Generally, the output device in a graphics system is a video-monitor which is based on the Cathode-Ray Tube(CRT).
Let us first see, the working of the CRT.
Cathode Ray Tube(CRT) :- It is an special vacuum tube which is used for producing images. When the electron beam are focused on the phosphor-coated screen, it emits a small spot of light at each position by the electron beam. Because the light emitted by the surface emits very rapidly , there need to be a some mechanism to keep the screen picture intact.
One way is to redraw the picture very quickly as soon as the images starts disappearing. This is also known as Refresh CRT.
The major component in the CRT is the heated metal cathode and focusing anode(control grid).
- Function of cathode :- When the heat is supplied to the cathode by directing the current through filament , it causes negatively charged particles to get accelerated toward the phosphor coating screen.
- Function of control grid :- The control grid control the intensity of the electron beam on the phosphor screen. when high negative voltage is applied to the , the control grid will shut off the beam by repelling the electrons. Since , the intensity of the light depends on the number of electrons emitted , the control grid can control the brightness of the light that need to be produced.
Different kinds of phosphor are available which is used in the CRT. One major feature of phosphor is that , besides color , is that how long the phosphor is able to emit the light which is also called persistence. Persistence is also defined as the time it takes the emitted light to decay from the screen to one-tenth of its original intensity.
- The intensity in the CRT is greatest at the center of the spot and decreases as going toward the edges.
- The maximum number of points/spots that can be displayed without overlapping on a Cathode Rat Tube is called resolution of CRT.
- Another important property is Aspect Ratio.
RASTER SCAN DISPLAY:-
The most common display employing a CRT is Raster Scan Display. It is based on the television technology.
- In this type of system , the electron beam is swept across the screen (from left to right), one row at a time from the top to bottom.
- The intensity of the light varies to create a pattern on illuminated spots.
- The picture definition is stored in a memory area of the system called frame buffer/ refresh buffer .
- This refresh buffer stores the intensity values for all the points.
- It follows horizontal retrace as well as vertical retrace.
- Refreshing on Raster scan system is carried out at the rate of 60 to 80 frames per second, although this is the minimum refresh rate.
- In this type of system , the refresh process is independent of the complexity of the image.