In this paper, an improved method for detecting light-flicker from multiple flicker sources which has a significant detrimental effect on power quality for power systems is proposed. The proposed method calculates the instantaneous flicker value directly from the frequency spectrum of the voltage signal unlike the standard IEC-61000-4-15 flickermeter known to give erroneous results in case of deviation of the fundamental frequency. Besides, owing to spectral corrections, this can also obtain the correct flicker value when the fundamental frequency deviates. Tests with synthetic signals have been generated in the presence of a single interharmonic frequency for different values, multiple frequency components and fundamental frequency deviations respectively, and then the instantaneous flicker values calculated by this method have been compared with IEC-flickermeter results. The method has been also used to calculate the flicker of field data obtained from busbars feeding electric arc furnace plants in the Electricity Transmission System of Turkey and the flicker-severity results have been compared with the results of the IEC-flickermeter. The advantage of the method is that it can swiftly and precisely estimate the flicker because of the reduction of it to arithmetic operations on Fast Fourier Transform(FFT) samples already calculated in many power quality analyzers.