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Choosing a display has never been so difficult as with LCD screens. There are so many acronyms, so many differences, and so little information. Recently, my wife and I decided to purchase a laptop for her to do her work and school on. Of course e-mail was a necessity as she and her brother's wife spend ungodly amounts of time talking on the phone, and speaking of which, my wife and her have the strangest rules of engagement for e-mail I have ever witnessed. One of them will send an e-mail in the normal e-mail sending fashion, and then for some unknown reason, some unforeseen force, some primal urge, they will pick up the phone and proceed to tell the recipient exactly what was contained in the e-mail. I'm confused! Maybe I missed something? Possibly there is an RFC that discusses this phenomenon. But, I digress. Let's get back to LCD screens.

LCD stands for Liquid Crystal Display and you've seen the technology hundreds, if not thousands of times while looking at your digital watch. How it works is both simple but very interesting. A liquid containing crystalline structures is sandwiched between two sheets of glass and the crystals react to electrical pulses by changing their position. Depending on the position of the crystal, it either allows light to pass or blocks it much like a shutter in a camera. Of course, the LCDs used in computer screens are a much more complex technology due to the need for color and the the ability to view the screen from the widest possible angles.

Let's start off by classifying the types of LCD technology, and then we'll move into what each offers.

There are two main types of LCD displays called Passive-Matrix and Active-Matrix.

Passive Matrix

  • CSTN - Color Super Twist Nematic
  • DSTN - Double-layered Super Twist Nematic
  • TSTN - Triple Super Twist Nematic
  • HPA - High Performance Addressing

Active Matrix (synonymous with TFT)

  • No major differences.

Active Matrix is currently the king for LCD displays as it offers the brightest overall picture and the widest viewing angle. However, this performance comes at a price. The reason for this is the main difference in Active Matrix and Passive Matrix screens. Passive Matrix screens access the pixels on the screen via a column and row type of power system.

Let's say the screen size is 800x600 pixels, and to access any particular pixel you activate it by sending an electrical pulse to the column and row that the pixel resides on. This is done with the use of transistors at the heads of each column and row. Because our screen is 800 x 600 we need 1,400 total transistors to do this job. Active Matrix screens differ in that every single pixel has between one and four transistors each. This means that for an 800 x 600 screen you would have at a minimum of 480,000 or a maximum of 1,920,000 transistors per screen.

The high cost is a result of low yields in the manufacturing process, because if a certain number of those transistors were to fail (either constantly bright pixels or constantly dark ones) you would have a worthless screen. Notice I said "a certain number of failed transistors". Depending on the manufacturer's quality control guidelines, there are certain criteria for sellable and non-sellable screens. They differ from manufacturer to manufacturer, but basically state that only a certain number of dark pixels and a certain number of bright pixels within a certain distance from each other can exist in a sellable screen. This is why it is imperative that you ensure there is a no questions asked return policy for the screen/laptop you purchase. While it may pass the QC criteria at the manufacturer level, it can be an extremely aggravating and annoying thing to actually have to look at while using the screen. A test you should perform before purchasing that LCD screen is to do this:

  1. Right click on a blank area of the desktop.
  2. On the Background tab, select "None" and click Apply.
  3. On the Appearance tab, Item dropdown box, select Desktop
  4. To the right of that, change the color to Black and click OK
  5. Close any open windows.
  6. Look closely at the screen, are any pixels not black?
  7. Move the icons that are on the screen to a different area and check the screen where the icons were.
  8. Go back to step 4 and make the color White, then do steps 5 - 7 except you are looking for non-white pixels

You should probably try to set everything back to the way it was before your test, so as not to make the salesperson angry :^). If you noticed no incorrect pixels, then that screen is good, however that may not be the case with your actual purchase. Again, check for a no questions asked return policy. Chances are that you will most likely not be able to find a screen that does not have some defect, but it's your screen and you make the decision.

As for Passive Matrix screens, the order in which I listed them above goes from worst to best. Passive Matrix screens used to be a no-no for the serious LCD user, but recently new advances in the ability to twist the lightwaves provides a brighter screen and a wider degree of viewability. They still can't match an Active Matrix, but they can still be a very viable alternative for someone looking to cut costs on their purchase. The only real test is to try and see the different types next to each other before buying them.

One other item worthy of note is that Active Matrix screens use more power than their Passive Matrix counterparts.

A real nitty-gritty resource for those of you who wish to delve deeper into the mysteries of LCD technology should go to: http://www.sharp.co.jp/sc/library/lcd_e/indexe.htm



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