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Shadow Mask Technique in Computer Graphics

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Pre-requisites: CRT 

A CRT Monitor displays color pictures by using a combination of phosphorus that emits different colored lights. By combining the emitted light from the different phosphorous, a range of colors can be generated.

There are basically two techniques for producing color displays with a CRT the:

  1.  Beam-penetration method 
  2.  Shadow mask method 

In this article, we will purely discuss the Shadow Mask Technique.

Shadow Mask Technique:

Shadow Mask Technique in Computer Graphics is commonly used in raster-scan systems ( including color TV) because they produce a much wider range of colors than the Beam Penetration Method.

 It has three phosphor color dots at each pixel position. The first phosphor dot emits a red light, the second one emits a green light and the third emits a blue light. The phosphor transforms the Kinetic Energy of the electrons into Light Energy

 The Shadow Mask Technique involves the usage of three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen. 

Shadow-mask technique

 

Types of Shadow-Mask Technique: 

1.  The below figure illustrates one of the shadow-mask techniques that is, the delta-delta shadow mask technique commonly used in color CRT systems. the electron beams are diverted and concentrated as a group on the shadow mask, which contains a sequence of holes lined up with phosphor-dot patterns. When these three beams cross through a hole in the shadow mask, they animate a dotted triangle, that appears as a small color spot over the screen. The phosphor dots on the triangle are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.

2.  Another configuration for three electron guns is an in-line arrangement in which the three electron guns and the corresponding red-green-blue color dots on the screen, are aligned along a single scan line rather than in a triangular pattern. This type of arrangement of electron guns is convenient to keep in alignment and is generally used in high-resolution color CRTs. 

We get color variations in a shadow mask CRT by varying the intensity levels of the three electron beams. By turning off red and green guns, we get only the color coming from the blue phosphor. Other combinations of beam intensities generate a small light spot for each pixel position since our eyes tend to merge the three colors into one composite. The color we see depends on the excitation of the red, green and blue phosphorus. A white (grey) is the result of activating all three dots of equal intensity. Yellow is generated with the green and red dots only, magenta is generated with the blue and red dots and cyan shows up when blue and green are activated uniformly.

Advantages of the Shadow Mask Technique: 

  • The image is more real as compared to other RGB filtering methods.
  • Tailored, that is this technique is customizable. 
  • And is relatively less expensive compared to other techniques. 

Disadvantages of the Shadow Mask Technique:

  • Use of very limited colors, only three to four colors usage.
  • Other methods available provide much better results in terms of the quality of the picture.

Last Updated : 30 Nov, 2022
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