The devices which can display the signals generated by the computer in the form of an image. In general terms, a display device is known as monitor. Different types of display devices based on different technologies:
Plasma display device
Parameters for measuring performance of display device
Invented by karl ferdinand braun ,in 1897.
Electron gun -- rear side
Inner side coated with phosphor
Phosphor coating glows and can see on the front side.
Electrostatic deflection of e- beam
Direct the beam toward specified positions on the screen.
Phosphor emits a small spot of light at each position contacted by the e- beam.
Properties of Phosphorus coating
Components of electron gun
Heated metal cathode
Heat is supplied to cathode by filament
Causes e- to boiled off
Accelerated anode can be used to produce high +ve voltage.
Control grid controls Intensity of e- beam.
Techniques for producing images on CRT screen
Vector/random scan display
Raster scan display
RASTER SCAN DISPLAY
Most common method.
Beam is swept across the screen , one row at a time from top to bottom(pixel by pixel).
Screen is refreshed.
RANDOM SCAN DISPLAY
One line at a time.
End to end point.
The components lines of picture can be drawn and refreshed by a random scan system in any specified order.
Only area where picture is drawn refreshed.
Example-hard copy device.
Difference between Raster & Random scan display Raster scan display Random scan display Pixel by pixel Line by line Picture definition is stored in memory area called refresh/frame buffer. Stored as a set of line-drawing commands in memory area called refresh display file/display program. Whole screen is refreshed. Only area where picture is drawn is refreshed. More memory/time. Less memory/time. Lower resolution Higher resolution Zigzag Smooth
Color CRT monitors
Displays color picture by using a combination of phosphorus that emit different colored light.
It generates a range of colors by combining the emitted light from different phosphorus.
There are two basic techniques used for producing color displays:
Shadow mask technique
Used with random scan monitors
Two layers of phosphors usually red and green, are coated in to inside of CRT screen,.
Cheapest method to produce colors.
Picture quality not good.
Support only four colors(R,G,Y,O).
No more used because of limitation of colors.
Tough to control voltage settings.
Shadow Mask CRT
Used in Raster scan systems
Types- inline and triangle
Alternative method of maintaining image.
Two e- guns used- Primary gun(picture pattern),Flood gun(maintains the picture display)
Storage grid-It stores picture information as a charge distribution just behind the phosphor –coated screen.
It can let the picture display for long time
Refreshing is not required.
Very complex pictures can be displayed at very high resolution. Without flicker.
It is good for static picture only.
It has flat screen.
Not suitable for dynamic images.
Do not display colors.
To make a small change in pic. There is need to redraw entire picture.
Flat Panel Display
It refers to a class of video devices that have reduced volume, weight and power requirements compared to a CRT.
Thinner than CRT’s.
Flat Panel Display
Emissive display :
Example: Plasma panels , thin film electro luminescent displays, and L.E.D .
Nonemissive displays :
Plasma panel display
Consists of two plates.
Constructed by filling the region b/w two glass plates with a mixture of gases (neon,xenon or helium).
This material is sensitive to light.
Frame buffer/refresh buffer: stores pixel value.
Refreshing is not required.
Produces a very steady image, totally free of flicker.
Less bulky than CRT.
Available in black and white as well as color displays.
Hanged on a wall.
Wear them on wrist.
Selective parts of image can be erased or update.
Refresh rate is only 60 bits/second, so not suitable for animations.
They are slow devices.
Costlier than CRTs.
It requires complex addressing and wiring.
Earlier plasma panels were suitable for black and white concepts but now we can use them for RGB applications also where segments of conductor plate can be painted with RGB pattern.
The region between the plates is filled with phosphor, such as zinc sulfide doped with manganese , instead of a gas.
Requires more power than plasma panels and good color and gray scale displays hard to achieve.
A matrix of diodes is arranged to form the pixel positions in the display .
The diodes are like bulb placed over display in a matrix form
Example: LED computer monitors,LED laptop screens as well as LED T.V’s
Compounds have a crystalline arrangement of molecules ,yet they flow like a liquid.
Consists of two glass plates ,each containing light polarizer at right angles to the other plate .
Rows of horizontal transparent conductors are built into one glass plate, and columns put into other plate.
The intersection of two conductors defines a pixel position.
LCD Two states: ON state -polarized light is twisted. To turn OFF the pixel, we apply a voltage to the two intersecting conductors to align the molecules so that the light is not twisted(passive matrix) For constructing LCDs ,place transistor at each pixel location, using TFT technology Transistors are used to control voltage at pixel locations . To prevent charge from gradually leaking out of liquid-crystal cells. (active matrix).