This was one the earlier CRTs to produce color displays. Coating phosphors of different compounds can produce different colored pictures. But the basic problem of graphics is not to produce a picture of a predetermined color, but to produce color pictures, with the color characteristics chosen at run time.
The basic principle behind colored displays is that combining the 3 basic colors –Red, Blue, and Green, can produce every color. By choosing different ratios of these three colors we can produce different colors – millions of them in fact. We also have basic phosphors, which can produce these basic colors. So, one should have a technology to combine them in different combinations. Two basic methods for producing color displays are –
1. Beam Penetration CRT
This CRT is similar to the simple CRT, but it makes use of multi-colored phosphorus of a number of layers. Each phosphorus layer is responsible for one color. All other arrangements are similar to simple CRT. It can produce a maximum of 4 to 5 colors.
The organization is something like this – The red, green and blue phosphors are coated in layers – one behind the other. If a low-speed beam strikes the CRT, only the red colored phosphorus is activated, a slightly accelerated beam would activate both red and green (because it can penetrate deeper) and a much more activated one would add the blue component also.
But the basic problem is a reliable technology to accelerate the electron beam to precise levels to get the exact colors – it is easier said than done. However, a limited range of colors can be conveniently produced using the concept.
- Less expensive
- Quality of images are not as good as compared with other methods
- Four colors are allowed only
2. Shadow – Mask CRT
This works, again, on the principle of combining the basic colors – Red, green and Blue – in suitable proportions to get a combination of colors, this principle is much more sophisticated and stable.
The shadow mask CRT, instead of using one electron gun, uses 3 different guns placed one by the side of the other to form a triangle or a “Delta” as shown. Each pixel point on the screen is also made up of 3 types of phosphors to produce red, blue and green colors. Just before the phosphor screen is a metal screen, called a “shadow mask”.
This plate has holes placed strategically, so that when the beams from the three electron guns are focused on a particular pixel, they get focused on particular color producing pixel only i.e. If for convenience sake we can call the electronic beams as red, blue and green beams (though in practice the colors are produced by the phosphors, and until the beams hit the phosphor dots, they produce no colors), the metal holes focus the red beam onto the red color producing phosphor, blue beam on the blue producing one etc. When focused on to a different pixel, the red beam again focuses on to the red phosphor and so on.
Now, unlike the beam penetration CRTs where the acceleration of the electron beam was being monitored, we now manipulate the intensity of the 3 beams simultaneously. If the red beam is made more intense, we get more of red color in the final combination etc. Since fine-tuning of the beam intensities is comparatively simple, we can get much more combination of colors than the beam penetration case. In fact, one can have a matrix of combinations to produce a wide variety of colors.
The shadow mask CRT, though better than the beam penetration CRT in performance, is not without its disadvantages. Since three beams are to be focused, the role of the “Shadow mask” becomes critical. If the focusing is not achieved properly, the results tend to be poor. Also, since instead of a one-pixel point in a monochrome CRT now each pixel is made up of 3 points (for 3 colors), the resolution of the CRT (no. of pixels) for a given screen size reduces.
Another problem is that since the shadow mask blocks a portion of the beams (while focusing them through the holes) their intensities get reduced, thus reducing the overall brightness of the picture. To overcome this effect, the beams will have to be produced at very high intensities, to begin with.
Also, since the 3 color points, though close to each other, are still not at the same point, the pictures tend to look like 3 colored pictures placed close by, rather than a single picture. Of course, this effect can be reduced by placing the dots as close to one another as possible The above displays are called refresh line drawing displays, because the picture vanishes (typically in about 100 Milliseconds ) and the pictures have to be continuously refreshed so that the human persistence of vision makes them see as static pictures. They are costly on one hand and also tend to flicker when complex pictures are displayed (Because refreshing because of the complex).
These problems are partly overcome by devices with inherent storage devices – i.e. they continue to display the pictures until they are changed or at least for several minutes without the need of being refreshed.
- Produce realistic images
- Also produced different colors and shadows scenes.
- low resolution
- electron beam directed to the whole screen
Difference between Beam Penetration and Shadow Mask method
|Beam Penetration method||Shadow Mask method|
|Where Used||It is used with Random Scan System to display color.||It is Used With Raster Scan System to display color.|
|Colors||It can displays Only four colors i.e. Red , Green, Orange and Yellow.||it can display Millions of colors.|
|Color Dependency||Less colors are available because the colors in Beam Penetration depends on the speed of the electron beam.||Millions of colors are available because the colors in Shadow Mask depends on the type of the ray.|
|Cost||It is Less Expensive as compared to Shadow Mask||It is More Expensive than other methods.|
|Picture Quality||Quality of picture is not so good i.e. Poor with Beam Penetration Method.||Shadow Mask gives realism in picture with shadow effect and millions of color.|
|Resolution||It gives High Resolution.||It gives Low Resolution.|
|Criteria||In Beam Penetration method, Color display depends on how far electron excites outer Red layer and then Green layer.||In Shadow Mask Method, there are no such criteria for producing colors. It is used in computers, in color TV etc.|