display principles

7/28/2019 display principles

1/25

DisplayPrinciples Seminar1


2/25

Contents

1. Whats Liquid Crystals (LC)

2. Introduction to Liquid Crystal Displays

3. Operating Principle

4. Display Addressing

5. ApplicationsA) Thin Film Transistor (TFT)B) Alpha-numeric DisplayC) Back Lighting System

6. Reference

2


3/25

1. Whats Liquid Crystals (LC)

intermediary substance between a liquid andsolid state of matter.

e.g. soapy water light passes through liquid crystal changes

when it is stimulated by an electrical charge.

3


4/25

Examples of LCs

4


5/25

2. Introduction to Liquid Crystal

Displays

Consists of an array of tiny segments(called pixels) that can be manipulated to

present information.Using polarization of lights to displayobjects.

Use only ambient light to illuminate thedisplay.

Common wrist watch and pocketcalculator to an advanced VGA computerscreen

5


6/25

Different types of LCDs

Thermotropic and Lyotropic

Liquid crystalsNematic liquid crystals

Smectic liquid crystals

6


7/25


The parallel arrangement of liquid crystalmolecules along grooves

When coming into contact with groovedsurface in a fixed direction, liquid crystalmolecules line up parallel along the grooves.

7


8/25


Molecules movement

Offline (no voltage isapplied)

Along the upper plate :Point in direction 'a'

Along the lower plate :Point in direction 'b

Forcing the liquidcrystals into a twistedstructural arrangement.(Resultant force)

8


9/25


Light movementOffline (no voltage is applied)

Light travels through the

spacing of the moleculararrangement.

The light also "twists" as itpasses through the twistedliquid crystals.

Light bends 90 degrees as it

follows the twist of themolecules.

Polarized light pass throughthe analyzer (lowerpolarizer).

9


10/25


Molecules movement

Online (voltage is applied)

Liquid crystal molecules

straighten out of their helixpattern

Molecules rearrangethemselves vertically (Alongwith the electric field)

No twisting thoughout the

movementForcing the liquid crystalsinto a straight structuralarrangement. (Electric force)

10


11/25


Light movement

Online (voltage is applied)

Twisted light passesstraight through.

Light passes straightthrough along thearrangement of molecules.

Polarized light cannotpass through the loweranalyzer (lower polarizer).

Screen darkens.

11


12/25


Sequences of offline andonline mode

Offline

1. Surrounding light is

polarized on the upperplate.

2. Light moves along withliquid crystals andtwisted at right angle.

3. Molecules and lights are

parallel to the loweranalyzer.

4. Light passes through theplate.

5. Screen appeartransparent.

12

Offline Online


13/25


Sequences of offline andonline mode

Online

1. Surrounding light is

polarized on the upperplate.

2. Light moves along withliquid crystals whichmoves straight along the

electric field.3. Molecules and lights are

perpendicular to thelower analyzer.

4. Light cannot pass

through the plate.5. Screen appear dark.

13

Offline Online


14/25


Polarization of light

When unpolarized light passes through a polarizingfilter, only one plane of polarization is transmitted.

Two polarizing filters used together transmit lightdifferently depending on their relative orientation.

14Online Offline


15/25


Construction of

Liquid Crystal Display

Two bounding plates (usuallyglass slides), each with atransparent conductive coating(such as indium tin oxide) thatacts as an electrode;

A polymer alignment layer :undergoes a rubbing process asgrooves.

Spacers to control the cell gap

precisely; Two crossed polarizers (the

polarizerand the analyzer);

Polarizers are usuallyperpendicular to each other.

15


16/25


Properties of LCD Display Small footprint (approx 1/6 of CRT)

Light weight (typ. 1/5 of CRT)

power consumption (typ. 1/4 of CRT)

Completely flat screen - no geometricalerrors

Crisp pictures - digital and uniform colors

No electromagnetic emission

Fully digital signal processing possible

Large screens (>20 inch) on desktops

High price (presently 3x CRT)

Poor viewing angle (typ. 50 degrees)

Low contrast and luminance (typ. 1:100)

Low luminance (typ. 200 cd/m2) 16

Maximum luminosity : 50%

of CRT as 50% of light is

blocked by the upperpolarizer.


17/25


Advantage of LCD over CRT

Smaller sizeAMLCDs occupy approximately 60percent less space than CRT displaysan

important feature when office space is limited. Lower power consumptionAMLCDs typically

consume about half the powerand emit muchless heat than CRT displays.

Lighter weightAMLCDs weigh approximately70 percent less than CRT displays of comparablesize.

No electromagnetic fieldsAMLCDs do notemit electromagnetic fields and are notsusceptible to them. Thus, they are suitable foruse in areas where CRTs cannot be used.

Longer lifeAMLCDs have a longer useful lifethan CRTs; however, they may require

replacement of the backlight. 17

Maximum luminosity : 50%

as 50% of light is blocked by

the upper polarizer.


18/25


Addressing is the process by which pixels are turned onand off in order to create an image.

There are two main types of addressing, direct andmultiplexing.

Direct addressing is convenient for displays where thereare only a few elements that have to be activated. Withdirect addressing, each pixel in the display has i ts ow ndrive ci rcui t. A microprocessor must individually apply a

voltage to each element. A common application of directaddressing is the t radi t ion al seven segment l iquidcrys tal display, found in wristwatches and similardevices.

18


19/25


In multiplex addressing, a larger number of pixels areinvolved. When the elements are in a regular order, theycan be addressed by their row and columninstead of

each element being driven separately. This reduces th ecomplex i ty of the ci rcu i t rybecause each pixel nolonger needs i ts own dr iver c i rcu i t.

If you have a 10x10 matrix of pixels, with directaddressing, you need 100 individual drivers. However, if

you use multiplex addressing, you only need 20 drivers,one for each row and one for each column.

This is a tremendous advantage, especially as displaysbecome larger and larger.

19


20/25


Optical Response

twisted nematic displays can switch between light and dark states, orsomewhere in between (grayscale).

Electro-distortional curve is shown as follows :

the electro-distortional response determines the transmission of lightthrough the cell.

Different light intensity of an image projected on the screen is determinedby different voltage supply. Thus the level of blocking of light may vary.

20


21/25

21


22/25

5. Applications

A) Thin Film Transistor (TFT)

Constructed on a glass surface using a photolithographicprocess.

The source and gate are the control electrodes. The drainelectrode connects to the liquid crystal pixel. The thin layerof amorphous silicon is the semiconducting material thatallows the TFT to function. The capacitor is attached to thepixel electrode, but is not an integral part of the TFT.

22


23/25

5. Applications

B) Alpha-numeric display

Digital letters can be displayed by blocking the lights indifferent plates we place.

For applications such as digital watches and calculators,a mirror is usedunder the bottom polarizer. With novoltage applied, ambient light passes through the cell,reflects off the mirror, reverses its path, and re-emergesfrom the top of the cell, giving it a silvery appearance.

When the electric field is on, the aligned LC molecules donot affect the polarization of the light. The analyzerprevents the incident l ight from reachin g the mirror

and no ligh t is ref lected, causing the cell to be dark.When the electrodes are shaped in the form ofsegments o f numbers and let tersthey can be turned on

and off to form an alpha-numeric display.

23


24/25

5. Applications

C) Back lighting systems

Alpha-numeric displays are not very bright because thelight must pass through multiple polarizers whichseverely cut down on the intensity of the light, in

addition to the various layers of the display which are onlysemi-transparent. Therefore a more intense source isemployed in the form of a back lighting system.

For brighter displays

Light bulbs mounted behind

At the edges of the display replace the reflected ambientlight.

Disadvantage : very power intensive. Back lightingsystems are used in more complex displays such as laptopcomputer screens, monitors, LCD projectors, pda,digital devices such as digital camera and DV.

24


25/25

6. Reference

Reference webpages

LCD Principlehttp://infochem.hanyang.ac.kr/researches/researches_lcd_english.html

Sharphttp://www.sharp.ca/lcd_principles.html

Liquid Crystalshttp://www.ee.calpoly.edu/~dbraun/courses/lcd.html

PC Technology Guidehttp://www.pctechguide.com/07panels.htm

Casio official homepagehttp://www.casio.co.jp/edu_e/product/2line/

25
http://infochem.hanyang.ac.kr/researches/researches_lcd_english.htmlhttp://infochem.hanyang.ac.kr/researches/researches_lcd_english.htmlhttp://www.sharp.ca/lcd_principles.htmlhttp://www.ee.calpoly.edu/~dbraun/courses/lcd.htmlhttp://www.pctechguide.com/07panels.htmhttp://www.casio.co.jp/edu_e/product/2line/http://www.casio.co.jp/edu_e/product/2line/http://www.pctechguide.com/07panels.htmhttp://www.ee.calpoly.edu/~dbraun/courses/lcd.htmlhttp://www.sharp.ca/lcd_principles.htmlhttp://infochem.hanyang.ac.kr/researches/researches_lcd_english.htmlhttp://infochem.hanyang.ac.kr/researches/researches_lcd_english.html

Documents

display principles