Colour CRT Monitors | Colour Generating Techniques in Computer Graphics

Colour CRT Monitors

Color tubes use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes or clusters called “triads” (as in shadow mask CRTs) as shown in figure 1.7.

The three beams in color CRTs would not, of themselves, strike the screen at the same place at the same time. Uncorrected, the three colors in a typical image would be unacceptably misaligned. Elaborate measures are needed to make the beams converge acceptably over the entire screen.

Convergence in Color CRTS For one. static convergence brings the beams together, while dynamic convergence (such as from auxiliary coils near the deflection yoke) maintains convergence over the whole screen.

Delta-gun CRTS required the electronically driven convergence coils placed on a “triangle” device on the deflection yoke, powered by the deflection oscillators through a complex set of tuneable coils, capacitors, and resistors. This set had around 15 points of manual adjustment to align the color and was usually located on a board from the side of the TV case, which could be accessed without dismounting the rear cover of the TV, and easily accessed while viewing the test grid picture on the screen.

Colour Generating Techniques

A color CRT monitor displays color pictures by using a combination of phosphors that emit different colored light. By combining the emitted light, a range of colors can be generated. Two basic methods for producing color displays are:

1)     Beam Penetration Method: Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit).

Each gun’s beam reaches the dots of exactly one color; a grille or mask absorbs those electrons that would otherwise hit the wrong phosphor. Since each beam starts at a slightly different location within the tube, and all three beams are perturbed in essentially the same way, a particular deflection charge will cause the beams to hit a slightly different location on the screen (called a ‘sub pixel’).

Color CRTs with the guns arranged in a triangular configuration are known as delta-gun CRTS because the triangular formation resembles the triangular shape of the Greek letter A (delta). While having deep color reproduction, CRTS can often exaggerate red.

The advantage of this method is that it is less expensive.

The disadvantages of this method are:

i)                    Quality of images is not good as compared with other methods.

ii)                  Four colors are allowed only. 

2)    Shadow-mask Methods: These are commonly used in raster-scan systems (including color TV) because they produce a much wider range of color than the beam penetration method.

A shadow-mask CRT has three phosphor color dots at each pixel position One phosphor dot emits red light, another emits green light, and the third emits blue light. This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen. Figure 1.7 illustrates the delta-delta. a shadow-mask method is commonly used in color CRT systems.

The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen the phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.

Advantages

i) Produce realistic images.

ii) Produced different colors.

iii) Shadows scenes.

Disadvantages

i) Low resolution.

ii) Expensive.

iii) Electron beam directed to the whole screen. Low resolution.