Flicker refers to the subjective impression of light density changes or an impression of unsteadiness of visual perceptions, caused by luminous stimuli with temporal fluctuations of the light density or the spectral distribution. From a technical perspective, voltage variations cause light density changes in lamps, which can result in visual perceptions referred to as flicker. From a certain threshold value the appearance of flicker can be disturbing. The disturbing effect of voltage variations depends here on the extent of the repetition rate and the curve form of the change in voltage. The short-term flicker strength and long-term flicker strength are defined measures of the disturbing effect.
Voltage variations, caused by individual devices (on the low voltage network), are permissible if the resultant flicker disturbance factor is not greater than 1. The long-term flicker disturbance factor averaged from twelve values must not exceed a value of 0.65.The most simple method for evaluating the value is the = 1 p.u. curve. P.u. stands here for the "unit of perception" and is the maximum tolerance level for the interference sensitivity of the human eye with regards to its perception of light fluctuations. It is also not permissible to exceed the value = 1 p.u. in combination with all interferers.