How Do LCD Screens Work?
As liquid crystal display would indicate, LCD screens use liquid crystals to switch pixels on and off to reveal a specific color. Liquid crystals are like a mixture between a solid and a liquid, where an electric current can be applied to change their state in order for a specific reaction to occur.
These liquid crystals can be thought of like a window shutter. When the shutter is open, light can easily pass through into the room. With LCD screens, when the crystals are aligned in a special way, they no longer allow that light through.
It's the back of an LCD screen that's responsible for shining light through the screen. In front of the light is a screen made up of pixels that are colored red, blue, or green. The liquid crystals are responsible for electronically turning a filter on or off in order to reveal a certain color to or keep that pixel black.
This means that LCD screens work by blocking light emanating from the back of the screen instead of creating the light themselves like how CRT screens work. This allows LCD monitors and TVs to use much less power than CRT ones.
LCD vs LED: What's the Difference?
LED stands for light-emitting diode. Although it has a different name than liquid crystal display, it's not something entirely different, but really just a different type of LCD screen.
The major difference between LCD and LED screens is how they provide backlighting. Backlighting refers to how the screen turns light on or off, something that's crucial for providing a great picture, especially between black and colored portions of the screen.
A regular LCD screen uses a cold cathode fluorescent lamp (CCFL) for backlighting purposes, while LED screens use more efficient and smaller light-emitting diodes (LED's). The difference in the two is that CCFL-backlit LCDs can't always block out all the black colors, in which case something like a black on white scene in a movie may not appear so black after all, while LED-backlit LCDs can localize the blackness for a much deeper contrast.
If you're having a hard time understanding this, just consider a dark movie scene as an example. In the scene is a really dark, black room with a closed door that's allowing some light through the bottom crack. An LCD screen with LED backlighting can pull it off better than CCFL backlighting screens because the former can turn on color for just the portion around the door, allowing all the rest of the screen to remain truly black.
Not every LED display is capable of dimming the screen locally like you just read. It's usually full-array TV's (versus edge-lit ones) that support local dimming.