Flicker in the LED light explained

Do you know almost every LED light bulb or fixture is flickering when powered ON? But most of the times you won’t not even noticed the flickering at all.

What is flicker ?  Per IEEE1789 , it says flicker is :variations in luminance over time. According to CIE17.443e-ILV, the definition of flicker is the impression of unsteadiness of visual perception induced by a light  stimulus whose luminance or spectral distribution fluctuates with time.  Have you hear about Temporal Light Artefacts ( TLA)?  TLA are undesired effects in the visual perception of a human observer induced by temporal light modulations. Most of time, TLA and flickering in lighting used interchangeably.

The flicker(TLA) can be divided into Visible flicker and Stroboscopic(mostly Invisible) flicker.

Visible flicker is easy to understand light fluctuation at relative lower frequency (< 80 Hz) at smaller modulation level. This flicker is typically irritating to average person when perceived directly or indirectly via a reflectance surface.

Stroboscopic effect is the  Luminous flux modulation made perceptible by the motion of objects, when the observer’s eye is still. The moving object is illuminated by modulated light at somewhat higher frequencies (>80 Hz and up to 2000Hz ) and larger modulation levels.

Is the flicker in lighting an issue?

There are studies suggested working under flickering lights could cause neurological problems like epileptic seizure, it could also cause some persons headaches, fatigue, blurred vision, eyestrain , Migraines. It can cause distraction and reduce visual task performance. What make it worse is longer exposure time and brightness of the flicker light source.

What could cause led lighting flicker ?

LED as an electronic component that can be powered by DC supply and maintain a light output steadily. So LEDs  is not be blame for the flicker, but the power supply ( led drivers) should be. If the driver output current is a modulated waveform, the light output will simply reproduce the modulated light output. There are multiple ways to driver LEDs from Constant current reduction to PWM(pulse width modulation ) and even PFM (pulse frequency modulation). PWM and PFM both can introduce flicker(TLA).  The Power supply sometimes does not have output buffer capacitors to create a larger current ripple on its output which will create worse flicker. It could also come from the power line voltage/current transcend fluctuation due to apply or remove loads in the local power network. There is a big challenge on flickering when the dimmable led fixture has a compatibility issue with the dimmer (such as phase cut dimmers)used together.

The lighting industry has established multiple metrics for flicker measurement, from Flicker Index to Flicker Percentage.

Here are some of the standardized test and measurement methods:

Measurement of Pst and optionally testing effect of mains voltage fluctuations or dimming: see IEC TR 61547-1, edition 3;

Measurement of SVM(Stroboscopic effect Visibility Measure), and optionally testing effect of dimming: see IEC TR 63158;

TLA: Test Methods and Guidance for Acceptance Criteria, see NEMA 77-2017.

In North America lighting market,
the TLA method proposed by NEMA in 2017 is using measure of both Pst and SVM. 

NEMA 77-2017 uses Pst standard
(flicker meter) below 80Hz, and Values above 1.0 considered acceptable; It also
use SVM predicts visibility of strobe effect based on wave shape and duty
cycles, above 80 Hz. SVM value of 1 means for this waveform, standard observers
will be able to see flicker 50% of the time. <1 is harder to see; >1 is
easier to see.

 

There are also other  flicker standards
from California CEC title 24 and IEEE P1789-2015 Standard. Here is a great one page summary created from PNNL Naomi Johnson Miller in 2018. None of these standards address the flicker health concern perfectly, it seems that NEMA77-2017 and IEEE P1789-2015 both getting more attentions from the lighting industry.

Energy star Luminaire specification 2.2 version is also using the NEMA77-2017.

It always come down to the lighting manufacturer to assume the responsibility to deliver a flicker less version lighting fixture to the market and the lighting specifier to start asking more questions about whether the lighting application need a good performance on flicker to avoid any health related problems for end users.

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