Flickering Lights Some changes or modulations of the voltage that aren’t large enough to be consider sags may not seem to effect equipment operation. However, these anomalies can result in quality variations in extrusion and textile processes and in flickering lights that can cause human discomfort. In the case of flicker, the frequency of the modulation is critical as to whether it will be noticeable to the particular susceptibilities of the human eye and brain. For example, it would only take about 0.3 volts of modulation at 9Hz on a 120V system for most people to notice the flicker in a 60W light bulb. However, at 1 Hz, it would take nearly 10 times the modulation to be noticeable.