in, the numbers show up as dark. How does that work?

There is no backlight on simple LCD displays but there is a mirror on the back.
With the liquid crystal in one state you can see the room light reflected from the back, in the other state the the liquid bocks the reflected light - hence black.