SUBLIMINAL FLICKER
Part I:
Computer screens, TV's and Flicker Sensitivity
by Richard Conrad, Ph.D. Revised 05/04/09
Subliminal: below the threshold of conscious
perception; inadequate to produce
conscious awareness but able to evoke a response.
The light emitted
from computer screens and TV's is not steady, but has flicker.
This is true for all monitors, of all types. The flicker is usually invisible, at least to the conscious
mind. Flicker is invisible when it
consists of pulses or waves of light that repeat one after the other so rapidly
that they appear to fuse together into steady light. Our flicker fusion frequency (the frequency above which we no
longer consciously see flicker) ranges from about 25 to 55 Hz (Hz means times
per second). Flicker fusion frequency
varies with the person, with the intensity and color of the light, and also
depends on where the light falls on the retina. Optic nerve signals proportional to flicker at frequencies far
above the conscious flicker fusion frequency do reach our brain from the eye
(as shown by EEG and other studies). Any invisibly flickering light that affects
the brain is what I call subliminal flicker.
Subliminal flicker from computer and TV screens is at a
particular frequency or frequencies. It
is analogous to a tone, for example a loud hum or a dial tone, that goes on and
on incessantly in your ear and very quickly causes irritation. Subliminal flicker can have effects on the
brain and body in a similar manner. A
significant percentage of people who have chemical sensitivities are also
sensitive to subliminal flicker, sometimes severely so. The
symptoms caused by flicker can include any of the following: a feeling of being unable to focus on the
screen, disorientation, confusion, attention deficit/brain fog, irritability,
headache, migraine, eye or neck pain, dizziness, queasiness, or an
uncomfortable feeling down through the chest.
An extreme sensitivity to subliminal flicker is probably due to prior neurological
damage.
In some cases subliminal flicker affects us and produces
its symptoms by inducing partial complex seizures in a particular area of the
brain. Sometimes sensitization to
particular flicker frequencies develops over time. The degree of effect of subliminal flicker depends on many
factors, which include:
1. the frequency of the optical flicker - slow flicker/low refresh rate is
worse for most people; in computers the
flicker frequency is set by the vertical refresh rate of the screen (the higher
the refresh rate the better), and in TV's depends on whether interlaced
("i") or progressive ("p") scan is used - "i" has
more low frequency flicker than "p", so "p" is best
(computers use the equivalent of "p", see the paragraph on "TV
flicker" near the end of this article);
2. the brightness of the light coming from the
screen (brighter is worse);
3. the angle that the edges of the screen
makes with the eye, which is a function of the ratio of the size of the screen
to the distance between eye and screen (the periphery of the retina is more
sensitive to flicker than the center of the retina), this means that the larger
the screen and/or the closer you are to it, the worse the effect of the
flicker;
4. the percent
modulation of the flicker (the “depth” of the waves); and
5. the color of
the light (a complex subject, use trial and error).
The screen can appear steady, and yet while looking at it,
neurological symptoms develop either immediately or after watching it for a
period of time. Sound familiar? Instead of being due to the wrong
eyeglasses, or to the screen glare or poor posture that most ophthalmologists
and optometrists attribute computer screen symptoms to, these symptoms are
usually due to subliminal flicker (or for some people, due to any combination
of subliminal flicker, EMF from the computer or monitor, and chemical
offgassing from the monitor). A
sensitivity to fluorescent room lighting (see Subliminal Flicker Part II) makes
it likely that you also have some sensitivity to subliminal flicker from your
monitor.
A method to
determine whether symptoms are due to subliminal flicker, to EMF or to
offgassing is as follows. First, with your computer
screen on (and set so that it does not go into a dark/sleep mode) sit at your
normal working position with a dark towel placed over the face of the screen so
that you cannot see any light from it.
Be sure not to obstruct any ventilation openings. You could pass the time by reading a book or
talking to a friend (but not on the phone because this could introduce another
EMF variable). If the towel eliminates
the symptoms, then you know that they were due to subliminal flicker. If it does not, the next step would be to
either wear a good activated carbon mask, or to exhaust from the room all
offgassing vapors that could possibly be coming off the monitor. If that does not cure the problem, the
culprit is most likely a sensitivity to EMF from the monitor, computer, and/or
keyboard and mouse. To determine the
worst source of EMF, the next step could be to remain sitting at your
workstation with the computer on (and not in sleep mode) but with the monitor
off. Next, with the computer and
monitor on, sit in the same position but with the keyboard and mouse pushed
away from you, etc. Be sure to change
only one variable for each experiment.
The old fashioned
cathode ray tube (CRT) type of monitor has much more subliminal flicker and potentially more EMF
and offgassing than the new liquid crystal display (LCD) monitors, but one can
adjust the frequency of the CRT flicker to be so fast that it has less effect
on the brain. In your system software
settings, set the vertical refresh rate (of the signal that the computer sends
to the CRT monitor) to as high a frequency as it will allow. (The video graphics card and the resolution
setting (and the particular CRT monitor) will limit the maximum vertical
refresh rate that can be set - the lower the resolution setting, the higher the
allowed vertical refresh rate.) The
minimum for long-term comfort for normal people is between 72 and 85 Hz. A flicker sensitive person may have to set
the frequency even higher. Sometimes it
is necessary to buy a faster video graphics card for the computer and/or a
faster CRT monitor in order to accomplish this.
In the case of a CRT monitor the flicker has approximately
90% modulation (i.e., the light intensity is 9 times brighter at its highs than
at its lows). It is like a strobe light
flashing in your face. Even though the
screen may be comfortable at for example a 90 Hz refresh rate, attempts to rapidly
proofread text on such a screen will sometimes result in missing typographical
errors. This is because the screen will
be almost dark between its flashes of light, and as the eye scans quickly
across a line of text, a character or word at the location that the eye is
scanning past in a dark moment can be missed.
Thus it is more accurate to proofread from a printout than from a CRT
screen. This is yet another reason to
raise the vertical refresh rate of a CRT monitor as high as possible. For more discussion of minimizing flicker
from CRTs, go to http://www.displaymate.com/flicker.html.
LCD monitors are a
whole different story. The propaganda on the street
says that LCDs (Liquid Crystal Displays) have no flicker and that the light
from a LCD screen could never bother anyone.
Wrong! It is true that the
percent modulation of the flicker of LCDs is on the order of only about 1%, as
compared to 90% in a CRT, but for a flicker sensitive person, even 1% can be
devastating if it is at a frequency of less than 85 Hz. The vertical refresh rate and therefore the
flicker frequency of most LCDs is fixed at the rather low frequency of 60
Hz. This is true whether the LCD is in
a stand-alone monitor, a laptop or a projector. The native 60 Hz frequency does not change even when the monitor
is fed input signals from the computer that have higher refresh rates. The
specifications given for these monitors are misleading because they list
refresh rates accepted, not displayed.
Even worse, some LCDs also have a significant amount of 30
Hz flicker due to the way the pixels are refreshed (see footnote). Additionally, in many LCDs that are 15”
diagonal and less, “dithering” is used to generate the illusion of more colors
while using fewer signal channels.
Dithering is a method of alternating back and forth between two colors,
for example a light and a dark green, to produce the impression of a medium
green. Dithering introduces additional
low frequency flicker.
If a particular LCD monitor that is 17” or larger does not
solve your problem, try other brands - it is a crap shoot (until someone at
Consumer Reports becomes flicker sensitive and starts to report comparative
measurements taken with a photodetector and a spectrum analyzer). You may do better with a CRT monitor running
at a very high refresh rate, provided that you can tolerate or adequately
shield its EMF and radio frequency (RF) emissions. (Although LCDs usually have less low frequency EMF than CRTs,
they can have as much or more RF emissions.)
Don’t waste money on "EMF neutralizers" or "shielding
diodes" etc. Based on my reading
the marketing literature of these devices, and on my background in the workings
of EMF, my opinion is that the only way they could work is via a placebo
effect. See my article "EMF
Scams".
Make sure that you are not experiencing image jitter in
addition to flicker. Jitter can occur
from improper synch settings, or from using a video cable between the computer
and the monitor that is either faulty, not well shielded, or is too long. DVI cables transmit a more stable video
signal than do VGA cables, especially over longer distances, but they radiate
more EMF than a VGA cable does.
To reduce the
effects of screen flicker: one can sit further away from the
screen and/or experiment with the color of the text, background and buttons,
etc. Sometimes increasing the room
brightness can help. In the case of CRT
monitors, set the vertical refresh rate as high as possible. In some CRTs the flicker in the blue has a
greater % modulation than the other colors, and so sometimes decreasing the
amount of blue on a CRT screen can
help. Decreasing the screen brightness
always helps to reduce the effects of flicker, but unfortunately in most LCD
monitors any setting of brightness below the maximum level introduces
considerably more EMF emissions because of the chopping mechanism that is used
to dim the backlight. Therefore better
methods of reducing the brightness of a LCD screen would be to wear sunglasses,
or to place a transparent anti-glare/E-field shielding filter in front of the
screen. (No filter can reduce the
percent modulation of the flicker). The
herb bilberry and for the author also
the supplement carnosine taken before
being exposed to flicker can reduce sensitivity to flicker significantly. (Carnosine is a dipeptide found naturally in
the body; the one available from
www.integratedhealth.com is the best I have found.)
You are probably wondering if the new LCDs that are
backlit by LEDs (Light Emitting Diodes) have less subliminal flicker and/or
less EMF than the fluorescent tube backlit type. The answer is no. The
flicker rate is determined by the vertical refresh rate of the pixels, and is not
caused by the backlight. Concerning
EMF, the LED backlight is usually operated in a pulsed mode by a very
electrically noisy/EMF-producing power supply similar to the one that runs
fluorescent backlights. Also, the EMFs
in LCDs come not only from the backlight power supply, but also from the main
power supply, the electronics circuit board, and from the front of the screen
itself due to the high frequency of the addressing and refreshing of the
individual liquid crystal pixels. An
EMF filter placed in front of an LCD screen can only reduce the emitted fields
slightly because the EMFs leak through and around the filter plate. A true very low EMF monitor is very
difficult and expensive to achieve.
Greater distance from the screen reduces EMF exposure. (Note that the plasma type of screen has
been reported to have extremely high EMF emissions.)
Monitors that have a
vertical refresh rate of 120 Hz and even 240 Hz, a welcome recent development: Flicker at 120 Hz is so fast that it has little effect on the
brain. Mitsubishi, NEC and Sharp (and
probably others) now have new LCD TV's that have a vertical refresh rate of 120
Hz, and may also be usable as computer monitors. Samsung has 61" and 67" DLP TV's that refresh at 120
Hz. Note that a refresh rate of 120 Hz
is not always a guarantee that there is no flicker at lower frequencies, since
electronic engineers sometimes use various tricks to adjust brightness or color
that introduce lower frequencies. But
120 Hz is a good start. They are using
120 Hz because it enables faster gaming and/or 60 Hz stereovision (don't use
the stereo feature, or you will be running at 60 Hz per eye). Some of the latest HDTV's refresh at 240
Hz! Make sure you have a 30 day money
back guarantee so you can try out your choice.
And watch out for the EMF emissions of these large monitors.
A data projector can be employed as a monitor by
projecting from a distance onto a rear projection screen. This removes the EMF source much further
away from the user, but most projectors have far more intense EMF emissions
than does a direct view LCD monitor due to their high-wattage lamp
switching-type power supplies. LCD
projectors have the same flicker problems as direct view LCD monitors. DLP type projectors, with the possible
exception of the very highest speed color wheel types, are not suitable because
they have a large amount of subliminal flicker and various types of flicker
motion artifacts. (For some persons DLP
may be satisfactory for TV use, but not for text, data or CAD use). A very few (not all) of the LCOS projectors
flicker only at 120 Hz. These can
potentially solve flicker, EMF, and offgassing problems for some people, but
such setups are custom and expensive. I
have designed and am using as my monitor a special LCD projector with low EMF
electronics. It utilizes a dither-free
LCD projection panel that I drive at an actual displayed vertical refresh rate
of 85 Hz (in the case of the LCD type display, 85 Hz is a high enough frequency
to avoid the effects of subliminal flicker on the brain). I may soon be able to build this projector
for others on a custom basis.
TV flicker: TV video displayed on a TV or computer screen usually has
a frame rate of 30 Hz. The TV video
data can be displayed via either an interlaced ("i") or progressive
("p") scan. On older TV's the
scan is a fixed 480i. This interlaced
scan is a combination of 30 Hz and 60 Hz vertical refresh rates; a summed vertical resolution of 480
horizontal lines, displayed as sequential sets of the odd field (240 odd lines)
alternating/interlaced with the even field (240 even lines). The total field rate is 60Hz, and the frame
rate is 30 Hz (a frame, or complete image, is made up of an odd field followed
by an even field). Newer TV's and DVD
players provide the option of a progressive scan setting, for example 480p,
where the odd and even field data are both stored for a short time and then
displayed as integrated into a single frame, with a display rate of 60 Hz
achieved by displaying each frame of data twice in a row.
Progressive scan TV has less flicker
than interlaced, and thus 480p has less flicker than 480i. Higher resolution settings are often also
available, including 720p, 1080i, and 1080p.
They provide high definition (HD) TV with clearer picture and more
visual detail. The best is 1080p. In the case of 1080i, although it interlaces
odd and even fields, the lines are so close together that the subliminal
flicker is often not effectively worse than progressive scan. Motion artifacts (due to fast motion across
the screen such as a football player running, etc.) are another problem, which is best solved by the TV's that have
the newest and best processors.
DLP-type TV's tend to have more flicker and motion artifacts, but the
newest ones that refresh at 120 Hz might be satisfactory. LCD TV's and LCD TV projectors that refresh
at 120 or 240 Hz and run at 1080p generally have the least flicker and are the
best visually (EMF can still be a big problem). For additional methods of reducing TV flicker, follow the
principles outlined in the paragraph above that begins with: “To reduce the
effects of screen flicker”.
FOOTNOTE: In most current LCD panels, each pixel is refreshed 60
times a second. In other words, each
pixel is hit by a very brief voltage update 60 times a second. (Some of the newest LCD TV's refresh 120 or
240 times per second.) The brightness
of a particular pixel is proportional to the voltage across it at the moment,
independent of the polarity of this voltage.
The voltage that the pixel feels at any one moment is the voltage output
of its capacitor which stores the voltage between updates. Capacitors leak over time, and in
one-sixtieth of a second this leakage causes a droop in the voltage of between
1/2% and 1%. This is the cause of the
60 cycle flicker of LCD panels.
Furthermore, the updates are not all of the same polarity; if they were, the liquid crystals would
begin to pile up against one side of the pixel cell and the image would
stick. Alternate updates are of
alternate polarities, that is, one update is a positive voltage, the next is a
negative voltage, the next is positive, etc.
If the input image data does not change, the update voltages should all
be of the same voltage magnitude/level no matter whether they are positive or
negative. But even for non-changing
image input data, alternate polarity refreshes are often not of exactly the
same voltage levels. This is due to
imbalances in the driving electronics, including the fact that the resistances
of the transparent conductive surfaces on each side of the pixel cells are not
homogeneous. This alternating imbalance
results in an unpredictable amount of 30 Hz flicker.