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.

 

[astrokreng]

Yes, you are correct in your understanding of how polarised light is used in LCD screens. The liquid crystals in the display twist the polarised light, and then the second polaroid filter allows the twisted light to pass through and create the image on the screen.

As for displays that darken when turned on, this is due to the presence of a backlight in the LCD screen. When the display is turned off, the liquid crystals are in a relaxed state and the backlight is able to pass through easily, creating a pale color on the screen. However, when the display is turned on and a current is applied to the electrodes, the liquid crystals twist and block some of the light from the backlight, creating a darker image. This is why the numbers on your calculator appear dark when the display is turned on and a current is applied.

I hope this explanation helps clarify how the darkening effect works in LCD screens. Additionally, the amount of current applied to the electrodes can also determine the level of darkness on the screen, allowing for different levels of brightness in the display.