Pratibha Gopalam

EE 594

Liquid crystal displays

• Most common application of liquid crystal technology• Wrist watches, Calculators and Advanced VGA monitors• Manipulation of an array of tiny segments(pixels) to present

information• Important because

– of size – less power consumption

• Drawbacks – viewing angle– contrast ratio– response time

Liquid crystals

• Mesomorphic state• Affected by changes in mechanical stress , electromagnetic fields and

temperature• Three main types (phases)

– Nematic, Smectic, Cholestric

• Nematic – Cigar shaped molecules orient themselves with their long axes parallel to

each other– Can be aligned by electric/magnetic fields resulting an ability to be

electrically switched from clear to opaque– Technical applications - Image display systems

Twisted Nematic liquid crystals

• Nematic liquid crystal sandwiched between two plates of glass.

• NLC on which a twist is imposed.• A special surface treatment is given to the

glass.• Sets up a 90 degree twist into the bulk of

the liquid crystal.• Hence the name of the display.

The twist is visible in the following animation.

Optical Properties of Twisted Nematic liquid crystals

• Acts locally as a uniaxial crystal, with the optic axis parallel to the molecular direction

• The twist angle varies linearly with z,

» where is the twist coefficient

• The phase retardation coefficient(retardation per unit length) is

= (ne - no ) ko

z?? ? ?

?

Optical Properties of Twisted Nematic liquid crystals

• is much greater than than .• It can be shown that if the incident wave at z = 0 is linearly polarized

in the x direction, then when >>– the wave maintains its linearly polarized state– but the plane of polarization rotates

• This polarization rotation property of the twisted Nematic liquid crystal is principal behind the working of the LCD systems.

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• The black lines represent crossed polarizer • As light enters the cell, its polarization

rotates with the molecules. • Passes through the second polarizer.• A mirror is placed at the bottom of the cell • Polarization twists as the light traverses the

sample• Emerge from the top of the cell.

Optical Properties of Twisted Nematic liquid crystals

A schematic of a TN cell

• When electric field is applied in the direction of twist, the molecules tilt toward the field.

• When the tilt is 90o , the molecules lose their twisted character

• The polarization rotatory is deactivated.• The light is cancelled by the second

polarizer.• Regions where an electric field is applied

appear dark against a bright background.

Optical Properties of Twisted Nematic liquid crystals

Light traversing a cell with an applied electric field

Liquid Crystal Display Construction

• a liquid crystal display is composed of multiple layers.– Electrode– A polymer alignment layer

– The two glass sheets that are sealed with epoxy.

– LC injected under vacuum

– Polarizers

– Connections to the driving circuitry.

Sectional View of a LCD

Optical Response Of TND

• Electro-distortional curve.– The tilt of the molecules out of the plane

of the glass slides in measured as a functionof the applied voltage.

– Percentage transmission as a function of voltage is shown in the second graph.

– Difference in the OFF and ON voltages must be very small.

– Hence the Super Twisted Nematic Crystal displays.

Electro-distortional Curve

Super Twisted Nematic Displays• Twist with an angle of 270o instead of 90o

as compared to the TND cell.– Change in the tile angle becomes

very abrupt as the twist angle increases– OFF and ON voltage are much closer

together.– Commercial STN devices use a

twist angle of 210o .– Undesirable coloration resulting from a

shifted transmission spectrum of the device.– Solved by adding a second layer with the

opposite twist sense to the cell. – Double-STN devices

Electro-distortional Curve

Display Addressing• Addressing is a process by which pixels are

turned ON and OFF in order to create an image.• Passive Matrix Displays

– Addressed by a set of multiplexed electrodesperpendicular to one another

– The pixel is addressed when there is a sufficientvoltage across it.

– A pixel has a short turn-on time to become opaque

– When the voltage is removed the molecules become transparent.

Passive Matrix Displays

Display Addressing• Active Matrix Displays

– In high end laptop computers.

– The addressing take place entirely behind the LC film.

– The front surface of the display is coated with a continuous electrode

– The rear surface electrode is patterned intoindividual pixels.

– A TFT(thin film transistor) acts as a switchfor each pixel.

– Can be viewed up to 45o and has a contrast ratio of 40:1

Active Matrix Displays

Color Displays

• In order to achieve color, it is first necessary to have a display which is either black in one state and white in another state.

• To get full color, each individual pixel is divided into three subpixels, Red, Green and Blue(RGB),That is to say for each full color pixel, three distinct pixels are employed.

• These subpixels are created by applying color filters.• With a combination of red, blue and green subpixels of various

intensities, a pixel can be made to appear any number of different colors.

Ferroelectric Liquid Crystal Display

• This is what a ferroelectric liquid crystal display looks like!

• Flat Panel Display Systems

• “Picture on the wall” television sets.

References

• Fundamentals of photonics (Saleh & Teich)• http://plc.cwru.edu/tutorial/• http://bly.colorado.edu/lc/• http://www.britannica.com• http://www.atip.or.jp/fpd/src/tutorial/