How Designers Can Use Flicker-Safe Dimming To Offset Dynamic White ‘Natural Lighting’ Schemes
By Gé Hulsmans
We're able to rethink lighting design thanks to the dynamic atmosphere control and freedom LEDs afford. Unlike incandescent, fluorescent or halogen solutions, LED systems enable precise, subtle, instant adjustment of brightness and color.
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Lighting designers enjoy emulating natural light—from changing the color and brightness of sunlight to creating the dawn, midday and dusk transitions to merging into the muted, yellow-orange tones of firelight. Human-Centric and Dynamic White lighting both deliver on these design philosophies. They're also both dependent on LED drivers and control systems. By dimming across two or more LED channels, you can manage precise and simultaneous control of brightness and color temperature.
Flicker’s Unintended Side Effects
While dynamic white light is supposed to emulate natural light, the installation brings out common LED dimming side-effects. The most troubling result is flicker—which has harmful effects on health and comfort. Our industry's response includes the development of the IEEE 1789 technical recommendation. Today, this is the go-to framework for determining the safety of lighting products and preventing flicker.
Another issue with flicker in dynamic lighting is cross-dimming. This usually occurs in single-color dimmable LED Systems. Cross-dimming occurs when multiple LED channels are individually dimmed at different frequencies. As a result, this produces multiple flicker sources—which makes the viewer disoriented.
After reading all of this you're probably asking yourself, "Why is flicker a problem in dimmed LED systems? What can lighting specifiers do to ensure that their carefully crafted dynamic white lighting schemes are not disfigured by flicker?"
Why Conventional LED Dimming Systems Create Flicker Effects
CCR VS. PWM
To dim an LED, the driver has to reduce the average current. There are two conventional techniques for dimming: constant current reduction (CCR) and pulse width modulation (PWM) (see Figure 1).
- Constant Current Reduction (CCR): The simple method where continuous current is supplied to keep the LED on. Reducing the current also reduces the output and perceived brightness.
- Pulse Width Modulation (PWM): The current supplied to the LED is continually switched between on and off. The longer the LED is fully off, the more its output is dimmed.
Fig. 1: the dimming current waveform of a PWM-controlled driver (left) and of a CCR driver (right)
Neither of the techniques mentioned produce acceptable performance in dynamic white lighting systems.
Why PWM Is Problematic
PWM is the most used dimming technique since it produces accurate regulation of the drive current down to low dimming levels. However, PWM is highly problematic for two reasons:
- The potential to generate electrical noise. This disrupts the operation of nearby electronic circuits
- Unpleasant and/or harmful flicker. This depends on the frequency and amplitude that the driver switches the LED supply.
Why CCR Is Problematic
On the other hand, CCR doesn't switch—so there's smooth dimming without flicker or electrical noise. Additionally, CCR maintains higher LED efficacy at low dimming levels since the LED operates at a low drive current.
Unfortunately, this still doesn't make CCR the ideal dimming technique. This is because the CCR control is not precise enough to effectively regulate LED brightness at dimming levels at 10% or lower. Our eyes are acutely sensitive to intense, deeply dimmed light—even the small CCR current variation can produce a visible change in brightness.
What's equally concerning for lighting designers is some LEDs suffer from non-uniform color shifts. These shifts occur when the drive current varies from its nominal value. As a result, it impairs the color tuning operation in a dynamic white lighting fixture's system.
Neither the PWM or the CCR technique is able to implement controlled dim-to-dark functionality. While the human eye can perceive a graduated change in brightness as the dimming level drops from 1% to 0.1%, the PWM and CCR techniques perform poorly when dimming below 1%.
Why Should The Lighting Industry Be Concerned With Flicker?
Accurate dimming at levels of <10% is an essential requirement of a tunable white lighting system. The fixtures used in dynamic white lighting today mostly implement PWM rather than CCR control. This is extremely problematic because of flicker.
Why The Lighting Industry Implemented IEEE 1789
The main concern with flicker is to do with comfort and health. Research has suggested associations between flicker and various health effects:
- Headaches, fatigue, blurred vision and eye strain
- Neurological problems such as epileptic seizures
- Increased prevalence of autistic-like behaviors—especially in children
Additionally, flicker produces banding and other unwanted visual artefacts in video and still images. Since people use video as a primary form of communication, resolving flicker is all-the-more imperative.
While 'natural' lighting's demand grows, natural lighting technology appears to be responsible for the unnatural and unhealthy responses! This effect is exacerbated in dynamic white systems comprised of multiple LED channels. In these cases, the 'cross-dimming' effect produces clashing sources of flicker at different frequencies—resulting in unstable and disorienting series of high-frequency shifts in color and intensity.
The harm associated with flicker is not limited to the low-frequency flicker that's generally visible. Studies have shown that a minority of the population—approximately 8%—display unusually high sensitivity to flicker. Flicker which is usually noted as invisible is detectable by highly sensitized people—causing the same levels of harms and discomfort as visible flicker causes to the larger.
The Intent Of Flicker Regulations
EN61547 & IEC61547 Discrepancies
IEEE 1789 provides clear guidance to designers and specifiers and empowers them to quantify the risk of harmful flicker in lighting installations. Other norms and standards—such as the European regulation EN61547's 'P' metric for the 'perception of short-term light modulation'—are cited by some suppliers as evidence for flicker regulation compliance.
The issue with the P metric is it's only concerned with flicker associated with the oscillation of AC mains power supplies. It doesn't provide any measurement for important LED dimming operation effects.
The international industry standard IEC61547—the basis for EN61457—includes a 'Stroboscopic Visibility Measure'. SVM is what provides a single value calculation method to determine whether a light's flicker may be certified as visible or non-visible.
In addition, the SVM measure is a misleading simplification of flicker. It's only measured at one operating condition –usually when the light is fully on. This is an issue since the most visible and harmful flicker occurs during light dimming. As a result, the SVM measure normally fails to provide even a partial or incomplete validation of a light's flicker performance. In most cases, any flicker that a light produces will be completely missed by the single SVM score.
IEEE 1789 As The Ideal Flicker Standard
The issue that really matters to users, specifiers and designers is not compliance with the law. It's the potential to harm human health and cause short-term or lasting discomfort and/or pain.
IEEE 1789 is the only regulation that provides the comprehensive framework for evaluating the detectable and undetectable effects of flicker produced by an LED light on human health and comfort. Ultimately, making IEEE 1789 the gold standard for flicker measurement.
How IEEE 1789 Measures Harmful Flicker
The other factor—flicker percentage—is an intensity deviation measure between the high and low points of the light's switching cycle. If a high-power LED is switched from full power to zero and back, the visual impact is much greater than if it is switched from 55% of full power to 45% and back.
The IEEE 1789 specification determines which flicker frequency and percentage combinations are detectable and the methodology for plotting a light's various dimming levels on a three-color graph. Here's the meaning of the three colors:
- Green: Safe
- Yellow: Moderate Harm/Risk
- White: Detectable By Human Eye
Figure 2 shows the IEEE 1789 graph for a frequently used PWM LED driver. It shows that LEDs powered by this driver emit harmful flicker at various dimming levels. There's even several that emit flicker in the undesirable yellow zone. These results are typical of the test scores produced by many PWM drivers on the market today.
Fig. 2: IEEE 1789 flicker safety graph for a PWM driver in common use in LED lighting installations today
Hybrid HydraDrive: A Flicker-Safe Dimming Technique
There is a third technique that meets all the requirements for natural dimming in dynamic white lighting systems—with no drawbacks. eldoLED's Hybrid HydraDrive uses a reduced current in combination with a variable frequency to achieve natural dimming to dark and optimize flicker performance.
Fig. 3: the current waveform of the Hybrid HydraDrive dimming technique used in eldoLED drivers
Figure 3 shows the current waveform of Hybrid HydraDrive in dimming mode. Lowering the modified PWM current reduces the flicker percentage. The modified PWM scheme gives fine-grained control of the average current supplied to the LEDs. This also gives excellent control of regulation and accurate brightness down to deep dimming levels. The PWM scheme's variable frequency reduces the noise impact on external devices, while maintaining a high duty cycle to give a high flicker frequency across the dimming range.
As a result, eldoLED's proprietary Hybrid HydraDrive technology produces accurate dimming, with full dim- to-dark (0.1% dimming) capability. The flicker remains in the IEEE 1789 green 'safe' zone at all dimming levels, as shown in Figure 4.
Fig. 4: IEEE 1789 flicker safety graph for the eldoLED SOLOdrive LED driver over a dimming range from 100% to 1%
Hybrid HydraDrive technology gives the control and deep dimming achieved with PWM and the smooth dimming, low noise and flicker-safe operation that results from CCR.
Test results performed according to the requirements of IEEE 1789 are published in the public product specifications for all eldoLED dimming drivers.
Ideal Cross-Dimming Performance In Color & Natural Light
The same Hybrid HydraDrive dimming technology is also implemented in the DUALdrive range. This product range encompasses two-channel drivers for use in color-tunable and dynamic white lighting fixtures. The DUALdrive drivers are ideal for the new generation of natural lighting products which dynamically adjust both color and brightness to emulate the effects of sunlight or firelight.
A DUALdrive driver implements the hybrid dimming technique across both LED channels—controlling the flicker frequency and flicker percentage globally. Ultimately, ensuring the light output complies with the IEEE 1789 specifications for cool-white channels, warm-white channels and across the mixed light output.
Luminaires equipped with a DUALdrive driver guarantees a safe and stable smoothly dimmed light. Lighting designers and specifiers can rest assured that their luminaires are safe at all dimming levels—and that they'll maintain a uniform beam at the chosen color and brightness.
This is why eldoLED's recommendation to designers and specifiers is to include this statement in all project specifications that require dimming or color tuning:
"LED drivers shall conform to IEEE P1789 standards. Alternatively, manufacturers must demonstrate conformance with product literature and testing which demonstrates this performance. Systems that do not meet IEEE P1789 will not be considered."
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