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Subject: TWL: Pulsing LEDs
LRZeitlin@XXX.XXX
Date: Wed Jan 01 2003 - 11:25:06 EST
Bob Austin writes
<<Interestering concept of pulsation of LED's--and in fact I understand that
they can be driven as strobe lights--perhaps what you are refering to , but
I usually think of strobs as a bit faster rate. I have always felt that
1/30 of a second was the speed at which human eye/brain began to precieve
steady state/vs pulsing. >>
I'm not talking about strobing the LEDs per se, but taking advantage of the
perceptual attention grabbing capability of a light at the approximate
flicker fusion frequency of the human eyeball. That is the point at which the
average person cannot tell if a light is continuous or flickering. Few of us
have any difficulty telling when a light appears steady or flashing when the
on/off rate is below 10 cycles a second or above 30 cycles a second. It is in
the middle range between 12 to 24 cps that confusion occurs. Early movies
used a 16 cps frame rate but when projector lamps increased in brightness,
many members of the audience complained of headaches after attending the
"flicks". The rate was raised to 18 cps for amateur films and 24 cps for
films shown in theaters. US television uses a 30 cps frame rate. In the UK it
is 25 cps and some flicker is noticable on brightly illuminated scenes.
Fluorescent lights in the US flicker at 60 cps and for most of us the light
appears to be continuous.
At or near the flicker fusion rate, a light appears to be more brilliant than
it really is. Certainly it looks more intense than a comparison light source
being fed from a steady state electrical supply. It does not appear to be
flashing like a strobe, just brighter, attention getting, and somewhat
disturbing - a fact well known to creators of illuminated advertizing
displays. So rapidly pulsing running lights and anchor lights might be just
the thing for attracting attention on those dark, overcast nights when
visibility is most important.
Larry Z
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