SSL Flicker Meter

Comparison of SSL Spectroradiometers SPECTRI 80 Models

What is Flicker?

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 frequency below which light flicker appears is different in different conditions but is usually between 60 and 400 Hz.

Flicker can be both invisible and innocuous but depending on the flicker frequency and the depth of modulation, flicker can sometimes present a serious hazard in workplaces and elsewhere. A piece of rotating machinery can appear to be static if illuminated by a lamp that suffers from flicker at the wrong frequency and depth of modulation. In addition, flicker is associated with several health concerns, particularly to those who suffer from photosensitive epilepsy. Flicker can also lead to eye strain, headaches and can trigger migraines.

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 1^st September 2021, the European Union requires that the flicker level be measured and reported for luminaires. Commission Regulation (EU) 2019/2020 of 1^st 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 (shaded red).

Stroboscopic Effect Visibility Measure (SVM)

The SVM measures the visibility of flicker.

The Stroboscopic Visibility Measure (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, flicker is not visible, if SVM=1 flicker is just visible and if SVM>1 flicker is visible.

Short Term Perceptibility of Light Modulation (PstLM)

PstLM also measures the visibility of light flicker. Higher PstLM values mean more visible flicker.

Commission Regulation (EU) 2019/2020 of 1^st 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).

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 an 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. P_r0 and P_r1 are the aforementioned values scaled to the human eye sensitivity to frequency.

JEITA flicker is thus calculated according to:

Flicker_JEITA = 20 log₁₀ (P_r1/P_r0) (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.

Flicker_VESA = Flicker_JEITA + 20 log₁₀ (2^-½) (dB)

Datasheets

• SSL Resource Spectri 80 Brochure

Manuals

• SSL Resource Spectri 80 Manual