The importance of choosing flicker-free LED drivers
Most modern light sources include an element of flicker, which can have serious health implications for all living things, and LED light sources are certainly no exception. epileptic seizures. Long term exposure to imperceptible flicker can trigger serious ailments, such as headaches, eye strain, malaise, visual impairment and even epileptic seizures in a small number of cases. For this reason, IST's iDrive® R&D team made development of flicker-free LED drivers a high priority.
1. What causes flicker in lighting?
Flicker can be defined as the perception of visual unsteadiness induced by a light source which fluctuates with time for a static observer in a static environment. LED Flicker is caused by voltage ripple at the output of the AC power supply. Furthermore, a stroboscopic effect is the change in motion perception induced by a light stimulus whose luminance or spectral distribution fluctuates with time for a static observer in a non‐static environment.
The frequency range in which the stroboscopic effect is typically experienced is from 50 Hz up to 3000 Hz. Stroboscopic effect may occur as a result of product properties or system‐level interactions, such as lamp/dimmer interactions and is especially important when considering lighting for industrial markets, where high speed machines are operating or in garages at home as the lighting stroboscopic effect can make high speed moving parts appear static when they are not.
The degree and effects of flicker depend on a number of variables
• Frequency of the voltage change – Hz.
• How much of a voltage change occurs.
• The type of light source (LED, incandescent, fluorescent, HID, etc.)
• The gain factor of the lamp (gain factor is a measure of how much the light intensity changes when the voltage fluctuates – [% relative change in light levels] divided by [% relative fluctuation in voltage]).
• The amount of natural ambient light within the lit area.
• Mains frequency: the frequency of the flicker is typically equal to either the mains frequency or double the mains frequency (in Europe this would mean flicker is seen at 100Hz or 120Hz in the USA).
2. Why and in which environments is flicker a problem?
According to various scientific studies, a certain number of people are highly susceptible to flashing lights cycling in the 3 to 70 Hz range, which can result in epileptic seizures. Much flicker is invisible to the human eye, but can still affect health. It is the imperceptible flicker (100Hz -150Hz), which can trigger serious ailments as a result of long term exposure, such as headaches, eye strain, malaise and even visual impairment.
Since many people spend between 80-90% of their time indoors in today’s society, it is very difficult to avoid exposure to this persistent frequency flickering. Understandably, it is more important in certain environments to avoid/reduce flicker, such as:
• TV & film studios
• Industrial units with moving equipment
• Tunnel lighting
The health effects of flicker depend on a number of physical and physiological factors, as well as the function of frequency, including:
• Position of the light source relative to the centre of the retina
• Spectral characteristics of the light (e.g. bright light is worse than dim; red and alternating red and blue light is the worst kind
• Dimming pattern/degree of difference between light and dark, i.e. lights that go completely dark during the ‘off’ cycle are worse than lights which only dim partially
A recent 118 page study by the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) provided its opinion on emerging or newly-identified health and environmental risks associated with artificial lighting and makes an interesting read. Click here to download SCENIHR study.
3. The danger zone versus the iDrive® safe zone!
Most LED drivers operate at 100Hz or 120 Hz, the range, as shown in the graph (Fig.1) below. Certain LED fixtures, e.g. LED MR16s are particularly susceptible to flicker, due to the size limitations of the LED drivers powering them. By taking simple precautions at the design stage, however, LED drivers can significantly reduce detectable flicker in LED lighting.
IST’s iDrive® range of drivers operate at 85,000 Hz and between 0.05 and 0.2 Flicker index, so are one of the safest LED drivers available and are well within the suggested safe zone, as demonstrated in the diagram (Fig.1) below:
4. How to combat flicker
As far as IST Ltd. is concerned, it is a myth that to eliminate imperceptible flicker and perceptible flicker automatically means huge cost implications; it is a simple matter of using good quality electronic design and know-how. IST has made it simple for everyone in the lighting supply chain to eliminate lighting flicker and the associated health impacts and has completely solved the flicker issue.
One of the most commonly used methods of dimming LEDs is by modulating the driving current and voltage of the LEDs attached to the driver. Pulse Width Modulation (PWM) is such a technique, but this type of modulation scheme has significant drawbacks from a healthy lighting perspective, including:
• The flicker index is always set at a maximum 100% because the forward current is switched between 100% and 0% for some point in time, which places PWM drivers in the danger zone
• The frequency of the flicker is usually limited to a few hundred Hertz.
• If the application requires high quality dimming, which does not have steps at the low intensity levels, then the flicker frequency has to be reduced further in order to increase the dimming resolution making it difficult to achieve high quality healthy lighting
The best way to combat flicker is to avoid the use of PWM type dimming drivers and specify:
• Two stage, high frequency LED drivers such as those in the iDrive® centralised LED driver range
• LED drivers which dim using either DC or Constant Current Reduction
• LED drivers which dim using Pulse Amplitude Modulation (PAM)
As flicker can have a detrimental effect on health, it is important to pick LED drivers with no flicker when the lighting is dimmed. IST’s iDrive® range of LED Drivers significantly reduces flicker because the output stages run at 85,000 Hz, which is well away from the danger zone and is the reason why no flicker is detected in the following flicker-free video link:
The iDrive® range of LED drivers has been used in the global lighting industry for many years and our innovative technology, is covered by several patents. Click here to view patent portfolio.
5. Attempts to outline safe flicker levels
A new publication from the Institute of Electrical and Electronic Engineers (IEEE) was announced recently, entitled: "Recommended Practice for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers" (IEEE Std 1789-2015). The paper sets out to define acceptable flicker limits for LED-based lighting systems, defining key metrics and offering solid-state lighting (SSL) product developers practical guidance on how to ensure that LED-based products present no danger to humans.