1. The minimal frequency of flashes of light per second below which an intermittent light stimulus is no longer seen as a continuous visual sensation.

2. The refresh frequency of a displayed image at which flicker is perceived by the operator. It depends upon the brightness of the display, the angle subtended at the eye, and on the persistence of the phosphor. This frequency varies between individuals, but for typical cathode-ray tubes flicker is generally accepted as being perceptible to less than 5% of users when the CFF exceeds 80 hertz.