cathode-ray tube

A Closer Look

Cathode-ray tubes (CRTs), also called electron-ray tubes, provide the visual display in such devices as conventional television sets, computer monitors, hospital heart monitors, and laboratory oscilloscopes. CRTs are generally made of funnel-shaped glass vacuum tubes. At the larger end of the tube is a phosphor-coated screen, and at the other end is an electron gun. The gun consists of a heated cathode, or negative electrode, which emits electrons, and a control grid, which controls the intensity of the beam of electrons to vary the brightness of the image. The gun directs the electron beam, or cathode ray, toward the screen, where a positively charged anode attracts the electrons. Outside the tube, coils creating a magnetic field or plates creating an electric field both focus and steer the beam. Wherever the beam strikes the screen, it causes the phosphors to glow. Shapes and images can be formed by manipulating the beam so that its focal point on the screen sweeps across it in various paths and with different brightness. In most CRTs, the beam follows a zigzag path that covers the entire screen many times per second. Color screens use three separate beams that strike three individually colored phosphor cells (having the three primary colors red, blue, and green) that are very close together. The color combinations appear to the eye (at a distance to the screen) as one point of a single color.