An apparatus for measuring the power frequency of a light source includes a photo-sensitive transistor, a modulators and a logic unit. The photo-sensitive transistor generates an electrical signal that is responsive to light incident thereon from the light source. The modulator generates a modulated signal based on the electrical signal that toggles at a rate substantially proportional to the power frequency of the light source. The logic unit is coupled to receive the modulated signal and determine its toggling frequency.