Flicker (screen)
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Flicker is visible fading between image frames displayed on cathode ray tube (CRT) based monitor. Flicker occurs when the monitor's CRT is driven at a low refresh rate, allowing the screen's phosphors to lose their excitation between sweeps of the electron gun.
For example, if a computer monitor's vertical refresh rate is set to 60 Hz, most monitors will produce a visible "flickering" effect. Most people find that refresh rates of 85 Hz and above enable flicker-free viewing on CRTs. Refresh rates above 120 Hz are uncommon, as they provide no noticeable flicker reduction. On LCDs, lower refresh rates (around 75 Hz) are often acceptable.
The exact refresh rate necessary to prevent the perception of flicker varies greatly based on the viewing environment. In a completely dark room, a sufficiently dim display can run as low as 30 Hz without visible flicker. At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable.
Another factor in detecting flicker is peripheral vision. The human eye is most sensitive to flicker at the edges of our field of view, and least sensitive at the center of gaze (the area being focused on). As a result, the greater portion of our field of view that is occupied by a display, the greater is the need for high refresh rates. This is why computer monitor CRTs usually run at 70 to 80 Hz, while TVs, which are viewed from further away, are seen as acceptable at 60 or even 50 hz. (see PAL and NTSC tv)
Refresh rate vrs Frame rate
In evaluating flicker, it is important to distinguish between refresh rate - how often a screen is illuminated, and frame rate - how often the image being displayed can change. Motion picture film has a standardized frame rate of 24 hz, meaning that there are 24 photographs projected in each second. However, to reduce the flicker that such a low refresh rate would cause, each image is illuminated twice before the film advances to the next frame. The result of this is that the movie screen is refreshed with light 48 times per second, even though there are only 24 unique frames in that second.
Television sets never vary their refresh rate - an NTSC set is always 59.94 hz. If the source material is slower, such as film, then the film images are repeated as needed to produce the NTSC refresh rate. Thus a low frame rate cannot cause flicker on TV, but can cause jerky motion, as is the case of silent films that run at 12-20 hz.
Other uses
Flicker can also refer to the phenomenon which occurs on computers and gaming consoles when many objects must be displayed and hardware limitations prevent all of them from being rendered simultaneously. In these cases, one screen refresh may display the first set of objects while the next screen refresh will display the remaining ones. When observed in real time, the viewer will see all of the objects, though they appear and disappear rapidly.
Flicks is an old english expression referring to the Cinema or building where films are shown. Example usage, "Are you going to the flicks tonight?". Less often used today. More recently replaced with the words cinema or "The Pictures" (which is in turn derived from "the picture house").