speed [AS, spedan, to hasten] , 1. the rate of change of position with time. Compare velocity. 2. See amphetamines. 3. a reciprocal of the amount of radiation used to produce an image with various components of an x-ray imaging system, such as screens, film, and image intensifiers. There is often a tradeoff between radiation dose to the patient and the overall image quality. Thus a system using little radiation is “fast,” whereas one requiring more radiation is “slow.” 4. the amount of exposure of film to light or x-rays needed to produce a desired image. X-ray film speed usually is indicated as the reciprocal of the exposure in roentgens necessary to produce a density of 1 above the base and fog levels. See also fogged film fault.
