Persistence of Vision
Fascinations'
new XP3™ Clock uses "Persistence of
Vision" technology to create images
floating in mid air. We live in a
world of both blinking and
continuous lights. The phenomenon
known as the "persistence of vision"
causes many of the flashing lights
we see to appear continuous. This
phenomenon has been recognized for
over 350 years.
Sir
Isaac Newton in Book I of his
treatise on Optics wrote "When a
coal of fire moved nimbly in the
circumference of a circle makes the
whole circumference appear like a
circle of fire, is it not because
the motions excited in the bottom of
the eye by the rays of light are of
a lasting nature, and continue till
the coal of fire in going round
returns to its former place?" 1
Today,
it is understood that once light has
struck the retina, all the
detection, processing, and
transmission of the neural signal
are chemical. The retained image is
a result mainly of the time required
for the production and decay of the
photosensitive retinal molecules. 2
The
continuous flow of retinal
information received by our brain is
interpreted in discreet packets.
This is because some time is
required to collect enough data for
interpretation. The collection time
is referred to as the "integration
time". Amazingly, our brain is able
to automatically adjust for
different light intensities. For
in-stance, when less light is
available, our brain requires more
time to collect sufficient data for
interpretation so it automatically
selects a longer integration time.
The
XP3™ clock offers an excellent
demonstration of the "persistence of
vision".
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The XP3™ clock’s wand
oscillates back and forth;
in a plane that is typically
nearly perpendicular to
one's line of vision,
approximately 16 times per
second. Eight LEDs (light
emitting diodes) are
embedded along a line near
the tip of the wand. Because
the flashing LEDs remain on
for only .185 milliseconds,
only one point on the retina
is stimulated so our brain
interprets the information
as a point of light. As the
oscillating wand passes
across one's field of
vision, the eight LEDs are
programmed to blink, such
that a pattern is produced,
which is interpreted by our
brain as a character.
Some of the LEDs need to
blink more than once in
order to produce a
character. For instance, the
top LED on the wand blinks
five times in order to
produce the horizontal line
of the top portion of the
number "3". Since up to 12
characters can be displayed
at one time, if the number
'3' were repeated 12 times
across the display with no
spaces between the numbers,
the top LED would flash on
and off 60 times per sweep
of the wand. Amazingly,
since the wand makes 16
sweeps per second, the top
LED in this example would be
required to flash 960 times
per second to create the
perceived pattern. Due to
the 'persistence of vision',
the brain does not perceive
that the points of light are
being repainted 16 times per
second. |
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However, since our eye's
integration time is only
slightly greater than the
wand's sweep time, some
flickering is noticeable.
One way to reduce the
flicker would be to operate
the clock in a darker
location. This works because
our brain, in order to
gather enough light
information from the retina,
automatically shifts to a
longer integration time in
reduced light. Another way
to prevent flicker would be
to increase the wand's sweep
rate. It is interesting to
note that movie film, which
runs at only 24 frames per
second, would produce some
noticeable flicker were it
not for the fact that the
theater's movie projectors
use a triple shutter. By
shuttering each frame three
times, a flash rate of 72
times per second is
achieved. This rate is well
above our brain's
integration time, thereby
eliminating the flicker
problem. Computer monitors
also avoid this flicker by
flashing approximately 75
times per second. |
Due to
the fact that alternating current is
used to power most of the lights
around us, you might be surprised to
learn how many of these lights,
which seem to be continuous,
actually blink. In order to discover
which lights do blink, try the
following experiment: Attach a
mirror to a small round stick. Look
at the reflection of a point light
source while rotating the stick back
and forth around its major axis with
the palms of your hands. If the
light source is continuous, the
point of light will become a solid
line, but if it is blinking, the
light will appear as a dotted line.
References 1. 'Brewster and
Wheatstone on Vision' by Nicholas
Wade, Academic Press 1983. 2.
'Molecules' by R. W. Atkins,
Scientific American Library
