seminar on lcd :

1. seminar on lcd :

2. Lcd introduction : A liquid crystal display (LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector.

3. Specifications : Resolution: The horizontal and vertical size expressed in pixels . Dot pitch: The distance between the centers of two adjacent pixels. Viewable size: The size of an LCD panel measured on the diagonal . Response time: The minimum time necessary to change a pixel's color or brightness. Refresh rate: The number of times per second in which the monitor draws the data it is being given. Matrix type: Active or Passive. Viewing angle: Color support: How many types of colors are supported . Brightness: The amount of light emitted from the display. Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark. Aspect ratio: The ratio of the width to the height Input ports:

4. history : 1888: Friedrich Reinitzerdiscovers the liquid crystalline nature of cholesterol extracted from carrots. 1904: Otto Lehmann publishes his work "Liquid Crystals". 1911: Charles Mauguin describes the structure and properties of liquid crystals. 1936: The Marconi Wireless Telegraph Company patents the first practical application of the technology, "The Liquid Crystal Light Valve". 1968: NCR's John L. Janning invented liquid crystal displays (LCD). 1970: The twisted nematic field effect in liquid crystals was filed for patent by Hoffmann-LaRoche. 1972: The first active-matrix liquid crystal display panel was produced in the U.S. by T. Peter Brody.

5. Color display : In color LCDs each individual pixel is divided into three cells, or sub pixel’s which are colored red, green, and blue. Each sub pixel can be controlled independently to yield thousands or millions of possible colors for each pixel. Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through sub pixel rendering. To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used.

6. Passive and Active matrix Addressed : Small monochrome displays screens have a passive-matrix structure . Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. The pixel must retain its state between refreshes without the benefit of a steady electrical charge. Number of pixels increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix addressed LCDs.

7. Continue….: • High-resolution color displays use an active matrix structure. • A matrix of TFTs is added to the polarizing and color filters. • Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation.

8. Continue : Active-matrix addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images.

9. Active matrix technology : Twisted nematic (TN): It contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved. In-plane switching (IP): It aligns the liquid crystal cells in a horizontal direction. In this the electrical field is applied through each end of the crystal, but this requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. This results in blocking more transmission area, thus requiring a brighter backlight, which will consume more power, making this type of display less desirable for notebook computers.

10. Continue…: Vertical alignment (VA): In VA the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors . When no voltage in liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide an improved viewing angle and improved black level.

11. Disadvantage : CRTs are capable of displaying multiple video resolutions without introducing artifacts, LCDs produce crisp images only in their "native resolution" and sometimes, fractions of that native resolution. Attempting to run LCD panels at non-native resolutions usually results in the panel scaling the image which introduces "blockness" and is susceptible. Many LCDs are incapable of displaying very low resolution screen modes due to these scaling limitations. LCDs have more vibrant images and better "real-world" contrast ratios than CRTs, but do not have lower contrast ratios in terms of how deep their blacks are. LCDs can have "backlight bleed" where light leaks out and turns black into gray. LCDs have longer response times than plasma and CRT . when images rapidly change. Some LCDs have significant input lag. If the lag delay is large enough, such displays can be unsuitable for fast and time-precise mouse operations compared to CRT displays or smaller LCD panels with negligible amounts of input lag.

12. Contineu…: LCD panels have a limited viewing angle . This reduces the number of people able to conveniently view the same image . This lack of ambient radiation is reduced power consumption in comparison to CRTs and plasma displays. Consumer LCD monitors more fragile than their CRT counterparts. The screen may be especially vulnerable due to the lack of a thick glass shield as in CRT monitors. Dead pixels are a common occurrence. Horizontal and/or vertical banding is a problem in some LCD screens. This flaw occurs as part of the manufacturing process, and cannot be repaired. The degree is determined by the manufacture's quality control procedures.

13. Quality control : Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels . LCD panels with a few defective pixels are usually still usable. The maximum acceptable number of defective pixels for LCD are 11 allowed . To regulate the acceptability of defects and to protect the end user, ISO released the ISO 13406-2 standard. LCD panels also have defects known as mura, which look like a small-scale crack with very small changes in color.

14. Display applications : Television and digital television . Liquid crystal display television (LCD TV). LCD projector. LCD monitor .

15. Manufacturers : • International Display Works • JVC • LG.Philips LCD • Panasonic • Samsung Electronics • Sharp Corporation • S-LCD • Sony • Toshiba • Viewsonic • Acer • BenQ • Epson • Fujitsu • HP

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