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".
| 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
