A year from now, at the end of 2021, the new Commission Regulation EU 2019 2020 (Ecodesign) enters into force. The specific requirements for electrical equipment including LED light sources are changing. In addition to the new energy efficiency classes, completely new minimum requirements for flicker and the so-called stroboscopic effect are introduced. Lamp manufacturers, importers and other stakeholders will need to take care of their products to comply with the new regulations. Here is the least what we need to know about the new optical parameters.
New requirements, old problem
LEDs are semiconductor sources that immediately respond to changing power supply parameters. In conventional sources, thermal inertia significantly slowed down changes in the level of light output under changing power supply conditions, so these changes were not so visible. In the case of LEDs, when selecting an appropriate power supply or designing a new driver system, it is necessary to take into account not only electrical issues but also optical parameters. The class of the power supply used will have a direct impact on the lighting performance parameters.
Flicker and stroboscopic effect
Flicker is a periodic, rapid and directly visible change in brightness of a light source, its photometric values (luminous flux), which can be due to fluctuations of the light source, which is varying (aperiodic). Flicker can cause visual discomfort at work. It can also cause migraines and even epileptic seizures.
The stroboscopic effect means a change in the perception of movement by the observer in lighting conditions changing over time. These changes can be periodic and non-periodic and can be caused by the light source itself, the power source. In an industrial environment it can cause accidents involving people, because the stroboscopic effect caused by light changes leads to a disturbance in the perception of the speed of rotating objects, such as machine parts.
New measures: PstLM and SVM
PstLM is a flicker metric introduced in the Regulation, where “st” (short term) means short term and ‘LM’ the method of measuring flicker, as defined in the standards. A value of Pst LM = 1 means that the probability that the average observer will notice flicker is 50 per cent; Table 4 of the Regulation specifies the required level of Pst LM <= 1 at full load and applies to LED and OLEDs.
SVM is a new Stroboscopic Visibility Measure – it is a new measure of the probability of a stroboscopic effect. The threshold of visibility of a stroboscopic effect is the value of SVM equal to or greater than 1. If the value is less than 1, the stroboscopic effect will not be visible for the observer. The minimum required level of SVM at full power is 0.4.
Measurement
Measurements of these quantities must be made with photometric instruments. Such instruments consist of a high-performance photo-sensitive system (photodiode with transimpedance amplifier) supplemented by a V-lambda optical filter to adapt the sensitivity curve of the system to the characteristics of the human eye (Figure 1).
Figure 1: Diagram of a light flicker measuring system.
Light flicker meters vary in measurement times, sampling rates, maximum transfer rate and frequency resolution depending on the FFT size. High-end devices sample the signal with a high oversampling rate 10x higher than the highest expected frequency of the measured signal, which is important to reproduce the shape of the flicker signal. Measurements are taken in a photometric darkroom by placing the meter at a certain distance from the light source. The measuring instrument and the light source should be stable because the vibrations can generate additional low frequencies in the signal. It is important to remember to stabilize the source before the measurement at least 25 minutes. The PN semiconductor connector of the LED has a negative temperature coefficient, so the value of current flowing through the diode will increase with its heating. Another method is to use a meter system with an integrating sphere, which allows for verification of flicker characteristics when measuring the luminous flux.
Measurement instruments
GL Optic is a Europe based company that has been manufacturing complex systems for measuring optical light sources, optical components and lamps and luminaires for over 10 years. For the measurement of flicker and stroboscopic effect, two high class measuring devices are currently offered. The devices have been positively verified in cooperation with Philips Research and highly rated by the American Department of Energy DOE.
GL PHOTOMETER 3.0 LS + FLICKER is a laboratory grade photometer. It has been designed for fast luminaire photometry and its electronics have been optimized for fast integration and a large measuring range. It is a USB device used for laboratory measurements on an optical bench or as part of a measuring system with an integrating sphere or goniophotometer. Measuring range 0.0001 lx to 1,000,000 lx, resolution, 125 kHz sampling rate filter class A < 3 %, and 18 bit A/D converter
GL SPECTIS 1.0 Touch Flicker is a portable “smart” spectroradiometer. It combines light flicker measurement with spectral light analysis. No specialist knowledge is required to use it. One button starts the measurement, presenting key data: light intensity; CRI; TM-30; colour temperature, CCT; and others. When the “Flicker” function is selected and the measurement button is pressed again, the spectrometer immediately shows the light intensity, PstLM and SVM data. The device has a convenient touch screen, intuitive Android operating system, automatic data saving, data transfer via USB to PC and WiFi.
Relevant organizations, norms and standards
- Commission Regulation (EU) 2019/2020 of 1 October 2019 laying down ecodesign requirements for light sources and separate control gears pursuant to Directive 2009/125/EC of the European Parliament and of the Council and repealing Commission Regulations (EC) No 244/2009, (EC) No 245/2009 and (EU) No 1194/2012
- DIN EN 12464-1 Light and lighting – Lighting of work places – Part 1: Indoor work places
- CIE TN 006:2016 Visual Aspects of Time-Modulated Lighting Systems – Definitions and Measurement Models
- IEEE 1789-2015 – IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers
- ASSIST: Alliance for Solid-State Illumination Systems and Technologies
- National Electrical Manufacturers Association (NEMA) – Temporal Light Artifacts (Flicker and Stroboscopic Effects)
- International Commision on Illumination
- Illuminating Engineering Society (IES),
- Lighting Handbook 10th Edition (2011)
Summary
When designing power supply and control systems for LEDs, one should take into account the light flicker parameters discussed in the article. In many cases it is enough to follow good design rules, but the increasing demands on lighting parameters make accurate measurements of light modulation parameters becomes a necessity.
WANT TO KNOW MORE?
Check out previous post about flicker.
We explained the current state of flicker metrics and suggested best practices for taking measurements and characterizing the phenomenon –> HERE
Flicker and Stroboscopic Visibility Measure (SVM) introduced by the revised Ecodesign Regulation for light sources –> HERE
