Liquid Crystal Displays

IQxplorer

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LCD Overview

LCDs are a type of light modulating display device. LCD technology does not directly emit light in order to create visualizations, it rather modifies the transmission of light to represent images. On the other hand Plasma displays, CRT and OLED devices are all light

emitting display devices. LCDs are rapidly becoming the most popular display device

available. 2006 - Nearly 33% of all TVs sold in North America were LCDs 2007 - Expected to surpass 50%

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History and Development of LCDs Discovery of liquid crystals

1888 Friedrich Reinitzer discovers liquid crystals

1904 Otto Lehmann publishes work on liquid crystals

Innovation 1963

Richard Williams and George Heilmeier suggest using

liquid crystals for making a display device 1968

First operational LCD created by RCA using Dynamic Scattering Method (DSM)

1971 First LCD using Twisted Nematics (TN) produced

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History and Development of LCDs (cntd) Applications

1970’s First commercial LCDs came into existence. The technology was first used

in the quartz watch and in early calculator displays. 1990’s

Early use of colour LCDs for digital cameras and computer monitors 2000’s

Technological improvements led to larger LCDs suitable for home theatre use.

Full HD 1080p displays produced Currently the largest LCD panel is 108”

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Basic operation of a TN (twisted nematic) LCD The panel consists of a number of layers:

Liquid crystal layer 2 transparent conductors 2 perpendicular polarizing filters Colour filter

In their natural state the liquid crystals align themselves in a helical structure. The nature of the molecules rotates the polarization of light as it passes through.

How LCDs work

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How LCDs work When an electric field is applied to the liquid crystals the twisted structure gets

distorted to varying degrees depending on the voltage applied. The change in the structure subsequently causes a change in how drastically the light is rotated. This ultimately decides how much light will pass through the filters and determine the brightness of the sub-pixel.

When light passes through the first polarizing filter, half of it is blocked. The remaining light normally gets rotated by the liquid crystals and can pass through the second perpendicular polarizing filter. If it isn’t rotated at all, then no light will pass.

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Types of LCDs Direct Electrical Connections vs Multiplexing

In small LCD devices, such as watches with 7-segment displays, it is possible to have a direct connection to control each segment.

This is not feasible for larger displays, like colour monitors. Millions of individual connections would be required. Therefore the pixels must be controlled by a multiplex array. Multiplexing reduces the connections required substantially.

There are associated problems with multiplexing since some of the inactive pixels will still be effected by the voltage running through their row or column. This was solved through “switching” where thin film transistors are used in the multiplex array, ensuring only activated pixels are effected.

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Types of LCDs Passive vs Active Matrix

Passive Each pixel must retain it’s state without a steady electric charge Scanned one pixel at a time Poor contrast, and very slow response times

Active Electric charged is stored between refreshes Scanned one row at a time

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Types of LCDs A light source is required for LCDs Types of lighting

Transmissive Backlight is used to light the display

Poor performance outdoors or in bright light. Solutions: Brighter backlight or less reflective surface

Transflective Backlight and a reflective mirror surface are both used

Good overall performance in all lighting conditions

Reflective Only ambient light is used via a mirror surface

Ie. Watches

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LCDs Advantages

thin and lightweight low power consumption

Plasma: 200-1000W+ LCD: 100-200W relatively cheap and getting cheaper higher resolutions for less

1080p size range lifespan

Disadvantages poor contrast ratio

“Backlight bleed” response times viewing angles

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Future LCD will continue to close the gap with Plasma

Technological advances will further improve the contrast ratio and response time of LCDs

LCD will likely be the prominent display device for the foreseeable future 3D LCD

Stereoscopic vision without the need for glasses or other aids Projects a different image to each eye

Sharp Dual Directional Viewing Using similar technology to the 3D LCD, two distinct images can be displayed to

two different viewers Triple viewing Privacy

OLED Displays Can be put onto flexible surfaces Better colours, contrast, brightness, more efficient Lifespan issues though

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References P. Engardio, E. Woyke, M. Ihlwan, K. Hall. 2007. Flat Panels, Thin

Margins. Business Week. Issue 4023. pg. 50-51. Jeffrey A. Hart. Technology, Television and Competition.

Cambridge University Press, 2004. D.J. Channim, A. Sussman. Liquid-Crystal Displays, LCD. Topics

in Applied Physics Volume 40: Display Devices. Ed. J. I. Pankove. New York, 1980.

Liquid Crystal Display http://en.wikipedia.org/wiki/Liquid_crystal_display