Explain this optical illusion


#1

This is something that I’ve noticed all my life and have never gotten the right answer.
It does not bother me, but it’s just always wondered whenever I noticed it.

When I was little and watching Westerns as a kid (in the 50s and 60s it was westerns or doctor shows)
I always noticed that when a wagons wheels reached a certain speed…the spokes appeared to be going backwards, or fluctuating between forward or backwards. You knew those wheels were going fast, but at a certain speed, they looked as if you were looking at them with a strobe light.

I worked with strobe lights while running a printing press when I was younger, so I know how that works.

For some time I thought it might be the speed that the camera’s shutter, but that cant be either.
I noticed that even with the naked eye this illusion will happen with certain automotive wheels.
There I got automotive into the conversation!!!

So can anyone tell me why this illusion happens with the naked eye???

Yosemite


#2

I think you started to answer your own question. It would have to do with a strobe effect.
Suppose a wagon wheel turns so that a particular spoke is at the “12 o’clock” position 24 times per second (I think that’s the frame rate for old movies). Then, each time you see a frame, you would be observing that same spoke, and it would appear to be standing still (at 12 o’clock). If the speed of the wagon wheel would change, the spoke would appear to move forward or backwards. Much the same as using a timing light on an engine and observing marks on the crankshaft pulley.


#3

With car wheels you’ll usually see that at night, it has to do with the type of street lights in the area.

Most people cannot notice the flicker in fluorescent lights or other types that use a ballast that have a flicker rate of 120 cycles per second (or 120 Hz). The light flicker may be detected by its stroboscopic effect. When objects move or rotate rapidly, they may be lit at or about the same position during each cycle or rotation.


#4

Tom has the right answer. The frame rate, whatever it is - I think 24 is correct - means you are seeing 24 snapshots flashed by each second to give the illusion of continuous movement.

If the wheel has 24 spokes, and it rotates at 1 revolution per second, the spokes appear to stand still. If we speed that to 2 revolutions per second, they still appear to stand still because the spoke is in the same position as one spoke before at one rev/sec, or 2 spokes before at 2 revs/sec. If the wheel varies between 1 and 2 revs/sec the spokes may appear to be going backwards at 1.1 to 1.49 revs/sec and going forward at 1.51 to 2.0 revs/sec. A longer explanation to expand on Tom’s post.


#5

I do understand and agree completely with your answers, whether it’s a strobe light, timing light or with the streetlight effect.

But I have noticed it in day light with the naked eye, and that is the effect that I need explained.

Yosemite


#6

Depends on where you’re watching it. Old Film movies are usually (but not always) 24fps, but when you watched them on TV in the old days, they were conerted to 29.97fps via a very-imperfect process, which introduced jerkiness and made the wheel problem even worse.

Modern movies are usually 60, unless the director is “artistic” in which case it’s anywhere from 24 to 48 to 60 to 120. Modern TV is anything from 23.976 for “film look” programs all the way up to 300fps.

Dealing with a jillion different framerates is just one reason video editors are often grumpy old farts. :wink:


#7

Your eyes have a flicker rate too. It works much like a digital video camera in that it takes an electronic image from your retina and passes it through the optical nerve to the brain. In very young children, the optical refresh rate, aka flicker rate can be as high at 60 fps or higher. No one knows for sure.

BTW, the flicker rate for most ballasts is only 60 Hz because most of them only conduct on one half cycle. They rarely use a full wave rectifier due to costs. CFL’s for some reason, not sure why but the bulbs fluorescence is more persistent, similar to an incandescent bulb so it doesn’t turn completely off. Some LEDs filter the pulses into a DC so they stay on, some flicker at 60 Hz like most long tube fluorescents.

Anyway, back to the subject, your brain can freeze frame something for about a half second or less so a ceiling fan at low speed and momentarily freeze the blade in position. This happens more as you grow older as your flicker rate slows down with age.

The slowing of the flicker rate is why older drivers slow down. Things move more between each refresh of their eyes so things appear to be moving faster to them. Your reaction time slows down as your age partly because of the lower flicker rate.

Young to middle age adults typically have a flicker rate of around 30 fps, children and teens can have a flicker rate of around 40 fps. Film movie cameras took movies at 24 fps, as fast as they could go when the technology was first developed, (8mm filmed at 16 fps). The movies would appear to flicker badly when played back on a projector that projected at the same rate. Movie projectors were changed to project each image twice to give a 48 image per second rate which is higher than most people internal flicker rate so the movie appears to flow smoothly. TV vidicon cameras, analog recording on magnetic tape, filmed at 30 fps, analog TVs would again project each image twice but offset a half line so there is no visible flicker and no lines on the screen. Old movies were televised using a special projector that would alternate showing each frame of the film, with 3/2. That is the first frame would project 3 times, the next 2 times getting the 24 fps movie to keep time on a 30 fps TV.

Now to go off track a bit. I developed a theory when working with schizophrenic patients that they seemed to be very sensitive tot he flicker of fluorescent lights. They would calm down (somewhat) almost immediately when going outdoors or when the fluorescent lights were turned off and would become more agitated when exposed to fluorescents almost immediately. My theory is that they may have much higher flicker rates than most of us.

Long exposure to flicker can also cause something called flicker vertigo. This commonly affects helicopter pilots. Long term flicker vertigo can lead to brain damage.

From this, I came up with another theory. Babies are believed to have much higher flicker rates (theory only) which could make them sensitive to fluorescent lights used in hospitals. I wonder if the rapid increase in children with ADHD could not be caused by that early exposure to fluorescent lighting.

Also schools now have windowless classrooms with fluorescent lighting and I wonder if that is contributing to kids having more learning disabilities.

Just something to think about.


#8

Interesting hypotheses, although I would point out that ADHD and other learning disability diagnoses have increased at least in part because we are seeing more things as being disorders.

i.e. 40 years ago a 2nd grader who wiggled around in his seat and was easily distracted would have been called “a boy,” not “an ADHD patient.”


#9

+1 to @shadowfax 's comment!


#10

Every old movie I’ve watched on my VCR and “single-framed” simply repeated every 4th frame to adapt to 30fps.

My guess is the “naked eye effect” @Yosemite refers to is the result of reflections between or through the spokes.


#11

There are top people with explanations (seriously):

“We propose, that a motion aftereffect is superimposed on the moving stimulus, sporadically allowing motion detectors for the reverse direction to dominate perception.”
(from the second link below)


#12

As often said “there’s more than meets the eye”.


#13

Thats true and I agree but I do think that it is either getting worse or teachers and parents just don’t handle it as well as in the past. But I do think that hospitals should not use fluorescent lights that flicker at 60 Hz around babies, not 24/7 like they do.


#14

Florescent lamp ballast are not rectifiers, the lamp still gets AC and the current flows in both directions giving you 120 Hz flicker. A ballast is a special transformer that has a high flux leakage between the primary and secondary, this gives a drooping voltage output with increased current so that the current is stable, sort of a constant current regardless of the voltage across the electric arc in the tube. Works like a tiny welding transformer.
It’s LED lamps that flicker at half wave frequency, unless there is a full wave rectifier feeding it. LED lamps also need a ballast to give a constant current regardless of the voltage. In cheap flashlights, the ballast is often a simple resistor in series with the LED.


#15

Fluorescent lamp tubes have a cathode at one end and an anode at the other. It acts like a rectifier and it only conducts on one half wave. 60 Hz, not 120.

Edit: correction, newer tubes have 2 pins on each end and conduct by directionally so they are 120 Hz. When I went to school, most fluorescent tube had only one pin on each end and conducted in only one direction.

I still would not use fluorescent tubes in the maternity ward and I would either get rid of them in schools or go to the higher frequency CFL’s


#16

Thank you all for your input on this subject.

I think @keith pretty much answered my question. I never knew that our eyes have a flicker rate too.

Yosemite


#17

I’ve noticed that effect in broad daylight too. When I asked a university professor about it, they said it was thought to be due to a sampling effect done in the eye/brain interface, similar to what Keith mentions above. In other words the brain cannot process all the information at once that it receives from the eye, so it breaks the job down into time segments. It sounds like not all the scientific types agree with that idea however, based on Waterbuff’s post. So Yosemite, there’s more work to do. Put your forge down and break out the research papers … lol …


#18

That makes sense too @George_San_Jose1

It was just something that I noticed often enough to “Wonder Why”.

But I’m not that concerned that I’d go back to Readen and Writen all day.
@Keith 's answer gave me a headache just reading all that, but it was a good read!!!

Yosemite


#19

I have used the strobing of florescent lamps and a stop watch to asses how fast induction motors were turning under load, to see if the motor was being over loaded. For a 1800 rpm induction motor, I would draw a cross shape on the end of the shaft. Without a load, the florescent lamps would freeze that cross with the strobe. With a load, the cross would slowly spin backwards. Using a stop watch, I would time that slip speed. If the motor lost 40 revolutions in one minute of time, the shaft was spinning 1760 rpm, a typical full load rpm of a four pole induction motor.


#20

I never knew our eyes flickered either. 'splains a few things as Ricky Ricardo would say. Must have something to do with seizures from strobes, not to turn this into doctor talk.