I observed this while doing my homework. I have a habit of shaking my pen between my fingers vigorously while thinking something. My PC's screen is right in front of me (on my study table itself).

There was a white background on the screen. Whenever I shook the pen with the screen behind it, I could see 8-10 blurred distinct images in the region instead of a continuous one which I generally see when the background is dark. Could anyone explain the reason for this?

closed as unclear what you're asking by Norbert Schuch, John Rennie, Kyle Kanos, Bruce Lee, AccidentalFourierTransform Aug 18 at 22:52

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    Your eyes and visual system don't have an FPS rating. – StephenG Aug 12 at 7:50
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    To the OP. What you see is not your eye fps, but the flicker of the screen. You may be able to change it a bit in display properties. It was a bigger issue with CRT, but with modern LCD the flicker is less noticeable and annoying. With CRT 60 Hz was just visible, so the VESA standard prescribed the minimum of 70 Hz if I recall correctly. You can see the flicker of some other light sources too exactly the way out described, except modern bulbs use much higher frequencies than in the past. – safesphere Aug 12 at 8:16
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    @safesphere It's not really a physics subject but my understanding of human vision does not equate to what you're saying. Do you have a reference explaining the basis for this 10 fps figure, out of curiosity ? – StephenG Aug 12 at 9:14
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    @safesphere, more specifically, where did you get the idea that individual receptors and neurons in your visual system work together in some way to collect coherent image "frames" at some periodic rate? – Solomon Slow Aug 12 at 13:19
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    @safesphere, You said, "individual cells have different refresh rates," and, "the signal passed from the eye to the brain...is not a bitmap." So, why describe human vision using the language of digital image processing, in which frames/bitmaps are delivered at one synchronous, constant refresh/frame rate? IMO, if you want to help someone understand that two superficially similar systems actually work in different ways, then you should use different words to describe them. "Effective frame rate" and "Effective megapixels" suggest frames and pixels, neither of which exist in human vision. – Solomon Slow Aug 12 at 20:44
up vote 2 down vote accepted

The picture you observe when you are shaking a pen in front of a computer or TV screen is a result of the averaging action of the eye.

When you are shaking a pen in front of a wall illuminated by natural light, you see a continuous blur. Both the wall and the pen contribute to the brightness of the blur. The brighter the wall in comparison with the pen, the greater its relative contribution - so you'll see more of the wall than the pen. If the pen has a bright band in the middle, the blur will have a corresponding bright band as well, i.e., you'll see more of the pen there.

When you are shaking a pen in front of a TV screen, what you see, similarly, depends on the brightness of the part of the screen behind the pen, the background.

If the background is black, you'll see a lot of the pen in the blur. If the background is bright, you'll see mostly the background itself, except at the moments, when the screen goes dark as part of the refresh cycle and, correspondingly, at those moments, you'll see mostly the pen and, overall, a darker image due to a lower average brightness.

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In summary, the blur is caused by the inertia (averaging action) of the eye, but the discrete nature of the image of the shaking pen in front of a TV screen is due to the refresh cycle and would be apparent even if the eye had no inertia.

  • This was exactly what I wanted. Just because I couldn't explain what I was asking and used terms like FPS it doesn't mean my question is supposed to be closed. – user64829 Oct 14 at 8:15
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    @user64829 there are presently 4 reopen votes (5 are needed to reopen it). After a couple of edits removed the stuff about FPS, it seems clear to me. Generally, if you disagree with the closing of a question, it's ok to go ask something our meta site, although I recommend you don't do that for this post (at least until another week passes) because it's almost reopened anyways. – Chair 3 hours ago
  • Oh, ok. Thank you for the edit and the information, I didn't know that. – user64829 3 hours ago

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