Understanding LCD Display Technology part 2
			The development of the first LCD (liquid crystal display) by RCA Laboratories in 1968 ushered in a new era of displays. Since then, LCDs have been incorporated into all types of digital devices from small watches and calculators to video displays and projection televisions. The first article in this series of articles that explore LCD display technology showed the basics of how LCD panels work, illustrated the liquid crystal panel and discussed how the polarized filters pass, or block, light. Part 1 LCD Display Technology This second article discusses how LCD pixels can be individually switched, how LCD technologies are changing and how color is reproduced.
Glen Kropuenske SENCORE, Inc. Application Engineer 1.800.736.2673 or 1.605.339.0100 mailto:ae@sencore.com http://www.sencore.com
LC Cell Construction The complete structure of a liquid crystal cell is illustrated in Figure 1. If you were to look at this structure, it resembles a sheet of glass in terms of transparency and thickness, but it is much more complex. This illustration shows the basic structure of a single liquid crystal cell.
	1. Backlight: light source, usually several fluorescent tubes, and a diffuser. 2. Polarizing filter: two polarizing filters establish the orientation of the light waves that are able to pass through the LC cell. 3. Glass substrate: provides mechanical support for the cell. 4. Transparent electrodes: conduct the control voltage to the liquid crystal. 5. Alignment layer: contains the fine grooves that align the liquid crystal molecules in a fixed direction. In dual scan screens, the orientation varies between 90° and 270°. 6. Liquid crystal: molecules realign to applied voltage, light waves follow molecules. 7. Spacer: maintains uniform spacing between glass plates. 8. Color filter: determines what color of light is transmitted by the LC cell.
							Figure 1. A liquid crystal cell resembles a sheet of glass, but is much more complex.
Addressing the Pixels To produce an image on the display, each of the subpixels must be made to turn on and off at the correct time. In addition, to produce shades of gray and different colors, the voltage applied to each LC cell must be variable. Since it is physically impossible to have a unique wire going to each of the 2.3 million pixels, another method of addressing the pixels is needed - LCD pixels are controlled using a matrix.
To understand how an LCD matrix works, consider the small 4 x 4 checkerboard illustrated in figure 2, which has a light bulb in each square. The bulbs in each horizontal row are connected together to a common wire, giving us 4 separate rows of wire. At the end of each wire attach a terminal so that we can connect a battery. Likewise, connect the bulbs in each vertical column to a common wire, for a total of 4 column wires. Now connect a battery between one of the row wires and one of the column wires – notice that only the bulb that corresponds to the intersection of the row and column wire is lit. By connecting to different combinations of row and column wires, we can individually turn on each light bulb.
						Figure 2. Connecting a battery to different combinations of row and column wires turns on each light bulb individually.
The matrix addressing in an LCD works just like this, except on a much larger grid. In a color LCD display, the intersection of each grid wire corresponds to a single LC cell, or subpixel (remember a 1,024 x 768 display contains 2,359,296 subpixels). The intersecting grid of horizontal and vertical conductors are etched onto the glass plates or substrates of the LC cell – the row conductors are etched onto one plate and the column conductors are etched on the other plate. These conductors are made of a transparent material, such as indium-tin oxide. Large scale integrated circuits (LSICs) control the timing of the signal voltage that is applied to each individual LC cell, one cell at a time. Connections to the transparent conductors are made through bonding pads that are located at the ends of the rows and columns on the LCD assembly.
Two different ways to apply drive voltages to an LCD screen: • Passive matrix • Active matrix		Depending on the applied voltage, the LC cell can either pass or block all light, or partially pass light. There are two different ways of applying the drive voltages to an LCD screen using this matrixing process: passive matrix and active matrix. Simple displays, such as those used in calculators or clocks, only need to address pre-defined patterns. These displays use segment drive where each pattern segment is addressed directly.
Passive Matrix LCD (PMLCDs) Passive matrix is the simplest way to address LCD pixels. The voltage on a conductor is applied directly to one plate of the LC cell, and the voltage on the other conductor is applied directly to the other plate of the LC cell. To maintain the on or off state, each cell must be addressed for more than one frame time. The effective voltage applied to the cell is an average of several signal voltage pulses, which results in a slow response (>150 msec), low brightness, and poor contrast ratio.
						Figure 3. The voltage is applied directly to each plate of the LC cell in a passive matrix.
Passive matrix addressing also produces ghosting or blurred images because some of the drive voltage spills over onto non-selected pixels. Consequently, passive matrix addressing is used for still image applications (i.e. calculators and word processors), but not for video or computer LCD monitors. Twisted Nematic (TN) and Dual-Scan Twisted Nematic (DSTN) use passive matrix addressing.
Active Matrix LCD (AMLCDs) Video and computer LCD monitors use active matrix drive. This method also uses an intersecting horizontal and vertical grid, but tiny transistors and capacitors are etched onto the glass substrate at the intersection of each row and column. It is these transistors and capacitors that actually control the charge on each liquid crystal cell. This means that the switching occurs right at the cell, rather than at the end of a long conductor. The result is faster response times, and less crosstalk between cells.
						Figure 4. Tiny transistors and capacitors control the charge on each liquid crystal cell in an active matrix.
Additionally, higher drive signals can be used which creates much brighter and higher contrast images. Because the transistors are fabricated directly on the cell’s substrate using thin film, these displays are often called thin-film transistor LCDs. Liquid crystal must be driven with an alternating current to prevent any deterioration of image quality resulting from dc stress. This is usually implemented with a frame-reversal drive method, where voltage that is applied to each pixel varies from frame to frame.
LCD Types The basic twisted nematic LCD technology has several limitations that prevent its use in video and computer monitor displays – primarily limited viewing angle, poor contrast ratio and slow speed. Manufacturers are constantly working to improve the performance characteristics of LCDs by modifying the common LCD technologies. For example supertwisted & dual supertwisted nematic (DSTN) twists the light 180 to 270 degrees to improve contrast and viewing axis shortcomings. In-Plane Switching (IPS) uses polarizing filters in a perpendicular arrangement to widen the viewing angle and improve contrast ratio. Vertical Alignment (VA) structure improves viewing angles, contrast, and color reproduction while using less power compared to IPS. Multi-Domain Vertical Alignment is similar to VA with ridges added to the glass surface to provide more uniform brightness over the 160 degree viewing angle. Note that different manufacturers use different trade names, and may have slight variations in their designs.
Producing Colors A liquid crystal cell by itself has no color and cannot differentiate between colors. In order to reproduce color the light that passes through an LC cell must pass through a color filter. Each pixel in a color LCD is made up of three subpixels. Each subpixel is simply a liquid crystal cell that has a red, green, or blue filter in front of it. This filter blocks all wavelengths of light except those within the range of that subpixel. The color filters are integrated into the upper glass, and the area in between the filters is printed black to increase contrast.
The subpixels are so close together that our eyes only see a mixture of the three colors. When the three subpixels of a color “triad” are all on, our eyes see white. The subpixels can be controlled to pass more or less light of each color. This allows the LCD to reproduce different color saturation levels, different tints, or a gray scale. (Color and gray scale reproduction in an LCD is similar to the three electron guns in a CRT; except that the LCD subpixels work by blocking light, rather than by creating light.)
					Figure 5. Each pixel in a color LCD is made up of three subpixels. Each subpixel is simply a liquid crystal cell that has a red, green, or blue filter in front of it.
Most active matrix panels use digital signal controllers. Since an 8-bit controller can produce 256 luminance steps, an LCD is capable of reproducing 256 shades of red x 256 shades of green x 256 shades of blue or 16,777,216 colors. While this sounds like a lot, it is considerably fewer colors and grayscale steps than CRT displays can reproduce.
Conclusion For more information on LCD Display Technology, or information on how your business can grow with the Sencore multimedia video generators or color analyzers, call your Sencore sales representative 1.800.736.2673 or outside of the U.S. 1.605.339.0100. Learn more - VP400 VideoPro family: http://www.sencore.com/vp400/index.htm Learn more - CP5000 ColorPro: http://www.sencore.com/products/cp5000.htm
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