Flicker is defined as a temporal variation in light intensity and is a characteristic of every light source or display, to a varying degree. A lamp that is driven directly from a mains electricity supply will suffer from an AC perturbation. Examples include incandescent lamps and fluorescent lamps, more so if the latter is equipped with old fashioned magnetic ballasts. Displays will also flicker at a frequency that depends on their design refresh rate.
Even LEDs can suffer from a surprisingly high level of flicker despite being nominally driven with a DC current. Modern semiconductor light sources have a lot shorter response time than traditional filament lamps. This makes LED-based lighting products more vulnerable to power-line flicker. In addition, fast temporal modulation of LED sources via pulse width modulation (PWM) is frequently used, for example when dimming.
Temporal light modulation becomes visible if the modulation frequency is below a certain frequency. Above this threshold, an intermittent light stimulus appears to be steady state. This visible light modulation is called light flicker or just flicker.
The Impact on Health of Flicker
When describing flicker as ‘visible’ it means that the observer is consciously aware of, and senses, the modulation of light output. This usually occurs when the frequency of the modulation is less than 60Hz. When ‘invisible’, the observer can still sense the flicker, but is not consciously aware of it. This effect can occur anywhere between 60-100Hz depending on the light source and the individual and is called the critical flicker fusion frequency (CFF). If someone senses flicker in an external source, even when not consciously, the neurological system responds. This can cause headaches, general malaise, eye strain, fatigue, migraines, and cause photosensitive epileptic seizures. It is estimated that 500,000 people have epilepsy in UK, of which 5% have photosensitive epilepsy and are most sensitive between 3 – 70Hz.
As well as the responses of the neurological system, flicker also poses a threat in industrial settings. The stroboscopic effect is an illusion that occurs when a moving object appears to be moving at a different speed, in an opposite direction or even stationary when illuminated intermittently. This can happen if the frequency of flicker from a light source is a multiple of the frequency of a moving object and can increase the risk of accidents in workplaces that use moving machinery.
How is Flicker Measured?
Flicker is measured using a fast photodiode or photometer. A spectrometer is not normally suitable for flicker measurements due to a more limited bandwidth. There are several metrics used to quantify the degree of flicker present in a display or light source.
As of 1st September 2021, the European Union requires that the flicker level be measured and reported for luminaires. Commission Regulation (EU) 2019/2020 of 1st October 2019 refers. The flicker metrics mandated by the EU regulation are the SVM and PstLM, details of which follow.
What are the Flicker Metrics for Lighting Products?
Flicker Frequency (Hz)
Describes the repetition speed of the light intensity variation. The frequency does not depend on the waveform shape. Flicker frequency is expressed in Hertz (Hz), the inverse of the time period of one cycle.
Percent Flicker
Percent flicker, also known as the modulation index, also does not depend on the waveform shape and is computed based upon the intensity maxima (A) and minima (B) in the waveform according to the formula below:
Percent Flicker = (A-B)/(A+B)*100%
This is shown in the diagram below. Percent flicker varies from 0% (no flicker) to 100%.
Flicker Index
The flicker index metric considers the shape of the waveform, as shown below. It compares the area in one cycle above the average light output to that below it, according to the following formula:
Flicker Index = Area 1/ (Area 1 + Area 2)
The IEEE publication 1789-2015 classifies flicker by percent flicker and flicker frequency in three categories: no observable effect level (NOEL, shaded green in the diagram below); low-risk (shaded yellow); and risk (red).
Stroboscopic Effect Visibility Measure (SVM)
SVM is a metric used to evaluate the visibility of the stroboscopic effect or flicker in lighting systems. The stroboscopic effect refers to the phenomenon where a moving object under a flickering light source appears to be moving at a different speed or even in the opposite direction. This effect is particularly noticeable in situations where the flicker frequency of the light source is close to the movement frequency of the object or the observer’s eye. SVM quantifies the potential perception of stroboscopic flicker by considering the characteristics of the light source and the human visual system.
SVM takes into account the effect on appearance of moving objects when illuminated with flickering light at up to 2kHz. It applies for illuminance levels greater than 100 lux and for motion speeds equivalent to those of a typical hand movement. A detailed description of SVM is presented in CIE technical note TN006 and IEC TR 63158. If SVM<1, stroboscopic flicker is not visible, if SVM=1 stroboscopic flicker is just visible and if SVM>1 stroboscopic flicker is visible.
Short Term Perceptibility of Light Modulation (PstLM)
PstLM is based on the human visual perception of flicker. It considers the fact that flicker is more noticeable and potentially more annoying when it occurs at lower frequencies and with larger amplitude variations. PstLM calculations involve averaging the flicker values over a specified time period to provide an overall assessment of flicker severity.
Commission Regulation (EU) 2019/2020 of 1st October 2019 and IEC TR 63158 have set the pass criteria for SVM and PstLM. The maximum allowed value is 0.4 for SVM and 1 for PstLM measured at full load (except for HID sources when the luminous flux used is more than 4,000 lumens and for light sources intended for use in outdoor applications, industrial applications or other applications where lighting standards allow a CRI< 80).
Flicker Meters from Pro-Lite Technology
What are the Flicker Metrics for Display Products?
The two most common metrics applied to the flicker produced by a display monitor are the contrast flicker and the flicker measured according to the JEITA method.
Contrast Flicker
In general, flicker for any source – display or lamp – is determined as the ratio of the magnitude of an AC perturbation compared to the DC or mean signal in the measured temporal luminance waveform. The contrast flicker for a display is calculated per:
Contrast Flicker = (max-min)/average*100% (%)
Contrast Flicker = 10 log10 (max-min)/average (dB)
JEITA Method
The Japan Electronics and Information Technology Industries Association (JEITA) publishes a flicker metric for LCD displays. The JEITA calculation is based upon a frequency domain calculation and uses a fast Fourier Transform (FFT) to determine both AC and DC levels of the measured luminance waveform and translates the signal into an FFT. In effect, you analyse the frequency component of the fluctuation and determine the flicker from the ratio of the DC and maximum AC components at up to 60Hz. A weighting factor is applied to allow for the human vision system’s sensitivity to frequency.
We define P0 as the DC level and P1 as the smallest detectable AC signal in frequency space. Pr0 and Pr1 are the aforementioned values scaled to the human eye sensitivity to frequency.
JEITA flicker is thus calculated according to:
FlickerJEITA = 20 log10 (Pr1/Pr0) (dB)
VESA Method
VESA (formerly the Video Electronics Standards Association) flicker metric involves essentially the same calculation as for the JEITA flicker metric, but with a small difference in the resultant flicker value arising due to the squaring of amplitudes in the FFT.
FlickerVESA = FlickerJEITA + 20 log10 (2-½) (dB)
Further Information
Pro-Lite specialises in equipment for measuring light and the optical properties of materials and supplies light metrology equipment on behalf of manufacturers such as Labsphere, Konica Minolta, JETI, Avantes, SSL Resource and Westboro Photonics – Pro-Lite Light Metrology Equipment
In addition, Pro-Lite also provides training in photometry, light metrology and display measurements – Photometry Training Workshop
