Frame rate
Frame rate, or frame frequency, is the measurement of how quickly an imaging device can produce unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems. Frame rate is most often expressed in frames per second (often abbreviated "fps", but not to be confused with FPS or first-person shooter) or, equivalently, Hertz (Hz).
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Flicker fusion frequency
The frame rate is related to but not identical to a physiological concept called the flicker fusion threshold or flicker fusion rate. Light that is pulsating below this rate is perceived by humans as flickering; light that is pulsating above this rate is perceived by humans as being continuous. The exact rate varies depending upon the person, their level of fatigue, the brightness of the light source, and the area of the retina that is being used to observe the light source. Few people perceive flicker above 75 Hertz or so.
These rates would be impractical for the actual frame rate of most film mechanisms so the shutter in the projection devices is actually arranged to interrupt the light two or three times for every film frame. In this fashion, the common frame rate of 24 fps produces 48 or 72 pulses of light per second, the latter rate being above the flicker fusion rate for most people most of the time.
Video systems frequently use a similar approach referred to as interlaced video. Broadcast television systems such as NTSC, PAL, and SECAM produce an image using two passes called fields. Each field contains half of the lines in a complete frame (the odd-numbered lines or the even-numbered lines). Thus, while only using the bandwidth of 25 or 30 complete frames per second, they achieve a flicker fusion frequency of 50 or 60Hz, at the expense of some vertical judder and additional system complexity. The "frame rate" of interlaced systems is usually defined as the number of complete frames transmitted each second (25 or 30 in most broadcast systems). However, since a conventional television camera will scan the scene again for each field, under some circumstances it may be useful to think of the frame rate as being equal to the field rate.
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Computer science
Frame rate is also a term used in realtime computer systems. In a fashion somewhat analogous to the moving-picture definition presented above, a realtime frame is the time it takes to complete a full round of the system's processing tasks. If the frame rate of a realtime system is 60 Hertz, the system reevaluates all necessary inputs and updates the necessary outputs 60 times per second under all circumstances.
The designed frame rates of realtime systems vary depending on the equipment. For a system that is steering an oil tanker, a frame rate of 1 Hz may be sufficient. For a realtime system steering a guided missile, a frame rate of 100 Hz may not be adequate. The designer must choose a frame rate appropriate to the application's requirements.
See also
Categories: Video and movie technology