How does a LCD display works

Question

working principle of LCD(liquid crystal display)

Answer 1

LCD display contains million of small LCD. Each LCD made of 3 small LCD. It has red,green and blue. As all red,green and blue light up,it mixes into white (fool human eye). Each this combined LCD called a pixel. It uses the same principle as a ordinary TV system except the picture tube replaces with solid state device.......an image is combined with many lines and line is combined with dots. Each dot is combine with three color dots to show most the colors. A huge IC  controls each dot,instructs it when and where it should light up or not.

Answer 2

The name "liquid crystal" sounds like a contradiction. We think of a crystal as a solid material like quartz, usually as hard as rock, and a liquid is obviously different. How could any material combine the two?

We learned in school that there are three common states of matter: solid, liquid or gaseous. Solids act the way they do because their molecules always maintain their orientation and stay in the same position with respect to one another. The molecules in liquids are just the opposite. They can change their orientation and move anywhere in the liquid. But there are some substances that can exist in an odd state that is sort of like a liquid and sort of like a solid. When they are in this state, their molecules tend to maintain their orientation, like the molecules in a solid, but also move around to different positions, like the molecules in a liquid. This means that liquid crystals are neither a solid nor a liquid. That's how they ended up with their seemingly contradictory name.

It turns out that liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature and why they are used to make thermometers and mood rings. It also explains why a laptop computer display may act funny in cold weather or during a hot day at the beach.

Hope the information helps you

Adieu

Answer 3

There's lots of different kinds, but the easiest to understand is the twisted nematic (TN) cell.  This is the cell that is in lots of active matrix LCD panels (that don't have a super-wide viewing angle).

Let's start from the back--the backlight.  The area of the LCD is lit from behind with some sort of light; fluorescent tubes, or an array of LEDs.  The light, as it is created, is randomly polarized.  Next you put on a linear polarizer.  This cuts half the light, and the result is linearly polarized.

Next, temporarily place another polarizer.  If the polarizer is oriented the same way as the first one, almost all of the light will go right through.  If the second polarizer is turned 90 degrees so that the polarizers are crossed, the second polarizer will stop all the light.  

This same effect can be observed by 'turning the light' instead of 'turning the polarizer'.  The polarization axis of light can be manipulated by using quarter-wave or half-wave (or whatever) retarders.  If you cross two polarizers, the light is blocked.  If you stick a half-wave retarder in the middle, the all the light goes through.  Less than half-wave means part of the light goes through.

So, to make a display, we need a means to vary or switch the amount that the polarization of light is changed as it passes from the back polarizer to the front polarizer.  We need to make a voltage-variable half-wave retarder.

Imagine stealing a case of spaghetti from the store, slitting it open, and sliding the box along the floor.  The strands spill out, but because they are long and stiff and they are lying against their equally long, stiff neighbors, they're all sort of pointing the same direction and lying against one another.  If you were to freeze them and cross section them you would get a regular pattern.  However, it is not possible to form a function that predicts where the endpoint of a spaghetti strand is using another spaghetti strand as the origin.  If you had a mythical super-microscope that could examine a nematic liquid crystal, this is what it would look like.  It is a 'liquid' in the sense that you can pour it (but commercial display LC's are very viscous).  However, it also has order, like a 'crystal'.  Furthermore, they want to line up together again if they are disturbed.

Commercial liquid crystals also have a dipole moment and 'dielectric anisotropy'.  This means that if you apply an electric field to them, they all line up together like a bunch of little compass needles with a magnet.  In addition, liquid crystals also affect polarized light...and how they affect it changes whether the light hits them parallel or perpendicular to the axis of the molecule.

As mentioned previously, liquid crystals like to line up in their ordered phase.  In addition, if they are placed in contact with something that has similar order to what the liquid crystal naturally wants to do, they will follow along.  So, if you have a solid surface that looks sort of like a liquid crystal, the liquid will all line up following the solid.  This is an 'orientation layer'.   For commercial TN cells, there are orientation layers on both sides of a thin (5 micron) layer of LC.  The orientation layer encourages the LC molecules to lie flat but twist like a spiral staircase from one surface to the other.

Imagine a capacitor made from liquid crystal with an orientation layer on the plates.  The voltage is off.  The liquid crystal is lying flat.  When the voltage is on, all the LC molecules turn up on end because the voltage overpowers the forces of the orientation layer.  When the voltage is turned off, the LC sees the orientation layer and lies down flat again.

Back to the display....starting from the back...backlight and polarizer.  Then lay down an array of capacitors with oriented liquid crystal.  Then put on another polarizer, oriented so that it is crossed (blocks light) with the first one.  The properties of the liquid crystal are designed so that when the capacitors have no voltage, the optical properties are a half-wave retarder.  When the voltage is fully on, the light hits the LC on-end, and there is no retardation.  At intermediate voltages, the forces from the electric field balance the forces from the orientation layer, and the polarization of the light is partially rotated.

Add color filters for a color display.