Thin Film Transistor - Liquid Crystal Display

LCD displays, namely TFT LCD, due to improvements made in failure rates, falling production costs, and the increasing importance of environmental protection as a factor (TCO 99 standards) in monitor production, are becoming more and more widespread. 

Add to this the increasing tendency for diversification in computers, it is clear that within the computer peripheral industry. LCD monitors will be the star product of the next century. 

Common belief is that when the price of an LCD monitor, compared to a traditional CRT, - assuming both have an equal sized viewable image area- becomes equivalent to about double, the LCD will replace CRT as the principally purchased monitor. They will not replace the CRT display monitor, but rather will provide and alternative choice.

Besides the issue of price, LCD monitors also have advantages over a traditional monitor, Specifically: the body size is smaller by 3/4, the weight is 2/3 lighter, and, energy consumption is only 1/5 of a CRT. Simply stated, the LCD has qualities that a normal CRT can never compete with.

On the other hand, CRT monitors have qualities, that to date, and for the foreseeable future for that matter, the LCD-TFT displays can only dream of.

While LCD manufacturers boast 16.7 million colors, current technology only allows for 2,600 colors to be displayed at any single moment.

Why is this so ?

Liquid crystals are not able to be controlled such as is associated with a CRT's Electron Beam.  Liquid Crystals are either ON or Off,  while a CRT's electron Beam is able to be controlled to allow for all the luminance's required to produce 16.7 million colors on screen at the same time.

Moreover, Refresh rates (the number of times a screen is drawn per second) is able to be changed to avoid flicker and eye strain. 

These are perhaps the two most significant inabilities of Digital signals to match Analog.

a) Light, slim, and space saving. 
b)Large viewable area, actual flat screen and true image
c)Low power consumption, no radiation or magnetic field interference. 
d)More function possibilities, higher productivity and so therefore an excellent return on your money for those frequent computer users. 

The Basic Working Principles of LCD
Before I introduce the specifications of a LCD monitor I want to first, using TFT LCD as an example, mention a few basic working principles. The TFT LCD is unquestionably more expense than the DSTN LCD, - about double in price -. It has, however, become the more commonly used model because of its bright, vivid colors, superior contrast and color grade resolution. An important thing to note is that liquid crystal itself cannot reflect light, for it to display as clearly in dark or low light conditions, as in other conditions, the LCD monitor liquid crystal face is installed with a background light. What needs to be clarified is how the LCD produces color as LCD itself only has black and white. The principle of color creation with LCD is the same as that with phosphor in a CRT monitor. The white back light passes through a crystal dot which is switched by a Thin Film Transistor (TFT), when the TFT is switched "ON" the crystal dot is non-transparent and black, when the TFT is switched "OFF" the crystal dot is transparent and white. After this process the light passes through a color filter and the color is displayed. Using a 15"LCD monitor as an example. The TFT controls a transistor switch, add the mix of the 3 primary colors and the result is the need for 1024x768x3=2359296 TFT on one liquid crystal spot. It If one transistor is broken will develop a flaw point so maintaining a very low failure rate is important. 

Specifications and Basic Functions

1. Model and Maximum Viewable Size
At present the most common and effective model is the TFT active matrix LCD. The panel size is usually measured from opposite corner to opposite corner of the screen. Unlike CRT monitors the figure given for panel size is equal to the figure for maximum viewable area. 

2. Resolution
LCD monitors, unlike CRT monitors, only have one "Pretty" (Native) resolution: the maximum resolution. If the monitor is set at a lower resolution, this resolution needs to be simulated and so the picture quality will suffer. If you take a 15" LCD as an example it has a maximum resolution of 1024x768, which is also its optimum resolution. The reason is that every liquid crystal spot, has built-into it an independent control mechanism for determining the brightness of a ray, the sum total of these spots immediately decides the resolution of the LCD. Therefore as the number of spots is fixed so is the resolution. 

3. Pixel Pitch
The smaller the pixel pitch the more detailed the image: the pixel pitch is usually related to the resolution and the LCD size. To give an example, a 15" LCD has a pitch of 0.3mm. 

4. Viewing Angle
This is the angle, using the center of the screen as our starting point, at which the image can be seen clearly, irrespective of whether one is viewing the angle from the top, bottom, left or right side. 

5. Color Depth 
For the full color specifications the display should be able to provide 16,770,000 colors. Providing it has a video card that supports this number of colors it is easy for a CRT Monitor to provide this kind of color performance. However, for a LCD Monitor to provide this kind of color performance it means that each crystal must be able to control the strength of the light rays passing through it, only when it is doing this is it able to show different levels of color tones. At present this is obtained by controlling the strength of the flow of electric that is flowing through the transistor and by using an express speed switch. The technology for this is exceptionally complex so most of the LCDs on the market only support 2,600,000 colors 

6. Brightness and Contrast Ratio 
These two specifications are particularly strong in LCDs, when assessing a monitor the higher the figure the better these values are. Most 15" LCDs have a brightness estimated at 200-300cd/m2 and a contrast calculated to be 200:1-300:1. This figure for the contrast has to be estimated because the human eye can only distinguish contrast up to about 100:1. 

7. Response Time 
When the resolution is changed the LCD will automatically change to a different preset value in order to show the picture, The time it takes from when the signal is received to its conversion Time, as an image displayed on the screen is the Response Time. For most LCD Modules the normal response time is 30-60ms.

8. Vertical Refresh Rate 
This figure is a measure of the number of times a screen can be completely redrawn in a second. The video card has an important role in determining this figure. However, one of the special characteristics of LCD is that it does not flicker so TCO99 regulations don't specify the need of LCD monitors to support the new, high frequency 85Hz Vertical Refresh Rate. A Refresh Rate of 75Hz is sufficient. 

9. Power Consumption Rates 
The LCD Monitor, because it does not use a cathode ray tube, is more energy efficient. The power consumption of the liquid crystal face is specifically related to its translucence, In other words the ability of light to pass through it and in what amount. The harder it is for light to pass through the liquid crystal, the higher is the wattage required from the background light, the lower are the energy savings, the greater is the amount of heat required and the shorter is the user life. Clearly , in order to save energy, and improve color brilliance and contrast the liquid crystal needs to be as translucent as possible. 

10. Pivot
Most LCD Monitors on the market provide landscape and portrait image styles. The specific style you desire is created by rotating the picture 90 degrees. In landscape the emphasis in the picture is on width, in portrait on height. 

11. OSD Setting 
Irrespective of if you buy a CRT Monitor or a LCD Monitor a built in On Screen Display to adjust monitor functions such as brightness and contrast is now common. 

12. Input Signal
LCD Monitors are divided into two categories, analog and digital. 

a) Analog Input Signal 
The analog signal is the same as a CRT Monitor. The interface it uses to connect with the host is a RGB analog signal; however, it then converts it to a digital signal for use inside the monitor. The most important advantage of this type is it is compatible with presently used popular operating systems. This makes the operation simpler when replacing a CRT monitor. Problems occur in the transformation from analog to digital that result in distortion, making it less stable and sharp when compared to a traditional monitor. Moreover, it is very inconvenient; every time you want to change the frequency, resolution or video card it is necessary to readjust the whole system. 

b) Digital Input Signal 
With a digital signal there is no A/D change so the picture is sharper and more stable, There is also no need to make readjustments making it much more convenient. However, it is not very compatible, it has to use specialist digital video cards. For a long time the 20 pin MDR ( Mini-D Ribbon) connectors were the mainstream digital type used for the digital signal of a DFP( Digital Flat Panel). In April '99, after the PanelLink-based DVI protocol issued by DDWG received support from VESA and some large manufacturers such as Compaq and HP, it is expected that this protocol will be adopted as the future mainstream. 

13. Color Profile
For a long time monitor user difficulties have revolved around differences in the system display, utilization and output of graphics. Recently, following the claims of WIN 98 that it has created a function that will manage these color related problems Microsoft has brought out the ICM color management file to assist the windows operating system. This acts as a bridge between the monitor, system, and image input/output equipment so the monitor image and printed materials are the same. 

Although LCD monitor interface standards have not yet been unified, and, prices are still rather high, new technology and levels of product quality are continually being advanced (e.g. color LCD, LPS-TFT, CGS-TFT) and commercialized. New and old manufacturers alike are all investing in LCD technology products.