Light flickers, as a result of rapid voltage fluctuations, are visible changes in the brightness of a lamp. Rapid voltage fluctuations is the result of a changing load current that occurs at subharmonic frequencies less than 25 Hz.

An incandescent lamp consists of a tungsten filament sealed inside a glass bulb with an inert gas. When a current flows through the filament, the temperature of the tungsten rises and reaches a value that is high enough to cause emission of light. If the temperature of the filament remains steady, the lamp emits light at a constant intensity. Voltage fluctuations cause the current in the filament to change, which leads to temperature variations. The resulting change in temperature causes fluctuations in the intensity of the light that the lamp emits.

Observable fluctuations in light intensity may be annoying and even lead to psychological stress for a human being. The frequency at which light changes is important for measuring flickers. If the frequency is too low, an observer notices the corresponding slow changes over time instead of perceiving the changes as flickers. If the frequency is too high, the light intensity appears continuous and the observer is unaware of the flickers.

A flicker curve shows voltage fluctuations that lead to observable changes in light intensity as a function of the fluctuation frequency. Flicker curves are derived from controlled experiments measuring the human perception of fluctuations in light intensity. The following figure shows the flicker curve of a 60 W and 120 V incandescent bulb.

In the previous figure, a voltage fluctuation of 0.2% at 8.8 Hz results in an observable flicker. Electrical utilities must ensure that the frequencies of voltage fluctuations are above or below the flicker curve for a specific light source. The critical flicker frequency corresponds to the frequency at which observers no longer perceive light as continuous but notice flickers.

Lamps that operate at a power that is higher than 60 W contain a thicker filament, which can support a higher current. Such lamps are less sensitive to voltage fluctuations because the lamps have a higher thermal constant and, consequently, are less likely to exhibit light flickers. These types of lamps include luminescent lamps and LEDs. Luminescent lamps use an electric current to ionize a gas or vapor, which emits light. Examples of luminescent lamps are fluorescent lamps, sodium-vapor lamps, and mercury-vapor lamps. Luminescent lamps usually are less sensitive to voltage fluctuations than incandescent lamps. The following figure shows the lamp-eye-brain response for an incandescent lamp, a magnetic ballast fluorescent lamp, and an electronic ballast fluorescent lamp.