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RBG
2005-Sep-08, 09:15 PM
Hey, I think I may have found a slight error or at least a significantly incomplete answer to the question:

#19: "Why does a wheel seem to move backwards as it speeds up?"

on the BA's 1996 MADSCI Q&A.

http://www.badastronomy.com/mad/1996/wheel.html

He rightly relates the phenomenon to the frame rate of film, but I'm surprised that there was no mention of this effect as seen in real life.

I always thought this was strictly a film-thing myself until I happened to see this a number of times, directly with my own eyes. No film.

Has anyone else seen this too? I'm sure it exists. And I would guess it is somehow caused by the number of spokes on the wheel in relation to the number of revolutions / minute.

Can anyone help me out here?

RBG

Gillianren
2005-Sep-08, 09:22 PM
yes. bear in mind that I'm no good at the code necessary to insert the URL, so if any mod sees this and can fix it, I'd really appreciate, huh?

http://www.straightdope.com/classics/a3_199.html

you can humour me and copy-paste this once, right? (you should know that I'm feeling fairly stupid right now, as this has been explained to me repeatedly. I guess I'll never remember.)

[edit--I didn't need to enter code, at least not in my browser! hurrah!]

Dark Helmet
2005-Sep-08, 09:29 PM
It could be the lighting.

Under flourescent lighting, you can get the same find of effect without a camera, due to the pulse-like nature of it, as opposed to sunlight or incandescent lighting.

Josh
2005-Sep-08, 11:21 PM
One explaination I've heard is to do with your eye/brain frame rate. After certain speeds you start seeing the wheel going backwards because your eyes aren't updating fast enough.

It's the same reason why you aren't loking at your computer screen scrolling and updating. we can't process the information fast enough.

Let's say you have a four spoked wheel. To begin with the spokes are in the shape of a "+".
For an average car say a wheel is 0.6m diameter.
so it's circumference is: 0.6*pi = 1.9m
if the car is travelling at say 32m/s therefore the wheel is rotating at: 30/1.9 = 17rev/s

that value would have to be faster than the rate at which our brains can process the data for this phenomenon to occur.

Now, the spokes have obviously completed some part or more of one revolution of the wheel. If your brain updates the information from the eye at a speed at which the spokes have seemingly travelled more than 45deg (i'm guessing - probably further) but less than 90deg from their original position, then it will look like the spokes have travelled backwards.

Champion_Munch
2005-Sep-08, 11:27 PM
One explaination I've heard is to do with your eye/brain frame rate. After certain speeds you start seeing the wheel going backwards because your eyes aren't updating fast enough.


I have also heard that somewhere. Seems to make sense. :P


Has anyone else seen this too? I'm sure it exists.

Yup, pretty much every day for the past dozen years or so. :D

wth regards

grant23
2005-Sep-09, 02:04 AM
Airplane props look like there moving backward also. Why i dont know.

ZaphodBeeblebrox
2005-Sep-09, 03:03 AM
Airplane props look like there moving backward also. Why i dont know.
Um ...

Same Phenomenon, perhaps?

It's Not Limited to Wheels, ya' know?

;)

Seeker of Knowledge
2005-Sep-09, 03:21 AM
I have found (from napping in the passenger seat; don't try this while driving :P) that if your head is touching the window, the vibration of the car (I think that's what it is) can produce the "backwards turning effect".

I have just spent ten minutes watching various fans and things, and with a ceiling fan swiching between super-high-speed and high speed under incandescent light, I see a ghostly "image" of the blades turning slowly, in addition to the blurry circle presented by the blades. It's only for a few second, and then it goes away as the speed increases. I've seen this far more clearly with helicopter rotor blades, but whether it was filmed or not I cannot remember.

Gillianren
2005-Sep-09, 03:47 AM
according to the article I cited, it's in part a function of the markings/spaces involved.

Enzp
2005-Sep-09, 04:09 AM
I can't speak to the brain scan rate. I would be surprised if the brain refreshes the whole image simultaneously at a regular rate, but what do I know.

But artificial lighting stobes with the power line frequency. I use this phenomenon when testing speeds of tape recording equipment. I have a spinning disc with lines on it, and they will strobe a pattern in the flourescent lighting. i am supposed to get out a neon strobe for this, but why bother... It is the exact same AC freq strobing either.

So if your spinning whatnot is indoors, chances are the lighting is strobing. Even putdoors, if there are artificial lights around, they will add their strobe effect to the otherwise steady solar lighting. Even on airport gateways there are high intensity lighting systrems running even in daylight often as not.

These effects can be seen in real life and are independent of any TV or film frame rates. Which have their own obvious effects.

Regardless of the source, the spacing of the spokes and their speed will have to interact with some form of strobe, whether it is film frames, flourescent lighting , or brain scanning. SO if you added more spokes, it would change the apparent speed of the phantom image, yes.

Fram
2005-Sep-09, 08:34 AM
In movies, it has to do with the beta movement, the phi phenomenon, and perhaps persistence of vision (this one is disputed nowadays).
An early researcher was Joseph Plateau (http://www.visual-media.be/plateau-intro.html) (with my apologies for the ironically bad visual layout of this site about visual media).

In real life, the reverse rotation effect (http://en.wikipedia.org/wiki/Reverse_rotation_effect), as it is called, is supposed to be rare, although my experience is that many people do experience it.

kucharek
2005-Sep-09, 08:44 AM
Sometimes I can get the effect by making humming noises. Maybe it gets the eyes vibrating, thus producing the interference.

skrap1r0n
2005-Sep-09, 01:58 PM
IIRC, your eye/brain can interpret 28fps.

William_Thompson
2005-Sep-09, 03:28 PM
Hey, I think I may have found a slight error or at least a significantly incomplete answer to the question:

#19: "Why does a wheel seem to move backwards as it speeds up?"

on the BA's 1996 MADSCI Q&A.

http://www.badastronomy.com/mad/1996/wheel.html

He rightly relates the phenomenon to the frame rate of film, but I'm surprised that there was no mention of this effect as seen in real life.

I always thought this was strictly a film-thing myself until I happened to see this a number of times, directly with my own eyes. No film.

Has anyone else seen this too? I'm sure it exists. And I would guess it is somehow caused by the number of spokes on the wheel in relation to the number of revolutions / minute.

Can anyone help me out here?

RBG

It seems to happen in real life because light bulbs run on alternating current and 60 times a second a light bulb is actually turning off. The effect is more apparent in florescent light because those bulbs do not warm down like incandescent light bulbs.

Probos
2005-Sep-09, 04:17 PM
Here there´s another example of this effect: The Technics desk.

It has a four dotted band along the edge of the plate. They all rotate at the same speed (from 33 to 45 rpm) but each band has different number and diameter of dots. 2 of them (I guess the bigger and more spaciated) go clockwise and the other 2 counterclockwise. It´s a fascinating view.

http://www.netmusic.pl/images/Kopia%20z%20deck.jpg

RBG
2005-Sep-09, 05:12 PM
I would think that the electical frequency in a florescent light would indeed produce a strobing effect, but I'm sure I recall seeing the phenomenon outside in pure sunlight.

I am very skeptical about a human vision update frequency, and would love to read about such evidence.

I think a lot of blurring that is being seen is related to "persistence of vision." This is what is responsible for movies looking like smooth action rather than discreet frames. Any image that hits the retina will retain for a moment.

I've seen this effect as a shadow that follows behind a dark moving vehicle when the background is light. Or more obvious: just staring at a light & then turning the room to pitch dark will produce an image that hangs around for a long time.

I'd be very surprised if "persistence of vision" has been "busted". That combined with the idea that there is a human vision frame rate would be a significant scientific revision & I'd like to see a paper on such.

RBG

RBG
2005-Sep-09, 05:21 PM
On the other hand, unless its possible to really analyze what I was seeing - maybe in every case there was some kind of florescent light involved which never made an impression upon me.

But I do recall pointing it out to a colleague of mine and both of us being surprised at what we were seeing - & both of us being film & video people.

No, this has to be real. Anybody else see this before? This has to be a well understood effect by lots of learned folks.

RBG

Sam5
2005-Sep-10, 02:31 AM
I spent half my live seeing the world through a video or film camera viewfinder, but I've never heard of the effect being seen directly by a human in sunlight.

Maybe in flickering streetlight, but not sunlight.

I read in an old astronomy book that a 18th Century scientist wondered if stars ever appeared to go completely "out" when they twinkled, and he figured out a way to see it. He observed them through a mirror and he saw them go out (dim to black) when he wiggled the mirror. Seems that with our "persistence of vision" we can't usually see this without a wiggling mirror.

Ricimer
2005-Sep-10, 05:18 AM
I've seen the effect in broad daylight. Then again, I'm not the best test case as I have nistagmis. This my eyes twitch rapidly about 1 mm to the sides...all the time.

My gf, who doesn't have such birth defects, has also seen it in daylight.

Sam5
2005-Sep-11, 01:59 AM
I've seen the effect in broad daylight. Then again, I'm not the best test case as I have nistagmis. This my eyes twitch rapidly about 1 mm to the sides...all the time.


That might be the reason you see wheels appearing to go backwards.

This is most noticeable in movies and on TV with wagon wheel that have spokes.

BA explained it fairly well.

A 16 mm movie camera runs at 24 frames per second and a TV camera photographs at 30 fps. A film camera has a shutter that is closed about half the time and open about half the time as the film moves inside the camera while the shutter is closed, stops while the shutter is open, then moves again while the shutter is closed. A TV camera has an “electronic” shutter that usually jumps from one video frame to another with no “closed shutter” time.

For film fps to be compatible with TV fps, some years back they invented a video device that photographs every 4th movie frame twice, and that converts films to 30 fps for TV. You can see that by still-framing a movie you record off TV, then advance the film one frame at a time.

If you have a wheel with four spokes, and the wheel makes 1/4 of a revolution while a movie camera shutter is closed, you will see the wheel as not turning at all, even though it is turning. Let’s say the shutter is open always when two spokes are vertical and two are horizontal. It will appear that way when you view the moving film and it will appear that you are seeing the same spokes always in the same horizontal and vertical positions. But actually you are seeing different spokes in the different positions. If the wheel doesn’t quite make a 1/4 turn during filming, then the spokes and the wheel will appear to be turning backwards.

Let’s say we take a four-spoke wheel and we paint the right horizontal spoke red, then we turn the wheel clockwise and photograph it with a film camera. If we turn it 1/4 turn in between each film frame, we will see the wheel appear to not turn at all, but we will notice that the red spoke seems to move around the wheel clockwise. The red spoke will first be seen as being at the horizontal right, then the vertical bottom, then the horizontal left, then the vertical top, then the horizontal right, etc. Of course, that reveals that the wheel is actually turning.

If we slow down the wheel a little, so that it doesn’t quite make a full 1/4 revolution turn while the shutter is closed, the wheel will appear to be turning backwards, however, we will notice that the red spoke goes from being horizontal right, to being almost vertical bottom, to being less than almost horizontal left, to being much less than horizontal top, to much much less than horizontal right. So that tells us the wheel is actually turning clockwise, but not quite 1/4 turn per movie frame.

Your eye twitching might also cause a similar effect when you see wheels turn in sunlight.

Extravoice
2005-Sep-11, 02:57 AM
Oooooh, finally a topic I'm familiar with.

The "wagon wheel effect" on film/TV has its basis in sampling theory. Do a Google search for "Nyquist's Theorem", if you want the details.

To keep it simple, you can think of filming a wheel as watching the wheel rotate while illuminating it with a strobe light flashing at 24 flashes per second. Imagine a wheel with one red spoke. If the wheel was rotating at 24 RPM, it would appear to stand still, because the red spoke would be in the exact same position each time the flash went off. If the flash was a little slower than the wheel, the wheel would appear to be rotating slowly in the proper direction because the red spoke would move slightly more than one revolution per flash. If the flash was a little fast, the wheel would appear to rotate backward, as the red spoke wouldn't quite make a full revolution for each flash.

If you have a ceiling fan in your house, you can duplicate the effect by laying on the floor and blinking your eyes while watching the fan rotate. With a little practice, you can make the fan appear to rotate forward, backward, or stand still by blinking faster or slower.

Normally, your brain/eyes do not sample signals the way film does, but vibration of your eyes will result in similar results. At a previous job where we designed Liquid Crystal Displays, we would routinely check for "flicker" by looking at the display while chattering our teeth.

Sam5
2005-Sep-11, 03:07 AM
Oooooh, finally a topic I'm familiar with.


At a previous job where we designed Liquid Crystal Displays, we would routinely check for "flicker" by looking at the display while chattering our teeth.

Wow, I love high-tech science. I think someone was chattering his teeth when he deisgned my Windows XP program.

dougreed
2005-Sep-11, 03:22 AM
hi, I have seen this effect in daylight with no possibility of any other light source...riding on interstate 10 thru west Texas on my beemer I saw many truck wheels with that effect. Maybe the vibration of the bike/pavement, whatever, it is definitly there! I can enhance it under any lighting with a trick I learned- I vibrate my eyeballs horizontally very rapidly. (weirdo) It's very usefull for watching rotating or vibrating objects. (aircraft props, orbital sanders) Maybe in direct sunlight it has to do with those strange shadow bands that become so visible during a solar eclpise, maybe they are always present causing a sort of strobe effect...that's all

RBG
2005-Sep-11, 05:19 PM
There is this majority of professional 35mm cinema people who insist that the historic 24 frames per second film frame rate is somehow a more perfect and beautiful image to watch. Usually they can not cite any psychological or biological reason for this and rely on the hypothesis that the eye & brain must therefore somehow have a similar, natural, preferred frequency rate of 24 frames per second.

It just occurred to me that such a natural frame rate of vision could explain some of the stobing-effects seen in pure daylight. Or, conversely, seeing such strobing might be evidence of this "natural" frame rate.

Except I truly believe such an idea is bunk.

Now I'm going to go into a long-winded background of this human frame rate idea, based not so much on science but my own observations & opinions of its history and politics. This is a bit of a tangent here, but it's an interesting tangent and, besides, you can't stop me. ;^) ;^)

As professional video quality has become more & more indistinguishable from 35mm motion picture quality, those who worked exclusively in the chemical-based motion picture format are being forced (my words) to work in video.

But they (most) are moving to video kicking & screaming (though they would never admit it).

In doing so, these powerful & influential Hollywood film people have spear-headed a "movement" towards *video* cameras that record at 24 frames per second. Normally & historically, video has been recording at 30 frames per second (or close enough for this discussion) based (originally) on the frequency of the power system.

Film on the other hand, has recorded at 24 frames since the early part of the 20th century - mostly because this is the slowest (and thus most economical) speed that can be run and still have intelligible sound right on the film itself.

As film people move into a video world, they have been attempting to make their cherished 24 frames per second the frame rate standard. And they just might do it, given the numbers of professional cameras now capable of 24 frames per second. Some might argue it is a done deal.

While there are lots of technical reasons why moving from the current 30 to 24 frames is a bad idea; the main reason cited for doing so continues to be this mysterious natural and emotionally beautiful frame rate that they believe is inherent in humans. It is a strong enough belief to change the course of motion picture video technology.

So, if this does exist, then it might explain the strobe-effect some of us claim to see in perfect daylight. On the other hand, one would then expect to see a host of interference patterns in anything that has a similar frequency to it, including monitors.

RBG

Sam5
2005-Sep-11, 09:42 PM
Hmm, interesting. The only thing I’ve ever noticed in real life that is equivalent to a visual “frame rate” is I’ve notice that some large rooms that are lit by florescent lights tend to “strobe” and flicker for me, and I don’t like it.

I wonder if we have any brain waves that are close to 24 or 30 cycles per second?

What I used to hear about the difference between 35 mm film compared to Video movies is that film has a softer contrast and more realistic color.

The original movie cameras shot at very slow speeds, such as 8 fps and 10 fps. The standard in Hollywood before sound was 16 fps. I used to read that they went to 24 fps only so the sound quality would be better (the faster the sound film moved the steadier the playback speed was). Like the old days of recording audio tape at 15 inches per second instead of 7 1/2 inches per second.

Back in the old days I always read semi-science articles about film that said humans don’t have a visual or mental fps rate and that we see light in nature on a continuous basis, and that early professional film at 16 fps was designed to stop the flicker effect of the slower speeds, since a movie camera shutter and a projector shutter was always closed about half the time. In other words, at 8 frames per second projection speed, the movie screen is black half the time each second, 8 times a second. So, 16 fps was fast enough so that no one noticed the flicker of the black screen. 24 fps was even better.

Video on the other hand has no “closed shutter”. One frame of video is on the screen until the next frame comes up, so there is no black screen between frames. However, there is the “scan rate” situation, which means each frame is scanned onto the TV screen and that does leave slightly dark bands on the screen (which can be seen if you photograph a TV screen with a still camera at a shutter speed of about 1/60 or 1/125 of a second. Or, just take an empty 35 mm camera with a shutter, take the lens off, and look through the back toward your TV screen and snap the shutter at high speeds and you’ll see the dark bands on the TV screen.

I’ve never heard the argument that 24 fps is more beautiful than 30 fps. I’ve always heard the movie/video argument in terms of color contrast and color saturation, so I don’t know about the 24 fps vs 30 fps debate.

There is a blur effect that people see in real life. This is why many of the best movie cartoons try to simulate this blur effect in the various stills of the cartoon. And you’ve probably noticed that video cameras set to high “shutter speeds” appear to produce an unnatural image. For example, with falling now and rain or people running and dancing.

Maybe there is more natural-looking blur effect at 24 fps than there is at 30 fps.

RBG
2005-Sep-12, 02:24 AM
The blur effect we see when objects go into motion, I would assume, is an artifact of our "persistence of vision". An image will "burn" into the retina and remain for a short moment. (In the extreme, looking at the sun for a moment will cause its image to remain in the field of vision for a long time.)

The "beautiful" image I refer to is creative for "optimally esthetically pleasing, all else being equal - not in terms of contrast or saturation. Such that a two frames per second image would be less pleasing to watch.

Traditional television video doesn't have frames per se. It is structurally made of "fields". Each field contains half of the information found in a frame. Each field presents its information on-screen on alternative scan lines from top to bottom. This would be different than a scan rate.

RBG

Extravoice
2005-Sep-12, 11:51 AM
There is a blur effect that people see in real life. This is why many of the best movie cartoons try to simulate this blur effect in the various stills of the cartoon.

I'm drifting a little off-topic, but when I first saw "The Lion King" I was absolutely "wow'd" by the opening scene where the "camera" focuses on ants crawling on a branch in the foreground and then refocuses on the amimal parade in the background.



And you’ve probably noticed that video cameras set to high “shutter speeds” appear to produce an unnatural image. For example, with falling now and rain or people running and dancing.


I've seen it, and in general I don't like it. It is used extensively for sporting events where I assume they need it for high quality slow-motion playback.

Sam5
2005-Sep-12, 04:43 PM
I'm drifting a little off-topic, but when I first saw "The Lion King" I was absolutely "wow'd" by the opening scene where the "camera" focuses on ants crawling on a branch in the foreground and then refocuses on the amimal parade in the background.




I've seen it, and in general I don't like it. It is used extensively for sporting events where I assume they need it for high quality slow-motion playback.

Yeah, I don't like it either. It's like watching everything under a flashing strobe light. I see on local news that some photographers like to use the higher shutter speeds in snow storms and I don't light that either, because every snowflake seems to go downward in a jump, jump, jump manner rather than in a nice smooth downward flow.

NEOWatcher
2005-Sep-12, 07:15 PM
Film and lights in my mind are a given. Even in daylight, there are sometimes enough lights to give some of the effect (since the 60Hz is governed at the power station, a whole city is synchronized)
Now for the non-artifical daylight effect. I have 2 off-the-cuff notions..
1) Shadows, as the wheel is moving the shadow is moving slightly differently. This could cause optical illusions.
2) Your eye is trying to focus on a portion of the wheel, but since it is moving, it is drawing your eye around with it. The constant "rebounding" of the eye back to the first spot may have some effect.

Swift
2005-Sep-12, 07:31 PM
Hopefully this isn't too off topic, but I just read about an interesting study as to why our brain normally doesn't "notice" blinking. That is, even though people normally blink 15 times a minute, we are not consciously aware of the world getting dimmer/darker.

This (http://www.physorg.com/news5431.html) link is from the physorg.com website, but I also read about it in the September issue of Photonics Spectra magazine.

Blinking temporarily switches off parts of your brain, according to a study published in the latest issue of Current Biology. The University College London (UCL) team found that the brain actively shuts down parts of the visual system each time you blink, even if light is still entering the eyes. Their findings could explain why you don’t notice your own blinks.

Ricimer
2005-Sep-12, 09:03 PM
of course, the moment you say that, i notice my blinking. Then I think about the other involontary motions we never notice, like breathing. but now that I've noticed, i have to control it...and I do a worse job than if i left it well enough alone.

Gahh!

Lets see if this works: Who yawns after reading this (or feels like yawning?).

NEOWatcher
2005-Sep-13, 12:33 PM
of course, the moment you say that, i notice my blinking. Then I think about the other involontary motions we never notice, like breathing. but now that I've noticed, i have to control it...and I do a worse job than if i left it well enough alone.

Gahh!

Lets see if this works: Who yawns after reading this (or feels like yawning?).
Thanks alot, now I need to get my mind off of what my body's doing. (pant, pant, blink, pant, huff, yawn, pant, blink...)

Sam5
2005-Sep-13, 05:45 PM
I noticed something interesting several years ago when I was trying to identify some of the Mexican music tunes in “Treasure of the Sierra Madre”. After some practice, I was able to mentally tune out the loud dialogue of the film and concentrate on the background singing. Now I can do either, tune out the music and listen to the dialogue, or tune out the dialogue and listen to the music. But I have trouble now listening to both the dialogue and the background music at the same time, as I originally heard them together the first dozen or so times I saw the film.

jkmccrann
2005-Oct-21, 05:06 PM
Stifled a yawn after I read that Rici! hehehe

JohnD
2005-Oct-21, 06:10 PM
All,
returning to the original theme, I see another ?allied? phenomenon on my TV. It is positioned so that walking by the room, I can see the screen through a slit between the open door and the door frame. Walking by, or standing still and moving my head, I can see vertical black lines on the screen, that are either truly vertical or sloped, according to how fast the eye is moving, side to side.

Try it yourself! How does this happen? The lines painted on the screen by the electron beam are horizontal.

John

Ken G
2005-Oct-21, 07:07 PM
2) Your eye is trying to focus on a portion of the wheel, but since it is moving, it is drawing your eye around with it. The constant "rebounding" of the eye back to the first spot may have some effect.
I like this idea, that's where my money is. There's a natural frequency at which your eye moves back to the original location as you watch a propeller. I would think that persistence of vision would be a gradual rather than sudden fading, so it would be harder to pick out a specific frequency, but maybe that could work too.

dzohar
2005-Oct-31, 11:18 AM
I searched internet and didn't find any video example of this effect (reverse rotating wheel). Does anybody know where I could find it?

The Mangler
2005-Nov-01, 02:31 AM
On the freeway when you're driving next to a big truck. Or when the light turns green and the car next to you is accelerating faster (or slower) than you.

Jens
2005-Nov-01, 02:37 AM
I like this idea, that's where my money is. There's a natural frequency at which your eye moves back to the original location as you watch a propeller. I would think that persistence of vision would be a gradual rather than sudden fading, so it would be harder to pick out a specific frequency, but maybe that could work too.

I think this is a wonderful hypothesis, and my initial feeling was that it's true. There's one doubt I have, though. The sample may be too small, but I turned on a fan in my office and asked fellow workers whether they saw the same effect. And they all did. But the thing I wonder is, if the effect is a natural speed at which the eye moves, wouldn't that speed vary from person to person, depending on their age or muscle strength, for example? Why would all humans move their eyes at the same speed? Maybe they do, and there's some reason for it. But I would recommend that other people try spinning something until you see it go backwards, and then ask people around you whether they see the effect at the same speed or not.

01101001
2005-Nov-01, 03:27 AM
The sample may be too small, but I turned on a fan in my office and asked fellow workers whether they saw the same effect.
Fluorescent lights (which strobe) or incandescent lighting?

Ken G
2005-Nov-02, 08:42 PM
Important question, you always have to watch out for the light source. Best is to turn off the fluorescent lights and use daylight or light bulbs. I remember seeing some interesting effects for propeller planes starting up, in broad daylight, but now I can't remember exactly what I saw. Can anyone describe an effect of this type they've seen in daylight? Jens' point is interesting, you might not expect everyone to see the same thing at the same time if it was physiological.

Ehmie
2008-Mar-15, 08:56 PM
We've all seen it.
It is not exclusive to video, or film, or any kind of recorded medium.
When you are on the freeway looking at the cars going by, sometimes the wheels can indeed "look" as though they are going backwards.

There have been plenty of explanations typed up on the "retro-grade" effect we see when this happens on film. Basically, if a wheel is filmed at 24 frames per second, and it is also spinning at that exact speed, it will "go recorded" as if it were still.

Some might say that the wheel in the recording "is" still. After all, every 1/24th of a second, we see the exact same picture of the wheel.

This happens in other mediums as well.
Sound:
It has been observed that different animals all have different sensitivities to different areas of the sound spectrum. As we can imagine, cats seem to tend toward being very good at hearing high-pitched sounds, say, those made by a mouse rustling across a leaf. etc. the main idea being that it is a trait that usually can be connected with survival...go figure, Darwin.

At any rate (excuse the pun), Humans generally hear best within the range of 500-5000 Hertz, but as we go away from that range, things begin to get quieter and quieter until we just can't hear them at all. This, they say, in general is 20-20,000 (vibrations measured in units invented and named after a guy called Hertz)

But if you notice, CD digital recordings are done at a bit over 40,000 vibrations (44.1khz actually). Why?

Well if you record a tone that goes on and off (so to speak, they are really just peaks of vibrations) but you only capture the parts when it is "on", what you end up hearing on the recording is very similar the wheel "being" still on the film. What you hear is a "constant" tone. This can be heard in poor quality digital Mp3's etc as little high pitched "tin can" tones and sounds morphing and changing in the background of music, so to speak.

The reason we sample at twice the rate of what the human can actually hear is to account for this. Just as if we recorded the wheel at twice the frame rate, we would have twice as much recorded on film, of what the original situation "really" was.

But let's go back to the situation of seeing it with our bare eyes. What are we seeing?
Through inference we believe that the wheel is in fact turning. But our perception mechanism tells us that it is still.

It makes me think of black holes.
Here is a situation where we know we can't see something.(knowledge of no-knowledge) It is a limitation of our own perception. We see light from stars behind the black hole disappear as the go behind it and then as they come out the other side of the black hole, the light reappears.

All of this seeks to question the nature of the reality we perceive vs. a human-invented (and assumed) story-term called actual "reality"....which, according to definition, we have NEVER received, unfiltered by the former.

My question is, since we can only have perceptions in this world through our individual perception mechanisms, what are WE doing?


What if you could speed up your frame rate? To see into the gaps of time in-between what everyone else were seeing. would you?
What if you saw something that only blinked "on" at a rate that just always happened to be when all humans perceptions were "blinked off"? (or the parts that humans noticed were culturally rejected as "uncontrollable", "mystery" or "random"). If you told people, what would they say? better yet, What would they hear? how could you tell them?

A light bulb is actually a fast flicker, so fast, we don't see the "off" blinks.

What we are talking about here is what I'd call "other dimensions" that, unbenounced to us, could exist, (in my mind, naturally-MUST-exist) at the same time and in the same space as our "proper reality".

Past that, I don't believe that science in it's communal-aspect can study this much further. It becomes what some people call Quantum. (others call miscommunication).

Yet to study this for one's self becomes a spiritual journey where it's an entire world anew to explore without any history.

It begs the question: Does it exist if we don't notice it?

I recently almost died. In fact, I was so close that I had to psychologically accept it.....as "reality". The world was,( at least as far as I would be concerned), as I had known it. I would be getting nothing new from my perception-mechanism. During that "time" it seemed to me that my whole life was simply one "blink-on" of the light. One frame.

It turned out that the "reality" of the situation was not as I had perceived (haha the irony). I live on, and think about that wheel and it's recording.

My best goes out to you all for what it's worth.
Hope all is "really" well.

Ken G
2008-Mar-15, 10:01 PM
I recently almost died. In fact, I was so close that I had to psychologically accept it.....as "reality". The world was,( at least as far as I would be concerned), as I had known it. I would be getting nothing new from my perception-mechanism. During that "time" it seemed to me that my whole life was simply one "blink-on" of the light. One frame.
Welcome to the forum Ehmie, we are glad you didn't die! As for the issue of "what is real" and "what is time", the attitude normally taken here is that the words are so general that one could take a philosophical or poetic or any kind of "take" on them, but here we use the scientific take. So if we want to understand time, we need to ask how to measure it. I would submit that the "snapshot" of a life you refer to can be measured with two clocks, one embedded in the memory that recorded each "tick" of your life, and the one that was ticking while you were conceptualizing that life. The former clock had millions of ticks, but the latter one was just a fraction of a tick, into which all those former ticks were compressed. This is a lot like relativity, where any number of ticks on one clock can be compressed into a single tick of another clock. It's all a matter of reference frame. But the concept of "reality" survives in the way we know how to transform between the frames-- we know (some do anyway) how considering a whole life in an instant will appear to turn it into a snapshot, or how looking at a turning wheel with a given frame rate will make it look stationary. In other words, we have gone one better than "seeing is believing".

pg_chelsea
2008-Mar-16, 01:42 PM
Hi all,

Something I noticed years ago learning to play the bass: If you pluck a string and look at it with a TV in the background, you see sine waves. Pretty neat. BTW, this also works with the guitar but the effect is more noticeable with lower tones.

I told myself that one day I would figure to tune the instrument using this phenomenon...sigh...never did.

cheers

neilzero
2008-Mar-17, 02:18 AM
Floresent tubes drop to about half brightness 120 times per second. About half brightness for USA television, but 60 times per second. Computer screens, and compact floresents lights have refresh rates up to about 200 times per second, so all behave like a strobe. persistance of the phosphor (and some other factors) detemines how close to black you get when the voltage crosses zero. The incandesent bulb filliment only cools a few degrees during the near zero voltage millisecond, so the strobe effect of an incandecent is negligible, unless the power is considerably less than 50 or 60 hertz which is standard, except many airplanes use 400 hertz to reduce the weight.
I have not personally observed the eye sample rate perhaps because my left eye vision is poor. Neil

Van Rijn
2008-Mar-17, 07:57 AM
Floresent tubes drop to about half brightness 120 times per second. About half brightness for USA television, but 60 times per second. Computer screens, and compact floresents lights have refresh rates up to about 200 times per second, so all behave like a strobe.


CFLs usually have electronic ballasts (http://en.wikipedia.org/wiki/Electrical_ballast#Electronic_ballasts) these days. So do modern tube installations:


An electronic lamp ballast uses solid state electronic circuitry to provide the proper starting and operating electrical condition to power one or more fluorescent lamps and more recently HID lamps. Electronic ballasts usually change the frequency of the power from the standard mains (e.g., 60 Hz in U.S.) frequency to 20,000 Hz or higher, substantially eliminating the stroboscopic effect of flicker (100 or 120 Hz, twice the line frequency) associated with fluorescent lighting (see photosensitive epilepsy).

Extravoice
2008-Mar-17, 12:38 PM
The incandesent bulb filliment only cools a few degrees during the zero voltage millisecond, so the strobe effect of an incandecent is negligible...

I have a small 4W "night light" in my bathroom that is controlled by a photocell such that it only comes on at night. This light definately exhibits a stroboscopic effect. I suspect that the electronics in the photocell-control include half-wave rectifiers that result a more pronounced flicker.

That said, none of the other incandescent bulbs in my house exhibit flicker.

Exposed
2008-Mar-17, 03:00 PM
IIRC, your eye/brain can interpret 28fps.

Actually no, the eye and brain do not work in this manner. This is one of the most common misconceptions spread about, somehow pegged to fact that NTSC and PAL standards have both low framerates.

The human eye is capable of detecting extremely short bursts of light (fractions of a second), as studies have shown, and can distinguish minute changes in high frequency environments.

For example, you will definitely notice a difference in moving images involving a CRT monitor with a 60Hz refresh rate (updates 60 times per second) compared to a CRT monitor with 100Hz refresh rate, or an LCD monitor with a 2-4ms response time compared to an LCD monitor with 16ms response time.

The PAL/NTSC framerates are low, but can seem fluid due to motion blur. This is where the eyes "have a 24-30 FPS limit" misconception typically derives from. A good experiment to bash this misconception is to play a PAL/NTSC movie side by side on two LCD monitors, one with a 60Hz refresh rate (typical for most early LCD's) and the other with 120hz refresh rate (new models). Both movies are at 24fps (assuming NTSC), but your eyes will definitely pick up less motion blur on the 120Hz screen.

EricM407
2008-Mar-18, 05:42 PM
The PAL/NTSC framerates are low, but can seem fluid due to motion blur. This is where the eyes "have a 24-30 FPS limit" misconception typically derives from. A good experiment to bash this misconception is to play a PAL/NTSC movie side by side on two LCD monitors, one with a 60Hz refresh rate (typical for most early LCD's) and the other with 120hz refresh rate (new models). Both movies are at 24fps (assuming NTSC), but your eyes will definitely pick up less motion blur on the 120Hz screen.

I'm not sure that's a good explanation of that test. You'd actually be testing the ability of the LCD to change its pixels quickly, and what you'd see are pixels in transition, not one image followed by another distinct image 24 times per second.

Two LCDs both at 60 Hz could have very different amounts of this kind of motion blur. Or, put another way, one screen capable of multiple refresh rates will have the same amount of this blur no matter its refresh rate. 24 FPS material played at 24 Hz will look identical to the same material played at 120 Hz. There is no difference between a frame held on the screen for 1/24th second and the same frame refreshed 5 times in 1/24th second, since nothing is actually changing in those refreshes. The response time of the LCD is only a factor when you go to the next frame, and that will happen 24 times a second, regardless of refresh rate.

RBG
2008-Mar-27, 11:16 PM
EricM407, you might be right about that, mainly because I don't know enough about the decay physics of LCDs.

But consider this: When you watch a movie at a theater, they are shown at 24 frames a second (exotic digital possibilities aside). But it is my understanding that the projector film gate artificially disrupts the projection of each frame so that the eye is given 48 images to watch. A shutter runs through each frame. The purpose of this is to up the flicker rate to make the strobe effect of 24 frames less noticeable. They do this in spite of the fact that the human eye has its own persistence of vision, much like a poor LCD pixel. (That's why you can see a trailing image of your arm if you move it quickly in front of your body.) Likewise, home NTSC TV introduces that beneficial shutter effect by running half a frame information on one field, followed by the other half on a subsequent interlaced field. Each field is up for 1/60th of a second.

A low response rate of an LCD pixel comes into play when it does not refresh the image very quickly. That is, a bright pixel from one frame could be on-screen and still decaying even as the frame has already been updated with a new, perhaps darker, scene.

Certainly, a movie cannot be updated with new frames any quicker than was originally filmed (ie: 24 frames per second) nor any quicker than encoded onto a DVD (ie: 24 frames... or even 30 frames) but a high screen refresh rate will at least rid the viewer of the irritating strobe effect that can often be seen in bright NTSC sources and, for me, all PAL sources.

I think any image would benefit from a flicker rate higher than 60 cycles per second.

RBG

skrap1r0n
2008-Mar-27, 11:43 PM
holy thread resurrection batman...

Anyway, what i mean by 28fps is that much slower and it's not interpreted as smooth motion. Of course you don't actually see in FPS, theres not a flickering shudder in your eye blocking light. A camera or animation, however, is different in that it assembles a series of stills in an progressive order that simulates motion.

Jeff Root
2008-Mar-28, 12:22 AM
Acknowledged that this is a very old thread.

I didn't see what I'm sure is the correct answer to the original question:

The sun is essentially a point source. When it reflects spectrally from
a flat surface, the light is reflected in a specific direction. If your eye
is in that direction, you will see the light, otherwise, you won't. If the
surface is curved instead of flat, the light will be more or less spread
out, but it will still go in specific directions.

When a fan or a fancy hubcap rotates, the parts of the fan blades or
parts of the hubcap that reflect sun to your eyes change constantly.
First one fan blade will reflect light to your eyes, then it will move to
a different position and you will no longer see that light, but the next
blade will move into the position formerly occupied by the first blade,
and you will see the light reflected from it in the same place that you
saw the light from the first blade. Same with fancy hubcaps.

-- Jeff, in Minneapolis

RBG
2008-Mar-28, 04:49 AM
Jeff Root #52:

I think you may really be onto something there. A brighter spot or different light texture that is reflected to the eyes in a strobe-pattern related to however many "spokes" or patterned reflective surfaces are on the wheel.

That might explain why the wheel will appear to stop, but does it then explain why it might then appear to rotate the opposite way? I would speculate that this phenomenon has something to do with the rate of "flashes" as it relates to the number of "spokes" as well as the relation to the rate of revolutions of the wheel. It's an interference pattern that is generated from all that.

RBG

Ehmie
2008-Mar-28, 05:53 AM
That's a really good point.
If I'm getting your drift correctly. You're bringing emphasis to the fact that there is something of a flicker or blinking flashes observed by the viewer, due to the shininess?

That's a brilliant observation and it seems to be true to me, as, when a bright light shines in my eyes, it can be so bright, that it's blinding.

So then we'd be getting more of an zoetrope effect from shinny things.

Jeff Root
2008-Mar-28, 02:19 PM
That might explain why the wheel will appear to stop, but does it then
explain why it might then appear to rotate the opposite way?
Good question. I should have addressed that.

I think the spinning thing will only appear to rotate in the opposite
direction -- or rotate slowly forward -- if the light source is moving or
the relative positions of the rotating object and observer are changing.
The part of the disk that is reflecting light to your eye will then also be
changing.

If you are looking at a ceiling fan, and sunlight reflected from a sidewalk
outside hits the undersides of the fan blades, you might see that light
reflected from the fan blades when they are at the 3 O'clock position.
Move a bit to one side, and the reflection is at the 2 O'clock position.
This is what would happen with fancy hubcaps on a car that is moving
at high speed relative to the road, but at low speed relative to you in
an adjacent lane.

Pure Euclidean geometry!

(By "fancy" I just mean that the hubcap must have some angled or
colored surfaces that vary circumferentially. And of course they need
to be evenly-spaced. Simple holes to get at the wheel bolts will do if
the edges of the holes reflect light.)

-- Jeff, in Minneapolis

EricM407
2008-Mar-31, 05:46 PM
EricM407, you might be right about that, mainly because I don't know enough about the decay physics of LCDs.

But consider this: When you watch a movie at a theater, they are shown at 24 frames a second (exotic digital possibilities aside). But it is my understanding that the projector film gate artificially disrupts the projection of each frame so that the eye is given 48 images to watch.

They have to do this, or you'd see the film strip itself moving across the screen (think of a slide projector if you spun the thing really fast). They want light going through frame 1, then light off while the film advances, then light going through frame 2, etc. So you have light cycling on-off, which causes flicker. And to get rid of that they do open and close the shutter multiple times per frame.

But there is no mechanical reason for a blank period on an LCD, and there isn't one. They don't go:

frame 1, blank, frame 2, blank, frame 3, blank (like a projector)

They go:

frame 1, frame 2, frame 3

No blank period, no light cycling, no flicker. I've seen 24 fps material on an LCD refreshing at 24 Hz, and it doesn't strobe in any way. It also looks no different whether the display refreshes at 24 Hz, 48 Hz, 72 Hz, etc.


Likewise, home NTSC TV introduces that beneficial shutter effect by running half a frame information on one field, followed by the other half on a subsequent interlaced field. Each field is up for 1/60th of a second.

There's really nothing beneficial about that effect on a television, and LCD's aren't interlaced and don't display things in that fashion.


A low response rate of an LCD pixel comes into play when it does not refresh the image very quickly. That is, a bright pixel from one frame could be on-screen and still decaying even as the frame has already been updated with a new, perhaps darker, scene.

That's right, but the rate of these changes is the rate the material was captured at, not the refresh rate of the screen. If it's 24 fps material, it looks like this:

(120 Hz display)
frame 1, frame 1, frame 1, frame 1, frame 1 -> frame 2

(24 Hz display)
frame 1 -> frame 2

Frame 1 is on screen for 1/24 second in both cases. Every 1/24 second, there will be a change to a new frame, so both LCDs will be changing their pixels every 1/24 second. One of them may have a faster pixel response time, but that's a separate issue. The required response time (for it to look pleasing to you) is the same in either display.


Certainly, a movie cannot be updated with new frames any quicker than was originally filmed (ie: 24 frames per second) nor any quicker than encoded onto a DVD (ie: 24 frames... or even 30 frames) but a high screen refresh rate will at least rid the viewer of the irritating strobe effect that can often be seen in bright NTSC sources and, for me, all PAL sources.

I think any image would benefit from a flicker rate higher than 60 cycles per second.


It definitely would if there is any cycling happening. I couldn't stand anything less than 75 Hz on my old CRT monitors, but I'm looking at an LCD at 60 Hz right now... no flicker.

RBG
2008-Apr-03, 11:16 PM
EricM407
"They have to do this, or you'd see the film strip itself moving across the screen"

But what I am talking about is, while the frame is solidly pinned and motionless in the gate, a shutter is moved quickly through the middle of the momentarily *still* projected image purely to introduce a 2X flicker effect. This is especially beneficial when viewing a bright source.

"A simple experiment will show that the flicker rate must be of the order of 50 per second for it not to be obvious. For much of the 'silent' period, films were shot at roughly 16 frames per second and shown on a projector with a three-bladed shutter. Each individual frame was shown three times, so around 48 screen images were projected every second"
http://tinyurl.com/369mxn

From "Wikipedia NTSC": "In the complete raster ... to yield a *flicker-free* image at the field refresh frequency of approximately 59.94 Hertz."

Wikipedia "Flicker (screen)": "if a CRT computer monitor's vertical refresh rate is set to 60 Hz, most monitors will produce a visible "flickering" effect, unless they use phosphor with long afterglow. Most people find that refresh rates of 70-80 Hz and above enable flicker-free viewing on CRTs."

As I say, many people (ie: me) (especially NTSC-watchers) can see the strobing on British PAL 25 fps which is on the minimum edge of obvious flicker when field rate is doubled to 50.

Trumbull's Showscan film format projected beautifully, if inefficiently, at 60 frames per second.

According to the Wikipedia item, LCDs have reduced flicker perception due to the transistorized "keep" state of each pixel, but the backlight must nevertheless still have a high refresh rate - higher than the pixel refresh rate. That appears to add the required high flicker rate that 35mm movies added mechanically.

RBG

EricM407
2008-Apr-08, 03:28 PM
EricM407
But what I am talking about is, while the frame is solidly pinned and motionless in the gate, a shutter is moved quickly through the middle of the momentarily *still* projected image purely to introduce a 2X flicker effect. This is especially beneficial when viewing a bright source.

I know exactly what you're talking about. The thing is, they only do this because there is no mechanical way to avoid the light pulsing, so they have to make it pulse at a rate high enough for humans not to see flicker.

There is not light pulsing on an LCD in the way it refeshes the screen, no flicker, so no visual reason to refresh at any rate higher than the capture rate of the media.


"From "Wikipedia NTSC": "In the complete raster ... to yield a *flicker-free* image at the field refresh frequency of approximately 59.94 Hertz."

That standard had to conform to the existing CRT televisions of the time.


Wikipedia "Flicker (screen)": "if a CRT computer monitor's vertical refresh rate is set to 60 Hz, most monitors will produce a visible "flickering" effect, unless they use phosphor with long afterglow. Most people find that refresh rates of 70-80 Hz and above enable flicker-free viewing on CRTs."

All true. Yet I'm looking at 60 Hz on my LCD monitor here, and it doesn't flicker one bit.


As I say, many people (ie: me) (especially NTSC-watchers) can see the strobing on British PAL 25 fps which is on the minimum edge of obvious flicker when field rate is doubled to 50.

That can only be because you're watching it on a CRT. If you didn't have blanks between each refresh of the screen, there would be no strobe. You need light on-light off-light on-light off to have strobe, right? If you just have constant light, no strobe.


According to the Wikipedia item, LCDs have reduced flicker perception due to the transistorized "keep" state of each pixel,

This means that whether you refresh 24 FPS material at 24 Hz, 48 Hz, 72 Hz, etc. - it will look exactly the same. Not one single pixel on your screen will change until it's time for the next frame of new data. And when it is time for the next frame, it comes immediately after the previous frame. The screen doesn't go black between the two frames, as on a projector or CRT. That black period alternating with light periods is what gives you the strobe effect.


but the backlight must nevertheless still have a high refresh rate - higher than the pixel refresh rate. That appears to add the required high flicker rate that 35mm movies added mechanically.


No, the backlight frequency has nothing to do with what we're discussing. Projectors bulbs run at high frequency too. It's not related to the frame rate they're projecting or whether you see flicker.

RBG
2008-Apr-09, 12:06 AM
Projector bulbs are like CRTs with long afterglows. Watch an incandescent light as it turns off to see that effect as the filament energy drains to zero. That is why you can't see the strobing 60 cycle current. Then watch a set of bright LED Christmas lights and many people, including me, can see a slight, annoying, flicker. That's because the light does not decay into subsequent cycles. It goes to "off" as the polarity changes. The florescent light from an LCD screen is similar and thus must be increased in frequency to avoid the perceived power-cycle flicker. I believe the backlight runs about 100 cycles / second for that purpose. Of course, when you are looking at any part of an LCD image that is white (to highlight the most obvious color), you are basically looking directly at the florescent light. It would drive you nuts if it was running at 24 cycles per second, in synch with the frame rate. So LCD technology as they are generally built presently, still have to deal with refresh rates, only in a different way from CRTs & film projection.

Here is a link to an article that actually refers to the LCD pixel panels themselves exhibiting flicker: "LCD Screens Don't Flicker, Or Do They?" Last para: "Contrary to popular belief, LCD panels do exhibit flicker."
http://tinyurl.com/44qy6n
"Active Matrix LCD displays often have some flicker because their refresh rates are reduced for performance reasons." Likewise gas plasma & other LCDs have reduced flicker. http://www.displaymate.com/flicker.html

I suppose if you remove all the technical artifacting from projection, televisions, interlaced monitors, progressive monitors and LCDs, yeah, you probably would get an acceptable image at a true 24 progressive frames per second. But even then, you'd have to deal with the choppiness inherent in most computer rendered video: a visible gap that can be seen between the moving object and its afterimage in the eye. "A culture of competition has arisen among game enthusiasts with regards to frame rates, with players striving to obtain the highest fps count possible."
http://en.wikipedia.org/wiki/Frame_rate

RBG

XweAponX
2008-Apr-09, 12:25 AM
Hey, I think I may have found a slight error or at least a significantly incomplete answer to the question:

#19: "Why does a wheel seem to move backwards as it speeds up?"

on the BA's 1996 MADSCI Q&A.

http://www.badastronomy.com/mad/1996/wheel.html

He rightly relates the phenomenon to the frame rate of film, but I'm surprised that there was no mention of this effect as seen in real life.

I always thought this was strictly a film-thing myself until I happened to see this a number of times, directly with my own eyes. No film.

Has anyone else seen this too? I'm sure it exists. And I would guess it is somehow caused by the number of spokes on the wheel in relation to the number of revolutions / minute.

Can anyone help me out here?

RBG

Does it have anything to do with the doppler effect? Or is that sound-only?

RBG
2008-Apr-09, 05:06 AM
Hey...hey, that's a bit too on-topic, don't you think?

Doppler Effect is light as well, that's why it pertains to the red shift of the expanding Universe.

But even if you could detect such a shift in a rotating wheel, presumably it would only affect the color of the bits moving toward you and away. But since you don't see a red shift in the color of a train or jet as it races by & away, this can't be applicable. Though theoretically, I would think there must be some infinitesimally small amount of physical red shifting. There, back off-topic. I knew I could do it.

RBG

Kaptain K
2008-Apr-09, 06:40 PM
I just finished rereading this thread. Kucharek (humming) and extravoice (chattering teeth) illuminated the heart of the matter in re apparent motion of wheels in daylight. When driving down the road in traffic, every little bump moves your eye in random direction. Each little jiggle moves the image to a different place on the retina. Our brain interprets those discrete images as stills and sees them as a strobe effect! you can get a similar still image effect by flicking your vision to and away from the wheel.

EricM407
2008-Apr-10, 12:55 AM
Projector bulbs are like CRTs with long afterglows. Watch an incandescent light as it turns off to see that effect as the filament energy drains to zero. That is why you can't see the strobing 60 cycle current. Then watch a set of bright LED Christmas lights and many people, including me, can see a slight, annoying, flicker.

I'm going to suggest to you that these annoying Christmas lights are made to flicker. Because I don't really think driving an LED without flicker is a difficult task to accomplish. I don't have a lot of experience with Christmas lights though, I'll admit.


That's because the light does not decay into subsequent cycles. It goes to "off" as the polarity changes. The florescent light from an LCD screen is similar and thus must be increased in frequency to avoid the perceived power-cycle flicker. I believe the backlight runs about 100 cycles / second for that purpose.

I think it's much higher than that on a CCFL. Do you think you can detect this flicker? It has absolutely nothing to do with the refresh rate or the frame rate anyway...


Of course, when you are looking at any part of an LCD image that is white (to highlight the most obvious color), you are basically looking directly at the florescent light. It would drive you nuts if it was running at 24 cycles per second, in synch with the frame rate.

If the frequency of the backlight was 24 Hz, yes, that would probably drive you nuts. But to do that something would have to be broken, and that's not the refresh rate anyway. The refresh rate could be 24 Hz, and it would look splendid with 24 FPS material.


So LCD technology as they are generally built presently, still have to deal with refresh rates, only in a different way from CRTs & film projection.

Well, for the backlight, they have to deal with the nature of AC the same way every fluorescent light you come across does. And they do this independent of the refresh rate.


Here is a link to an article that actually refers to the LCD pixel panels themselves exhibiting flicker: "LCD Screens Don't Flicker, Or Do They?" Last para: "Contrary to popular belief, LCD panels do exhibit flicker."
http://tinyurl.com/44qy6n

Your ability to find that one sentence and ignore everything else is unique. You really only had to go one sentence further: "Simple potentiometer adjustments can be made to minimize the effect since LCD flicker arises from an offset of the common voltage, not a refresh signal. "

So unfortunately, refreshing an LCD at 120 HZ or even 1200 Hz isn't going to help with the form of flicker being discussed in that article. But fortunately, you can't see it anyway.


I suppose if you remove all the technical artifacting from projection, televisions, interlaced monitors, progressive monitors and LCDs, yeah, you probably would get an acceptable image at a true 24 progressive frames per second.

I know you can get an acceptable image at 24 Hz with nothing more exotic than the technology that was common several years ago, because I've seen it. And it's better than any 60 Hz TV I've ever seen, because it doesn't have that lopsided judder. But go to an electronics store and look for yourself.


But even then, you'd have to deal with the choppiness inherent in most computer rendered video: a visible gap that can be seen between the moving object and its afterimage in the eye.

Well, yeah, 24 FPS is choppy... even if you refresh it at 120 Hz on an LCD. In fact, I would say it is precisely as choppy at 120 Hz as 24 Hz. There is no, none, nada, zero difference.

RBG
2008-Apr-10, 04:26 AM
EricM407:

LED lights that flicker 60 times a second are not doing it for effect. Most people can't perceive 60 cycles. But google "LED Christmas light flicker 60 Hz" and you'll find a sample of at least a hundred folks it drives bananas.

Florescent lighting used to similarly really irritate me. "Older fluorescent fittings using a magnetic mains frequency ballast do not give out a steady light; instead, they flicker (fluctuate in intensity) at twice the supply frequency. While this is not easily discernible by the human eye..."
http://en.wikipedia.org/wiki/Fluorescent_lamp

"LCD flat panels do not seem to flicker at all as the backlight of the screen operates at a very high frequency of nearly 200 Hz"
http://en.wikipedia.org/wiki/Flicker_fusion_threshold

I can actually see the high flicker on my own modern HPw2207 LCD as I write this. It's hard to believe this light could be running at 200 Hz. But that is precisely why they needed run them so high. It's related to human Flicker Fusion Threshold, the reason movie projectors, TVs & CRTs had to artificially introduce higher flicker frequencies.

My point re LCD flicker, of course, is that... LCDs flicker. (From more than one source as you acknowledge)... not whether it is related to a refresh signal. In fact my whole premise is that an LCD image has its own flicker problems that must be addressed; similarly film projection, television and CRTs. In each case, the solution involves provision of an acceptable flicker rate.

This thread started with the point that somehow LCDs were immune from flicker. That there was "no light cycling" involved with LCDs. Well of course there is. And your 24fps LCD "acceptable image" is only acceptable because the technology deals with LCD flicker.

That aside, agreed, most computer rendered video at 24 fps with an ultra-high refresh would look just as choppy. You need to up the frame rate itself for an improvement there.

"It is argued that games with extremely high frame rates "feel" better and smoother than those that are just getting by (referred to as "buttery smooth" by devoted gamers)."
http://en.wikipedia.org/wiki/Frame_rate

As I mentioned above, I would describe Doug Trumbull's 60fps Showscan as "buttery smooth" as well.

RBG

RBG
2008-Apr-10, 04:56 AM
Kaptain K:

I too have the "power" to momentarily freeze the rotation of certain objects (ie: fan blades) with the flick of an eye. So I'm almost ready to accept your explanation. The problem remains that those eye flicks whether purposeful or through car bumps, etc. would only produce one freeze frame, so to speak. And to get a rotating object to appear to reverse itself would require a whole long series of precisely timed eyeball-produced freeze frames. I read somewhere (above?) one interesting solution that builds on your idea and that is that some people can cause their eyeballs to vibrate (purposely or unconsciously I presume). The interference between the two frequencies could be responsible for the phenomenon.

But even after all the above, until I see & study the effect again, I have to admit to some creeping skepticism - whether I have ever seen it happen without any of the usual explanations.

RBG

Kaptain K
2008-Apr-10, 05:08 AM
An out of balance wheel, slightly warped brake disk, flat spotted tire or worn CV joint could cause a slight vibration that even when unnoticed could be enough to cause a wheel to appear to slow, stop or reverse.

EricM407
2008-Apr-10, 10:58 AM
EricM407:

My point re LCD flicker, of course, is that... LCDs flicker.(From more than one source as you acknowledge)... not whether it is related to a refresh signal.

Actually, that wasn't your point at all. Your point was this: "a high screen refresh rate will at least rid the viewer of the irritating strobe effect that can often be seen in bright NTSC sources and, for me, all PAL sources."

Which was wrong with respect to LCDs (or any other modern digital display). And since then you've spammed me with a bunch of increasingly irrelevant links that I don't think you're even bothering to read. I'm going to do what I should have done about five posts back.:lol:

RBG
2008-Apr-10, 08:14 PM
I've made a clear case, citing references, for the need and mechanism for ridding the viewer of irritating strobe effects first with film, then TV, and then LCD panels when you brought it up. That should have been enough.

RBG

Disinfo Agent
2008-Apr-22, 11:51 AM
This seems to be a recurring question. It's been discussed before, here (http://www.bautforum.com/general-science/65875-what-causes-seeing-fast-moving-objects.html) and here (http://www.bautforum.com/general-science/63880-why-do-fast-turning-car-wheels-appear-turn-slowly.html).

RBG
2008-Apr-25, 02:23 AM
That was great, actually. Especially the second one. Thanks.

RBG

NardK
2008-Jul-08, 01:58 AM
Hey, I think I may have found a slight error or at least a significantly incomplete answer to the question:

#19: "Why does a wheel seem to move backwards as it speeds up?"

on the BA's 1996 MADSCI Q&A.

http://www.badastronomy.com/mad/1996/wheel.html

He rightly relates the phenomenon to the frame rate of film, but I'm surprised that there was no mention of this effect as seen in real life.

I always thought this was strictly a film-thing myself until I happened to see this a number of times, directly with my own eyes. No film.

Has anyone else seen this too? I'm sure it exists. And I would guess it is somehow caused by the number of spokes on the wheel in relation to the number of revolutions / minute.

Can anyone help me out here?

RBG

I've seen this for years!:doh:
I've always been fascinated with the event, since a kid. The closest I can figure so far to what is happening is this:

Talking to optometrists and opthomalogists, I've heard that the eyes receive images in 'bursts' so to speak. That is to say what the eye sees (and it sees everything upside down to make things difficult, and the brain turns things right-side up).. is not a continous non- broken stream of images. What I have not found out is the number of images a eye receives per, say, a second.

What I think is happening is that the hub-cap wheel, when it is spinning at the same rate that the eye receives images, the wheel cover appears to stop moving, turning slowly, as it were, left or right. Furthermore, I understand, there is a extremely small fraction of a second when the eye sees 'nothing'. That in itself is interesting.

That's about the best I can figure out so far.

Extravoice
2015-Mar-09, 01:26 PM
Thread necromancy alert.
I was sitting at the stop sign at a T intersection the other evening and noticed the wheel stroboscopic effect in full force as cars passed in front of me...which reminded me of this thread.

We know that "sampling" causes the stroboscope effect, but my car has incandescent headlights, so I doubt any flickering came into play.
I think the cause was the vibration of my car. The engine was cold, and the 4-banger on my wife's Mini already creates a generous amount of vibration.

For full scientific rigor, I should go back to the intersection and repeat the experiment with the car engine running and stopped.
If I get around to it, I'll report the findings here.

profloater
2015-Mar-09, 04:28 PM
Thread necromancy alert.
I was sitting at the stop sign at a T intersection the other evening and noticed the wheel stroboscopic effect in full force as cars passed in front of me...which reminded me of this thread.

We know that "sampling" causes the stroboscope effect, but my car has incandescent headlights, so I doubt any flickering came into play.
I think the cause was the vibration of my car. The engine was cold, and the 4-banger on my wife's Mini already creates a generous amount of vibration.

For full scientific rigor, I should go back to the intersection and repeat the experiment with the car engine running and stopped.
If I get around to it, I'll report the findings here.evening... street lights? they will flicker.

Extravoice
2015-Mar-09, 05:50 PM
evening... street lights? they will flicker.

Yeah, that's a possibility in this particular case because it was around twilight and I didn't check for streetlights.

I've seen the effect in daylight, so I still think vibration may play a part.

I'm going to have to identify potential variables and take them into account. Maybe I'll present my findings in ATM ;)
(Or not, I don't think I could survive a full review by EigenState)

malaidas
2015-Mar-09, 09:42 PM
Firstly I would doubt that the vibration from the roads surface could cause this, it simply isn't going to cause a consistent vibration, because the surface is never evenly uneven. If it's vibration however it could come from the engine itself, running at the correct rpm, this would be smoothly transmitted through the chassis to the occupants.

Jeff Root
2015-Mar-10, 05:42 AM
malaidas,

Extravoice said he was sitting at a stop sign when he noticed
the effect, so the vibration would neccessarily be from the engine.

I think the explanation I tried to give in posts 52 and 55 are most
likely the correct explanation. But I may not have explained the
explanation all that well....

At the time, I developed and tested my explanation by watching
a ceiling fan turning at a moderate or low rate which allowed me
to see exactly what was happening to the light reflected from it.
The fan blades had a satin or glossy finish, allowing significant
spectral reflection. In other words, light from a small source was
reflected by the surface finish on the blades as if from a mirror,
so that my eyes had to be in the right place to see it. As the fan
turned, the locations of the reflected spots of light moved across
the blades. If I then walked past the fan while watching the
reflections, I would see the light spots moving backward, giving
the impression that the fan blades themselves were moving
backward.

Nothing to do with flickering lights or how human eyes work. Just
ray tracing. Simple geometry. The illusion requires the rotating
object to be spectrally reflective and have radially symmetrical
features like spokes or blades, it requires a smallish source of
light, and it requires relative motion between the rotating object
and the observer. So a flat or hemispherical hubcap won't do it,
a matte white or matte chartreuse fan won't do it, and light from
the whole sky won't do it, and even those fancy rotating hubcaps
some guys put on their cars won't do it unless the car is moving
past you or you are moving past the car.

-- Jeff, in Minneapolis

.

malaidas
2015-Mar-10, 02:53 PM
It's a reasonable theory Jeff, my question would be that this appears to rely on very specific factors which don't seem to explain the commonality of the experience