cathode ray tube
A vacuum tube that displays still or moving images (such as for a television), by controlling the direction of a cathode ray emitted towards the front of the tube. The front is coated by a layer of fluorescent material, so that it emits light when struck by the beam.
cathode ray tube: electron beam steered to paint a picture
A cathode ray tube, or CRT, is an evacuated glass vessel containing an electron gun at the rear and a phosphor-coated screen at the front. The electron gun heats a filament to around 1000 degrees Celsius, releasing electrons that are accelerated through a potential difference of 10 to 30 kilovolts toward the screen. Magnetic or electrostatic deflection coils steer the beam left, right, up, and down, sweeping it across the phosphor layer in a precise grid pattern. Where the beam strikes the phosphor, light is emitted for a fraction of a millisecond, then fades. Persistence of vision binds these brief flashes into a continuous image.
CRTs dominated display technology for nearly a century because they were efficient, gave good color fidelity, and could be made in large sizes. Television receivers, oscilloscopes, radar displays, and computer monitors all relied on them. Monochrome CRTs used a single phosphor; color tubes contained three phosphors (red, green, blue) arranged in dots or stripes, with three electron guns firing in sync to produce any color. Typical screen sizes ranged from 5 inches in portable oscilloscopes to 40 inches in large television sets.
The core limitation of any CRT is the need to redraw the image repeatedly, called the refresh rate. Television standards operated at 50 or 60 hertz; computer monitors required 70 hertz or higher to avoid flicker. The electron beam had to be fast enough to trace an entire frame in one refresh cycle, which constrained tube length and power consumption. Horizontal and vertical synchronization signals told the deflection coils when to reset the beam to the top-left corner.
Failure modes and decline
CRTs suffered from several wear-out mechanisms. The cathode gradually depleted over thousands of hours, reducing brightness. Leaks in the tube seal broke the vacuum, causing the gas to ionize and scatter the beam. The phosphor coating degraded with age, especially in high-brightness operation. Screen burn-in occurred when a static image stayed on the phosphor for extended periods, depleting local phosphor faster than surrounding areas and leaving a ghost image. By the 1990s, flat-panel displays using liquid crystals and then LEDs offered smaller depth, lighter weight, and longer service life, driving CRTs from consumer markets by the 2010s.
CRTs remain in specialized industrial use: radar displays in maritime and air traffic control, high-bandwidth oscilloscopes, and some medical imaging systems value their unmatched real-time performance. A quality CRT oscilloscope can acquire and display signal transients faster than most digital instruments. The high voltage and hot cathode inside a CRT require careful power management and shielding; live CRTs can deliver lethal shocks even when powered off if the screen capacitance is not discharged safely.