1. Causing one complete horizontal or vertical traverse of the spot of light on the screen of a cathode-ray tube (CRT), i.e. one sweep of the screen, in response to a voltage generated by a timebase circuit. If the deflection extends beyond the usable physical dimensions of the phosphor on the screen, overscanning occurs; if it is less than the usable size underscanning results. In the former case information can be lost from the ends of the scan; in the latter the image does not fill the screen. A particular type of scanning sometimes used with a CRT is spiral scanning. In this case the electron beam is made to execute a spiral trace on the screen.
2. Exploring a particular area, volume, or frequency range in a methodical manner in order to produce a variable electrical signal whose instantaneous values are a function of the information contained in the small section examined at each instant. The information scanned may then be reproduced by a suitable receiver. The technique is most often used in television, radar, and fax.
The most common type of scanning used in television and fax is rectilinear scanning, in which the target area is rectangular and scanning proceeds in a set of narrow parallel strips (see also television).
High-velocity scanning is a type of electron scanning, i.e. scanning of the target with a beam of electrons, in which the energy of the electrons in the beam is sufficient to produce a secondary-emission ratio at the target that is greater than unity. If the energy of the electrons is less than the minimum velocity required to produce a secondary-emission ratio of unity, the scanning is termed low-velocity scanning.
Coarse scanning is frequently used in order to produce a ‘rough’ picture of the target before carrying out a more detailed investigation. In this case the size of the scanning spot is comparable to the image detail and is the diameter of the electron beam, light beam, or beam of radiowaves used to carry out the scan. Coarse scanning is most often used in radar systems.
Radar systems may also employ circular scanning, in which one complete scan is a horizontal rotation of the radar beam through 360°, or conical scanning, when the major lobe of the transmitted antenna pattern generates a cone.
Most forms of scanning assume that the scanning speed is uniform throughout each scan. In some applications however it is useful to employ a variable speed of the scanning spot if one section of the scanned area is to be investigated in more (or less) detail than the rest. Some form of distribution control is used to achieve a variable speed distribution during the trace interval by controlling the output of the timebase circuit used.