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LED lighting flicker and potential health concerns: IEEE standard PAR1789 update


Abstract and Figures

The IEEE Standards Working Group, IEEE PAR1789 “Recommending practices for modulating current in High Brightness LEDs for mitigating health risks to viewers” has been formed to advise the lighting industry, ANSI/NEMA, IEC, EnergyStar and other standards groups about the emerging concern of flicker in LED lighting. This paper introduces power electronic designers for LED lighting to health concerns relating to flicker, demonstrates that existing technologies in LED lighting sometimes provide flicker at frequencies that may induce biological human response, and discusses a few methods to consider when trying to mitigate unintentional biological effects of LED lighting. The paper represents on-going work in IEEE PAR1789 that is vital to designing safe LED lamp drivers.
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LED Lighting Flicker and Potential Health Concerns:
IEEE Standard PAR1789 Update
Arnold Wilkins
Dept. of Psychology
University of Essex; UK
Jennifer Veitch
National Research Council Canada
Ottawa; Canada
Brad Lehman
Dept. Elect. & Comp. Eng
Northeastern University; USA
Abstract -- The IEEE Standards Working Group, IEEE
PAR1789 "Recommending practices for modulating current in
High Brightness LEDs for mitigating health risks to viewers"
has been formed to advise the lighting industry, ANSI/NEMA,
IEC, EnergyStar and other standards groups about the
emerging concern of flicker in LED lighting. This paper
introduces power electronic designers for LED lighting to
health concerns relating to flicker, demonstrates that existing
technologies in LED lighting sometimes provide flicker at
frequencies that may induce biological human response, and
discusses a few methods to consider when trying to mitigate
unintentional biological effects of LED lighting. The paper
represents on-going work in IEEE PAR1789 that is vital to
designing safe LED lamp drivers.
Index Terms-- LED, health risk, flicker, lighting, power
electronics, ergonomics, drivers, headache, seizure, standards
Flicker: a rapid and repeated change over time in the
brightness of light.
Modulation: a measure of light variation that is often
applied to periodic oscillations. This report refers to
modulation as variation in luminance as a proportion of the
average luminance (commonly referred to as Percent Flicker,
Peak-to-Peak Contrast, Michelson Contrast, or Depth of
Modulation). For a time-varying luminance with maximum
and minimum values:
Modulation = (Lmax - Lmin) / (Lmax + Lmin)
(Lighting Design Glossary)
Visible Flicker: Flicker that is consciously perceivable by
a human viewer.
Invisible or Imperceptible Flicker: Flicker that is not
consciously perceivable by a human viewer.
The effects of flicker can range from decreased visual
performance to non-specific malaise to the onset of some
forms of epilepsy.
This paper summarizes a public report created by the IEEE
Standards PAR1789 group on LED lighting that is examining
biological effects of flicker in emerging LED lighting
technologies. (The full length version of the report can be
found at ) The intention
of this document is to provide an objective summary of the
effects on human health for both visible and invisible flicker
and to draw attention to implications for the design of LED
lighting. Specifically, contributions of this paper include
making the reader aware of
1. Risks of seizures due to flicker at frequencies within the
range ~3- ~70Hz;
2. Human biological effects due to invisible flicker at
frequencies below ~165Hz;
3. The differences between “visible” flicker and “invisible”
flicker and any relation to health risks;
4. A few, typical driving approaches in LED lighting that
may produce flicker.
This report does not attempt to make recommendations on
safe flicker frequencies or modulation depths for LED
lighting. Its purpose is to describe possible health
implications of flicker. By bringing these issues to the power
electronic and lamp designers now, it will permit better
ethical discussions and decisions to be made on development
of future LED lamps as the market continues to have
explosive growth. This report endorses no technology for
driving LED lamps. Specifically, Section III of the report
gives tutorial surveys on health effects of flicker. Section IV
introduces a few typical LED driving methods that introduce
flicker in frequency ranges of interest.
The health effects of flicker can be divided into those that
are the immediate result of a few seconds’ exposure, such as
epileptic seizures, and those that are the less obvious result of
long-term exposure, such as malaise, headaches and impaired
visual performance. The former are associated with visible
flicker, typically within the range ~3- ~70Hz, and the latter
with invisible modulation of light at frequencies above those
at which flicker is perceptible (invisible flicker). Human
biological effects are a function of flicker frequency,
modulation depth, brightness, lighting application, and
several other factors.
A. Photosensitive Epilepsy
About one in 4000 individuals is recognized as having
photosensitive epilepsy. Repetitive flashing lights and static
repetitive geometric patterns may induce seizures in these
individuals, and in perhaps as many again who have not been
diagnosed and may be unaware that they are at risk.
The seizures reflect the transient abnormal synchronized
activity of brain cells, affecting consciousness, body
movements and/or sensation. The onset of photosensitive
epilepsy occurs typically at around the time of puberty; in the
age group 7 to 20 years the condition is five times as common
as in the general population. Three quarters of patients remain
photosensitive for life (Harding and Jeavons, 1994; Wilkins,
1995; Fisher et al. 2005). Many factors [see Fisher et al.,
2005 for extensive reference list] may combine to affect the
likelihood of seizures including:
Flash Frequency. Any repetitive change in a visual
stimulus within the frequency range 3 Hz to 70 Hz, is
potentially a risk and the greatest likelihood of seizures is for
frequencies in the range 15 Hz to 20 Hz, see Fig. 1. The
flashes do not have to be rhythmic.
Brightness. Stimulation in the scotopic or low mesopic
range (below about 1 cd/m2) has a low risk and the risk
increases monotonically with log luminance in the high
mesopic and photopic range.
Contrast with background lighting. Contrasts above
10% are potentially a risk.
Distance between the viewer and the light source and its
location which determine
Total area of the retina receiving stimulation. The
likelihood of seizures increases according to the
representation of the visual field within the visual cortex of
the brain. The cortical representation of central vision is
greater than that of the visual periphery, and so
Location of stimulation within the visual field is
important: Stimuli presented in central vision pose more of a
risk than those that are viewed in the periphery, even though
flicker in the periphery may be more noticeable.
Wavelength of the light. Deep red flicker and
alternating red and blue flashes may be particularly
Open or closed eyes. Bright flicker can be more
hazardous when the eyes are closed, partly because the entire
retina is then stimulated. However, if flickering light is
prevented from reaching the retina of one eye by placing the
palm of a hand over that eye, the effects of the flicker are
very greatly reduced in most patients.
Figure 1. Percentage of patients with photosensitive epilepsy exhibiting
epileptiform EEG responses to the flicker from a xenon gas discharge lamp
shown as a function of flash frequency. After Harding and Jeavons (1994).
Figure 2. Escalator stair tread
In addition, a substantial minority of patients (usually
those who are sensitive to flicker) are sensitive also to spatial
patterns; see Fig. 2 for an example. About one third of
patients are sensitive to patterns even when there is no
flicker, and most are more sensitive to flicker if it is patterned
(Harding and Jeavons, 1994; Wilkins, 1995; Fisher et al.,
2005; Wilkins et al. 1979). The worst patterns are those of
stripes in which one cycle of the pattern (one pair of stripes)
subtends at the eye an angle of about 15 minutes of arc.
B. Imperceptible Flicker
The frequency of the alternating current electricity supply
is 60Hz in America and 50Hz in Europe; in Japan, both 50Hz
and 60Hz are used in different regions. The circuitry in older
fluorescent lamps with magnetic ballasts operate so as to
flash the lamps at twice the supply frequency (100Hz or
120Hz). However, as the lamps age, the flashes that occur
with one direction of current may not equal those that occur
with the other direction, and the lamps may emit flicker with
components at the frequency of the electricity supply. It has
been determined that photosensitive seizures should not occur
if fluorescent lamps are operating properly. However, when
the lamps malfunction giving flicker below 70Hz,
electroencephalographic recordings indicate a risk of seizures
(Binnie et al., 1979). Nevertheless some photosensitive
patients do complain of normally functioning (older)
fluorescent lighting (with magnetic ballasts).
Measurements of the electroretinogram have indicated that
modulation of light in the frequency range 100-160Hz and
even up to 200 Hz is resolved by the human retina although
the flicker is too rapid to be seen (Burns et al. 1992, Berman
et al., 1991). In an animal (cat), 100Hz and 120Hz
modulation of light from fluorescent lamps has been shown
to cause the phase-locked firing of cells in the lateral
geniculate nucleus of the thalamus, part of the brain with
short neural chains to the superior colliculus, a body that
controls eye movements (Eysel and Burandt, 1984). There are
several studies showing that the characteristics of human eye
movements across text are affected by modulation from
fluorescent lamps and cathode ray tube displays (see work of
Wilkins, 1986; Kennedy and Murray, 1991), and two studies
have shown impairment of visual performance in tasks
involving visual search as a result of flicker from fluorescent
lamps (e.g. Jaen et al., 2005). Under double-masked
conditions the 100Hz modulation of light from fluorescent
lamps has been shown to double the average incidence of
headaches in office workers, although this effect is
attributable to a minority that is particularly affected (Wilkins
et al., 1989).
Sensitivity effects due to flicker at frequencies above
perception have also been observed in normal people with
good vision and health. A substantial decrement in sensitivity
to visible flicker at 30 Hz, used as a testing condition, occurs
in normal observers when there is a prior exposure of only 2
minutes duration with flicker frequencies about 20% above
the observers critical fusion frequency (CFF) (Shady et al.
Computer monitors and backlights
When making a rapid jerk (saccade), for example when
reading, the eyes move at a velocity of about 180 degrees per
second. As a result, any intermittently lit contour is displaced
at a succession of retinal positions during the flight of the eye
and can sometimes be seen as a set of repetitive targets. The
LED rear lamps of motor vehicles can produce such an effect.
Some displays on netbook computers have LED backlights
and exhibit significant flicker at 60Hz. Their flicker also
results in the perception of multiple images during a saccade.
It is possible that this effect is responsible for the known
disturbance of ocular motor control by high frequency flicker,
a disturbance which, in its turn, may be responsible for the
known impairments in visual performance.
Modulation depth and the Fourier fundamental.
The effects of flicker depend not only on the frequency of
the flicker but also on the modulation depth. For visible
flicker, the amplitude of the Fourier fundamental predicts
flicker fusion (de Lange Dzn, 1961). For imperceptible
flicker the effects of different waveforms have not been
studied in detail. The peak-trough modulation depth of the
100-120Hz flicker from older fluorescent lamps with
magnetic ballasts varies with the component phosphors, some
of which exhibit persistence, varying the chromaticity of the
light through its cycle (Wilkins and Clark, 1990). The peak-
trough modulation depth known to induce headaches from
fluorescent lighting at 100Hz is about 35% (Wilkins et al.,
1989). The IEEE Standards PAR1789 working group is
developing new measures/definitions of flicker that rely on
Fourier series of the flicker. The present definitions for
modulation do not distinguish the difference between low
frequency and high frequency modulation. But for
sufficiently high flicker frequencies, there are limited human
biological effects.
C. Summary of Biological Effects
The obvious biological effects occur: 1) immediately from
2) flicker that is visible. The risks include seizures, and less
specific neurological symptoms including malaise and
headache. Seizures can be triggered by flicker in individuals
with no previous history or diagnosis of epilepsy.
The less obvious biological effects occur: 1) from flicker
that is invisible 2) after exposure of several minutes. Invisible
flicker health effects have been reported to include headaches
and eye-strain.
The table in the Appendix summarizes and categorizes the
types of flicker and the biological effects they cause.
(c) Simulation of current through HB LEDs.
Luminous intensity is proportional to current,
causing lamp to flicker at twice the line frequency
(shown periodic every 1/120 sec)
LED string
AC 50-60Hz
AC 50-60Hz
(a) Rectify AC and send to LED string
(b) Directly power two LED strings with opposite
Anode/Cathode connections
Or a
Figure 3. Two methods to drive LEDs at twice line frequency: (a) Full bridge
rectification, (b) Opposite connected parallel strings, and (c) Current/Luminous Output in
D. A Few General Implications for Practice
Visual flicker is an undesirable attribute of any lighting
system. The Appendix Table summarizes research suggesting
that, for both visible and invisible flicker (in the mentioned
frequency ranges), there may be a special at-risk population
for which flicker is more than just annoying and can be a
potential health hazard. Any hazard will, however, depend
on modulation depth, ergonomics, flicker parameters and
their relation to perception and the ability to
measure/determine the influence of these parameters with
human diagnostics. These topics are beyond the scope of this
paper and will be covered in future IEEE PAR1789
documents. However, it is possible to make general
comments about the research cited in the Appendix Table:
1. Frequency. Normally functioning fluorescent lighting
controlled by magnetic ballasts has been associated with
headaches due to the flicker produced. LEDs driven so that
they flicker at a frequency twice that of the AC supply may
have a depth of modulation greater than that from most
fluorescent lamps. The effects of the flicker are therefore
likely to be more pronounced in these cases.
2. Field of view. Point sources of light are less likely to
induce seizures and headaches than a diffuse source of light
that covers most of a person’s field of vision. Flicker from
LEDs used for general lighting is therefore more likely to be
a health hazard than that from LEDs used in instrument
3. Visual task. The invisible flicker described in the
Appendix Table is more likely to cause problems when the
visual task demands precise positioning of the eyes, as when
4. Spatial distribution of point sources of light. Spatial
arrays of continuously illuminated point sources of light have
the potential to induce seizures in patients with photosensitive
epilepsy when the field of view is large and when the arrays
provide a spatial frequency close to 3 cycles/degree (e.g.
large LED public display boards viewed from close
There are several common methods that are used to drive
LEDs that can operate with frequency of modulation in the
ranges discussed in the above table (below 120Hz, including
frequencies in the vicinity of 15Hz.) For example,
commercially available LED lamps have been reported (Rand
et al., 2007; Rand, 2005) to malfunction and produce visual
flicker in the 15Hz range when connected to a conventional
residential dimmer.
Below, we present only a few driving approaches that
modulate in frequency ranges from zero to 120Hz. The list is
not exhaustive, and the discussions are only meant to
demonstrate typical driving LED currents with frequencies in
this range.
A. LED Driving Current Frequencies in Range: ~100Hz–
(1) Full Wave Rectifier Connected to LED String
In this approach, the AC input source is sent into a full
wave rectifier, causing the (approximate) absolute value of
the input voltage to be sent to the load. In this case, the
current through the LEDs has a waveform shape similar to a
scaled absolute value of a sine wave. That is, the rectified
sine wave may be of the form, where Vp is
wave and ω is the
the amplitude of the sine
angular frequency in radians ω = 2*π*f. In this case, the LED
current is of similar shape, as Fig. 3 shows. In a first
approximation, the LED current is equal to a scaled rectified
voltage, with the additional deadtime (zero current) caused by
the LED bias voltage. Thus, when properly functioning, the
direct full wave rectifier driving approach modulates the
LEDs at twice the line frequency, which in North America
leads to 120Hz modulation and in Europe leads to 100 Hz
modulation. As Fig. 3(a) shows, often a resistor is added in
series with the LED string for current limiting protection.
(2) Directly Drive Two Parallel LED Strings with
Opposite Anode/Cathode Connections
A second LED driving method that doubles line frequency
is shown in Fig. 3(b). Two strings of LEDs are powered in
parallel, with anode of one paralleled string connected to the
cathode of the other parallel string. When the AC line voltage
is positive, energy drives one of the LED strings. When the
AC line voltage is negative, the other paralleled LED string is
driven. At most, one of the LED strings has current through
it. The net effect is that the effective LED driving current is
modulating at 120 Hz in North America or 100 Hz in Europe.
Thus, for both driving methods illustrated in Fig. 3, the
LED current modulates at twice the line frequency. Since the
intensity of the LEDs is (ideally) proportional to the current
through the LEDs, this causes the LEDs to flicker at
frequency equal to twice the AC line frequency, i.e.
100Hz~120Hz. There are many variations of the approach in
Fig. 3 that are not shown here.
(3) Simple Dimming Pulse Width Modulated (PWM)
It is common to dim LEDs by pulsing the current through
them intentionally.The luminous intensity of the LED can be
adjusted by varying the length of time that the LED current is
High or Low,. Thus, PWM dimming circuits may be designed
to operate at any frequency, whether the input is DC or AC.
(It should be noted that it is not uncommon for LED drivers
using AC residential phase modulated dimmer circuits to
attempt to “emulate” PWM type signals with frequency
120Hz. That is, when the AC dimmer shuts off, no current is
sent to the LEDs.)
It should be mentioned that there are alternative
approaches to dimming, such as amplitude dimming, in
which the current through the LED is continuous and not
|)sin(| tVp
pulsing. By reducing the value of this continuous current
(amplitude), the brightness is dimmed. This approach does
not use flicker to adjust brightness and therefore, should not
induce flicker related health risks.
(4) Power Factor Correction Circuitry
Even when sophisticated high frequency switching power
supplies with power factor correction circuits are used to
drive LEDs from AC mains, there is commonly a frequency
component in the current (and luminous intensity) of the
LEDs at twice the line frequency. Depending on the design of
the circuitry, the harmonic content of this flicker may vary
from being small and unnoticeable to being significant in
B. LED Driving Current Frequencies in Range: 3Hz~70Hz
(1) Failures in rectification or LED strings: 50Hz ~ 60 Hz
In either of the two methods of Fig. 3, there is risk of
failure that can cause LED current modulation at AC line
frequency, thereby entering the range of frequencies that may
induce photosensitive epilepsy. For example, if one of the
legs of the full wave rectifier bridge fails, then it is common
that the leg becomes an open circuit. Open circuits prevent
current flow, and therefore, the LED modulation frequency
may change. This single diode failure in the rectifier will
cause the output voltage for the full wave rectifier to become
the input voltage for half the AC line cycle, and then 0 volts
for the remaining half line cycle. This means that if the AC
Mains line frequency is f and the period is T=1/f, then non-
zero voltage is applied to the LEDs for 0.5*T seconds and
then is zero for the next 0.5*T seconds, causing the LED
current to modulate at line frequency.
Similarly, when the two strings of LEDs are connected in
parallel with opposite anodes and cathodes in each string, a
failure in one string of the LEDs may cause an open circuit to
occur in that string. The net effect is the same as before: the
current is modulating at line frequency, i.e. 50Hz ~ 60Hz.
This low frequency driving current leads to brightness flicker
in the LEDs at 50Hz~60Hz, since the current in the LEDs is
proportional to their intensity. This is in a range of
frequencies that are at risk of causing photosensitive epilepsy.
(2) Residential Dimmer Switches Can Cause Low
Frequency Flicker (~3Hz – 70Hz)
Residential dimmers for incandescent bulbs primarily
utilize phase modulating dimming through triac switches to
control the power sent to the bulb. These dimmers control the
RMS voltage applied to the bulb by suppressing part of the
AC line voltage using a triac. The effect is a chopped sine
wave as shown in Fig.4. Thus, as the dimmer switch is
manually adjusted, the value of the off-time, α (often referred
to as the phase delay) changes. As α is increased in Fig. 4,
less power goes to the incandescent bulb and brightness is
Some LED lamps and their associated drivers may not
perform properly with residential phase modulated dimmers.
Often on the LED bulb application notes or on the lamp’s
manufacturer web sites there are warnings to the user that
their bulbs may not work properly when used with residential
dimmer switches. Rand’s work (Rand et al., 2007; Rand,
2005) explains the causes of these failures and shows that low
frequency flicker may occur.
Dimmer Voltage
Figure 4. Residential dimmer and its output voltage sent to
the driver (Rand et al., 2007).
Fig. 5 illustrates how one type of commercially available
LED lamp flickers in the noticeable visual range when
connected to a dimmer switch. The particular lamp involved
has a common LED driver configuration (further discussed
below) of a full bridge rectifier with capacitor filter within
their Edison socket, described in more detail by Rand et al.,
2007; 2005). The results presented in the figure may be
typical of similar driving configurations. The circuit will
continuously peak charge the filter capacitor to the peak
voltage of the input waveform, i.e. 169Vdc for standard
120Vac line voltage. This high level DC voltage may then be
fed into a large string of LEDs in series. For example, some
typical lamps may have parallel strings of many Red, Blue,
Green LEDs, in series attached through a current limiting
resistor to the high level DC voltage. The particular lamp
tested utilized a combination of 64 Red, Green and Blue
LEDs to produce white light.
Figure 5. Commercial LED lamp flickers at 3.15Hz when
connected to typical residential dimmer switch.
The experimental data in Fig. 5 represents the voltage of a
photo-sensor placed directly underneath the LED lamp.
Specifically, a photoresistor circuit is used to generate a
voltage proportional to the light intensity shining on it. As the
experimental voltage shows, the bulb malfunctions when
connected to (phase modulated) residential dimmer switch. It
produces a noticeable visual flicker. In particular, the flicker
varies between around 3.0Hz and 3.3Hz, with average over
many cycles of 3.153Hz. This frequency is in the range that
has been shown to be a risk for causing photosensitive
epileptic seizures.
The flicker illustrated in Fig. 5 is typical of a few LED
lamps on the market when connected to a dimmer. However,
the precise flicker frequency is hard to predict, as it may
either be higher or lower depending on various factors such
as number of lamps on the dimmer, position of the dimmer
switch (the value of desired off-time α), and/or internal
characteristics of the lamp. However, as the experimental
oscilloscope plot shows, the flicker frequency may be in the
range that induces photosensitive seizures.
The reasons that the dimmer switch may fail when
connected to LED lamp bulbs are given in (Rand et al., 2007;
(3) Uneven Brightness in Different LED Strings When
Connected as in Fig. 3(b)- With Strings Having Opposite
Anode/Cathode Connections
Consider the circuit in Fig. 3(b). Notice that each LED
must have the same dynamic characteristics (forward voltage
and dynamic resistance) in order for the current to be
perfectly balanced in each alternating illuminated string. If
for some reason this does not occur (aging, temperature
gradients, poor design), then the current through the strings
will not be identical each cycle.
Figure 6. Unbalanced LED Current in Each String of
LEDs Using Driving Method in Fig. 3(b). The unbalanced
driving will cause uneven luminous output in the lamp and
low frequency flicker.
For example, suppose over time, aging causes degradation
of one of the two strings in Fig. 3(b) such that its string
resistance increases by 50%. This could also be caused by
improper design of each string in Fig. 3(b) so that the current
in each string is not balanced. This is quite possible since
LEDs are binned by different voltages, and further, each
string may be composed of different color LEDs that have
different nominal voltage drops for the same current. Then,
the effective LED current through the bulb will look as in
For example, the effective DC LED current in the
numerical simulation of Fig. 6 has average value of around
233mA. However, the Fourier component at 60 Hz (taking
FFT) is 80mA and the Fourier component at 120Hz is nearly
240mA. Thus, in this example the low frequency component
of 60Hz represents over 33% of the DC component, while the
120 Hz component represents 100% of the DC current.
Higher frequency components of the LED current in the
above figure are also present in multiples of 60Hz. However,
the typical analysis above indicates that LED lamps may
demonstrate flicker frequency at line frequency, similar to
older fluorescent lamps (previously discussed) that aged
unevenly: the flashes/brightness with one direction of line
current may not equal those that occur in the other direction.
The above example also illustrates that it is possible for
flicker in a lamp to have harmonics with multiple low
frequency components, here at both 60Hz and 120Hz.
The purpose of this paper is to make lamp and power
electronic designers aware of biological effects of flicker and
to introduce the reader to a few LED driving methods that
will have flicker. The LED driving approaches described in
this paper are not exhaustive and are only meant to introduce
the reader to a few common approaches. Other
approaches/applications of LED lighting that may also have
flicker include, but are not limited to, pulse amplitude
modulation driving, triangle wave currents through LEDs,
using LED flicker for wireless communication (see IEEE
Standard 802), beat frequencies created through the
interaction of different lamp flicker frequencies, etc.
This paper assigns no health risk to the biological effects
of flicker in the various LED lamps. The hope is that by
discussing the issue of flicker within the power electronic
community, it will be possible to decide as a community
whether or not standards or recommended practices are
necessary. We do not attempt to do so here. However, we do
offer simple suggestions as to what should be considered
when designing lamps, such as flicker frequency, angle of
viewing, task being performed, spatial distribution, AC
dimmer flicker, etc. Further, it is not difficult to create shut-
down or other safety prevention circuits that prevent
flickering in the 3Hz-70Hz range when the lamp is in failure
mode. This is the flicker range that has risk of photosensitive
epilepsy for small minority of the population.
This paper was assembled with input from many members
of IEEE Standards PAR1789 who at the time of writing
consisted of (including observers):
Maurizio Acosta, Ian Ashdown, Rolf Bergman, Sam
Berman, Anindita Bhattacharya, Subramanyam Chamarti,
Clint Chaplin, Yvonne de Kort, Montu Doshi, Kevin
Dowling, John Halliwell, Mark Halpin, Steve Hayes, Chris
Horton, Wijnand IJsselsteijn, Michael Jennings, Jiao
Jianzhong, David Keeser, Faisal Khan, Thorbjorn Laike,
Brad Lehman, Ihor Lys, Theron Makley, Naomi Miller,
Lesley Murawski, Brandon Oakes, Yoshi Ohno, Steve
Paolini, Radu Pitigoi Aron, Michael Poplawski, Conor Quin,
Eric Richman, Anatoly Shteynberg, Michael Shur, Roger
Shuttleworth, David Sliney, Jennifer Veitch, Joachim
Walewski, Arnold Wilkins, Howard Wolfman, Wei Yan,
Regan Zane.
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PAR1789 website for additional references that were also used. The web site will
continuously be updated.)
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Digitally signed by
DN: cn=arnold
Date: 2010.07.12
14:39:21 +01'00'
Source of flicker Fre
ical effect Evidence
Sunlight through roadside trees
or reflected from waves
Various Seizures Clinical histories (Harding and
Jeavons, 1994)
Xenon gas discharge photo-
3-60Hz Epileptiform EEG in
patients with
photosensitive epilepsy
Many clinical EEG studies e.g
(Harding and Jeavons, 1994)
fluorescent lighting
Large 50Hz
Epileptiform EEG in
patients with
photosensitive epilepsy
(Binnie et al., 1979)
Television 50Hz and 60Hz
(discounting 25Hz
Epileptiform EEG in
patients with
photosensitive epilepsy
Many studies eg (Harding and
Harding, 2008; Funatsuka et al.,
Flashing televised cartoo
Seizures in children
with no previous
diagnosis of epilepsy
Major incident (Okumura et al,
Normally functioning
fluorescent lighting (50Hz
100Hz (small
50Hz component)
Headache and eye strain Many anecdotes.
Normally functioning
fluorescent lighting (50Hz
100Hz (small
50Hz component)
Headache and eye strain Double-masked study (Wilkins et
al 1989)
Normally functioning
fluorescent lighting (50Hz
32% modulatio
Reduced speed of visual
Two masked studies (Jaen et al.,
Normally functioning
fluorescent lighting (60Hz
Reduced visual
(Veitch and McColl, 1995)
Normally functioning
fluorescent lighting (50Hz
100Hz (minimal
50Hz component)
Increased heart rate in
agoraphobic individuals
(Hazell and Wilkins, 1990)
Normally functioning
fluorescent lighting (50Hz
Enlarged saccades over
(Wilkins, 1986)
Visual display terminals 70-110Hz raste
Changes in saccade size (Kennedy et al., 1998)
Visual display terminals ~70H
Many anecdotal reports of
prolonged photophobia
Normally functioning
fluorescent lighting
100Hz and 120Hz Phase-locked firing of
LGN neurons in cats
(Eysel and Burandt, 1984)
Various Up to 162H
signals at light
(Berman et al.,1991; Burns et al
Normally functioning
fluorescent lighting (50Hz
Inconsistent changes in
plasma corticosterone
levels in captive
(Maddocks et al., 2001)
Normally functioning
fluorescent lighting (50Hz
Mate choice in captive
(Evans et al., 2006)
... Further reductions in the amplitude of flicker can be achieved by voltage smoothing with capacitors (common in fluorescent lights) and "constant current" drivers [2] used to power large-wattage LED sources ( Figure 1). The "critical flicker frequency" [3] or frame rate of human vision is variable but is placed in the range of 3 to 70 Hz [4]. It is well established that lighting flicker within the range of visual perception can induce neurological symptoms like headaches, loss of attention and visual acuity, irritability, and in some cases, epileptic seizures [5,6]. ...
... It is well established that lighting flicker within the range of visual perception can induce neurological symptoms like headaches, loss of attention and visual acuity, irritability, and in some cases, epileptic seizures [5,6]. Most The "critical flicker frequency" [3] or frame rate of human vision is variable but is placed in the range of 3 to 70 Hz [4]. It is well established that lighting flicker within the range of visual perception can induce neurological symptoms like headaches, loss of attention and visual acuity, irritability, and in some cases, epileptic seizures [5,6]. ...
Full-text available
The VIIRS day/night band (DNB) high gain stage (HGS) pixel effective dwell time is in the range of 2–3 milliseconds (ms), which is about one third of the flicker cycle present in lighting powered by alternating current. Thus, if flicker is present, it induces random fluctuations in nightly DNB radiances. This results in increased variance in DNB temporal profiles. A survey of flicker characteristics conducted with high-speed camera data collected on a wide range of individual luminaires found that the flicker is most pronounced in high-intensity discharge (HID) lamps, such as high- and low-pressure sodium and metal halides. Flicker is muted, but detectable, in incandescent luminaires. Modern light-emitting diodes (LEDs) and fluorescent lights are often nearly flicker-free, thanks to high-quality voltage smoothing. DNB pixel footprints are about half a square kilometer and can contain vast numbers of individual luminaires, some of which flicker, while others do not. If many of the flickering lights are drawing from a common AC supplier, the flicker can be synchronized and leave an imprint on the DNB temporal profile. In contrast, multiple power supplies will throw the flickering out of synchronization, resulting in a cacophony with less radiance fluctuation. The examination of DNB temporal profiles for locations before and after the conversion of high-intensity discharge (HID) to LED streetlight conversions shows a reduction in the index of dispersion, calculated by dividing the annual variance by the mean. There are a number of variables that contribute to radiance variations in the VIIRS DNB, including the view angle, cloud optical thickness, atmospheric variability, snow cover, lunar illuminance, and the compilation of temporal profiles using pixels whose footprints are not perfectly aligned. It makes sense to adjust the DNB radiance for as many of these extraneous effects as possible. However, none of these adjustments will reduce the radiance instability introduced by flicker. Because flicker is known to affect organisms, including humans, the development of methods to detect and rate the strength of flickering from space will open up new areas of research on the biologic impacts of artificial lighting. Over time, there is a trend towards the reduction of flicker in outdoor lighting through the replacement of HID with low-flicker LED sources. This study indicates that the effects of LED conversions on the brightness and steadiness of outdoor lighting can be analyzed with VIIRS DNB temporal profiles.
... The variations in light (e.g., light flicker) can be characterized by the change in the amplitude and its frequency of occurrence [24]. This may cause irritation to the eye, causing what's known as photosensitive epilepsy [25]. Potential risk assessment and biological effects of the light flickers are discussed in [26]. ...
Full-text available
The research presented in this paper focuses on the impacts of fast-charging stations on three power quality phenomena, namely voltage magnitude variations, voltage unbalance, and voltage fluctuation. The Markov Chain Monte Carlo is proposed to estimate the energy requirements of each fast-charging station when it is utilized to charge electric vehicles, considering their sto-chastic parameters such as battery capacities, state-of-charge levels at time of arrivals, and time of arrivals at charging stations. Two charging methods are implemented in this work: charging with an estimated output power and charging with an actual output power. The results reveal that the impact of fast-charging stations on voltage fluctuation by either of these charging methods can lead to light flickers. When the estimated output power is utilized, the light flicker is higher compared to when the actual output power is utilized; the proposed mitigation method is to integrate distribution static compensators, which can effectively eliminate the light flickers, whether the output power of the fast-charging station is estimated or actual. Not only will the system’s power quality be improved by installing distribution static compensators, but also the electricity consumers will not be annoyed by the light flicker, even when the rated power of the fast-charging stations is increased; this can positively lead to a reduction in charging time and an increase in customer satisfaction with electric vehicles, lending them to be more widely adopted.
... In [23], MFTP is approximated as a time period within 5 milliseconds. Unregulated flickering VLC transmission could trigger illnesses such as photosensitive epilepsy in sensitive people [24]. The flickering rate of the transmission relies on the modulation scheme used for data transmission. ...
Full-text available
The light-based Internet of things (LIoT) concept defines nodes that exploit light to (a) power up their operation by harvesting light energy and (b) provide full-duplex wireless connectivity. In this paper, we explore the LIoT concept by designing, implementing, and evaluating the communication and energy harvesting performance of a LIoT node. The use of components based on printed electronics (PE) technology is adopted in the implementation, supporting the vision of future fully printed LIoT nodes. In fact, we envision that as PE technology develops, energy-autonomous LIoT nodes will be entirely printed, resulting in cost-efficient, flexible and highly sustainable connectivity solutions that can be attached to the surface of virtually any object. However, the use of PE technology poses additional challenges to the task, as the performance of these components is typically considerably poorer than that of conventional components. In the study, printed photovoltaic cells, printed OLEDs (organic light-emitting diodes) as well as printed displays are used in the node implementation. The dual-mode operation of the proposed LIoT node is demonstrated, and its communication performance in downlink and uplink directions is evaluated. In addition, the energy harvesting system’s behaviour is studied and evaluated under different illumination scenarios and based on the results, a novel self-operating limitation aware algorithm for LIoT nodes is proposed.
... Humans perceive flicker above Critical Fusion Frequency (CFF) up to 500 Hz [75]. This invisible flicker also impacts human health causing headache and eye strain [76]. Moreover, concerns on flicker involve not only static lighting sources, such as public lighting. ...
Full-text available
The Sustainable Development Goals (SDGs) aim at providing a healthier planet for present and future generations. At the most recent SDG summit held in 2019, Member States recognized that the achievements accomplished to date have been insufficient to achieve this mission. This paper presents a comprehensive literature review of 227 documents contextualizing outdoor lighting with SDGs, showing its potential to resolve some existing issues related to the SDG targets. From a list of 17 goals, six SDGs were identified to have relevant synergies with outdoor lighting in smart cities, including SDG 3 (Good health and well-being), SDG 11 (Sustainable cities and communities), SDG 14 (Life below water) and SDG 15 (Life on land). This review also links efficient lighting roles partially with SDG 7 (Affordable and clean energy) and SDG 13 (Climate action) through Target 7.3 and Target 13.2, respectively. This paper identifies outdoor lighting as a vector directly impacting 16 of the 50 targets in the six SDGs involved. Each section in this review discusses the main aspects of outdoor lighting by a human-centric, energy efficiency and environmental impacts. Each aspect addresses the most recent studies contributing to lighting solutions in the literature, helping us to understand the positive and negative impacts of artificial lighting on living beings. In addition, the work summarizes the proposed solutions and results tackling specific topics impacting SDG demands.
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Kurzfassung Im Rahmen einer Pilot-Studie mit 30 Probanden (720 Messungen) soll die subjektive Flicker-Wahrnehmung einer kleinen Lichtquelle in Abhängigkeit von der Modulations-tiefe gemessen werden. Dazu wurde ein Aufbau erstellt, der ermöglicht im Frequenz-Bereich von 1Hz bis ca. 80Hz die Modulationstiefe zu messen, bei der das Flimmern zentral und peripher nicht mehr wahrnehmbar ist. Die Ergebnisse sollen im An-schluss mit vorgeschlagenen Grenzwerten der IEC (st LM) und der IEEE verglichen werden. Verbesserungen für weitere Messungen werden diskutiert.
In this paper, a bicycle light-emitting diode (LED) headlamp is design for the purpose of meeting the regulation and removing the unwanted reflection of some specific parts. In addition to the optical design of the headlamp, a three-dimensional mask is designed to block the LED light source to remove the specific unwanted light on the front tyre of the bicycle and then reduce the glare effect to the rider. The experimental results show that the 3D mask can effectively eliminate 97.7% of the unwanted light on the wheel of the bicycle and keeps 98.6% of the original illuminance at the A point of the K-mark regulation. The proposed mask approach will be helpful to the design by using LED as a light source.
Light emitting diodes (LED) is a promising device in visible light communication (VLC), enabling simultaneous illumination and data transmission via visible light. However, the generation of modulated visible light signals causes variation to the LED intensity in intensity modulation/direct detection (IM/DD) transmission, which may cause flickering — the fast blinking of the LED (when signals are below 200 Hz). Flicker may cause migraines, headaches, and even repetitive behavior among persons with autism. Therefore, flicking mitigation approaches must be applied for VLC with simultaneous illumination scenarios. While the optimization of the LED driving circuit and encoding can be employed to reduce flickering, driving circuit dramatically increase the system cost and communication requirements are usually excluded. As to encoding methods, they will result in reduced data rate and hardware overhead, leading to performance bottlenecks. In this paper, we proposed a signal processing method in the frequency domain to mitigate LED flickering. Low-frequency components (3-70Hz) of modulated signals are extracted by using Fourier transform and inversely restored for transmission. Besides flickering, it is shown that interference caused by other artificial light sources can also be mitigated with the proposed method. Simulation results show that by applying the proposed frequency domain signal processing technique, communication performance of the system is enhanced with the cancellation of interference while flicker is reduced significantly. Under the condition of a 13 dB signal-to-noise (SNR) ratio, the bit error rate can reach the order of 10−3.
Visible light positioning is expected to become an effective means of indoor localization, but most existing methods require the capture of direct light, which is a significant limitation. In this paper, we propose a novel localization method based on received signal strength, which does not need to use direct light signals but instead uses reflected light from the floor. Our method is based on two observations. First, assuming a flat floor, the reflected light from the light source decays according to a gradient model whose peak is just below the light source. Second, the decay can be estimated effectively even for a floor surface several meters away from the light source. Inspired by these observations, we propose a method for estimating the coordinates of the floor surface and the two-dimensional coordinates of the light receiver (a camera). The method uses pattern matching within the distribution of signal decay measurements obtained via photographs of an arbitrary floor surface. Although the proposed method is vulnerable to shadow effects such as those caused by the camera tripod used in our experiments, we achieved a 90th percentile of less than 32 cm in our offline experiments. After removing the tripod shadows from the captured video manually, the same technique achieved a 90th percentile of 22 cm. To investigate the efficiency of the pattern matching, we also conducted experiments on the relationship between pixel utilization and localization results. In this paper, we also discuss camera posture estimation and power consumption issues.
The goal of the research was to prototype a power supply unit for a lighting system for an indoor hydroponic plantation that will allow the future process of plants’ dyes excitation ratio optimization. Plant growth is based on photosynthesis, which converts light, in fact, its narrow absorption spectra, into chemical energy for the synthesis of organic compounds. By considering the usage of Light-Emitting Diodes (LED), which are characterized by low energy consumption, a lighting system for selected light spectra, in specific preferable by plants, was designed. However, considering the optimization process of the dyes’ excitation ratio, it was necessary to design a custom power supply unit to provide viable parameters for the future optimization process. In the presented paper, a set of cheap, commercially available LEDs is proposed to fit the plants’ dyes absorption spectra. And the design process of a dedicated power supply unit for optimization of the dyes’ excitation ratio has been presented and discussed.
Full-text available
Transient clouds cause rapid changes in the power output of Photovoltaic (PV) solar systems. These ramp rates may lead to power quality problems, such as voltage fluctuations, in the low-voltage (LV) electricity grid. This paper firstly assesses the impact of a growing number of distributed PV systems on the voltage profile in a LV grid by considering PV penetration rates of 40%, 70% and 100% of the local rooftop capacity. Next, the potential of active power curtailment, grid reinforcement and supercapacitors to prevent or mitigate these voltage fluctuations are examined. The experiments in this study are based on simulations run with a two-second time resolution for an urban LV grid located in Utrecht, the Netherlands. This study identifies that problematic fluctuations occur already at a 40% PV penetration rate and are expected up to 7.4% of time for a 100% PV penetration scenario. Additionally, the local deployment of either active power curtailment or supercapacitors are identified as adequate strategies to regulate the occurring voltage fluctuations. Finally, the most stable voltage profile and the lowest number of problematic voltage fluctuations are found in case of adopting supercapacitors as part of the PV system.
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The weekly incidence of headaches among office workers was compared when the offices were lit by fluorescent lighting where the fluorescent tubes were operated by (a) a conventional switch-start circuit with choke ballast providing illumination that pulsated with a modulation depth of 43-49% and a principal frequency component at 100 Hz; (b) an electronic start circuit with choke ballast giving illumination with similar characteristics; (c) an electronic ballast driving the lamps at about 32 kHz and reducing the 100 Hz modulation to less than 7%. In a double-blind cross-over design, the average incidence of headaches and eyestrain was more than halved under high-frequency lighting. The incidence was unaffected by the speed with which the tubes ignited. Headaches tended to decrease with the height of the office above the ground and thus with increasing natural light. Office occupants chose to switch on the high-frequency lighting for 30% longer on average.
Full-text available
Two experiments are described in which eye movements were monitored as subjects performed a simple target-spotting task under conditions of intermittent illumination produced by varying the display-screen frame rate on a computer VDU. In Experiment 1, subjects executed a saccade from a fixation point to a target which appeared randomly at a fixed eccentricity of 14 character positions to the left or right. Saccade latency did not differ reliably as a function of screen refresh rate, but average saccade extent at 70 Hz and 110 Hz was reliably shorter than at 90 Hz and 100 Hz. Experiment 2 examined the same task using a range of target eccentricities (7, 14, and 28 character positions to the left and right) and across a wider range of screen refresh rates. The results confirmed the curvilinear relationship obtained in Experiment 1, with average saccade extent reliably shorter at refresh rates of 50 Hz and 125 Hz than at 75 Hz and 100 Hz. While the effect was greater for remote targets, analyses of the proportional target error failed to show a reliable interaction between target eccentricity and display refresh rate. In contrast to Experiment 1, there was a pronounced effect of refresh rate on saccade latency (corrected for time to write the screen frame), with shorter latencies at higher refresh rates. It may be concluded that pulsation at frequencies above fusion disrupts saccade control. However, the curvilinear functional relationship between screen refresh rate and saccade extent obtained in these studies differs from previously reported effects of intermittent illumination on the average size of "entry saccades" (the first saccade to enter a given word) in a task involving word identification (Kennedy & Murray, 1993a, 1996). This conflict of data may arise in part because within-word adjustments in viewing position, which are typical of normal reading, influence measures of average saccade extent.
Full-text available
The effects of fluorescent light spectral composition and flicker rate on visual performance and visual comfort were studied on 48 undergraduate students using two different rates of flicker: conventional low-frequency flicker (120 Hz) and high frequency flicker(between 20-60 kHz); and three different light sources; full-spectrum lamps, cool-white lamps, and filtered-cool-white lamps. The design was a 2×3 (Flicker rate × Light source) mixed within-between ANOVA. Visual performance and time on visual performance task were assessed using a Landolt ring task. Visual comfort was assessed by self-report after a period of reading difficult text. Visual performance scores of 18-24-year-old male and female university students were significantly higher in the high-frequency flicker condition than the low-frequency flicker condition. There were no other statistically significant effects. Health status was unrelated to visual performance. Neurophysiological explanations are discussed. The finding that an energy-efficient means of driving fluorescent lamps also can improve visual performance provides added impetus to adopt this new technology.
It is known that temporal modulation of the source illuminating a scene can affect visual performance. Nevertheless, there is a lack of evidence showing this in a natural setting with a simple, natural task. We devised a simple test of visual function using a pencil and paper visual search technique − identifying low-contrast printed numbers on the sheet of paper. We measured the effect of the temporal modulation of the fluorescent lighting upon visual performance on this task. We constructed two office environments lit with both tubular and compact fluorescent lamps, completely identical except for the lamp ballasting. Office A equipped with electronic ballasts, producing a light emission mainly modulated at 60 kHz having a modulation percentage of 3 percent for 100 Hz component; office B equipped with magnetic ballasts, producing a temporally modulated light which fundamental component—100 Hz frequency—has a modulation percentage of 32 percent).
The temporal modulation of light from halophosphate, triphosphor and multiband fluorescent lamps (controlled by a conventional choke circuit) was measured as a function of wavelength. Within each category, all lamps had similar functions for peak-peak modulation. At the short-wavelength end of the visible spectrum all lamps showed a modulation near 100%. Halophosphate and multiband lamps had a low modulation at the long-wavelength end of the spectrum and gave the lowest overall modulation. Certain deluxe lamps had a modulation greater than 80% throughout the spectrum. The modulation of photopic energy, and energy transduced by the photoreceptors was calculated. Triphosphor lamps gave greater modulation than halophosphate, the lowest modulation being from warm-white halophosphate lamps.
With two electrical analogs of the brightness system it is shown that at a low luminance level (<5 photons) and in the low-frequency region (<2 cps), when under the special visual conditions no attenuation in the system occurs and with a symmetrical luminance variation such as a sinusoidal modulation or a square-wave modulation with a 1:1 on-off ratio, half the crest-to-trough value of the percentage variation at flicker fusion equals the internal threshold value r0; but with an asymmetrical variation such as a square-wave modulation with a 1:3 on-off ratio, it is the crest value of the periodical percentage variation above the mean luminance level which at flicker fusion equals the internal threshold value r0. At high luminance levels an overshoot in the low-frequency region occurs with both forms of square-wave modulation in accordance with the shape of the attenuation characteristic (AC) of the system under the experimental circumstances. At the steep slope of the ACs for the whole luminance range in cone vision, half the crest-to-trough value of the Fourier fundamental percentage variation at the site of the threshold mechanism located anywhere in the system, equals the internal threshold value r0 at flicker fusion.
Office Workers Visual Performance and Temporal Modulation of Fluorescent Lighting Jaen M *, Sandoval J *, Colombo E *, and Troscianko T ** Abstract—It is known that temporal modulation of the source illuminating a scene can affect visual performance. Nevertheless, there is a lack of evidence showing this, in a natural setting with a simple, natural task. We devised a simple test of visual function using a pencil and paper visual search technique – identifying low-contrast printed numbers on the sheet of paper. We measured the effect of the temporal modulation of the fluorescent lighting upon visual performance on this task. We constructed two office environments lit with both tubular and compact fluorescent lamps, completely identical except for the lamp ballasting. Office A equipped with electronic ballasts, producing a light emission mainly modulated at 60 kHz having a modulation percentage of 3 percent for 100 Hz component; office B equipped with magnetic ballasts, producing a temporally modulated light which fundamental component—100 Hz frequency—has a modulation percentage of 32 percent). Fifty students aged 20 to 22 years performed the same simple visual search tasks (errors were not allowed) under two lighting conditions. This permits evaluation of visual performance by analysis of a single dependent variable, the time taken to complete the task. An analysis of variance (considering only the complete data of 39 subjects) shows that the “modulation” factor is statistically significant (F(1,37)�10.21, p �0.00285). At the end of the session, the observers performed a subjective appraisal of the lighting, by means of a simple questionnaire for each lighting condition. A within-subject ANOVA shows that mean values for each factor in both offices are indistinguishable within a statistical significance level of 5 percent. These results confirm the hypothesis that the observers, even though they cannot visually discriminate between the two light modulation conditions, show significantly higher visual performance when performing the task in low modulation condition. Additionally it was suggested that the method would be capable of identifying the more sensitive individuals in the group. * Departamento de Luminotecnia, Luz y Vision, Facultad de Ciencias Exactas y Tecnologıa, Universidad Nacional de Tucuman, Argentina - ** Department of Experimental Psychology, University of Bristol, Bristol, UK. LEUKOS VOL 1 NO 4 APRIL 2005 PAGES 27– 4 6 ©2005 The Illuminating Engineering Society of North America doi: 10.1582/LEUKOS.01.04.002 Keywords—temporal light modulation, high frequency, flicker sensitivity, visual performance, visual search task.
Conference Paper
This paper discusses the difficulties in dimming Edison socket LED lamps directly from residential phase modulated dimmer switches. In order to explain these difficulties PSpice models for the dimmers are proposed that necessarily include diac characteristics to improve accuracy. A method to dim the LEDs from the residential dimmers is discussed.
The visual systems of birds are hypothesized to have higher temporal resolution than those of humans, suggesting that they may be able to perceive the flicker emitted from conventional low-frequency fluorescent lights (LF; 100 Hz in Europe, 120 Hz in the U.S.A.). These lights are commonly used in the housing of captive birds and this may affect both their welfare and performance in experiments. We carried out mate choice experiments on European starlings, Sturnus vulgaris, under both low- and high-frequency fluorescent lights (HF; >30 kHz, at which flicker is imperceptible). Indicators of male condition and size, together with the reflectance spectra and length of the males' throat feathers, were also recorded to ascertain which variables correlated with female preference. Females ranked males consistently under HF, but not LF, lighting, and individual females chose different males under the two lighting types. Under HF lighting, females chose to spend more time with males that had longer throat feathers. The flicker rate of the light clearly affected the choices made by the females, possibly because of nonspecific stress effects or decreased discrimination ability. Our results imply that careful interpretation of mate choice experiments is needed, especially with regard to the lighting types used, to elucidate the real cause behind any variation shown.