principles of display

7/28/2019 principles of display

1/25

Principles

of DisplaySeminar

1


2/25

Contents

1. Whats Liquid Crystals (LC)

2. Introduction to Liquid Crystal Displays3. 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 and

solid state of matter.e.g. soapy water

light passes through liquid crystal changeswhen 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 topresent information.

Using polarization of lights to displayobjects.

Use only ambient light to illuminate thedisplay.

Common wrist watch and pocket calculatorto an advanced VGA computer screen

5


6/25

Different types of LCDs

Thermotropic and Lyotropic

Liquid crystals

Nematic liquid crystals

Smectic liquid crystals

6


7/25

3. Operating Principle

The parallel arrangement of liquid crystal

molecules along grooves When coming into contact with grooved

surface in a fixed direction, liquid crystal

molecules line up parallel along the grooves.

7


8/25


Molecules movement

Offline (no voltage is applied)

Along the upper plate :Point in direction 'a'

Along the lower plate :Point in direction 'b

Forcing the liquid crystalsinto a twisted structural

arrangement. (Resultant

force)

8


9/25


Light movement

Offline (no voltage is applied)

Light travels through thespacing of the moleculararrangement.

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

Light bends 90 degrees as itfollows the twist of themolecules.

Polarized light pass throughthe analyzer (lower polarizer).

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 themovement

Forcing the liquid crystals intoa straight structuralarrangement. (Electric force)

10


11/25


Light movement

Online (voltage is applied)

Twisted light passes straightthrough.

Light passes straightthrough along thearrangement of molecules.

Polarized light cannot passthrough the lower analyzer(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 and twistedat right angle.

3. Molecules and lights areparallel to the loweranalyzer.

4. Light passes through theplate.

5. Screen appear transparent.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 which movesstraight along the electricfield.

3. Molecules and lights areperpendicular to the loweranalyzer.

4. Light cannot pass throughthe 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 a

transparent 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 gapprecisely;

Two crossed polarizers (thepolarizerand 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 geometrical errors

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 wherethere are only a few elements that have to be activated.With direct addressing, each pixel in the display hasi ts own dr ive c ircu i t. A microprocessor must

individually apply a voltage to each element. A commonapplication of direct addressing is the t radi t ion al sevensegment l iqu id crystal display, found in wristwatchesand similar devices.

18


19/25


In multiplex addressing, a larger num ber of pixelsare involved. When the elements are in a regular order,they can be addressed by their row and c olumninstead of each element being driven separately. Thisreduces the complexi ty of the ci rcui t rybecause eachpixel no longer needs i ts own d river ci rcui t.

If you have a 10x10 matrix of pixels, with directaddressing, you need 100 individual drivers. However, ifyou use multiplex addressing, you only need 20 drivers,

one for each row and one for each column. This is a tremendous advantage, especially as displays

become 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 light

through the cell. Different light intensity of an image projected on the screen is

determined by different voltage supply. Thus the level of blocking of lightmay vary.

20


21/25

21


22/25

5. ApplicationsA) Thin Film Transistor (TFT) Constructed on a glass surface using a photolithographic

process.

The source and gate are the control electrodes. The drainelectrode connects to the liquid crystal pixel. The thin layer

of amorphous silicon is the semiconducting material that

allows the TFT to function. The capacitor is attached to the

pixel electrode, but is not an integral part of the TFT.

22


23/25

5. ApplicationsB) Alpha-numeric display Digital letters can be displayed by blocking the lights in different

plates we place.

For applications such as digital watches and calculators, amirror is us edunder the bottom polarizer. With no voltageapplied, ambient light passes through the cell, reflects off the

mirror, reverses its path, and re-emerges from the top of the cell,

giving it a silvery appearance.

When the electric field is on, the aligned LC molecules do notaffect the polarization of the light. The analyzer prevents th einc ident l ight f rom reaching the m irror and no l ight is

ref lected, causing the cell to be dark. When the electro des are

shaped in the form o f segments of num bers and let tersthey

can be turned on and off to form an alpha-numeric display.23


24/25

5. ApplicationsC) Back lighting systems Alpha-numeric displays are not very bright because the light

must pass through multiple polarizers which severely cutdown on the intensity of the light, in addition to the various

layers of the display which are only semi-transparent. Thereforea more intense source is employed in the form of a backlighting system.

For brighter displays

Light bulbs mounted behind

At the edges of the display replace the reflected ambient light. Disadvantage : very power intensive. Back lighting systems are

used in more complex displays such as laptop computerscreens, monitors, LCD projectors, pda, digital devices suchas digital camera and DV. 24


25/25

6. ReferenceReference webpages LCD Principle

http://infochem.hanyang.ac.kr/researches/researches_lcd_

english.html Sharp

http://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

principles of display

Documents