Origin of Flicker
The voltage of public power networks is changing over time. These changes originate in the voltage drop generated over the source impedance of the grid by the changing load current of an equipment or facility.
The fluctuations of the load in time generate flicker, i.e. changes in the luminous flux of a lamp. The effects of flicker can range from disturbance to epileptic attacks of photosensitive persons.
The requirements of a flicker measurement equipment are defined in the standard  IEC 61000-4-15. The flicker meter is subdivided in several function blocks which simulate a 230 V/60 W incandescent lamp (reference lamp) an the human perception system (eye-brain model).
From the resulting momentary value of flicker the short term flicker value Pst is generated in the last block according to a statistic process over a predefined observation interval. Long term flicker Plt is calculated as the cubic average of several Pst values.
In the standard  IEC 61000-3-3 the observation intervals and the limiting values for Pst and Plt are specified:
|Value ||Observation Interval ||Limiting Value |
|Pst ||10 min ||1.0 |
|Plt ||2 h ||0.65 |