[PD] Spectrum graphing amplitude problem

Andy Farnell padawan12 at obiwannabe.co.uk
Tue Oct 23 22:48:03 CEST 2007

On Mon, 22 Oct 2007 22:48:32 -0500
"Charles Henry" <czhenry at gmail.com> wrote:

> > > > That won't work in sunlight for example.
> > >
> > > Haven't you ever seen it?  (in sunlight that is)
> As implied, I'm 99% positive I've seen it before.  You might still be
> able to convince me that I haven't ;)

I think Martin is correct. It's one of those things we've seen so often
in films as an artifact that we become convinced its a phenomena in
reality. Thinking about it, I've never seen that in direct sunlight,
the wheel just becomes blurred.

The optical (and other sensory) neurons are unlike a digital
system. For a Nyquist effect it would require some other separate
and internal timebase. The spiking neuron model behaves more like
a switch debouncer. If two stimuli arrive too close together the
persistance of the electrochemical mechanism make them seem like 
one slightly stronger stimulus. 

There are some other interesting effects if watching a spinning wheel
made of black and white spokes, some people see colour flashes at
certain speeds where the cones are tricked into firing instead of the
rods. But I've never seen the wheel spin backwards in natural light.

> > i think, that is the point that martin is trying to make.
> >
> > i am not an expert either, but when i encountered this effect (watching
> > the frontwheel, while cycling at night, for instance), there was always
> > artificial light involved. to be more precise, there was always kind of
> > a fluorescence or any other kind of 'gas discharge lamp' (is that the
> > correct expression?) involved. it doesn't seem to work with common light
> > bulbs, since their 'afterglow time' is too long.
> >
> > for me, it seems, that it is rather related to the light source and its
> > frequency and not to a property of the eye.
> hey, what do you know, there's even a wiki page on it:
> http://en.wikipedia.org/wiki/Wagon-wheel_effect
> Vision is in fact not purely analog.  Although I don't necessarily
> think the "discrete frame" theory mentioned in the page is true, there
> is a counterexample.  When your eyes make a saccade (fast movement),
> there is no blurring.  There is an inhibition of the movement in
> between the two visual scenes.
> There is also nystagmus.  Even when images appear perfectly clear, the
> eye moves in random oscillations.  When researchers (I forget the
> citation/can't find it) used an eye camera to track random
> oscillations and corrected for the movement (creating a static
> retinotopic image), the perceived image disappears.  There may be
> something involved in correcting for nystagmus that renders an image
> in a series of apparently still frames.  I don't know for sure.
> Chuck
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