Reply by August 13, 20032003-08-13
Hello,
a bit late to comment on this thread, but still...

The aliasing phenomena observer with the human vision
is not only depending on the light sources.

It is well known that the brain operates at a few diffrent
frequencies, where (approx) 16 Hz is one of the major
fundamentals. 

Also as mentioned in an earlier post, the central area of the
visual field is optimised for detail but less for movements
while the outer part is optimised to catch movements.

so, the aliasing effect, with wheels spinning backwards
or standing still can be observed in fine sunlight.

-j
Reply by Eric Jacobsen July 19, 20032003-07-19
On Sat, 19 Jul 2003 12:30:56 +0200, "raymund hofmann"
<filter001@desinformation.de> wrote:

>> > It isn't because the brain is sampling, but because the movie or TV >camera >> > is sampling. >> However, you can also observe the same thing when you stare at a >> ceiling fan, or a car driving down the road with spokes wheels, etc. >> So in that instance, it can be argued that your brain is "sampling".
I'd join in disagreeing with this notion. I've yet to see an instance where this effect can't be otherwise explained. I've seen no evidence from this effect or others to indicate that the brain samples as revealed by any aliasing effects. ...
>And also other things may play important roles here: > >- Mechanical resonances of our eyes and the related body.
Not only resonance but the effects of external vibration sources. I think IEEE Spectrum or some related publication had a lot of discussion a few years ago about the well-known "Frito effect" which is experienced by examining an oscilloscope trace while munching on Fritos. The mechanical vibration transmitted from your teeth to your eye sockets will do all kinds of wacky things to your perception of the oscilloscope trace. Note: The young whipper-snappers in labs equipped with only the latest equipment may miss out on this since it won't work unless the scope trace is swept. Some of the new all-digital machines don't do this. An old boss of mine (the founder of the company, actually) used to tease new technicians when they had difficulty with lab measurements. He would very carefully explain to them with the usual air of authority and knowledge that they could sometimes improve their measurements by rapping the top of their head with their knuckles while observing oscilloscope traces (this works with quickly swept spectrum analyser traces as well). We'd always know if he'd been around if we saw a confused tech sitting at a bench bonking himself on the head while staring at a scope. The effect is the same as Frito chewing, it jitters your eyeballs and screws up your perception. I've always been amazed at how much deviation in the scope trace you can observe but the rest of your field of view is stable. Human vision must have some kickass image stabilization to do that, but the fact that you can see a continuous (but very wiggly and sometimes highly deviated) scope trace is further evidence to me that the brain doesn't sample. If it does it's at some incredibly unbelievably high rate, which I don't think is the case.
>- Periodical Muscle movement of our eyes > >It's a very complex and fed back signal processing system ... > >Raymund Hofmann > >
Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by Eric Jacobsen July 19, 20032003-07-19
On Fri, 18 Jul 2003 18:54:46 -0400, Jerry Avins <jya@ieee.org> wrote:

>Eric Jacobsen wrote: >> >> On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org> wrote: >> >> >Bhanu Prakash Reddy wrote: >> >> >> >> > >> >> > It isn't because the brain is sampling, but because the movie or TV camera >> >> > is sampling. >> >> > >> >> > Yes, it is aliasing just like is often described here. >> >> > >> >> > Also note that the aliased motion may appear to be reversed. >> >> > >> >> > -- glen >> >> >> >> Even if u see really (not in a movie) a fast moving vehicle, u observe >> >> the same situation with its tyres.In that case who is sampling...brain >> >> only right??? >> >> >> >> -bp$ >> > >> >No. You won't see the effect in uniform light. At night, reflections >> >from parts of the wheel when they assume specific ankles can act like a >> >synchronized stroboscope, making the wheel appear stationary. >> > >> >Jerry >> >> At night artificial lighting creates a shutter effect since the lights >> turn off 120 times a second with 60Hz power and 100 times a second >> with 50 Hz power. This creates a visual sampling system because you >> only get to sample the object when the lights are actually on. >> Although one doesn't perceive the flicker, one does perceive the >> aliasing effects that can happen. >> >> I've personally witnessed this visual aliasing many times, understood >> it, and therefore never gave it much more thought beyond initial >> interest. Once a few years ago while driving home in the daylight I >> observed the same visual aliasing in the wheel of the car in my side >> rear-view mirror. It took me a while to wrap my brain around that >> one since it was natural daylight illumination. I was beginning to >> worry that I was on "The Truman Show" for a while until I realized >> that the mechanical vibration in the rear-view mirror was producing >> the same shuttering effect achieved with AC electric artificial >> lighting at night. >> >> Eric Jacobsen >> Minister of Algorithms, Intel Corp. >> My opinions may not be Intel's opinions. >> http://www.ericjacobsen.org > >I see that too, but people claim to see the effect with illumination by >headlights! Then it's reflection at specific orientations. The >explanation I rejected was that the eye itself sampled. Tain't so. If it >were, we would observe beats. > >Jerry
Agreed, and the phenomena would be as repeatable in natural daylight as it is at night, which it isn't. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by raymund hofmann July 19, 20032003-07-19
"Craig" <crrea2@umkc.edu> schrieb im Newsbeitrag
news:82396605.0307160454.51e76aab@posting.google.com...
> "Glen Herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:<Tr6Ra.74831$ye4.51585@sccrnsc01>...
> > "Bhanu Prakash Reddy" <itsbhanu@yahoo.com> wrote in message > > news:28192a4d.0307152153.355783d1@posting.google.com... > > > HI all, > > > > > > Thank you very much for the participants in " Negative Frequencies" > > > Thread. Here is another question from me. > > > > > > Yesterday I was watching a movie. In that, hero was chasing villain in > > > a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly > > > and suddenly it appeared that the tyres of the car are rotating at > > > only 8 miles/hr(I didnt > > > measure..felt that its RPM is very less).The reason i feel is because > > > of Aliasing. My eyes are sampling the scene at less than the Nyquist > > > rate. Our CPU (Brain) samples through the sensor called Eye at a > > > particular rate..because it has to spare some time for other 4 sensors > > > also( In some cases 5 sensors ;) > > > > It isn't because the brain is sampling, but because the movie or TV
camera
> > is sampling. > However, you can also observe the same thing when you stare at a > ceiling fan, or a car driving down the road with spokes wheels, etc. > So in that instance, it can be argued that your brain is "sampling".
I guess our Visual System is not a perfect low-pass without aliasing. As i think most of our nerves are operating in some kind of "pulse frequency modulation". Meaning: Information is not processed continuously "analog" but somehow "quantized". This includes the first nerve cells in the eye detecting light and continues with the nerve cells feed by them and the whole brain. Maybe the details of this are already worked out ... But something else is dangerous here: Our Consciousness ! As already discovered our Consciousness is only a very small fraction of our "Brain Power". Only a fraction of our "sensoric stimulation" gets into our consciousness already "symbolized" by our "cognitive filters" which are inherited/trained since we live. And also other things may play important roles here: - Mechanical resonances of our eyes and the related body. - Periodical Muscle movement of our eyes It's a very complex and fed back signal processing system ... Raymund Hofmann
Reply by Steve Underwood July 19, 20032003-07-19
Glen Herrmannsfeldt wrote:
> "Col Brown" <cb135@hotmail.com> wrote in message > news:a254af6b.0307180751.43c50302@posting.google.com... > > >>They do put flourescent lighting in workshops and such (at least I >>have) it's just not recommended to wire them with a single phase (all >>lights pulsing at the same frequency). Usually, people install 2-3 >>phases (120 degrees apart) throughout the building, so that you avoid >>the strobascopic effect.
Almost all fluorescent light fittings for office environments have three tubes, but in much of the world they are all wired to the same phase. What a waste. When they are wired to the three phases, the overall flicker is pretty minimal (as opposed the often bloody awful flicker of a single tube). You still get people whining about it, though. I guess it shows how much of the complaint about flicker is psychological (or is it just the lazy, looking for an excuse?).
> Some two lamp balasts run the two lamps 90 degrees out of phase to minimize > the overall flicker.
That is also to keep the power factor reasonable. In some places I think it is a requirement. Regards, Steve
Reply by Jerry Avins July 18, 20032003-07-18
Eric Jacobsen wrote:
> > On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org> wrote: > > >Bhanu Prakash Reddy wrote: > >> > >> > > >> > It isn't because the brain is sampling, but because the movie or TV camera > >> > is sampling. > >> > > >> > Yes, it is aliasing just like is often described here. > >> > > >> > Also note that the aliased motion may appear to be reversed. > >> > > >> > -- glen > >> > >> Even if u see really (not in a movie) a fast moving vehicle, u observe > >> the same situation with its tyres.In that case who is sampling...brain > >> only right??? > >> > >> -bp$ > > > >No. You won't see the effect in uniform light. At night, reflections > >from parts of the wheel when they assume specific ankles can act like a > >synchronized stroboscope, making the wheel appear stationary. > > > >Jerry > > At night artificial lighting creates a shutter effect since the lights > turn off 120 times a second with 60Hz power and 100 times a second > with 50 Hz power. This creates a visual sampling system because you > only get to sample the object when the lights are actually on. > Although one doesn't perceive the flicker, one does perceive the > aliasing effects that can happen. > > I've personally witnessed this visual aliasing many times, understood > it, and therefore never gave it much more thought beyond initial > interest. Once a few years ago while driving home in the daylight I > observed the same visual aliasing in the wheel of the car in my side > rear-view mirror. It took me a while to wrap my brain around that > one since it was natural daylight illumination. I was beginning to > worry that I was on "The Truman Show" for a while until I realized > that the mechanical vibration in the rear-view mirror was producing > the same shuttering effect achieved with AC electric artificial > lighting at night. > > Eric Jacobsen > Minister of Algorithms, Intel Corp. > My opinions may not be Intel's opinions. > http://www.ericjacobsen.org
I see that too, but people claim to see the effect with illumination by headlights! Then it's reflection at specific orientations. The explanation I rejected was that the eye itself sampled. Tain't so. If it were, we would observe beats. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by Eric Jacobsen July 18, 20032003-07-18
On Wed, 16 Jul 2003 15:12:02 -0400, "Clay S. Turner"
<physicsNOOOOSPPPPAMMMM@bellsouth.net> wrote:

>This brings to mind several interesting effects. The first is the speed of a >nerve impulse depends on its strength. So trying to catch a ball when it is >dark causes us to miss the ball since its true position is ahead of its real >position. Remember playing baseball late during the summer and trying to >catch a fly ball. By the time you manage to get the glove up, the ball has >already hit you in the face.
I take it you weren't often picked for the all-star team? ;) Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by Eric Jacobsen July 18, 20032003-07-18
On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org> wrote:

>Bhanu Prakash Reddy wrote: >> >> > >> > It isn't because the brain is sampling, but because the movie or TV camera >> > is sampling. >> > >> > Yes, it is aliasing just like is often described here. >> > >> > Also note that the aliased motion may appear to be reversed. >> > >> > -- glen >> >> Even if u see really (not in a movie) a fast moving vehicle, u observe >> the same situation with its tyres.In that case who is sampling...brain >> only right??? >> >> -bp$ > >No. You won't see the effect in uniform light. At night, reflections >from parts of the wheel when they assume specific ankles can act like a >synchronized stroboscope, making the wheel appear stationary. > >Jerry
At night artificial lighting creates a shutter effect since the lights turn off 120 times a second with 60Hz power and 100 times a second with 50 Hz power. This creates a visual sampling system because you only get to sample the object when the lights are actually on. Although one doesn't perceive the flicker, one does perceive the aliasing effects that can happen. I've personally witnessed this visual aliasing many times, understood it, and therefore never gave it much more thought beyond initial interest. Once a few years ago while driving home in the daylight I observed the same visual aliasing in the wheel of the car in my side rear-view mirror. It took me a while to wrap my brain around that one since it was natural daylight illumination. I was beginning to worry that I was on "The Truman Show" for a while until I realized that the mechanical vibration in the rear-view mirror was producing the same shuttering effect achieved with AC electric artificial lighting at night. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by Jerry Avins July 18, 20032003-07-18
Col Brown wrote:
> > Lasse Langwadt Christensen <langwadt@ieee.org> wrote in message news:<3F15E268.4030901@ieee.org>... > > Paavo Jumppanen wrote: > > > > > snip > > > > > > > > > Yes, it is aliasing and mechanical engineers may also refer tp it as > > > the strobascopic effect. > > > > > > As for why ti appears to be moving backwards, that depends upon the > > > proximity of the rotational speed to the frames per second. If > > > identical or harmonically related the tyre will appear stationary. If > > > the rotational speed is slower it will appear to be rotation in the > > > right direction but at a speed that is the difference between the > > > frame rate and the rotational speed. If the rotational speed is > > > slightly faster it will appear to be rotating backwards with speed > > > that is the difference between the rotational speed and the frame > > > rate. > > > > > > > I beleive that's why you can't have flourecent lights in places with > > rotating machines, someone might think it isn't rotating or rotating > > very slow and grap it... > > > > And I'm sure that illusion of the thing rotating backwards can be > > somehow related to negative frequencies > > > > -Lasse > > They do put flourescent lighting in workshops and such (at least I > have) it's just not recommended to wire them with a single phase (all > lights pulsing at the same frequency). Usually, people install 2-3 > phases (120 degrees apart) throughout the building, so that you avoid > the strobascopic effect.
Not exactly. The single-lamp fluorescent fixtures I have in my garage operate from the only phase I have available there, and they show a stroboscope effect. I use an incandescent gooseneck near the lathe, but I would have no problem otherwise. In larger spaces, I would use or recommend two-lamp fixtures. Those have ballasts so designed that the bulb currents are in quadrature. Running some fixtures from each of three phases in a large shop balances the load, but individual fixtures are usually far enough apart so than any one area is illuminated mainly by a single one. Most of those fixtures are two-lamp with quadrature balasts. That's common here in the US. Has anyone but me noticed that when one bulb in a two-lamp fixture burns out, the flicker becomes noticeable, and that unless both bulbs are replaced, residual flicker remains? Fluorescent lamps lose brightness as they age. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by Glen Herrmannsfeldt July 18, 20032003-07-18
"Col Brown" <cb135@hotmail.com> wrote in message
news:a254af6b.0307180751.43c50302@posting.google.com...

> They do put flourescent lighting in workshops and such (at least I > have) it's just not recommended to wire them with a single phase (all > lights pulsing at the same frequency). Usually, people install 2-3 > phases (120 degrees apart) throughout the building, so that you avoid > the strobascopic effect.
Some two lamp balasts run the two lamps 90 degrees out of phase to minimize the overall flicker. -- glen