Nonlinear loads including lighting fixtures generate harmonic currents and create distortions on the sinusoidal voltage of the power system. Harmonic field measurements have shown that the harmonic contents of a waveform varies with time. A cumulative probabilistic approach is the most commonly used method to solve time varying harmonics. In this paper the time varying harmonics of lighting loads are evaluated by international harmonic standards(IEC 61000-3-6).