How do LCD pixels work?

[Baronz]

I was wondering about this today. Anyways, since an LCD is just a display with dots on it that can change from black>white or vice versa depending on if they are being fed power or not, how does an LCD screen on a computer make the pixels change color? Isn't there just two states with LCD pixels?

 

[Stealth1024]

It all has to do with the polarization of light: http://www.howstuffworks.com/lcd.htm


There are two main types of LCDs used in computers, <B>passive matrix</B> and <B>active matrix</B>.
<B>Passive-matrix</B> LCDs use a simple grid to supply the charge to a particular pixel on the display. Creating the grid is quite a process! It starts with two glass layers called <B>substrates</B>. One substrate is given columns and the other is given rows made from a transparent conductive material. This is usually <B>indium-tin oxide</B>. The rows or columns are connected to <B>integrated circuits</B> that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, the integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the other. The row and column <B>intersect</B> at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that pixel.


<B>Active-matrix</B> LCDs depend on <B>thin film transistors</B> (TFT). Basically, TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a <B>gray scale</B>. Most displays today offer 256 levels of brightness per pixel.