page_title: Cathode-ray tube
section_title: Degaussing
hierarchical_section_title: Cathode-ray tube / Color CRTs / Degaussing
caption_attribution_description: English: A degauss in progress on a Dell Ultra Scan P991.
context_page_description: The cathode-ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam onto the screen to create the images. The images may represent electrical waveforms, pictures, radar targets, or other phenomena. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer.
  In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. In color devices, an image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color with a video signal as a reference. In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of electronic test instrument.
context_section_description: If the shadow mask or aperture grille becomes magnetized, its magnetic field alters the paths of the electron beams. This causes errors of "color purity" as the electrons no longer follow only their intended paths, and some will hit some phosphors of colors other than the one intended. For example, some electrons from the red beam may hit blue or green phosphors, imposing a magenta or yellow tint to parts of the image that are supposed to be pure red. (This effect is localized to a specific area of the screen if the magnetization is localized.) Therefore, it is important that the shadow mask or aperture grille not be magnetized. 
  Most color CRT displays, i.e. television sets and computer monitors, each have a built-in degaussing (demagnetizing) circuit, the primary component of which is a degaussing coil which is mounted around the perimeter of the CRT face inside the bezel. Upon power-up of the CRT display, the degaussing circuit produces a brief, alternating current through the degaussing coil which smoothly decays in strength (fades out) to zero over a period of a few seconds, producing a decaying alternating magnetic field from the coil. This degaussing field is strong enough to remove shadow mask magnetization in most cases. In unusual cases of strong magnetization where the internal degaussing field is not sufficient, the shadow mask may be degaussed externally with a stronger portable degausser or demagnetizer. However, an excessively strong magnetic field, whether alternating or constant, may mechanically deform (bend) the shadow mask, causing a permanent color distortion on the display which looks very similar to a magnetization effect.
  The degaussing circuit is often built of a thermo-electric (not electronic) device containing a small ceramic heating element and a positive thermal coefficient (PTC) resistor, connected directly to the switched AC power line with the resistor in series with the degaussing coil. When the power is switched on, the heating element heats the PTC resistor, increasing its resistance to a point where degaussing current is minimal, but not actually zero. In older CRT displays, this low-level current (which produces no significant degaussing field) is sustained along with the action of the heating element as long as the display remains switched on. To repeat a degaussing cycle, the CRT display must be switched off and left off for at least several seconds to reset the degaussing circuit by allowing the PTC resistor to cool to the ambient temperature; switching the display-off and immediately back on will result in a weak degaussing cycle or effectively no degaussing cycle.
  This simple design is effective and cheap to build, but it wastes some power continuously. Later models, especially Energy Star rated ones, use a relay to switch the entire degaussing circuit on and off, so that the degaussing circuit uses energy only when it is functionally active and needed. The relay design also enables degaussing on user demand through the unit's front panel controls, without switching the unit off and on again. This relay can often be heard clicking off at the end of the degaussing cycle a few seconds after the monitor is turned on, and on and off during a manually initiated degaussing cycle.
  At high refresh rates and resolutions, the deflection coil/yoke starts to produce large amounts of heat, due to the need to quickly move the electron beam (because the electron beam has to scan more lines per second), which in turn requires large amounts of power, for quickly generating strong magnetic fields. This makes CRTs beyond certain resolutions and refresh rates impractical, since the coils would need active cooling, to prevent the heat from the coils  from melting the glue that is used to attach them to the neck of the CRT.
caption: A degaussing in progress.
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