LCD (theory) FAQ

Contents:


  2.4) The TN Liquid Crystal Response Curve

The most prominent feature of the TN response curve is the central linear region between the two flat areas (Zone II). Zone I describes the white color of the display when no electric field is applied. In other words, the display will transmit virtually all the introduced light. On the other hand, in Zone III, the display will diffuse light and appear dark. The middle region can display gray scale or an image somewhere between White and Black. The key point here is that you must be able to very carefully control the voltage applied to the LC cell and maintain it for one duty cycle (before that pixel is addressed again) in order to produce accurate colors. For this reason, this type of LC material is primarily used for active matrix LCDs.

COMMONLY ASKED QUESTION NOTE: because the LC material is partially twisted in the gray scale area, when looking at a display at an off angle the colors tend to shift and sometimes invert due to birefringence.

COMPUTER APPLICATION NOTE: The TN response curve does not have to be utilized for gray scale, in order to make a simpler display, improve viewing angle, and use cheaper IC drivers; the Apple Powerbook 170's TFTs (thin film transistors) drive the TN response curve directly into region 3. This gives all the speed/contrast advantages of a TFT display and cheaper manufacturing cost, but provides no gray scale.


  2.5) The STN Liquid Crystal Response Curve

The Key to understanding the STN curve is simply that due to the addressing method applied, only a small amount of voltage is available to change the LC material from transmittance to a dispersion state. For this reason, the shape of the curve has nearly a 90 degree shift between Zone I and Zone II regions; in other words, it goes ballistic and nearly straight up ! This property allows the LC material to shift from white to black at its threshold voltage (VT) without being concerned with partial transmission (gray scale). Furthermore, the 90 Degree curve shape means that gray scale is not available from the LC material itself and the driving circuits must provide the necessary fixes for levels of gray.

STN displays inherently have a yellow on blue appearance (anyone remember the old Zenith Laptops ?). Because many individuals found the yellow and blue appearance undesirable, a number of techniques were developed to convert the STN image to a black on white scheme. DSTN, developed by Sharp Corporation, was the first commercial black and white conversion of the STN display and refers to Double Super Twisted Nematic. DSTN displays are actually two distinct STN filled glass cells glued together. The first is a LCD display as described previously, the second is a glass cell without electrodes or polarizers filled with LC material for use as a compensator which increases contrast and gives the black on white appearance. The drawbacks are a heavier module, a more expensive manufacturing process, and a more powerful backlighting system.

FCSTN is Film Compensated STN and is now the most commonly used STN display technology on the market. FSTN, monochrome STN, and Polymer film STN are all standard STN displays with a polymer film applied to the glass as a compensation layer instead of the second cell as in the case of the DSTN. This simpler and more importantly cost effective method provides the preferred black on white image for this display technology. However, once again, this design lowers the transmittance of light and requires a more powerful back lighting system.

COMMONLY ASKED QUESTION NOTE: Why are STN displays slow? Due to the method used to address passive matrix (STN/DSTN) displays and the high density of pixels required for standard VGA displays, the liquid crystal material must respond to an extremely small change in voltage. In developing these materials for this voltage characteristic, there was a reduction in the switching speed. A slow display can best be illustrated by the tendency of the cursor to "submerge" or disappear when rapidly moved across the screen. Another common example is the blurring of images when they quickly move across the display as in the case of high speed games. A fast display is less than 40 milliseconds, most STN type displays are between 200 and 250 milliseconds. However, some new LC mixtures are reaching 150 millisecond speeds.

COMMONLY ASKED QUESTION NOTE: What is Contrast ? Contrast is defined as the ratio of black to white, more simply put, how black is black when next to a white or clear pixel. In terms of numbers, passive matrix LCDs are usually able to produce a contrast ratio of approximately 13 - 20:1; in real terms you get a set of different grays and blues but no true blacks.


100%	^
	|                      Figure 6
T	|
R	|    ********
A	|            *
N	|             *
S	|<-Zone I----> *<-----------Zone II--------------------->
M	|              *
I	|              *
T	|              *
T	|              *     
A	|             *
N	|            *
C	|           *
E	|          *  
	|          *
	|          *
	|          *
	|           *
	|	      **************************
0%	------------------------------------------------------->
			Vt Threshold Voltage
	Applied Voltage

  2.6) Liquid Crystal Display Assembly

Once the switching devices or electrodes have been fabricated on the glass halves and the polyimide film has been applied & rubbed, spacer balls (usually 4 to 8 micrometers [1 X 10 - 6 meters] in diameter) are sprayed on one half of the display. Spacer balls are used to insure that the glass plates remain a certain distance apart over the entire area of the display; this is also known as cell gap. If the cell gap is not uniform, an image will appear different from one end of the display to the other. If the spacer balls are not applied correctly, they will collect and the user will be able see them as strange areas of non-uniform dust or distortion. (Single spacer balls are too small to see and they are not black dots.) If the Display has a very large cell gap, when you apply slight pressure to the display by touching it with your finger, you will see the image change and the LC material shift under the glass. Doing this does not damage the display, but take care when bringing any sharp objects, such as pen or pencils, near the screen; it is very easy to damage the polymer film and or polarizers on the display.

The two glass panels are then aligned and glued together with an epoxy. During panel assembly, if dirt is trapped between the two glass plates, you most likely will see these as annoying spots on the display. During the application of the glue, one corner is left open. In a vacuum chamber, the liquid crystal material is drawn into the display through the open corner. Upon completion, the remaining hole is filled with another epoxy. The LC material will align itself to the grooves in the polyimide and spread out around the spacer balls.

After final assembly, excess glass is cut and driver ICs are mounted. The finished display is mounted onto a backlight assembly (also known as an inverter assembly) and encased in metal. There are a number of methods for backlighting a LC display. STN displays usually have a side, top, or bottom lighting system. In simple terms, this is where the fluorescent tube is mounted. For example, in a side-lit display one or two fluorescent tubes will be located at the left and or right edges of the display. A fluorescent tube normally 4 mm in diameter is used. This is dispersed by a plastic plate around the entire area of the display. A dispersion plate looks like a white sheet with small holes; each of the holes provides a small point of light. On top of the dispersion plate, a diffuser is placed. A diffuser takes the numerous points of light and uniformly spreads it out over the entire area of the display. The net effect is providing a backlighting source around 4 or 5 mm thick!

An Active matrix display, especially color modules, transmit much less of the incident light and require more elaborate backlighting systems. An active matrix TFT display has a matrix fabricated on one piece of glass; the metal lines and transistor elements are not transparent and block a significant percentage of light. In order to obtain higher contrast, newer displays incorporate what is called a black matrix. This is a black film that surrounds the pixel elements (this can be on the matrix, but is usually around the color filters); although this yields higher contrast, it also reduces brightness. Further complicating this, the polarizers and the color filters reduce the output to less than 5% of the incident light. As a result, most backlighting systems designed for active matrix based displays usually consist of 4 or 5 four mm tubes placed directly behind the display with a diffuser plate to insure uniform irradiation. Therefore, they are called backlit. This method of lighting makes the display slightly larger, heavier, and greatly increases power consumption. The final metal encased display is called a display module or sub-assembly and this is what the end user or notebook manufacturer receives.


Chapter 3) Addressing and Color Technology


  3.1) General Overview [Passive and Active Matrix Displays]

Addressing describes the method employed to transfer charge (data or the display image) from the outside world to the display. Unlike a CRT, which is just a surface of phosphors scanned with a beam of electrons in a vacuum, a liquid crystal display is an array of conductors with metal (or metal like) lines running in both horizontal and vertical directions. For the case of a CRT, the electrons travel through a resistance free medium (vacuum) and deliver a clear consistent signal. The charge traveling through the metal lines of a LCD matrix is affected by the properties of the metal. As a result, the magnitude and waveform of the applied charge can vary from one end of the display to the other. This variation imposes limitations on display quality and capabilities.


  3.2) Addressing: Passive and Active Matrix Displays

There are distinct differences between active and passive matrix displays, but two factors make the greatest impact on potential customers. Active matrix displays can cost twice as much as an equivalent passive matrix display and add more than $1000 to the cost of a notebook form-factor computer. However, active matrix displays produce a stunning and bright image without ghosting or artifacts that rivals the quality of CRTs. Furthermore, even with the price differential, manufacturers are able to sell every active matrix color notebook they can produce.


  3.3) Liquid Crystal Cell Charging

In general terms (regardless of display type), in order to protect the liquid crystal material from deteriorating, cells are addressed by alternating current (AC), not direct current (DC). There is no resultant charge in the LC material following two addressing cycles; build up of charge in the LC material will permanently damage it. In other words, a positive and then an equal but opposite negative charge is applied to the LC material every other frame. By applying dual polarity addressing, the LC material changes twist direction every other cycle and the net charge is zero. Furthermore, since the liquid crystal material is changing twist directions every other cycle, screen savers or screen inverters are not required and in reality do absolutely nothing. Passive matrix displays utilize DIRECT ADDRESSING; the charge is applied directly from the drivers to the pixel element. Active matrix displays utilize INDIRECT ADDRESSING; the charge is "filtered" through a switch before reaching the pixel element.


  3.4) Driving Methods: Passive Matrix Displays

Passive matrix displays have rows of electrodes on one half of the display glass and columns of electrodes on the other. The electrodes are usually fabricated out of Indium Tin Oxide (ITO), which is a semi-transparent metal oxide. When the two pieces of glass are assembled into a display, the intersection of a row and column form a pixel element. Furthermore, if a pulse is sent down one row and a specific column is grounded, the established electric field can change the state of liquid crystal(from white to black). By repeating this process (display scanning) an image can be formed on the display. Problems arise as the number of rows and columns increase. With higher pixel density, the electrode size must be reduced and the amount of voltage necessary to drive the display rapidly increases. Furthermore, higher driving voltage creates a secondary problem; charging effects. Even though only one row and column are selected, the liquid crystal material near the row and column being charged are affected by the pulse. The net result is the pixel selected is active (dark), but the areas surrounding the addressed point are also partially active (grays). The partially active pixels reduce the display contrast and degrade image quality. A final problem is the speed of the STN material, a display must be able to react in less than 40 milliseconds for performance similar to a CRT. Most STN materials are between 150 and 250 milliseconds and can not switch from black to a white image that quickly. This problem results in disappearing cursors and blurred images when high speed graphics are utilized

COMMONLY ASKED QUESTION NOTE: What is STN Gray Scale? As discussed in part I section 1.24, the STN curve does not possess an intrinsic gray scale capability like the TN curve, therefore driving methods have been developed to create the illusion of true gray scale. Gray scale can be derived from frame-rate control and dithering/space modulation. Frame-rate control quickly switches on and off a pixel, the eye perceives this as gray. Dithering or space modulation is accomplished by alternately keeping some pixels black and some white in a checkerboard layout; when using this method, the layout is in a random order. If dithering is in a regular (repeating) pattern, it is detectable by the human eye. In real world applications, combinations of both technologies are applied to commercial displays. The result, however, is sometimes wavy or moving grays. ( The image appears to be moving in waves or a solid color appears to be in motion when a large area is set to a specific gray level.)

RECENT TECHNOLOGY NOTE: What are Dual Scan STN Displays? This is simply taking currently made color STN displays and applying some previously developed technology. Back in the late 1970's and early 1980's liquid crystal chemistry was not as advanced and in order to build high data content displays, manufacturers were forced to build two displays on one glass plate. A dual scan display utilizes similar technology. Instead of running the columns down the entire display, they are terminated in the center of the display, a small gap is left, and the line is continued to the bottom of the display. In reality, you now have two 640 X 240 displays on one glass plate. Therefore, if IC drivers are mounted on the top and the bottom of the display, the charge must only travel half the distance of a normal display. As a result, the effects of the contrast limitations discussed in section 2.12 can be reduced. The end result appears to be a brighter display, but in reality it is only improved contrast (the blacks appear darker). The dual scan STN display still suffers from the ghosting and artifact problems inherent in all slow STN displays.


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