Display devices

DISPLAY DEVICES

DONE BYJERIN . M2nd BCA 13BCA 4219

DISPLAY DEVICES

DONE BY JERIN . MBCA

A display device is an Output device

for presentation of information for

visual reception . The display systems

are often referred to as Video Monitor

or Video Display Unit (VDU) . Display

devices are designed to project , show ,

exhibit or display softcopy information .

Some of the display devices are

• Cathode Ray Tube (CRT)

• Raster Scan Display

• Random Scan Display

• Liquid Crystal Display (LCD)

• Light Emitting Diode (LED)

Cathode Ray Tube (CRT)

1. Electron Guns

2. Electron Beams

3. Focusing Coils

4. Deflection Coils

5. Anode Connection

6. Shadow Mask

7. Phosphor layer

8. Close-up of the

phosphor coated inner

side of the screen

Diagrammatic representation of CRT

A cathode ray tube or CRT is a specialized

vacuum tube in which images are produced

when an electron beam strikes a phosphorescent

surface .

It contains one or more electron guns , and a

fluorescent screen used to view images .

It also contains magnetic deflection coils and

focusing system .

Electron Gun

Heat is supplied to the cathode by the filament . The

free electrons are then accelerated toward the phosphor

coating by a high positive voltage . A positively

charged metal coating on the inside of the CRT

envelope near the phosphor screen . Intensity of the

electron beam is controlled by setting voltage level on

the control grid . A smaller negative voltage on the

control grid simply decrease the number of electrons

passing through .

Accelerating Anode

The accelerating anode is maintained at sufficient

high relative potential to accelerate the beam to

necessary velocity .

Focusing System

The focusing system is needed to force the electron

beam to converge into a small spot as it strikes

the phosphor .The electron beam will be focused properly only at the

center of the screen .

As the beam moves to the outer edges of the screen ,

displayed images become blurred .

Deflection Systems

The deflection system contains a set of coils mounted at

the neck of the tube . There are two sets of deflecting

plates . One pair of plates is mounted horizontally to

control the vertical deflection , and the other pair is

mounted vertically to control horizontal deflection .

Spots of light

Persistence : The time it takes the emitted light from

the screen to decay to one-tenth of its original

intensity .

Resolution : The maximum number of points that can

be displayed without overlap on a CRT .

Aspect Ratio : This numbers gives the ratio of vertical

points to horizontal points necessary to produce equal

length lines in both directions on the screen .

Refresh CRT : One way to keep the phosphor

glowing is to redraw the picture repeatedly by

quickly directing the electron beam back over

the same points .

Phosphors : Phosphors has a property to emit

light when electron comes and hits it . This

property is known as phosphorescence .

Raster Scan Display

In a raster scan system , the electron beam is

swept across the screen , one row at a time from

top to bottom .

As the electron beam moves across each row , the

beam intensity is turned on and off to create a

pattern of illuminated spots .

Picture definition is stored in a memory area called

the refresh buffer or frame buffer .

Refresh buffer or frame buffer : This memory area

holds the set of intensity values for all the screen

points .

Stored intensity values then retrieved from refresh buffer

and “painted ” on the screen one row (scan line) at a

time .

On a black – and - white system with one bit per

pixel , the frame buffer is called bitmap .

For system with multiple bits per pixel , the frame

buffer is called pixmap .

Refreshing on raster scan displays is carried out at

the rate 60 to 80 frame per second .

Horizontal retrace : The return to the left of the

screen , after refreshing each scan line .

Vertical retrace : At the end of each frame ( displayed

in 1/80th to 1/60th of a second ) the electron beam

returns to the top left corner of the screen to begin

the next frame .

Random Scan Display

Random scan monitors draw a picture one line at a

time (Vector display , Stroke –writing or calligraphic

displays) .

Refresh rate depends on the number of lines to be

displayed .

Picture definition is now stored as a line-drawing

commands an area of memory referred to as refresh

display file (display list) .

The component lines of a picture can be drawn

and refreshed .

Random scan displays are designed for line-drawing

applications and can not display realistic shaded scenes.

Differences between Random and Raster Scan Display

RASTER SCAN DISPLAY RANDOM SCAN DISPLAY

Picture with better contrast Cannot produce contrast , memory doesn’t store intensity value of pixel

Less resolution High resolution

Capable of producing curves better Smooth line drawings

Used in systems to display realistic images

Cannot draw realistic shaded scenes

Cost is less Cost is more

Mainly used for point plotting Line drawing , known as vector display

Ex . TV sets Ex . Pen Plotter

Liquid Crystal Displays (LCD)

Used in small systems , such as calculators , laptop

computers .

Produce a picture by passing polarized light (from

the surrounding or from an internal light source)

through a liquid-crystal material that can be

aligned to either block or transmit the light .

Two glass plates , each containing a light polarizer at

right angles to the other plate , sandwich the liquid

crystal materials .

Rows of horizontal transparent conductor & columns

of vertical conductors (put into glass plates) .

Polarized light passing through the material is twisted

so that it will pass through the opposite polarizer .

The light is then reflected back to the viewer .

Liquid crystal : These compounds have a crystalline

arrangement of molecules , yet they flow like a

liquid .

On State

Off State

Light Emitting Diode (LED)

An LED display is a video display which uses light-

emitting diodes . An LED panel is a small display , or

a component of a larger display . They are typically

used outdoors in store signs and billboards . LED

panels are sometimes used as form of lighting , for the

purpose of general illumination , task lighting , or even

stage lighting rather than display .

All early devices emitted low-intensity red light , but

modern LEDs are available across the visible ,

ultraviolet and infrared wavelengths , with very high

brightness .

LEDs are based on the semiconductor diode . When the

diode is forward biased (switched on) , electrons are

able to recombine with holes and energy is released in

the form of light . This effect is called

electroluminescence and the color of the light is

determined by the energy gap of the semiconductor .

LEDs present many advantages over traditional light

sources including lower energy consumption , longer

lifetime , improved robustness , smaller size and faster

switching . However , they are relatively expensive and

require more precise current and heat management than

traditional light sources .

Applications of LEDs are diverse . They are used as

low-energy indicators but also for replacements for

traditional light sources in general lighting and

automotive lighting . The compact size of LEDs has

allowed new text and video displays and sensors to be

developed , while their high switching rates are useful

in communications technology .

LED Display

Thank You.....