Modulation for perception (%)
LEDsmagazine.com APRIL 2017 55
light sources | FLICKER & AC DRIVE
Human vision is adversely affected by light fluctuations at frequencies up to 200 Hz, even though people can
only directly perceive fluctuations at frequencies up to about 70 Hz. The fundamentals of the sensitivity of the human eye to
rapidly changing light (transient light artifacts or TLAs) as a function of frequency have
been well known to science for a decade or
more. Despite this, the lighting industry has
so far limited itself to only characterizing
light sources over the range of frequencies
which the human eye can perceive directly.
This range is below 100 Hz. However, it is well
documented that human visual performance
is degraded by the presence of light fluctuations at frequencies in the range from 100 to
200 Hz. Here we will describe a new flicker
metric/tool that includes consideration of
higher frequencies and further discuss an
AC-LED light engine relative to performance
against the new metric.
Specifically, we will discuss a software
tool called CFD (compact flicker degree).
Mathematician Peter Erwin (co-author of
this article) first presented CFD at the LED
Professional Symposium (LPS) 2016 confer-
ence in Bregenz, Austria in September. The
CFD tool can characterize any light source
for its flicker acceptability, based on well-
known and published characterizations of
the human eye/brain interaction as a func-
tion of frequencies extended out to 2000 Hz.
This tool has met with widespread accep-
tance in Europe because of its consistency
with everyday common-sense observations,
and in particular has been adopted by the
German laboratory TÜV Süd as its criteria
for characterizing time variant light quality.
Indeed, we will chronicle the history and
mathematical basis of the new CFD measure
for flicker. Examples of the use of this tool
in application to AC-LED light engines, fluorescent lamps, and incandescent lamps will
then be detailed. Using this new tool, it can
be seen that some driverless AC-LED light
engines that use higher frequencies to control the light output have light quality for the
human eye which is comparable
to an incandescent light bulb.
Flicker research history
In recent years, there have
been several attempts to create a judgment standard for the
acceptability of light containing
TLAs. Examples of light containing TLAs are LED lights with
dimming, AC-LED light engines
with 120-Hz modulation, and
especially LED filament lamps.
The basic physiological data for
the sensitivity of the human eye
to light variations at a given frequency has been well known for many years.
For example, see the seminal work of Kelly
in 19601 that was funded by the Technicolor
company. Similar studies were reported as
part of a doctoral thesis by Farhang Ghasemi
Afshar in 20082. In 2016, Pierre Beeckman of
Philips provided the sensitivity versus frequency curves shown in Fig. 1, which he has
presented at various meetings including
Strategies in Light 2016.
In reviewing this data, the reader needs
to be aware that a great deal of statistical
variation is involved. For example, younger
subjects are generally likely to perceive low-
er-level and higher-frequency TLAs than older
observers can see. Ten different observers
will, in detail, produce ten different sensitiv-
ities for a given frequency data point. For this
reason, a commonplace procedure is to derive
a signal level that 50% of observers can dis-
cern. To add to the complication, the central
field of view of the human eye (the macula)
is less sensitive to TLAs than the outer parts
of the retina, which contribute to so-called
peripheral vision. For this reason, studies
sometimes show two response curves, one for
central vision and one for peripheral vision. In
this article, we shall by default mean central
vision when we refer to perception.
It is also important to realize that the
response of the human eye to higher frequencies can be augmented by stroboscopic
effects. A rapidly moving object — for example, a pencil, a wand, or a spinning disk with
radial markings on it — will show a series of
images in the presence of flicker at a frequency
that is too high for the human eye to observe
Understand a new flicker metric and its
application to AC-LED light engines
PETER ERWIN and PETER SHACKLE describe a new metric for characterizing the flicker of light sources and
illustrate how some popular sources, including AC-LED light engines, perform based on that metric.
PE TER ERWIN is the founder of Der Lichtpeter ( https://www.derlichtpeter.de/) and works as a
lecturer at Baden-Württemberg Cooperative State University. PETER W. SHACKLE is an inventor
with 60 issued US patents to his name and through Photalume ( http://photalume.com) works as a
consultant on LED circuits.
FIG. 1. Pierre Beeckman of Philips documented
flicker sensitivity relative to frequency at Strategies
in Light 2016.