Correspondence: On the state of knowledge concerning the effects of temporal light modulation

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Correspondence: On the state of
knowledge concerning the effects
of temporal light modulation
Temporal light modulation (TLM) is a change
in the luminous quantity or spectral distribu-
tion of light with respect to time of either a
light source or a lighting system. These
changes arise because of the device or system
design, including drivers and control gear, and
because of fluctuations in the electrical supply.
In former times, families of lighting products
all exhibited the same (or approximately the
same) TLM characteristics: For example, AC-
powered incandescent lamps shared the prop-
erty of TLM in a sinusoidal wave at twice the
mains frequency and 4%–10% modulation
depth. T8 fluorescent lighting systems with
electronic ballasts operated with a dominant
frequency between 20 kHz and 40 kHz and
little modulation depth. Previously, knowing
the lighting technology provided sufficient
information about the TLM properties of a
lighting system; today, this is impossible with-
out measuring the system directly.
Light-emitting diode (LED) light sources
and lighting systems exhibit a very wide
variety of TLM characteristics because of
the variety of ways in which the products can
be designed and because LEDs inherently
respond very quickly to the driving current.
1,2
Moreover, TLM is sometimes deliberately
introduced to LED lighting systems through
the use of pulse-width modulation (PWM) to
control the light intensity. When using PWM,
the light output exhibits a 100% modulation
depth at constant frequency but with variable
duty cycle depending on the desired intensity.
Most systems of which we are aware use
PWM at frequencies higher than twice the
mains frequencies (e.g., 300–400 Hz).
The importance of this phenomenon rests
in the fact that TLM can be a cause of adverse
visual, behavioural and health effects on
viewers.
2–4
The undesirable visual perceptions
are flicker, the stroboscopic effect and the
phantom array effect; these are collectively
known as temporal light artefacts.
1
Among
the unanswered questions is the relative
importance of the visual effects of TLM, as
compared to effects on health or task per-
formance.
5
Some argue that the visual effects
in common interiors are little more than an
annoyance provided that the light source does
not exhibit flicker; others argue that these
visual perceptions relate to more serious
phenomena, at least for some sensitive
people.
6
In 2015, the IEEE published a recom-
mended practice that included an in-depth
risk assessment of possible heath and behav-
ioural outcomes, taking into account both the
severity of the outcome and the probability of
its occurrence.
2
The risk assessment compo-
nent of the hazard analysis took into account
the strength of the evidence; notably, many of
the outcomes were judged to be in need of
more evidence to support evidence-based
guidance on limits for TLM. This also was
the conclusion drawn by the 2017 CIE stake-
holder workshop
3
and the 2019 ANSES
report.
4
Researchers have risen to these challenges
with several recent publications in this journal
and others, which is most welcome; nonethe-
less, there remains disagreement concerning
the meaning of these findings for recommen-
dations and regulations. We comment here,
Lighting Res. Technol. 2021; 53: 89–92
ßHer Majesty the Queen in Right of Canada. National Research Council of Canada, 2020 10.1177/1477153520959182

very briefly, on the state of knowledge, focus-
ing primarily on publications from Lighting
Research and Technology, and propose a few
priority topics for research attention.
It has long been the case that the visual
effects of TLM receive the most research
attention, and these were the basis for the
limits in the only institutional recommended
practice on the topic to date.
2
These effects
occur after very short exposures and are
therefore easy to test.
1
For example, Perz
et al. developed the Stroboscopic Visibility
Measure
7
to predict the stroboscopic effect
from a metric derived from the measured light
source TLM. This metric is founded on the
principle of threshold visibility and is normal-
ized so that the value of the SVM for threshold
visibility (50% likelihood of seeing the strobo-
scopic effect) of the average person is 1.0.
5,7
According to Scopus, the eighth-most-cited
paper from this journal (cited 141 times as of
this writing) is the 1989 paper by Wilkins,
Nimmo-Smith, Slater and Bedocs
8
in which
fluorescent lighting in an office was changed
between magnetic ballasts and high-frequency
ballasts. Complaints of headaches and eye-
strain were reduced by the high-frequency
operation of the linear fluorescent lamps, but
the effect was pronounced only for those with
a tendency to experience headaches and eye-
strain. That is, on average across the whole
sample the effects were marginal, but there
appears to have been a subpopulation of
sensitive individuals who experienced the
adverse effects of the TLM more powerfully.
This outcome occurred despite the large win-
dows in the offices, which would have the
effect of reducing the influence of the electric
lighting conditions during daylight hours.
That is, in the general population it can be
difficult to detect the effects of TLM. Veitch
and McColl
9
observed an effect of TLM (also
manipulated using fluorescent lighting bal-
lasts) only on a visually difficult task in a
repeated-measures experiment under restrict-
ive viewing conditions, and with young adult
participants. Sekulovski et al.
10
conducted a
field intervention study with conceptual simi-
larity to the original Wilkins et al.
8
paper, but
did not find that there was any difference in
headache incidence between LED lighting
with an SVM value of nominally 1.34 and
one of 0.47. They appear not to have had data
concerning the participants’ individual differ-
ences in sensitivity, so it is unknown whether
the most-sensitive individuals might have had
a different experience than the group overall.
Moreover, the experimental space had large
windows and included desks at varying dis-
tances from the window, so that there was
considerable variability in the effective SVM
of the conditions to which participants were
exposed both across the space and during the
day as well as an unknown additional vari-
ability in the SVM experience of individuals
as they moved throughout the space during
the day. Thus, there are several reasons why
the expected effect of SVM on headache
incidence was not observed.
Veitch and Martinsons
11
studied strobo-
scopic visibility and the acceptability of con-
ditions for a set of commercially available
lamps varying in SVM, focusing on young
adults (thought to be more sensitive) and
including a measure of sensitivity to visual
discomfort. The relationship between the light
source’s SVM value and participants’ report-
ing of stroboscopic visibility (namely, a rapid,
non-linear increase in stroboscopic visibility
as SVM increased above 0.4) were the same
for groups high and low in sensitivity to visual
discomfort, but the high-sensitivity group did
report that the conditions were more annoy-
ing than the low-sensitivity group when the
SVM value of the light source was 1.4 or 3.0.
Given the very short exposures, Veitch and
Martinsons argued that this finding merits a
greater focus in future research upon more
sensitive individuals. Individuals who are
more sensitive to visual discomfort also have
a higher TLM frequency threshold for detect-
ing the phantom array effect than those who
90 JA Veitch et al.
Lighting Res. Technol. 2021; 53: 89–92

are less sensitive (up to 11 kHz, vs. the 6 kHz
average threshold).
12
The renewed focus on effects beyond visual
perception seen in Sekulovski et al.
10
is
important. Zhao et al.,
13
with a small
sample size, examined brain activity, eye
movement, the stroboscopic effect and cogni-
tive performance in a repeated-measures
experiment with nine TLM conditions
chosen in relation to the IEEE 1789-2015
recommendations.
2
They found that the con-
ditions identified as being high-risk condi-
tions in IEEE 1789-2015 caused greater
cortical arousal and, in some cases, higher
ratings of task difficulty even when cognitive
performance was unaffected.
The rapid adoption of LED lighting has
created pressure to develop recommendations
to limit the risk of such adverse effects.
Whether in the form of recommendations
2
or regulations,
14
these are invariably contro-
versial.
5,15
Debate and discussion of these
various approaches to establishing the suit-
able metrics and the limit values on them are
important and necessary, of course; but more
and better information on which to base these
discussions would prevent a continual revisit-
ing of old disagreements. Specifically, the
world would benefit from more information
to address the following open questions,
among others:
What are the effects of TLM in the popu-
lation subgroup of people who are suscep-
tible to visual stress?
What are the effects of varying TLM
conditions on outcomes beyond visual per-
ception (i.e., physiological, behavioural and
health effects)?
Given that much emphasis has been placed
on dominant frequency and modulation
depth as predictors of outcomes, what are
the effects of other possibly influential par-
ameters, such as duty cycle and waveform?
It should go without saying that strong
research designs, adequate sample sizes, and
clean measurements of both stimulus condi-
tions and outcome measures will be required
to provide the strong evidence base upon
which to build future recommendations,
standards and regulations. Exactly what
those documents ought to say will be the
result of consensus processes in various
communities,
3
but more and better research
is needed as inputs to the discussions.
ORCID iDs
Jennifer A Veitch https://orcid.org/0000-
0003-3183-4537
Christophe Martinsons https://orcid.org/
0000-0002-2286-5991
Carsten Dam-Hansen https://orcid.org/
0000-0001-7518-4025
References
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(CIE). Visual aspects of time-modulated
lighting systems – Definitions and measure-
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2 IEEE Power Electronics Society. IEEE rec-
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high-brightness LEDs for mitigating health risks
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Institute for Electrical and Electronics
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2015.html.
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Jennifer A Veitch
a
,
Christophe Martinsons
b
, Steve Coyne
c
and
Carsten Dam-Hansen
d
a
National Research Council of Canada,
Ottawa, ON, Canada
b
Centre Scientifique et Technique du Ba
ˆtiment,
Saint-Martin-d’He
`res, France
c
Light Naturally, Brisbane, Australia
d
DTU Fotonik, Roskilde, Denmark
Address for correspondence: Jennifer A Veitch,
National Research Council of Canada, Ottawa,
ON K1A 0R6, Canada.
Email: jennifer.veitch@nrc-cnrc.gc.ca
92 JA Veitch et al.
Lighting Res. Technol. 2021; 53: 89–92