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Saturday, 15 February 2014

Computer Graphics Notes Lab 6 - Color CRT Monitors

COLOR CRT MONITORS

A CRT monitor displays color pictures by using a combination of phosphors that emit different colored light. It generates a range of colors by combining the emitted light from the different phosphors. There are two basic techniques used for producing color displays:
    Beam - penetration technique
    Shadow - Mask technique

Beam - penetration Technique
This technique is used with random-scan monitors. In this technique, the inside of CRT screen is coated with two layers of phosphor, usually red and green. The displayed color depends on how far the electron beam penetrates into the phosphor layers. The outer layer is of red phosphor and inner layer is of green phosphor. A beam of slow electrons excites only the outer layer. A beam of very fast electrons penetrates through the red layer and excites the inner green layer. At intermediate beam speeds, combinations of red and green light are emitted and two additional colors orange and yellow displayed. The beam acceleration voltage controls the speed the speed of the electrons and hence the screen color at any point on the screen.



Merits and Demerits
•       It is an inexpensive technique to produce color in random scan monitors.
•       It can display only four colors.
•       The quality of picture produced by this technique is not as good as compared to other techniques

Shadow - Mask technique

The shadow mask technique produces a much wider range of colors than the beam penetration technique. This technique is commonly used in raster-scan displays. In a shadow mask technique, CRT has three phosphor color dots at each pixel position. One phosphor dot emits a red light, another emits green light and the third emits a blue light. It has three electron guns, one for each color dot and a shadow mask grid just behind the phosphor coated screen.

The shadow mask grid consists of series of holes aligned with the phosphor dot pattern. As shown in the figure three electron beams are deflected and focused as a group onto the shadow mask and when they pass through a hole in the shadow mask, they excite a dot triangle. A dot triangle consists of three small phosphor dots of red, green and blue color. These phosphor dots are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. A dot triangle when activated appears as a small dot on the screen which has color of combination of three small dots in the dot triangle. By varying the intensity of the three electron beams we can obtain different colors in the shadow mask CRT.







Direct - view storage Tubes
In raster scan display we do refreshing of the screen to maintain a screen image. A DVST uses the storage grid which stores the picture information as a charge distribution just behind the phosphor coated screen.


It consists of two electron guns: a primary gun and flood gun. A primary gun stores the picture pattern and the flood gun maintains the picture display.

A primary gun produces high speed electrons which strike on the storage grid to draw the picture pattern. As electron beam strikes on the storage grid with high speed, it knocks out electrons from the storage grid keeping the net positive charge. The knocked out electrons are attracted towards the collector. The net positive charge on the storage grid is nothing but the picture pattern. The continuous low speed electrons from flood gun are attracted to the positive charge areas of the storage grid. The low speed electrons then penetrate the storage grid and strike the phosphor coating without affecting the positive charge pattern on the storage grid.

Advantages

1.     Refreshing of CRT is not required
2.     Very complex pictures can be displayed at very high resolution without flicker.
3.     It has flat screen.

Disadvantages

1.     They do not display colors.
2.     Erasing requires removal of charge on the storage grid. Thus erasing and redrawing process takes several seconds.
3.     Selective or part erasing of screen is not possible.

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