In modern lamps, electricity is used to produce light. In Finland and the EU countries the mains voltage operates at a frequency of 50 Hz. Lamps such as incandescent lamps, halogen lamps, and conventional fluorescent lamps, which operate with 50 Hz alternating current, produce light that flickers at a frequency of 100 Hz (Hz = cycle / sec), twice the mains frequency. In practice, the power is turned on and off 100 times a second. For this reason lamps that operate directly with mains current most often produce flicker at a frequency of 100 Hz. There may be 50 Hz flicker at the ends of a fluorescent tube, and the flicker frequency may become very high as the lamp ages. The control frequency in some fluorescent lamps has been raised so that the flicker does not have any harmful effects. However, even these lamps may produce flicker caused by the mains frequency.
In the newest lamps, the light is produced by LEDs. Typically, the intensity of the light produced by a LED lamp is controlled by feeding pulse-width modulated (PWM) current to the LED. As a result the LED is alternately on and off. This way the average intensity of light detected by the eye is lower. This PWM method is commonly used to control the intensity of light produced by a lamp and also the backlight of an LCD display. Thus, controlling the backlight causes the entire display to flicker. Although more and more direct current LEDs are entering the market, the 100 Hz flicker in the cheapest LEDs may comprise most of the light produced by the lamp. The most common reason for this is poor-quality power supplies.
Wherever lamps that operate on mains current are used, flicker may rise to a detrimental level. Malfunctioning or aging of the electronic components used in the lamps’ power supplies may also cause flicker. In some lamps, poor-quality power supplies may allow flicker caused by the mains voltage to leak through even when new. Natural flickering light is produced when the sun’s rays are reflected off of water or snow or when sunlight flickers through trees along the side of the road when riding in a car. However, sunlight itself is steady and flicker-free.