1. Explain refresh cathode ray tube?
- A beam of electrons, emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor coated area.
- The phosphor that emits a small spot of light at each position contacted by the electron beam
- One way to keep phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points
- This type of display is called is called refresh CRT
Primary components of electron gun are:
1. Heated metal cathode Heat is supplied to the cathode by directing the beam through a coil of wire called the filament inside the cylindrical cathode structure.
2. Control grid Intensity of the electron beam is controlled by setting the voltage levels on the control grid, which is a metal cylinder that fits to the cathode.
- Different kinds of phosphorus are available
- Besides color the major difference between phosphors is their persistence
- The maximum number of points that can be displayed without overlap on a CRT is referred to as resolution.
- Another property is aspect ratio
2. Explain color CRT monitors?
A CRT monitor displays color pictures by using a combination of phosphors that emit different colored light.
By combining the emitted light from the different phosphors, a range of colors can be generated
Two techniques
Beam penetration method
Shadow mask method
Beam penetration method
- Display color pictures
- Two layers of phosphors, usually red and green
- A beam of slow electrons excites the outer red layer
- Fast electron penetrates through the red layer and excites the inner green layer
- An intermediate beam speeds, combinations of red and green light
Shadow mask method
- They produce a wider range of colors
- Has three phosphor color dots at each pixel position
- One emits red light, another emits green light, and the third emits a blue light
- The three beams are deflected and focused as a group on to the shadow mask, which contains a series of holes aligned with a phosphor dot patterns.
- When the three beams passes through a hole in the shadow mask, they activate a dot triangle, which
- appears as a small color spot on the screen.
3. Explain direct view storage tubes and liquid crystal displays?
Liquid crystal displays
• Refers to the compounds having crystalline arrangement of molecules flow of liquid
• Two plates each glass plate contains a light polarized that are right angles to each other
Two types
Passive matrix LCD
Active matrix LCD
Direct view storage tubes
• Alternative method for maintaining a screen image
• Stores picture information as a charge distribution
• Very complex pictures can be displayed at very high resolutions
• To eliminate the picture section the entire screen must be erased
• The erasing and redrawing process can take several seconds for a picture
