Color CRT Monitors
Computer GraphicsColor CRT Monitors
A CRT monitor displays colour pictures by using a combination of phosphors that emit different coloured light. With the combination of phosphor a range of colours can be displayed. There are two techniques used in colour CRT monitors:
̇ Beam Penetration Method ̇ Shadow Mask Method
In beam penetration method two layers of phosphor, usually coated onto the inside of the CRT screen, and the displayed colour depend on how far the electron beam penetrates into the phosphor layers. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colours, orange and yellow. Beam penetration is an inexpensive way to produce colours as only a few colours are possible and the quality of picture is also not impressive.
Shadow mask methods can display a wide range of colours. In this technique each pixel position is made up of three phosphor dots called triads as shown in the following figure. Three phosphor dots have different colors i.e. red, green and blue and the display colour is made by the combination of all three dots. Three guns are used to throw beam at the three dots of the same pixel. By varying intensity at each dot a wide range of colours can be generated.
A shadow-mask is used which has holes aligned with the dots so that each gun can fire beam to corresponding dot only.
CRT Displays Advantages
- Fast response (high resolution possible)
- Full colour (large modulation depth of E-beam)
- Saturated and natural colours
- Inexpensive, matured technology
- Wide angle, high contrast and brightness
- Disadvantages
- Large and heavy (typ. 70×70 cm, 15 kg)
- High power consumption (typ. 140W)
- Harmful DC and AC electric and magnetic fields
- Flickering at 50-80 Hz (no memory effect)
- Geometrical errors at edges
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