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Author Topic: How quickly does light need to move to look like a blur?  (Read 1591 times)

Offline thedoc

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Andy Romano asked the Naked Scientists:
   Assuming you have a bar which is mounted on an axle (such as a propeller or fan blade) with a light (such as a bright LED) mounted at each end of the bar, assume we are viewing this thing against a dark sky background; approximately how fast (RPMs please) would the bar have to rotate on its axle (axis) before the light from the LEDs would appear to the human eye to blend or blur into a continuous circle (ring) of light? (RPMs)
What do you think?
« Last Edit: 08/12/2015 14:50:02 by _system »


 

Offline Thebox

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Re: How quickly does light need to move to look like a blur?
« Reply #1 on: 08/12/2015 15:59:40 »
I do not know the answer but I presume there is insufficient information to give an answer, such has length of the bar/circumferences.
 

Offline evan_au

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Re: How quickly does light need to move to look like a blur?
« Reply #2 on: 08/12/2015 21:10:32 »
Human persistence of vision is unable to track variations in brightness that happen more than 50 or 60 times per second. So the answer is 25 or 30 revolutions per second.

It is no coincidence that TV and cinema operates at these flicker rates (although some recent films have been shot at twice the normal rate, with claims of improved smoothness of motion).

See: http://en.wikipedia.org/wiki/Persistence_of_vision
 

Offline alancalverd

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Re: How quickly does light need to move to look like a blur?
« Reply #3 on: 08/12/2015 22:34:01 »
Helicopter rotor blades rotate at about 400 rpm (depending on the blade radius) , with the tips travelling at around 0.75 times the speed of sound. At night, tip lights appear to be a continuous circle.  With a 3-blade rotor this equates to 3 x 400 /60 = 20 cycles per second. Some old home movies gave an adequate impression of continuous movement at 12 frames/second and we sometimes refresh x-ray fluoroscopes at 8 fps or less. I think visual persistence is slightly longer at night.
 

Offline evan_au

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Re: How quickly does light need to move to look like a blur?
« Reply #4 on: 09/12/2015 08:39:17 »
Quote from: alancalverd
I think visual persistence is slightly longer at night.
Yes, optical systems need a longer integration time when light levels are low. That goes for the eyes as well.

In fact longer integration times are required in any system where you are trying to make measurements with a degraded signal-to-noise ratio.
 

Offline vhfpmr

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Re: How quickly does light need to move to look like a blur?
« Reply #5 on: 09/12/2015 13:00:01 »
I've seen an interesting effect with my persistence of vision, I don't know whether anyone else will recognise the same effect, I first noticed it at work whilst sat using an instrument on a trolley.

The lower shelf of the trolley had a rubber mat with parallel grooves moulded on it, and whilst my eyes were darting to and fro across the instrument panel I found that there were distracting moire fringes flickering in my peripheral vision created from the image of the grooves on the mat combining with the image of the grooves in my visual persistence. The effect disappeared if I looked directly at the mat, so it appears the persistence is greater in my peripheral vision than in the macula. I think it's also more pronounced when I'm tired. I noticed the same effect again just recently, but I've forgotten what the circumstances were now.
 

Offline RD

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Re: How quickly does light need to move to look like a blur?
« Reply #6 on: 09/12/2015 13:20:23 »
I've seen an interesting effect with my persistence of vision, I don't know whether anyone else will recognise the same effect ... flickering in my peripheral vision ... The effect disappeared if I looked directly ...

Flicker-fusion-threshold is higher in peripheral-vision ...

Quote from: wikipedia.org/wiki/Flicker_fusion_threshold
... flicker can be sensed in peripheral vision at higher frequencies than in foveal vision ...
https://en.wikipedia.org/wiki/Flicker_fusion_threshold
 

Offline Colin2B

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Re: How quickly does light need to move to look like a blur?
« Reply #7 on: 09/12/2015 13:59:09 »
I think visual persistence is slightly longer at night.
Yes, the rods have a flicker threshold lower than cones
 

Offline vhfpmr

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Re: How quickly does light need to move to look like a blur?
« Reply #8 on: 10/12/2015 17:29:18 »
flicker can be sensed in peripheral vision at higher frequencies than in foveal vision

Yes, I'd noticed that flickering lamps are more obvious in the periphery, and that it's curiously contrary to the moire effect I described.   ???
 

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Re: How quickly does light need to move to look like a blur?
« Reply #8 on: 10/12/2015 17:29:18 »

 

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