According to an example, a plurality of pixels of a modulator upon which an input light beam impinges may be modulated to apply a first asymmetrical attenuation pattern on the input light beam and to direct a first attenuated light beam from the modulator and a first power level of the first attenuated light beam may be measured. The plurality of pixels may be modulated to apply a second asymmetrical attenuation pattern on the input light beam and to direct a second attenuated light beam from the modulator, and a second power level of the second attenuated light beam may be measured. A difference between the first power level and the second power level may be calculated and a modified peak frequency for an attenuated light beam from the calculated difference may be calculated.