Flickering artificial light has been a longstanding problem with fluorescent lights, with the rapid phase-over from incandescent lights to LEDs this becomes an even greater issue.
Flicker free daylight is the best lighting solution; however this is not always available. IEEE 1789 made a beginning to define standards and criteria to measure flicker.
Visually perceptible flicker and strobe effects are an obvious deficiency of a lighting design. But also high frequency flicker, which cannot be immediately perceived, can have an influence on the human organism.
Flicker is fast blinking, from approx. 5 Hz, and may be visually perceptible up to approx. 80 Hz. Flicker can be experienced extremely disturbing. In exceptional cases it can even trigger photo-sensitive epileptic attacks.
When the change from direct current to alternating current (50/60Hz) occurred, this was not a problem, as the filament of lightbulbs had a high thermal inertia and would be too slow to follow the rapid changes in current.
However, with the introduction of gas discharge lamps with their low level of inertia of the gas, flicker became visible for sensitive persons, specifically with the older style magnetic ballasts. With the introduction of electronic ballasts (with the aim to reduce energy consumption) the problem of flicker was reduced, as high operating frequencies of 30 to 40 kHz were used.
With the introduction of solid state (LED) lighting the problem is here again. With the very quick, low inertia semi-conductors reacting in milliseconds to power changes, it is visible again that the luminaires operate on 50Hz alternating current.
Especially pulse-width-modulation (PWM) dimming control, even when operated at higher frequencies (400Hz+), creates a non-visible but nonetheless recognisable flicker, as the LED is switched off completely within a short period of time and is then switched on fully again. While not perceptible, because of the slower reaction of the human eye, an unwanted influence on the human organism may be created.
Low cost LED drivers that are used to operate LEDs typically operate with double the mains frequency, i.e. 100Hz, which is just above the threshold of perception.
In addition, poor design or faulty components (capacitors!) often add to flicker and strobe effects, too often regular flashes can be seen.
A new standard on “Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewer” has been published at the end of 2015: IEEE 1789-2015.
In this standard, it is shown that not only perceptible flicker below 80 Hz but frequencies up to 400 Hz can have a negative influence on the human organism. Recommendations have been made on operation frequencies.
Also the “Flicker Index” (FI) and the “Modulation” (Mod%) are introduced as additional criteria for evaluating flicker as both measurements have an impact on the severity of flicker.
The FI shows low fluctuations in the waveform. The FI expresses the percentage of operation which is above the median level.
Modulation, also known as the »Michelson Contrast«, describes the interval between the minimum and maximum of an oscillation.
- Rectangular wave forms (e.g. PWM control) with its rapid changes have a mod% of almost 100%.
- An incandescent lamp on mains voltage has a mod% of approx. 10%.
- LEDs operating on direct current have a mod % of almost 0%.
As described above, rapid changes (expressed as high mod%) will have a greater impact on the human organism.
The Effects of flicker on the human organism depend, of course, on individual circumstances. However, there are 3 broad categories:
- Visually perceptible flicker
- Unconsciously perceived flicker
- Strobe effects
Visually perceptible flicker with a frequency of approx. 4 – 80 Hz can cause headaches and impaired vision. For people suffering from photosensitive epilepsy (approx. 1‰ of the population) flicker can trigger convulsions and seizures, even after very brief exposure
Of course, many factors, such as colour temperature, field of vision, luminance levels etc. play a major role in the effects on the human organism.
Unconsciously perceived flicker, with frequencies above ca. 70Hz has, despite not “seen”, effects on the human neurological system. The human eye can detect light changes of up to approx. 180 Hz, depending on various factors. The effects are fatigue and general impaired vision as well as headaches and migraine attacks.
Strobe effects, i.e. flashes with a frequency of ca. 2-5 Hz are known also as ‘disco effects’. Strobes can affect the human organism even at very low frequency ranges, even below 2Hz.
While often used for desired visual effects, e.g. in a discotheque, in a factory or work environment this can be leading to confusion, as equipment may appear to stand still or even go backwards. In general, movements may be not recognisable, stationary objects seem to move, and instrument readings may be difficult. In darker environments, this can lead to disorientation.