Cad unit 1

DISPLAY DEVICES

DEFINITION A display device is a device for visual

presentation of images (including text) aquired,stored, or transmitted in various forms

Ex: computer monitor,TV screen etc. It is also called information display.

TYPES

Various technologies available are

CRT cathode ray tube LCD liquid crystal display Plasma technology

PLASMA TECHNOLOGY It is a type of flat panel display which is now

used commonly for large TV displays

CONSTRUCTION The xenon and neon gas in a plasma television

is contained in hundreds of thousands of tiny cells positioned between two plates of glass.

Long electrodes are also sandwiched between the glass plates, in front of and behind the cells

Cells are covered with a magnesium oxide protective layer.

CHARACTERISTICS The display panel is only about 6 cm (2½

inches) thick, and total thickness less than 10cm

Bright scenes will draw more power than darker scenes

Estimated life time is about 60000 hrs

ADVANTAGES Over all thickness Is less than 10cm and can be

installed on wall• Faster response time

DISADVANTAGES They are costly as compared to CRT,LCD

displays. Burn-in problem. Static images can leave a permanent mark

on the screen. This is why a plasma TV should not be used as a computer monitor or video game display

LCD TECHNOLOGY Liquid Crystal Display

Definition

Principle

Classification

DEFINITION A liquid crystal display is a thin, flat display

device made up of many number of pixels arrayed in front of a light source or reflector. It uses very small amounts of electric power.

Generally used in calculators,laptops,etc.

CONSTRUCTION Each pixel of an LCD consists of a layer of

molecules aligned between two transparent electrodes, and two polarizing filters,whose axes are perpendicular.

Orientation of the liquid crystal molecules is determined by the alignment at the surfaces.

CONSTRUCTION

1. Reflective twisted nematic liquid crystal display. filter film with a vertical axis to polarize light as it enters.

2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth.

3. Twisted nematic liquid crystal.4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the

horizontal filter.5. Polarizing filter film with a horizontal axis to block/pass light.6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light

source.)

WORKING Before applying Electric field, the molecules

arrange themselves in a helical structure, or twist.

When a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field , distorting the helical structure .

This reduces the rotation of the polarization of the incident light.

CLASSIFICATION Transmissive displays: It is illuminated from the back by a backlight

and viewed from the opposite side (front). Ex :Computer display.• Reflective displays: It is illuminated by external light reflected by a

diffusing reflector behind the display. Ex: Calculator.