23
2012
Technology Trends Part 3
An article by admin
Presentation Technology in LCD
Color LCDs are fairly new. Digital watches and calculations were the first commercial use of LCDs in large volume. Calculators and watches require displays with very few segments, which are easy to make. For TV or computer displays, a dot matrix displays is needed. Dot matrix displays require a much larger number of segments or pixels. To keep the electronics economical, each dot must be multiplexed at 100:1 or 200:1. This multiplexing of dots has the result of lowering the contrast of the display, since each dot is not individually driven. To improve contrast, LCD manufacturers have devised ways of improving the liquid crystal materials.
In a normal LCD, the liquid crystal material is placed between two planes of glass (one etched with row conductors, the other etched with column conductors) rotated 90 degrees against a polarizing film to cause a black and white difference. Different polarizing films can be used to get different color LCDs. In Super Twist LCDs, the liquid crystal material rotates 270 degrees.
In Double Super Twist displays, two layers of liquid crystal material rotate 270 degrees between three planes of glass or sometimes plastic.
This trick improves the contrast, but at the expense of response time. In a computer application, this gives the effect of a long persistence phosphor, or a smearing effect. In some cases, the response time is so long that fast moving elements (like a cursor) will completely disappear, until they stop moving. To combat this effect, the effective matrix was developed.
In an active matrix displays, a thin film transistor is actually deposited on the glass, thereby providing an individual driver transistor for each pixel. This improves contrast and response time dramatically at the expense of a more complicated glass process.
By far the most expensive element of an LCD display is not the cost of the glass but the cost of driver chips. A monochrome 640 ×480 display may have as many as 28 driver chips, each with 100 leads. Extrapolate this the color of LCDs: now you need three times the numbers of pixels- one each red, green, and blue. Next add a tremendously bright black light to be able to see through all the layers. Then add active matrix circuitry to prevent smearing of the videos. Making color LCDs is no simple process.
In the late 1980s, the first commercial use of color active matrix LCDs in a consumer product was for a small, the yields were sufficient to allow a cost-effective display for a consumer-period portable TV comparable to CRTs.
