One of the main problems in the area of power quality is voltage flicker, a variation in the root-mean-square (RMS) voltage, that causes flickering of incandescent bulbs. Varying loads can lead to such fluctuations of the supply voltage and for historical reasons, this type of disturbance is viewed as the principal cause of light flicker. With the constant increase of applications that employ power electronic devices, sub- and interharmonics are becoming more prevalent in the power system. These can also vary the instantaneous power supplied to incandescent lamps by beating and, thus, result in visible light flicker. In this paper, the special conditions under which light flicker occurs are derived qualitatively. Computer simulations that are based on a realization of the UIE/IEC flickermeter are then used to produce quantitative data.