Ken, Barb, and Janet,

The effects of fluctuating light you mentioned make a lot of sense,
especially when the light comes through a rotating fan (the big kind) or
through tree leaves as you drive by.   The light intensity fluctuations can
be bothersome.  Also, movies were called 'the flicks' for good reason.
 They used to disturb me too.

However, a little background about power-line-frequency fluctuations might
be helpful.

The human eye has enough persistence of vision that makes it unlikely that
it can **directly** perceive the fluctuation in light intensity due to the
60 Hz alternating current used in the U.S.  This current goes on and off
120 times a second---on when the current goes one way, then off, then on
when the current goes the other way, and then off again---that is on and
off twice, 60 times a second.   The filament of an incandescent light does
not cool down much in the less than 1/120 of a second that the current is
nearly off.  Thus the light from an incandescent bulb is for all practical
purposes constant in intensity.

Fluorescent lights on the other hand do go on and off, but the human eye
still can't respond fast enough to observe **directly** the 120 Hz
oscillation in intensity.  The 30 Hz frame rate of a TV is one fourth as
fast, and most people see the TV image as a mostly continuous thing.

However, one can easily see the wagon-wheel effect.  As the wagon in a
western movie speeds up or slows down, the wheel appears first to rotate in
one direction, then in the other.  This occurs because the moving-picture
frame rate goes in and out of synchronization with the rate of the spokes
of the wheel passing a given point.  In the case of lights at home, there
is the related, and somewhat disturbing, appearance of a ping pong ball
moving in a room lit only with fluorescent lights.  The ball seems to move
with a jerkiness.

But, I think that without something moving rapidly through the light, the
power-line frequency fluctuations in light intensity are not detectable by
the unaided human eye.

What really bothers me is the flicker from a poorly adjusted computer CRT
monitor.  I am not talking about the intensity fluctuations that occur at
the frame refresh rate of a well adjusted computer and monitor.  These can
be observed easily by holding your hand out with the palm nearly up to the
screen, and waving your hand from the left side of the screen to the right
and back.  Your fingers are like the spokes of the wagon wheel, and the
fluctuations in light intensity are very striking.  They could be very
disturbing to people that are borderline epileptic.  (Do not try it if you
are.)  Still as long as there is nothing moving near the light from the
screen, the fluctuations of light intensity from a properly adjusted
computer are not too bothersome.

However, if the refresh rate is not set correctly the light fluctuations
can drive you up the wall.  They are particularly disturbing when the en
tire screen is white, as occurs with a word processor.  The fluctuations
occur at a frequency equal to the difference between the actual refresh
rate and the correct one.  This can be in the neighborhood of 5 to 15 Hz.
 Perfect for getting to a PWP.

Baldwin, 62/4
----------


From:  janet paterson
Sent:  Friday, September 19, 1997 9:37 AM
To:  Multiple recipients of list PARKINSN
Subject:  Re: Internal Shakiness/pd/ep link?

Hi barb and ken {!!]

[good golly, where'd that capital H come from?]

barb wrote:

>Many times - but not every time - fluorescent lighting
>will trigger that "internal tremor" feeling.  And when
>I go to a movie, I spend the first 1/2 hour or so feeling
>like my insides have turned into jello!   Prior to my
>pallidotomy, I got TERRIBLE dyskenesia throughout any
>movie I sat thru

ken wrote:

>In the USA, as far as I know in all states, the electrical
>current pulsates at 60 Hertz, originally known as
>60 Cycles-per-second, so every appliance or device that..
>.. being interupted by the fan blade at a repetitive rate, or a
>light bulb being interupted by the fan blades. If you have 4
>blades blocking a light at 60 cycles per second, the eye sees
>15 flashes a second. parkinson tremors are said to be in the
>range of 8 to 15 cycles per second....... When my dad was able
>to walk around, he seemed to have less problem with freezing
>in the light of the sun, basically a non-pulsed source of light,
>then when inside under a 60 cycle driven lighting system.

i don't know if this has any relevance here
but my understanding is that there is definitely a relationship
between a certain rate/speed of flashing light i.e. 'strobe' lights,
or sun/shadows while driving e.g. down a tree-lined street
and the chance of 'triggering' a seizure in epileptics

i have had several eeg [electro-encephalo-graph] tests
over the years, to measure my interesting brain waves,
including at my pd diagnosis

a strobe light effect is part of this test
and almost always was guaranteed to get me so dizzy
that i asked the technician to stop

.. for what it's worth ...
.. planting more seeds in the archives ...

your brainy syber-sys

janet

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