CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [318]
Phosphors and Shadow Mask
All CRT monitors contain dots of phosphorous or some other light-sensitive compound that glows red, green, or blue (RGB) when an electron gun sweeps over it. These phosphors are evenly distributed across the front of the monitor (see Figure 19-6).
Figure 19-6 A monitor is a grid of red, green, and blue phosphors.
A normal CRT has three electron guns: one for the red phosphors, one for the blue phosphors, and one for the green phosphors. It is important to understand that the electron guns do not fire colored light; they simply fire electrons at different intensities, which then make the phosphors glow. The higher the intensity of the electron stream, the brighter the color produced by the glowing phosphor.
Directly behind the phosphors in a CRT is the shadow mask, a screen that allows only the proper electron gun to light the proper phosphors (see Figure 19-7). This prevents, for example, the red electron beam from “bleeding over” and lighting neighboring blue and green dots.
Figure 19-7 Shadow mask
The electron guns sweep across the phosphors as a group, turning rapidly on and off as they move across the screen. When the group reaches the end of the screen, it moves to the next line. It is crucial to understand that turning the guns on and off, combined with moving the guns to new lines, creates a mosaic that is the image you see on the screen. The number of times the guns turn on and off, combined with the number of lines drawn on the screen, determines the number of mosaic pieces used to create the image. These individual pieces are called pixels, from the term picture elements. You can’t hold a pixel in your hand; it’s just the area of phosphors lit at one instant when the group of guns is turned on. The size of pixels can change, depending on the number of times the group of guns is turned on and off and the number of lines drawn.
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NOTE Not all CRT monitors use dots. The Sony Trinitron line of CRT monitors uses bars of red, green, and blue instead of dots. The holes in the shadow mask have a rectangular shape. Many people feel this makes the monitor’s image much crisper and clearer. Somebody must agree with them because the Trinitron enjoys tremendous popularity. Even though the phosphors and shadow mask have a different shape, everything you learn here applies to Trinitrons also.
Resolution
Monitor resolution is always shown as the number of horizontal pixels times the number of vertical pixels. A resolution of 640 × 480, therefore, indicates a horizontal resolution of 640 pixels and a vertical resolution of 480 pixels. If you multiply the values together, you can see how many pixels are on each screen: 640 × = 480 307,200 pixels per screen. An example of resolution affecting the pixel size is shown in Figure 19-8.
Some common resolutions are 640 × 480, 800 × 600, 1024 × 768, 1280 × 960, 1280 × 1024, and 1600 × 1200. Notice that most of these resolutions match a 4:3 ratio. This is called the aspect ratio. Many monitors are shaped like television screens, with a 4:3 aspect ratio, so most resolutions are designed to match—or at least be close to—that shape. Other monitors, generically called wide-screen monitors, have a 16:9 or 16:10 ratio. Two of the common resolutions you’ll see with these monitors are 1366 × 768 and 1920 × 1200.
Figure 19-8 Resolution versus pixel size
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NOTE See the “Video Modes” section later in this chapter for the names of each resolution.
The last important issue is to determine the maximum possible resolution for a monitor. In other words, how small can one pixel be?