PstLM flicker index measurement

Methods for measuring the new PstLM flicker index according to the ecodesign requirements for LED lighting

The revised regulation of the European Commission on ecodesign for light sources introduces new requirements for light flicker indicators, including PstLM, which determines the resistance of the light source power supply system to network disturbances. PstLM stands for Short Term Perceptibility for light modulation. The new requirements enter into force in September 2021 and will apply to manufacturers and all the entities introducing lighting products on the European market.

by Miko Przybyla and Tomasz Pawlicki I GL Optic

Semiconductor light sources (SSL) can change light parameters very quickly because their response time to changing power conditions is immediate. This property means that any change in the supply current is directly translated into the luminous flux emitted by this source. As a result, LED and OLED lighting products are particularly susceptible to the so-called Temporary Light Artifacts (TLA Temporary Light Artefacts CIE TN 006: 2016) on AC power, dimming systems or mains voltage fluctuations.

The above factors forced the relevant regulations in the form of the Commission Regulation (EU) 2019/2020 of October 1st, 2019. According to which, from September 1st, 2021, new ecodesign requirements for light sources and separate control gear will apply.

As with any new provision, this regulation also raises many doubts. New requirements are introduced for the sake of consumer welfare and protection of the market against the influx of low-quality products. All the producers and marketers will be forced to adapt the entire range of their products to the new requirements. Not only does this mean additional costs and effort for business owners and engineering teams, but also the lack of adequate knowledge and information about these specific flicker parameters. For many, it is not clear how to measure these parameters and there are doubts as to which measuring equipment should be used. The below article will describe what the PstLM indicator is, how it should be measured and what measurement systems are available.

Light Flicker and PstLM

The rapid development of LED and OLED lighting technology has made the issue of flicker increasingly important due to the very short response time of this type of semiconductor light sources at the level of several dozen nanoseconds. In the case of incandescent sources, the time of the filament’s reaction to the change of supply was limited by the high thermal inertia of the filament. Therefore, flicker is more likely to occur in modern lighting installations than in conventional systems.

It is worth explaining that by light flicker we define periodic changes in the level of light intensity (or luminous flux) over time, resulting from disturbances. On the other hand, flicker is defined as periodic changes that result from the design of the device and are a feature, not a defect.

The PstLM parameter and testing method was introduced by the IEC 61547 standard. Now, a new 2020 edition of the standard for general lighting equipment – EMC noise immunity requirements is available. PstLM characterises the resistance of a light source, including lamp modules and luminaires, to flicker caused by voltage fluctuations and is a part of the electromagnetic compatibility (EMC) regulations. The technical report describes the flickering of light and the disturbing signals of AC voltage fluctuations. According to the regulation, only the product tested with PstLM ≤ 1.0 may be put on the EU market.

It should be noted that the PstLM measurements discussed in this article relate to actual measurements of the intensity of the light source under test. This should not be confused with flicker measurements according to the EU (Polish) standard EN6100-4-15. The old standard dealt with short-term measurements (Pst -10 minutes) and long-term (Plt index – 2 hours) voltage fluctuations in the power grid (120 V or 230 V, 50 or 60 Hz) caused by the time-varying power passive interfering receivers. The EU standard defined the measurement of mains voltage, and the current regulations require optical measurements, i.e. direct measurement of the flickering of the light source intensity.

How to measure PstLM?

Manufacturers should measure the flicker parameters of LED modules and lamps under laboratory conditions in order to declare these parameters before introducing the product on the market. Such tests should be performed in a darkened room where there are no other light sources that could disturb the measurement.

The basic device is the so-called flicker meter. It is a photometric device that uses a highly sensitive photodiode combined with fast reading electronics capable of registering very high frequencies. One should use meters with a sufficiently high dynamic range which should have a minimum measuring range 10 times higher than the frequency of the source. An example of a device that can be used for these measurements is the GL Photometer 3.0 + Flicker. The device has a sampling frequency of 125 Khz and a dynamic range from 100 µlx to 10,000Klx. This instrument determines the illuminance level with a resolution of four significant digits.

The light measuring device should be directed towards the light source and should be mounted on an optical bench or tripod, as hand vibration may cause additional low frequencies in the signal. The PN semiconductor LED junction of the LED has a negative temperature coefficient, so the current flowing through the LED will increase as it warms up. Thus, it is necessary to wait 15-30 minutes for stable temperature conditions of the module. Another method of measuring flicker is to install a meter in the integrating sphere. The integrating sphere with appropriate measuring instruments and software can create a measuring station.

In addition to the flicker meter, the right power conditions need to be created. In the case of PstLM, it is  dealt with the measurement of resistance to network disturbances, so an appropriate simulation of such disturbances in laboratory conditions must be created. For this purpose, it is necessary to use a programmable power supply capable of generating waveforms with appropriate resolution and speed.

The diagram below shows a diagram of the generated mains disturbance defined in IEC 61547. It consists in superimposing a square wave representing changes in mains voltage on a stable AC waveform.


PstLM flicker index measurement


The table below defines the frequency of rectangular component switching and the depth of its modulation for 230V and 50Hz mains conditions.

PstLM flicker index measurments

For each AC voltage and frequency, there are five predefined disturbance modes that characterise the number of voltage changes per minute and the percentage of voltage modulation.

It is required to simulate voltage disturbances which are defined for 120 V and 230 V and frequencies of 50 Hz and 60 Hz. To generate the required signals, the power supply must be able to switch between the two AC voltages in a precise phase with an accuracy better than 0.5 ms. Voltages must be modulated with a resolution of 10mV.

An example of a device that meets these requirements is the AC-WAVEFORM GENERATOR GL 750VA_M7622. It stores all the disturbance modes in its memory and executes them with appropriate parameters. It can also be controlled by the GL AUTOMATION software. This configuration allows you to create a fully automatic product testing process for PstLM.

Testing the flicker indicators required by the new regulations is not a simple process as it requires knowledge and appropriate measuring equipment. It is worth taking the advice of a measurement professional to be sure of its accuracy. The new EU regulation comes into force in September 2021 and will apply to manufacturers and all the entities that introduce lighting products on the European market.

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