Understanding LCD Display Technology part 1
			For many years CRTs have been the standard display device. 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. This is the first in a series of articles that explore LCD display technology. This article shows the basics of how LCD panels work, illustrates the liquid crystal panel and discusses how the polarized filters pass or block light.
Glen Kropuenske SENCORE, Inc. Application Engineer 1.800.736.2673 or 1.605.339.0100 mailto:ae@sencore.com http://www.sencore.com			Early LCD displays suffered from limited gray scale graduations, poorly saturated colors, narrow viewing angles, and slow response times. Today’s LCD panels have greatly improved, and are beginning to rival CRTs in most performance areas. But, in terms of size, weight and power consumption, LCD displays are far superior. This article introduces you to LCD technology and LCD displays.
How LCD panels work The basic function of the LCD is a light "valve", either blocking light or allowing light to pass through. LCD video displays are “transmissive” meaning that an LCD is an active filter that works by varying the amount of light (from a fluorescent light source called the backlight) that is able to pass through to the viewing screen. An LCD display is really a large collection of thousands of these small filters called liquid crystal cells. Arranged together in rows and columns, these cells form an LC panel. Cells are combined into groups of three containing a uniquely addressable “red”, “green” and “blue” cell. These cells or subpixels, working together, create one pixel or picture element.
A complete LCD display contains thousands of these tiny liquid crystal panels or cells. Its native resolution refers to the resolution that the LCD monitor is designed for (i.e. 800 x 600 or 1024 x 768) and is determined by the actual number of liquid crystal cells in the display. To produce color, every pixel location in an LCD display consists of three LC cells - one each for red, green, and blue. Each LC cell, or subpixel, can be individually addressed with a control voltage. This means, for example, that a 15" 1,024 x 768 video display contains 2,359,296 subpixels (1,024 x 768 x 3).
Fixed pixel displays can provide the best image only in their native resolution.		A  CRT monitor can easily display a variety of input resolutions without any loss in image quality; it is capable of producing a white or color dot at virtually any location on the screen. Fixed pixel displays such as LCD displays (all display types except CRTs are fixed pixel displays) can provide the best image only in their native resolution. While you can input an image having a resolution other than the display native resolution, the display can only reproduce a white or colored dot at the fixed physical pixel locations. To view an image at some other resolution, it needs to be scaled up or down in order to fill the screen. Fixed pixel displays are sometimes called "addressable displays" because each pixel can be addressed directly.
An LC panel is composed of several layers of different materials. The light from the backlight source passes through a layer of polarized glass. The polarized light then passes through liquid crystal cells. Electrical voltage applied to the liquid crystal causes them to reorient themselves up to 90° to either pass or block light. This liquid crystal layer is followed by a layer of red, green, and blue filters and then by a second layer of polarized glass. Basic principles to understand the operation of a LC panel:
	1. alignment of liquid crystal molecules can be controlled by a. fine grooves etched into a glass plate b. electric current/voltage 2. light follows liquid crystal molecules 3. polarizing filters block light
What is a Liquid Crystal? Liquid crystals have the physical properties of both a solid and a liquid. As a liquid they are able flow over and around small grooves and can change their position depending on applied voltage. However, liquid crystal also has the properties of a solid because light passing through it follows the alignment of the liquid crystal molecules.
						Figure 1. A liquid crystal panel with the liquid crystals aligning in a random tilt.
Aligning Liquid Crystal Molecules & Rotating Light The molecules in a liquid crystal are basically a rectangular shape, and without any outside influence they align themselves in a random tilt with their long axes parallel. When they come into contact with a grooved surface, the liquid crystal molecules easily orient themselves to be parallel to the grooves. This makes it possible to precisely control the “at rest” alignment of the molecules. Since the alignment of the molecules exactly follows the grooves, if the fine grooves are exactly parallel, the alignment of the molecules is also exactly parallel.
To make a basic “twisted” LC panel, liquid crystal is sandwiched between two transparent plates. Each plate contains very fine grooves, with the grooves in each panel placed exactly perpendicular. The liquid crystal molecules along the upper plate align in direction A, while those along the lower plate align in direction B. This is called a twisted structural arrangement. As light passes through liquid crystals, it follows the direction that the molecules are arranged. This means that as the light passes through the two panels, it twists 90° as it passes through the liquid crystals.
						Figure 2. As light passes through liquid crystals, it follows the direction of the crystals.
An important key to the operation of an LCD panel is that liquid crystal molecules can be easily rearranged by applying an external voltage to them. As voltage is applied between the two grooved panels, the liquid crystal molecules begin to rearrange themselves with the electric field. In this illustration they align vertically or stand up out of the grooves. As the voltage increases, the molecules rotate, until they are standing completely upward. Remember that as light passes through liquid crystal, it follows the direction that the molecules are arranged. So, with full voltage applied the light passes through the panels following the arrangement of the molecules without being rotated 90°. At lower voltages the molecules are not all fully aligned, so some light is directed at different directions.
						Figure 3. With full voltage applied, the light passes through the panels without being rotated 90º.
Blocking Light with Polarizing Filters Natural light travels in waves that are oriented at random angles. This is why an object has the same color and brightness when viewed from different angles (unless it has a shadow or other lighting differences). A polarizing filter is simply a set of extremely closely spaced parallel lines. These lines allow only the light waves that are parallel to them to pass through – light from all other directions is blocked. (This is how polarized sunglasses work.) If you were to lay two polarizing filters on top of each other with the lines in one parallel to the lines in the other, direct light would pass through each, with little loss. However, as you begin to rotate one filter, more and more light is blocked because only the light waves that have the proper orientation to both filters can get through. When you have rotated the filter so that the lines in one are perpendicular to the lines in the other, all light will be blocked.
Figure 4 illustrates how two polarizing filters can be used to block light. In both the left and right drawing, light is applied to the top of the filter pair. The incoming light has waves oriented at all directions, but for simplicity, only two orientations, “a” and “b” are shown. In the left diagram, the polarizing filters are oriented so that their “A” filter lines are parallel. Incoming light waves that have orientation “A” are able to pass through both the first and second filter, but light waves that are “B” orientation are blocked by both filters.
					Figure 4. Polarizing filters allow light waves that are parallel to them to pass through.
In the right drawing the two polarizing filters are oriented so the filter lines in one are perpendicular to the lines in the other. Again, light waves that are “A” orientation pass through the top filter, and “B” oriented waves are blocked. But in this case, when the “A” oriented waves reach the bottom polarizing filter, they are the wrong orientation compared to the filter lines, and they are unable to pass through. The result is: no light gets through the polarizing filters.
Controlling Light with an LCD Panel You cannot physically rotate polarizing filters to pass or block light in a video display, but we can use the properties of liquid crystal to rotate light. Let’s add some liquid crystal between the two polarizing filters, as illustrated in figure 5. In both of these illustrations the polarizing filters are oriented perpendicular, which as we saw in figure 4, prevents all incoming light from getting through. Also notice that a liquid crystal panel is inserted between the polarizing filters. This panel has a twisted structural arrangement (the molecules along the top plate are rotated 90° from those along the bottom plate).
Figure 5 left, illustrates how the liquid crystal allows light to pass through the polarizing filters. Light waves that have an “A” orientation pass through the 1^st (top) filter. These light waves are then rotated 90° by the liquid crystal in the twisted structural arrangement. Notice that this re-orientates the light waves so that they are now properly aligned to pass through the 2^nd (bottom) polarizing filter.
					Figure 5. Voltage applied to the liquid crystal causes the molecules to realign vertically.
In Figure 5 right, a voltage is applied to the liquid crystal, causing the molecules to re-align themselves vertically. Again light waves that have an “A” orientation pass through the 1^st filter, but this time they travel straight through the liquid crystal without being reoriented. Because they are perpendicular to the 2^nd polarizing filter, they are blocked and no light makes it through the liquid crystal panel. Note that varying the amount of voltage applied to the liquid crystal determines how much it twists. With a lesser voltage applied the molecules don’t fully rotate, and some light waves will be able to pass through the bottom polarizing filter. By changing the voltage in very small increments, LCDs can create a gray scale (or different intensities of R, G or B light). Most displays today offer 256 levels of brightness.
Conclusion This article showed the basics of how LCD panels work. The next article in the series discusses how LCD pixels can be individually switched, how LCD technologies are changing and how color is reproduced. 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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