Cathode Ray Tubes

The principles at work in a CRT monitor are the same as those used in televisions since the 1940s, although modern computer monitors are substantially more sophisticated than the TVs of yesteryear, or even modern analog TVs. Today, CRT monitors for computers are most often found on desktop computer systems. CRTs are simply too large and heavy for use in portable computers.

Basic CRT Design

A CRT monitor's most prominent feature is the picture tube. At the back of the monitor is a component called the electron gun that fires electrons toward the glass screen at the front of the monitor (see Figure 14.1). This beam sweeps along the screen from left to right in a tiny fraction of a second—roughly 0.00002s, depending upon the display's resolution. The beam then turns off and begins again at a slightly lower point to paint the next line of the display, and so on. Once the monitor's displayed an entire image (which typically takes 0.01-0.02s), the beam shuts off and the process begins again from the top of the display.

Chapter 14

Electron gun

Electron gun

Display front

Figure 14.1

The electron gun fires a moving beam of electrons at the display to "paint" an image on the screen one line at a time.

Display front

Figure 14.1

The electron gun fires a moving beam of electrons at the display to "paint" an image on the screen one line at a time.

The front of the display tube is coated with a phosphorous compound that glows when struck by the electrons from the electron gun. This coating is not uniform, however; it's broken into small units of red, green, and blue, which are arranged in patterns to allow the electron gun to strike each colored element precisely. To display a green object, the phosphor gun fires only when its electrons will strike the green phosphor element, for example. To display color other than red, green, or blue, colors are combined in various proportions. In this additive color scheme, white is a combination of all three colors, and black is the absence of all colors.

(Paints and inks use a subtractive color scheme, by contrast, in which merging all the colors 14

produces black.)

Persistence o

Two factors combine to make the display appear steady and flicker-free to most people: o

• Phosphor persistence Thephosphors used in monitors glow for a brief period after 1/1

having been struck by the electron beam. Ideally, they glow precisely long enough to remain lit until struck again, but no longer. In practice, the glow fades over time, which is why you may see a faint afterimage of a bright object after you've moved it away from a dark background.

Part III

• Human photoreceptor persistence The cells in your eyes that detect light also experience persistence. Where a monitor's phosphors continue glowing for a time after being struck by an electron beam, your photoreceptors continue firing electrical impulses to your brain after a light source has been removed. The human eye contains two broad classes of photoreceptors: rods, which are responsible for black-and-white vision, and cones, which are responsible for color vision. Rods are more common in your peripheral vision, and are generally more sensitive and have lower persistence.

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