The CRT is essentially a vacuum tube whose face, the glass front, is coated with phosphor compounds. In the CRT, negatively charged electrons are shot by a cathode toward the face of the tube, where they collide with the phosphor coating.

The coating converts the enormous energy of the electrons into light, creating the image you see on your screen.

The electron guns are one of the four main elements that determine the quality of the image.

The others are the shadow mask (replaced in some monitors by an aperture grill), the phosphors that make up the coating on the face of the tube, and the face itself.

The electron beam is one possible source of fuzziness. The larger the screen, the wider the angle from the absolute centre the beam must traverse in order to paint the edges of the display. The result is a distortion called astigmatism. If you see a monitor—especially a larger one (over 15 inches) — with fuzziness along the edges of the display, this is possibly what is happening. When the electrons hit the phosphor compound (simply called phosphors), the compound glows. This glowing, which happens for varying lengths of time and in different colour patterns, is essentially what you’re seeing when you look at your monitor. The glowing appears in different colours because a colour CRT contains not one but three separate phosphor compounds.

The three compounds are arranged together on the screen in three-dot patterns or three-stripe patterns, each dot consisting of a red, green, and blue dot and each stripe consisting of a red, green, and blue line. The three-dot patterns are called triads or, more popularly, picture elements. (The term picture element is frequently shortened to pixel or, in the television industry, pel.) To display red, green, or blue, the electrons strike only one dot in the triad. The space between dots of the same colour, which is consistent across the display face, is known as the dot pitch.

Among the most annoying and headache-inducing factors in working with monitors is flicker. The two factors involved in flicker are phosphor persistence and refresh rate. Phosphor persistence refers to the length of time the phosphor glows after the electron beam hits it. The electron beam paints the image to the screen at a rate of at least 60 times per second, or 60 Hertz— double the rate for TV. (If the glowing lasts too long, however, you get a ghost image that remains on the screen.)

Flicker is especially of concern with interlaced displays, in which the electron guns effectively cheat. In non-interlaced displays, the image is painted line-by-line from the top of the screen to the bottom, and then repainted again from the top. Interlacing involves painting the screen in two stages— the odd-numbered lines and then the even-numbered ones. Interlaced monitors can be made less expensively, because they demand less precision and less speed (and thus can contain less expensive components). But many people can perceive the trickery, in the form of flicker.

Flicker is only one possible factor that can undermine the quality of a display. Another is convergence, which derives its name from the trio of electron beams that hit each pixel precisely or converge on it. A monitor that is not manufactured properly, or not adjusted properly, can fail in this requirement. Consequently, the result will be images in which only partial pixels are illuminated. To the user, this means a fuzzy image. One should pay special attention to convergence claims while purchasing a monitor, because imperfect convergence can be the most significant problem in display clarity.

Test the monitor by running a program that displays text in your favourite typeface and font size, and carefully examine the clarity of the text in the centre of the display versus that as you near the corners. An even better test is to display a pattern of identical horizontal or vertical stripes and check the centre and edges that way. If the corners blur, then you probably have a convergence problem.