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    Casting new light on temporal light artefacts


    We live in revolutionary times. Revolutionary, that is, in terms of lighting design and the tunability of light.

    One of the main drivers of this revolution is the astonishing rate at which solid state light sources can change their intensity: the response time of a light source to the driving current can be as little as a couple of nanoseconds, almost instantaneously translating the driving current to the luminous output.

    A consequence of this is that the modulation of the driving current is directly transferred to a modulation of the luminous output. The modulation of the current can be intended (e.g. to control light intensity and color) or unintended (e.g. from mains power, or resulting from interactions between the driver and dimmer electronics), but one effect of the modulation of luminous output can be changes in visual perception for an observer in a certain environment. These changes are called 'temporal light artefacts', or TLAs.

    In some specific entertainment applications, a change of perception due to light modulation is desired, but for most everyday applications and activities involving artificial light, the change is detrimental and perceived as negative. At the simplest level, TLAs affect how we judge the quality of light in a space. Going further, the irregularity of light modulation can cause reduced performance, visual fatigue, epileptic seizures, headaches and migraine episodes.

    The potential negative impacts of TLAs have prompted lighting manufacturers, lighting application specialists, universities and governments to look for ways to measure the impact and come to a better understanding of the temporal quality aspects of lighting systems and reduce their presence and impact to the minimum. In this context, among the few bodies addressing this subject, the CIE formed Technical Committee (TC) 1-83 'Visual Aspects of Time-Modulated Lighting Systems', and also produced a significant paper: CIE Technical Note 006:2016, 'Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models'. Signify was a primary contributor to this paper.

    Understanding TLAs

    The two principal types of TLA are flicker and stroboscopic effects. Flicker is most simply defined as light fluctuation that is directly perceived. It is visible without motion of the illuminated object, or of the observing eye. So, if the light cuts out for a fraction of a second, then it's flicker. The perception of TLAs differs from one observer to the next. Flicker typically occurs between wavelengths of ~ 0.5Hz and ~ 80 Hz, but factors such as an individual’s age or level of tiredness can affect how acutely the flicker is perceived flicker.

    Unlike flicker, stroboscopic effects are not directly visible. As a result, not thoroughly investigated by many lighting professionals when selecting the light source. To experience and get affected by the effects, the illuminated object must be moving. Stroboscopic effects are defined as the misperception of motion (for example, where objects are perceived as moving in discrete steps rather than continuously, or where a rotating object is perceived as static). So if you move an object in front of the light and it seems to stutter, then it's a stroboscopic effect. Stroboscopic effects occur between ~ 80 Hz and ~ 2 kHz.


    How to eliminate flicker and stroboscopic effects

    As the lighting technology revolution continues, researchers, manufacturers and industry bodies are adding to the data available and refining their understanding of TLA’s. This has not only led to an improved understanding of the different effects but also a new set of national and international recommendations from industry bodies providing guidance to manufacturers and lighting professionals on how to define, measure and limit the effect of the disturbing artefacts.

    To achieve lighting design of the highest quality, manufacturers must set their sights higher, as Signify has done. It played a leading role in developing the new metrics that will be used as the basis for future TLA standards and regulation, and it ensures TLA performance across its businesses and their product portfolios.

    In seeking greater understanding of TLAs, and subsequently minimizing or eliminating them, we look at three key aspects when designing our driver and system designs at Signify.

    Modulation: The alteration of the amplitude or frequency of a wave. In modern LED lighting, modulation enables easy control of light output. It's used in full color control and in dynamic lighting, for example. The likelihood of a human to observe TLA is quantified by measures calculated from the light waveform emitted by a light source. The measures include IEC’s short-term flicker value (Pst), and CIE TN 006’s Stroboscopic Visibility Measure (SVM). 
    Frequency: The rate of recurrence of the cycle in a waveform. Where the 'period' is the time a waveform needs to repeat itself, the frequency is expressed as one divided by the period. Humans have different sensitivities to flicker at different frequencies – we are most sensitive at frequencies between 10 Hz and 20 Hz.

    Duty cycle: For square waveforms, the duty cycle is the ratio of the time that the light output is above a certain level – typically 10 percent – over the total period.

    By designing drivers with these considerations in mind, Signify ensures that its products will enable lighting professionals to create environments that deliver the greatest comfort and enhance human well-being. As an example, click here to learn how the Philips Xitanium Point LED drivers are prepared to meet all requirements and deliver the highest quality of light.

    In summary well-designed lighting enhances quality of life in every situation, and a proper understanding of TLAs is an essential part of the design process. Want to learn more about flicker? Click here to watch a webinar from Signify to get a better understanding about the artefact.