Hardware Concepts: Display devices(i)

Display devices

The display devices used in graphics system is video monitor. The most common video monitor is based on CRT technology.

Cathode Ray Tube (CRT)

−        CRT are the most common display devices on computer today. A CRT is an evacuated glass tube, with a heating element on one end and a phosphor-coated screen on the other end.

−        When a current flows through this heating element (filament) the conductivity of metal is reduced due to high temperature. These cause electrons to pile up on the filament.

−        These electrons are attracted to a strong positive charge from the outer surface of the focusing anode cylinder.

−        Due to the weaker negative charge inside the cylinder, the electrons head towards the anode forced into a beam and accelerated by the inner cylinder walls in just the way that water is speeds up when its flow though a small diameter pipe.

−        The forwarding fast electron beam is called Cathode Ray. A cathode ray tube is shown in figure below.

Figure :Cathode Ray Tube

−        There are two sets of weakly charged deflection plates with oppositely charged, one positive and another negative. The first set displaces the beam up and down and the second displaces the beam left and right.

−        The electrons are sent flying out of the neck of bottle (tube) until the smash into the phosphor coating on the other end.

−        When electrons strike on phosphor coating, the phosphor then emits a small spot of light at each position contacted by electron beam. The glowing positions are used to represent the picture in the screen.

−        The amount of light emitted by the phosphor coating depends on the no of electrons striking the screen. The brightness of the display is controlled by varying the voltage on the control grid.

Persistence:   

−        How long a phosphor continues to emit light after the electron beam is removed

−        Persistence of phosphor is defined as the time it takes for emitted light to decay to 1/10 (10%) of its original intensity. Range of persistence of different phosphors can react many seconds.

−        Phosphors for graphical display have persistence of 10 to 60 microseconds. Phosphors with low persistence are useful for animation whereas high persistence phosphor is useful for highly complex, static pictures.

Refresh Rate:

−        Light emitted by phosphor fades very rapidly, so to keep the drawn picture glowing constantly, it is required to redraw the picture repeatedly and quickly directing the electron beam back over the some point. The no of times/sec the image is redrawn to give a feeling of non-flickering pictures is called refresh-rate.

−        If Refresh rate decreases, flicker develops.

−        For refresh displays, it depends on picture complexity

−        Refresh rate above which flickering stops and steady it may be called as critical fusion frequency(CFF).

Resolution:   

Maximum number of points displayed horizontally and vertically without overlap on a display screen is called resolution. In other ways , resolution is referred as the no of points per inch(dpi/pixel per inch).