As festive, blinking Christmas lights surround us, it is difficult for us lighting nerds to avoid killing the buzz just a bit by thinking about Flicker… variation in the amplitude (brightness) of a light source over time. Flicker is important because of its potential consequences to human health that, as described in IEEE Standard 1789-2015, can include eye fatigue, headaches, loss of concentration and productivity, and in some people, nausea and epileptic seizures.
In general, flicker is caused by variations in the power that drives the light source. Incandescent lights flicker with the 50-60 Hz alternating current that drives them… a problem that has plagued both fluorescent and HID lighting in the past. LEDs – with their incredibly fast response time – are particularly sensitive to the quality of their drivers. LEDs can be dimmed to about 20% (which as you may recall from our September newsletter is fully 58% perceived level) by reducing the constant current that drives them. Truly deep dimming to 0.1% or even 1% requires us to modulate the light – to turn the LED on and off.
Characteristics of Flicker
Flicker has several relevant characteristics when it comes to human perception:
- Frequency – how fast the light is changing
- Amplitude or depth – the maximum percent difference between the peak and minimum light level
- Average intensity level
- Wavelength or wavelength range (i.e. color) of the light
- Angle – what part of the retina is receiving the light
- Light adaptation level, which affects both the light level sensitivity and time resolution of the eye
- Physiological factors such as age, fatigue, and genetically determined sensitivity
Critical Fusion Frequency (CFF)
Flicker may be visible or invisible. Our eyes have a limited response time called the Critical Fusion Frequency. If the modulation is fast enough, the pulses “fuse” into a single, average level. The “critical flicker fusion” (CFF) frequency is the threshold where flicker cannot be seen, and some people are far more sensitive than others. All other things being equal:
- Higher flicker rates are less visible and less health impacting.
- Higher modulation depth is more visible and health impacting.
- Dim light flicker is less visible and less health impacting.
- We perceive flicker more quickly in our peripheral vision.
- Fatigue diminishes our ability to perceive flicker.
IEEE Standard 1789-2015 is currently the best specification describing the health impacts of flicker, and how to avoid them. They have simplified the issue to just two parameters – frequency and modulation depth – and defined thresholds where flicker has “No Effect” and where it has a “Low Risk” of causing health impacts. These zones are shown in the following graph:
What IEEE has not yet accounted for are the other influencing factors, most notably absolute light level. What they cannot account for is individual sensitivities. Xicato’s recommendation is to take this standard seriously and minimize your risk by making sure that your LED driver stays in the No Effect zone to at least 2%, and in the Low Risk zone to at least 1%. The graph shows that Xicato’s own XIM driver is “No Risk to 1.2%, and Low Risk to 0.5%, with minimum settable level of 0.1% at 250 Hz (epilepsy safe) and dim-to-off at 0.012% (30 Hz – above CFF).