Identifying and Rectifying LED Flicker Problems

You may have noticed vertical lines in your smartphone camera images, experienced strobing in moving subjects, or been aware of the health hazards associated with LED flicker.
You may possess keen observational skills that enable you to perceive stroboscopic effects visually, particularly in relation to moving objects.
This article will elucidate the methods for assessing flicker levels in LED lighting and the corresponding remedial actions available.
Etiology of LED Flicker and Assessment
LED flicker fundamentally arises from the rapid alternation of the LED between on and off states. Despite being designed to flicker at an imperceptible rate, certain sensitive persons and applications may find the flicker intensity in some LED systems to be overwhelming and hazardous.

Assessment for flicker

The most scientifically precise method to assess the magnitude and intensity of flicker is to employ specialized instruments such as an oscilloscope or light meter. With purchase prices commencing at several hundred dollars per test unit, this is evidently impractical for the majority of consumers worried about flicker in their light bulbs.
We propose a rapid and efficient method to assess flicker with only a normal iPhone and its integrated camera application.
Open the camera application and slide right twice to activate the "SLO-MO" video mode. Subsequently, direct the camera towards the specified light source and initiate recording. Cease the recording after 2-3 seconds, and you are finished!
Now, meticulously observe the video you have just recorded. Significant flicker in the light source will manifest as a prominent flicker or recurrent flashing in the slow-motion movie. In the absence of flicker, the light source should exhibit stability and consistently emit uniform light levels throughout the slow-motion film.
Recent advancements in smartphone software have effectively eradicated the dark lines that formerly appeared in still photographs. This new breakthrough aids in photography but diminishes its efficacy in diagnosing flicker.

From whence is the flicker originating?

Flicker in an LED can originate from two sources: AC power or PWM. Identifying the source of the flicker is crucial for effective resolution.
The initial cause is the inherent fluctuation in alternating current electricity. Alternating current, as indicated by its nomenclature, oscillates between positive and negative voltage/current in a sinusoidal waveform with a frequency of 50-60 hertz. A 120 V AC input voltage refers to its root mean square (RMS) level; in actuality, it might vary between -170 V and 170 V.
LED devices are intrinsically built for direct current (DC) input, rendering the alternating current (AC) waveform fundamentally incompatible. Consequently, electrical components are required to "rectify" and stabilize the variable AC signal.
An LED system can operate with a basic rectified AC input, but will briefly stop or substantially diminish its light output at each of the two "valleys" occurring every cycle. An LED bulb may oscillate between its on and off states at a frequency of 100-120 times per second.
 

What measures may be taken to address the LED flickering issue?

We emphasized the necessity of identifying the source of the flicker (AC power or PWM) to ascertain which components of the lighting system require upgrading or replacement.
If the flicker originates from the LED bulb, it is necessary to replace it with a new bulb engineered to rectify the AC power and transform it into a genuinely steady DC current for the operation of the LEDs. When seeking an LED bulb, specifically identify those labeled as "flicker free" and examine their flicker metrics:
Flicker Percent - a percentage metric that quantifies the relative disparity between the maximum and minimum brightness levels (amplitude) during a flicker cycle. An incandescent bulb generally displays a flicker rate of 10%-20% due to its filament retaining heat during the troughs of an AC signal.
Flicker Index - a metric that quantifies the intensity and duration in which an LED produces light beyond its average output throughout a flicker cycle. An incandescent bulb possesses a flicker index of 0.04.
Flicker Frequency - the frequency at which a flicker cycle recurs every second, quantified in hertz (Hz). Most LED bulbs operate at a frequency of 100-120 Hz, corresponding to the frequency of the input AC signal. For bulbs with elevated frequencies, the impacts of comparable flicker percentage and flicker index values would be diminished due to their accelerated switching times.
The majority of LED bulbs flicker at a frequency of 100-120 Hz. At this frequency, the IEEE 1789 advises a safe ("low risk") flicker percentage of 8%, with 3% required to entirely eradicate flicker effects.
If the flicker originates from the PWM dimmer or controller, it will be necessary to replace the PWM dimmer unit. The positive aspect is that the flickering is probably not attributable to the LED strips or other components, hence your replacement expenses will be confined to the PWM dimmer/controller.
The sole significant parameter for PWM flicker is its frequency, as it typically exhibits 100% flicker. Therefore, while seeking a flicker-free PWM solution, ensure there is a specified frequency rating. For an entirely flicker-free PWM solution, we advise a PWM frequency of 25 kHz (25,000 Hz) or greater.
Standards like IEEE 1789 specify that PWM light sources operating at 3000 Hz effectively eliminate flicker effects. One advantage of raising the frequency above 20 kHz is the elimination of noticeable buzzing or whining sounds from power supply equipment. The audible frequency range typically peaks at 20,000 Hz for most individuals; thus, specifying a frequency of 25,000 Hz can mitigate the risk of irritating buzzing or whining sounds, which may be especially troublesome for those with heightened sensitivity or in sound-sensitive environments, such as recording studios.