A look at the health effects of flicker, measurements and metrics of temporal light modulation in traditional and modern lighting, and how it relates to some of the recent standards, regulations and proposals. This talk benefits exterior lighting designers, engineers, project managers, contractors, consultants and environmental. by Luke Price, Public Health England

1. Can the Adverse Health Effects of
Flicker from LEDs … Be Prevented?
Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019

2. Flicker is an old problem
• UK: 133 Hz used for very early electric lighting
(based on 8000 cpm, 2000 rpm)
• US: Westinghouse chose 60 Hz in 1888
Europe: AEG went from 40 to 50 Hz in 1891
• North Eastern Electric Supply Company’s 40 Hz
replaced by the National Grid 50 Hz in 1920
• This elephant wasn’t one of the animals electrocuted during
the War of Currents, and probably not by Edison.
2 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
Wikipedia: Press photo of the electrocution
of Topsy at Luna Park in 1903. Historians
point out that Edison was never at Luna
Park and it took place [over] ten years
after the War of Currents.

3. Flicker in energy-efficient lighting
• Light bulb PF = 9 to 12%
(incandescent)
• Fluorescent* up to 100%
(compact fl. lamp) * showing PF = 30%
• White LED up to 100%
PF = percent flicker = modulation depth = temporal Michelson contrast
PF = (Lmax - Lmin) / (Lmax + Lmin) , where L(t) is luminance over time
3 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
0
0.2
0.4
0.6
0.8
1
0.3 0.31 0.32 0.33 0.34 0.35
Voltage(a.u.)
Time (s)INC CFL LED

4. TLM: Temporal Light Modulation
TLA: Temporal Light Artefacts (some informal definitions)
• Flicker – a perceptible unsteadiness in light due to TLM
• Stroboscopic effect – a perceptible motion illusion due to TLM
• Phantom array – a perceptible trail of discrete images due to TLM
SVM = Stroboscopic Visibility Measure (a metric / model)
PPF = Physiological Percent Flicker (a metric / model)
Glossary
4 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
Price, L., 2017. Can the Adverse Health Effects of Flicker from LEDs and Other Artificial
Lighting Be Prevented?. LEUKOS 13(4), p.191.
Perz, M., et al. 2015. Modeling the visibility of the stroboscopic effect occurring in temporally modulated
light systems. Lighting Research & Technology, 47(3), pp.281.
CIE, 2016. CIE TN 006:2016 Visual Aspects of Time-Modulated Lighting
Systems – Definitions and Measurement Models. Vienna, Austria.

5. TLAs &other unwanted effects
Temporal Light Artefacts (TLAs)
• unwanted effects (due to TLM) that can be visually perceived by humans
“Other” unwanted effects
• unwanted effects (due to TLM) that cannot “be perceived by humans”
Two questions
• Is it really possible to have an effect that without visual perception?
• Why might it be unwanted?
5 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019

6. TLA Other Unwanted
• Retinal activity (via ERG responses)
• The exact cause(s) of myopia and macular degeneration are not known
• Epidemiological associations with the use of artificial lighting
• Possible explanations
• close focussing, reduced exposure (especially blue), extended day-lengths…
• …exposure to TLM
Effects of TLM on retinal photoreceptors
6 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
Up to 162 Hz, for example:
Berman, S., Greenhouse, D., Bailey, I.,
Clear, R. and Raasch, T., 1991. Human
electroretinogram responses to video
displays, fluorescent lighting, and other
high frequency sources. Optometry and
vision science, 68(8), p.645.

7. A model of retinal response
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• Cone rise time of 10 ms (from 10% to 90%)
suggesting 3 ms to 4.3 ms smoothing half-life
• 3 ms selected for proposal to regulate flicker:
• Safety margin
• Repetitive eye movements
• Parameter is not critical to proposal at higher
frequencies where the model predicts
TLM effects ∝∝∝∝ PF / frequency
ResponseAmplitude
Time (ms)
Figure showing the quicker of just
two subjects taken from:
Zele, A., Cao, D. and Pokorny, J.,
2008. Rod–cone interactions and
the temporal impulse response of
the cone pathway. Vision research
48(26), p.2593.
Unadapted
Adapted

8. PF:PPF ratio depends
on the waveform, not
on the type of light
Using the PPF model to calculate TLM severity
8 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
0
0.2
0.4
0.6
0.8
1
0.3 0.31 0.32 0.33 0.34 0.35
Voltage(a.u.)
Time (s)INC CFL LED
0
0.2
0.4
0.6
0.8
1
0.3 0.31 0.32 0.33 0.34 0.35
Voltage(a.u.)
Time (s)INC CFL LED
Time (s)
Smoothedsignal(a.u.)
Voltage(a.u.)
• Light bulb PF = 9.5% PPF = 3.3% < 3:1
• Fluorescent 30% 5% 6:1
• White LED 100% 20% 5:1
After applying the model, the photoreceptor response to TLM can be estimated

9. Effects & Artefacts of Temporal Light Modulation
TLA Other Unwanted
• Photosensitive epilepsy – seizures
• Flicker (perceived unsteadiness)
• Stroboscopic effect
• Phantom array
• Retinal activity (ERG)
• Visual discomfort and eye strain
• Impaired visual performance
• Migraine, headaches and nausea
• Impaired eye movements / reading
• Autism, and non-specific symptoms ? ?
9 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
Source: IEEE 1789:2015 review of the scientific evidence
Wording adapted and ticks and crosses added

10. IEEE 1789:2015 recommendations
IEEE Recommended Practices for
Modulating Current in High-
Brightness LEDs for Mitigating Health
Risks to Viewers
To create a comprehensive and precise set of
recommended practices, it was necessary to include in
the working group research experts in the fields of power
electronic drivers, risk analysis, photobiology, vision,
lamp design, psychology, LEDs, and many other areas.
The result was a diverse field of experts, able to interpret
scientific studies in medical fields, vision, electrical
engineering, hazard analysis, and lighting.
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Flicker
Stroboscopic
Phantom array
Stroboscopic
Flicker
TLM effects ∝ PF / f
Frequency (Hz)
Modulation(%)

11. 2015 sample of LED lighting
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12. IEEE 1789:2015 & CIBSE LED Sample
A sample of domestic LED lamps
purchased in 2015
100 Hz found to be the fundamental
frequency for all the TLM
Figure based on PF data from
CRCE-RDD 01-2016: Human responses
to lighting based on LED lighting solutions.
Commissioned by CIBSE and SLL.
[O'Hagan, Khazova and Price]
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Frequency = 100 Hz
2
2
1 3
1
B22
1
E27
4 4
2
GU10
2
Filament
(B22)
Lightguide
(B22/E14)
Pearl
(E27/B22)
Others
(GU10)
Onlineretailer
(B22/GU10)
AlldomesticLEDs
OfficeLEDpanels
LEDstreetlights
6
2
14
2
1
4
3
2
3
Modulation(%)undimmed
- - - -
-
incl.bothE14s

13. IEEE 1789:2015 & SVM
A proposal put forward by industry
(with PstLM at low frequency)
Majority of population not protected
even against stroboscopic effects
Ignores phantom array, eye strain,
retinal responses etc.
SVM = 1.0 revised to SVM = 0.4
SVM is only defined up to 2 kHz,
problems at 100 Hz and > 500 Hz
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100 Hz
Frequency (Hz)
Modulation(%)
50 Hz
SVM
1.0
0.4

14. IEEE 1789:2015 & PstLM
PstLM fails as a health or flicker
measure at 50 Hz (60 Hz in US) –
by far the most likely frequency
for flicker in the lower range
But there is an easy win here:
LED lighting that contributes to flicker
below twice the mains frequency
provides no engineering benefit
14 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019
50 Hz
Frequency (Hz)
Modulation(%)
50 Hz
PstLM values from https://www.derlichtpeter.de/en/light-flicker/
PstLM = 1

15. PPF =
3%
1%
IEEE 1789:2015 & PPF (Physiological Percent Flicker)
Thresholds agree to the IEEE limits
But PPF applies fairly to different
waveforms and duty cycles, so would
be appropriate for LEDs
In theory such LEDs would reduce or
remove the unwanted effects of TLM
There are effect data above 3000 Hz
up to 10 kHz to 11 kHz
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Frequency (Hz)
Modulation(%)

16. TLM interacts with
• source or object size and visual acuity
• speed of eye movements (up to 700 degrees per second1)
• contrast sensitivity
Crude example of how this could be implemented:
• Set PF = modulation depth, θ = minimum object size angle
• Require f >= 700 * ( PF / 100% ) / ( 3.3 * θ )
For reading 64 characters in 15 cm of text at a distance of 50 cm (θ = 0.14o
), say
• f >= 1.5 kHz for 100% modulation
• f >= 150 Hz for 10% modulation
• < 6.67% modulation at 100 Hz
Phantom array – another point of view
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1. E. Brown, T. Foulsham, Chan-su
Lee and A. Wilkins, 2019. Visibility
of temporal light artefact from
flicker at 11 kHz. Lighting Research
& Technology. doi
10.1177/1477153519852391
f = 11 kHz, PF = 100%, θ = 0.019o
These values are used to derive
the constant 3.3 in the equation.
Broadly equivalent to PPF = 2% or
midway between the IEEE limits

17. Stroboscopic effect – another point of view
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Adapted from:
Bullough, J., Hickcox, K., Klein, T., Lok, A.
and Narendran, N., 2012. Detection and
acceptability of stroboscopic effects from
flicker. Lighting Research & Technology,
44(4), p.477.
NOELLow
Risk
SVM 0.4
SVM 1.0
SVM does not match further
stroboscopic research data
above 500 Hz
IEEE approach and PPF are
simpler and more successful

18. PWM – Pulse Width Modula uses a rectangular wave to achieve dimming
PWM – Pulse Width Modula
f > threshold
PWM – Pulse Width Modulation (e.g. used for dimming LEDs)
f < threshold
With a consistent flicker metric that deals with high frequencies correctly (e.g.
PPF but not SVM), effect thresholds for PWM dimming can be chosen
Dimming LEDs – Pulse Width Modulation
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19. Dimming threshold and Duty cycle, D
To reduce brightness to 25% with PWM, turn the light off for 75% of the cycle
Duty cycle, D = 25%
Basic principle to avoid unwanted effects
TLM effect ∝∝∝∝ (1 − D) / f
So to improve the PWM dimming range, increase the modulation frequency
PPF = 3% D = 97.4% at 100 Hz, or D ≈ 21% at 10 kHz
PPF = 1% D = 99.1% at 100 Hz, or D ≈ 73% at 10 kHz
Conclude that dimming with PWM requires high frequencies
19 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019

20. Summary
At 100 Hz and above…
• LEDs of all types and fittings below NOEL (achievable in 2015)
• The IEEE report sets out a useful review of scientific evidence
…unwanted effects governed by three main features of TLM
• Modulation PF
• Frequency 1 / f
• Waveform, e.g. (1 – D)
…and PPF can be used as a regulatory metric to eliminate the unwanted effects
• Low risk PPF = 3% e.g. 8.7% sine wave at 100 Hz
• NOEL PPF = 1% e.g. 2.9% sine wave at 100 Hz
Below 100 Hz no contribution from LED lighting to periodic TLM (universal in 2015)
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all three are built into the PPF metric
& agree to many lines of evidence

21. Public Heath England
Centre for Radiation, Chemical and Environmental Hazards
Chilton, Leeds and Glasgow
Laser and Optical Radiation Dosimetry Group
Solar UVR Monitoring 1
Laser Protection 2
Artificial Optical Radiation Hazards
1. https://uk-air.defra.gov.uk/data/uv-index-graphs
2. https://www.phe-protectionservices.org.uk/nir/courses/
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CRCE

22. On health, lighting & safety
Khazova, M. and O'Hagan, J., 2008. Optical radiation emissions from
compact fluorescent lamps. Radiation protection dosimetry 131(4), p.521.
EC, 2011. A non-binding guide to the artificial optical radiation directive.
Baczynska, K. and Price, L., 2013. Efficacy and ocular safety of bright light therapy lamps.
Lighting Research & Technology 45(1), p.40.
CRCE-RDD 01-2016: Human responses to lighting based on LED lighting solutions.
Commissioned by CIBSE and SLL. [O‘Hagan, Khazova and Price]
O‘Hagan, J., Khazova, M. and Price, L., 2016. Low-energy light bulbs, computers, tablets and the blue light hazard.
Eye 30(2), p.230.
Price, L., 2017. Can the Adverse Health Effects of Flicker from LEDs and Other Artificial Lighting Be
Prevented?. LEUKOS 13(4), p.191.
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23. Thank you
Acknowledgements:
The IEEE 1789:2015 report diagram (Figure 18) in helping to illustrate the PPF proposal
and in many discussions on flicker.
Giorgio Pistelli (University of Pisa) during the course of completing his Masters, and my
PHE colleagues, for their patient assistance, including in matters relating to flicker and
lighting.
Prof. Arnold Wilkins for discussions on his latest published research.
23 Flicker from LEDs, Luke Price, ILP PROFESSIONAL LIGHTING SUMMIT 2019, 13 June 2019