The wide range of conditions
over which LCD monitors are used means that it is desirable to produce displays
whose luminance (brightness) can be altered to match both bright and dim
environments. This allows a user to set the screen to a comfortable level of
brightness depending on their working conditions and ambient lighting.
Manufacturers will normally quote a maximum brightness figure in their display
specification, but it is also important to consider the lower range of
adjustments possible from the screen as you would probably never want to use it
at its highest setting. Indeed with specs often ranging up to 500
cd/m2, you will
certainly need to use the screen at something a little less harsh on the eyes.
As a reminder, we test the full range of backlight adjustments and the
corresponding brightness values during each of our reviews. During our
calibration process as well we try to adjust the screen to a setting of 120 cd/m2
which is considered the recommended luminance for an LCD monitor in normal
lighting conditions. This process helps to give you an idea of what adjustments
you need to make to the screen in order to return a luminance which you might
actually want to use day to day.
Changing the display luminance
is achieved by reducing the total light output for both CCFL- and LED-based
backlights. By far the most prevalent technique for dimming the backlight is
called Pulse Width Modulation (PWM), which has been in use for many years in desktop
and laptop displays. However, this technique is not without some issues and the
introduction of displays with high brightness levels and the popularisation of
LED backlights has made the side-effects
of PWM more visible than before, and in some cases may be a source of visible
flicker, eyestrain, eye fatigue, headaches and other associated issues
for people sensitive to it. This article is not intended to alarm, but is
intended to show how PWM works and why it is used, as well as how to test a
display to see its effects more clearly. We will also take a look at the methods
some manufacturers are now adopting to address these concerns and provide
flicker-free backlights instead. As awareness grows, more and more manufacturers
are focusing on eye health with their monitor ranges.
What is PWM?
Pulse Width Modulation (PWM) is
one method of reducing the perceived luminance in displays, which it achieves by
cycling the backlight on and off very rapidly, at a frequency you can't
necessary detect with the naked eye, but which could lead to eye issues,
headaches etc. This method generally means that at 100%
brightness a constant voltage is applied to the backlight and it is continuously
lit. As you lower the brightness control the perceived luminance for the user
reduces due to a number of possible controlling factors:
1) Frequency - The backlight is cycled on and off very
rapidly, and this cycling typically occurs at a
fixed frequency (in Hz). How fast this cycling occurs can impact whether flicker
is visible or perceivable to the user, with higher frequencies being potentially
less problematic. PWM has been known to operate at low frequencies of 180 -
240Hz for example which are likely to be more problematic than higher
frequencies ranging up in to the Kilohertz range (e.g. 18,000Hz).
2) Modulation - The modulation of the cycling has an impact on
the perceived brightness, and this describes the difference between the
luminance in an "on" and in an "off" state. In some examples the backlight is
completely turned off during the cycle so it is literally being turned on/off
rapidly across the full brightness adjustment range. In those examples the
luminance output is controlled really by the duty cycle only (see point 3). In
other examples the backlight is not always being completely turned off but
rather the voltage applied to the backlight is being rapidly alternated,
resulting in less extreme differences between the on and off states. Often this
modulation will be narrow in the high brightness range of the display, but as
you reduce further, the modulation becomes wider until it reaches a point where
the backlight is being switched completely off. From there, the change in the
duty cycle (point 3) controls the further changes in the luminance output.
3) Duty Cycle - The fraction of each cycle for which the backlight is
in an "on" state is
called the duty cycle. By altering this duty cycle the total light output
of the backlight can be changed. As you reduce the brightness to reach a lower
luminance, the duty cycle becomes progressively shorter, and the time for which
the backlight is on becomes shorter, while the time for which it is off is
longer. This technique works visually since cycling the backlight on
and off sufficiently fast means the user cannot see this flickering, because it
lies above their flicker-fusion threshold (more on this later).
90% duty cycle
50% duty cycle
10% duty cycle
Above we can see graphs of a backlight's output
using "ideal" PWM for several cycles. The maximum output of this backlight in
the example is 100
and the perceived luminance for the 90%, 50% and 10% cases are: 90, 50 and 10
respectively. The modulation percentage is the ratio between the minimum and
maximum luminance during the cycle, and is 100% here, so it is being completely
turned on and off. Note that during the duty
cycle the backlight is at its maximum luminance.
The analogue (non-PWM) graphs
corresponding to these perceived luminance levels would appear as shown below.
In this case there is no modulation. This is the method used for flicker-free
backlights which we will discuss more a little later.
Why PWM is Used
The main reasons for the use of
PWM is that it is simple to implement, requiring only that the backlight can be
switched on and off rapidly, and also gives a large range of possible luminance.
CCFL backlights can be dimmed
by reducing the current through the bulb, but only by about a factor of 2
because of their strict current and voltage requirements. This leaves PWM as the
only simple method of achieving a large range of luminance. A CCFL bulb is in
fact normally driven by the inverter to cycle on and off at a rate in the 10's
of kilohertz and well outside the range of flicker visible to humans. However,
the PWM cycling typically occurs at a much lower frequency, around 175Hz, which
can produce artefacts visible to humans.
The luminance of LED backlights
can be adjusted greatly by altering the current passing through them, though
this has the effect of altering the colour temperature slightly. This analogue
approach to LED luminance is also undesirable since the accompanying circuits
must take into account the heat generated by the LED's. LED's heat up when on,
which reduces their resistance and further increases the current flowing through
them. This can quickly lead to runaway current use in very high-brightness LED's
and cause them to burn out. Using PWM the current can be forced to hold a
constant value during the duty cycle, meaning the colour temperature is always
the same and current overloads are not a problem.
Side Effects of
While PWM is attractive to
hardware makers for the reasons outlined above, it can also introduce
distracting visual effects if not used carefully. In order to understand what is
being seen we need to look at the flicker in real displays. Shown below is a
video of a CCFL backlight cycling 40x slower than normal, so that the flickering
can be seen more clearly. A plot of the luminance of RGB components over a
single cycle is shown just below it. This particular display is set to its
minimum brightness, where the flicker should be most pronounced. As seen in both
the video and accompanying plot, the overall luminance varies by about a factor
of 4 during each cycle. Interestingly, the colour of the backlight also varies
significantly during the course of each cycle as well. This is most likely due
to phosphors in the CCFL that have different response times, in which case we
can infer that the phosphor involved in producing blue can switch both on and
off faster than the other colours. The use of phosphors also means the backlight
will continue to emit light for a few milliseconds after the backlight power is
switched off at the end of a duty cycle, and gives a more continuous level of
light (lower modulation) than would otherwise be present. Note that the averaged
colour over time remains neutral.
Flicker from LED backlights is typically much more visible than for CCFL
backlights at the same duty cycle because the LED's are able to switch on and
off much faster, and do not continue to "glow" after the power is cut off. This
means that where the CCFL backlight showed rather smooth luminance variation,
the LED version shows sharper transitions between on and off states. This is why
more recently the subject of PWM has cropped up online and in reviews, since
more and more displays are moving to W-LED backlighting units now. As seen
below, there is no significant change in backlight colour during cycling.
Where the effect of flicker can really come into
play is any time the user's eyes are moving. Under constant illumination with no
flickering (e.g. sunlight) the image is smoothly blurred and is how we normally
perceive motion. However, when combined with a light source using PWM several
discrete afterimages of the screen may be perceived simultaneously and reduce
readability and the ability of the eyes to lock onto objects. From the earlier
analysis of the CCFL backlighting we know that false colour may be introduced as
well, even when the original image is monochromatic. Below are shown examples of
how text might appear while the eyes are moving horizontally under different
It is important to remember that this is entirely due to the backlight, and the
display itself is showing a static image. Often it is said that humans cannot
see more than 24 frames per second (fps), which is not true and actually
corresponds to the approximate frame rate needed to perceive continuous motion.
In fact, while the eyes are moving (such as when reading) it is possible to see
the effects of flicker at several hundred hertz. The ability to observe flicker
varies greatly between individuals, and even depends on where a user is looking
since peripheral vision is most sensitive.
So how fast is PWM cycling backlights on and off? This seems to depend on the
backlight type used, with CCFL-based backlights nearly all cycling at 175Hz or
175 times per second. LED backlights have been reported typically running from
180 - 420Hz, with those at the
lower end flickering much more visibly. Some have even faster frequencies of
>2000Hz so it really can vary. While this might seem too fast to be
visible, keep in mind that 175Hz is not much faster than the 100-120Hz flicker
observed in lights connected directly to the mains power.
100-120Hz flickering of
fluorescent lights has in fact been linked to symptoms such as severe eye strain
and headaches in a portion of the population, which is why high-frequency
ballast circuits were developed that provide almost continuous output. Using PWM
at low frequencies negates the advantages of using these better ballasts in
backlights because it turns an almost constant light source back into one that
flickers. An additional consideration is that poor quality or defective ballasts
in fluorescent backlights can produce audible noise. In many cases this is
exacerbated when PWM is introduced since the electronics are now dealing with an
additional frequency at which power usage is changing.
It is also important to
distinguish the difference between flicker in CRT displays and CCFL and LED
backlit TFT displays. While a CRT may flicker as low as 60Hz, only a small strip
is illuminated at any time as the electron gun scans from top to bottom. With
CCFL and LED backlit TFT displays the entire screen surface illuminates at once,
meaning much more light is emitted over a short time. This can be more
distracting than in CRTs in some cases, especially if short duty cycles are
The flicker itself in display
backlights may be subtle and not easily perceptible for some people, but the
natural variation in human vision seems to make it clearly visible to others.
With the use of high-brightness LED's on the rise it is becoming increasingly
necessary to use short PWM duty cycles to control brightness, making flicker
more of a problem. With users spending many hours every day looking at their
monitors, shouldn't we consider the long term effects of both perceptible and
Reducing the Effects
of PWM and Flicker Free Backlights
If you find PWM backlight
flickering distracting or just want to see if reducing it makes reading on a
monitor easier, I'd encourage you to try the following: Turn the brightness of
your monitor up to maximum and disable any automatic brightness adjustments. Now
use the colour correction available in your video card drivers or calibration
device to reduce the brightness to normal levels (usually by adjusting the
contrast slider). This will reduce the luminance and contrast of your monitor
while leaving the backlight on as much as possible during PWM cycles. While not
a long-term solution for most due to the decreased contrast, this technique can
help to discover if a reduction in PWM usage is helpful.
A much better method of course
would be to purchase a display not relying on PWM for dimming, or at least one
which uses a much higher cycling frequency. Few manufacturers seem to have
implemented PWM at frequencies that would limit visible artefacts (well above 500Hz for CCFL and above 2000 Hz for LED). Additionally, some displays
using PWM do not use a 100% duty cycle even at full brightness, meaning they
will always produce flicker. Several LED-based displays may in fact be currently
available which do not use PWM, but until backlight frequency and modulation
become listed in specifications it will be necessary to see the display in
person. Some manufacturers promote "flicker free" monitors in their range (BenQ,
for example) which are designed to not use PWM at all and instead use a Direct
Current (DC) method of backlight dimming. Other manufacturers such as Eizo talk about
flicker free backlights but also list a hybrid solution for their backlight
dimming, where PWM is used for some of the brightness adjustment
range at the lower end. In fact it seems an increasingly common practice for a screen to be PWM
free down to a certain point, and then fro PWM to be used to really drive down
the minimum luminance from there.
An easy method of measuring the
PWM frequency of a backlight would be ideal, and luckily it can be done using
only a camera which allows manual control of the shutter speed. This can quickly
and easily identify PWM frequencies in the lower range, but may not be suitable
for high frequency PWM. It should be able to detect PWM up to at least 500Hz
though, but anything above that may look like a solid block, suggesting no use
of PWM, when in fact it might be just using a higher frequency. Further more
complex methods such as our
oscilloscope setup would be needed to validate flicker-free status for
Set the monitor to the desired settings for
(Optional) Set the camera white balance by
getting a reading off the screen while displaying only white. If not possible,
then manually set the white balance to about 6000K.
Display a single vertical thin white line on a
black background on the monitor (1-3 pixels wide should be fine). The image
should be the only thing visible.
Here is an
example you may wish to save and use, show it full screen on your monitor.
Set the camera to use a shutter speed of 1/2 to
1/25 of a second. You may need to set the ISO sensitivity and aperture in
order to capture enough light. Make sure the line is in focus at the distance
you are holding it (lock the focus if needed).
Hold the camera about 2 feet in front of the
monitor and perpendicular to (looking straight at) the front. Press the
shutter button as you slowly move it horizontally across the screen (remaining
perpendicular). You may need to experiment with moving the camera at different
Adjust the captured image brightness so that the
pattern is easily visible.
Count the number of cycles visible in the
Multiply this count by the inverse of the
shutter speed. For example, if using a shutter speed of 1/25 of a second and 7
cycles are counted, then the number of cycles per second is 25 * 7 = 175Hz.
This is the backlight cycle frequency.
What we are doing with this
technique is turning a temporal effect into a spatial one by moving the camera
during capture. The only significant source of light during the image capture is
the thin line on the display, which is exposed onto consecutive columns on the
sensor. If the backlight is flickering, different columns will have different
brightness or colour values determined by the backlight at the time it was
A common problem when first
attempting this technique is that the image is too dark. This can be mitigated
by using a larger camera aperture (lower f/number) or increasing the ISO value.
The shutter speed is not a factor in the exposure since we are using it only to
control the total exposure time. The brightness of the image can also be
adjusted by changing the speed at which the camera is moved, with a fast speed
giving a darker image and more temporal resolution and a slow speed a brighter
image with lower resolution. Another problem encountered is unevenly-spaced
cycles in the final image, which is caused by the camera changing speed during
exposure. Continuing to move the camera before and after the exposure helps to
steady this. An image which looks particularly smooth may be due to it being out
of focus. This can sometimes be helped by pressing the shutter button halfway to
focus on the line target, then proceeding as normal.
Image too dark
Increase exposure after capture. Use larger lens aperture. Move camera more
Move camera at constant speed. Try to keep camera moving before and after
Image out of focus
Lock focus. Pre-focus by pressing shutter halfway. Ensure camera is
perpendicular to screen.
Depending on the monitor several additional effects may be visible. CCFL-based
backlights often show different colours at the start and end of each cycle,
which means the phosphors used respond at different rates. LED-based backlights
often use a higher cycling frequency than CCFL-based, and more rapid camera
movement may be needed to easily see them. Dark stripes between cycles mean that
the PWM duty cycle has been reduced to such an extent that no light is emitted
for part of the cycles.
The following are examples of using this
Brightness = 100
Brightness = 50
Brightness = 0
1/25 sec exposure we can clearly see 7 cycles, so the backlight uses a
175Hz cycling frequency. There is a small amount of flicker present even
at full brightness, though it is likely small enough to be unnoticeable. A
minor amount of flicker is introduced at half brightness, and by the time
the minimum brightness is reached there is significantly more flicker
along with colour shifting.
Brightness = 100
Brightness = 50
Brightness = 0
There is no
flicker visible at full brightness. Flicker and colour shifting has become
visible at half brightness. Stronger flicker and significant colour
shifting is present at minimum brightness. About 8 cycles are visible in
this 1/25 sec exposure, giving a frequency estimate of 200Hz. A longer
exposure measured it as exactly 210Hz.
LN40B550 Television (CCFL)
Brightness = Max
Brightness = Min
brightness adjustment cannot be turned off, so only the easily achievable
minimum and maximum levels are shown. No flicker is visible at full
brightness. At minimum brightness strong flicker and colour shifting are
present, where the colour shifting separates into yellow and blue
components. Only 6 cycles are visible in this 1/25 sec exposure, meaning
the backlight runs at 150Hz.
Brightness = 100
Brightness = 50
Brightness = 0
or colour shift is visible at any brightness using 1/25 sec exposures.
This display does not use PWM. Striations are from noise in the captured
MacBook Pro (LED)
Brightness = 100
Brightness = 50
Brightness = 0
flicker is visible at full brightness using a 1/25 sec exposure. A very
short duty cycle is used at 50 and 0 brightness, giving strong flickering.
A higher cycle frequency of 420 Hz is used with this LED backlight, but is
still far too low to eliminate flickering effects. There is no visible
colour shifting during cycles.
Advanced Oscilloscope Tests
Using our oscilloscope and photosensor equipment
it is possible to measure the PWM frequency and patterns far more accurately.
While the above photo method is certainly suitable for a casual user, an
oscilloscope can reveal more detail about the PWM operation and will be
featured in all our reviews moving forward. We measure the luminance output of
the screen at brightness settings of 100, 50 and 0%. This allows us to easily
identify the backlight dimming technique, and if PWM is being used we can work
out its frequency and comment on modulation, duty cycle etc.
Asus PA248Q - W-LED
backlight. At 100% brightness we see a constant luminance output and a
straight line, as there is no need for the backlight to be cycled. At 50%
you can see PWM controls the backlight on and off. The modulation is always
100%, but the luminance reduction is controlled by the duty cycle which
becomes progressively shorter. You can see much shorter "on" peaks in the 0%
brightness graphs. We measure the frequency at 180Hz which is fairly
BenQ GW2760HS -
W-LED backlight. At all brightness settings the luminance output is a flat
line, showing no PWM is being used. This is part of BenQ's flicker free
The oscilloscope graphs can also allow us to
examine the behaviour of the luminance output. Above is a typical W-LED
backlight dimmed to 0% where PWM is used. You can see the changes between on
and off are very steep and sudden, as the LED backlight is able to turn on and
off very rapidly. As we've already discussed this can lead to potentially more
noticeable flicker and associated issues as the changes are more pronounced.
The oscillographs for a typical CCFL display using
PWM at 0% looks like the above. You can see the transitions from on to off are
less sudden as the phosphors don't go dark as quickly as with LED backlight
units. As a result, the use of PWM may be less problematic to users.
As we said at the beginning,
this article is not designed to scare people away from modern LCD displays,
rather to help inform people of this potential issue. With the growing
popularity in W-LED backlit monitors it does seem to be causing more user
complaints than older displays, and this is related to the PWM technique used
and ultimately the type of backlight selected. Of course the problems which can
potentially be caused by the use of PWM are not seen by everyone, and in fact I
expect there are far more people who would never notice any of the symptoms than
there are people who do. For those who do suffer from side effects including
headaches and eye strain there is an explanation at least.
With the long term and proven success of a technology like Pulse Width
Modulation, and the many years of use in CCFL displays we can't see it being
widely changed at any time soon to be honest, even with the popular move to
W-LED backlit units. It is still a reliable method for controlling the backlight
intensity and therefore offering a range of brightness adjustments which every
user would want and need. Those who are concerned about its side effects or who
have had problems with previous displays should try and consider the frequency
of the PWM in their new display, or perhaps even try and find a screen where it
is not used at all in backlight dimming. Some manufacturers are proactively
addressing this concern through the use of flicker free backlights, and so
options are emerging which do not use PWM.