A CRT (cathode-ray tube) is a specialized vacuum tube used in television sets and computer monitors to produce images when an electron beam strikes a phosphorescent surface. The CRT in a computer display is similar to the "picture tube" in a television receiver. To produce an image on the screen, complex signals are applied to the deflecting coils, and also to the apparatus that controls the intensity of the electron beam. This causes the spot to race across the screen from right to left, and from top to bottom, in a sequence of horizontal lines called the raster. The CRT works by heating a cathode which causes electrons to flow. Accelerating and focusing anodes turn the electrons into a fine beam that is directed to the phosphors by magnetic fields that are generated by steering coils. CRT monitors have the ability to display all of the 16.7 million colors used in 24-bit displays, whereas common LCD monitors are unable to display all of these colors as they use TN technology. Although CRT technology offers many benefits, it also has some disadvantages. For example, CRT monitors are heavy and cumbersome.