Optical Flicker — the impression of unsteadiness of visual perception induced by a light stimulus whose luminance or spectral distribution fluctuates with time, or the rapid change in light source output over time — is present, to some degree, in nearly all light sources. However, the degree to which flicker can be perceived, or is acceptable, can vary. If present at high level, and according to individual perception, optical flicker may cause discomfort among users and make a lighting installation unacceptable from a quality perspective.