I notice that when I look at a point about 1 foot to the side of my computer, I can see the screen flicker. I don;t see this when I look straight at it. Also, if I wave a pen back and forth in front of the screen, I see 7 static shadow forms of the pen. Why?
I also notice that I can see these flickerings when I chew something crunchy. But what is really wied is that when I eat something crunchy, the numbers on digital clocks jump around independently. I have a feeling that this is all related. Anyone know why?
Can’t answer the first part of the first question, but I can tell you that if you increase your refresh rate the flickering will become less apparent.
As far as why you see several shadows of the pen when you wave it across the screen…each “shadow” (more like a silhouette - think of standing in a darkened room with a lamp at one end and a friend holding his hand in front of the lamp) coincides with a refresh of the screen. So if you had your horizontal refresh rate set to 60hz, and you waved your pen in front of the screen for 1 second, you’d theoretically see 60 “shadows”. In actually you’d see far fewer because AFAIK your eye can’t discern that many images that quickly.
[quote] (A)I notice that when I look at a point about 1 foot to the side of my computer, I can see the screen flicker. I don;t see this when I look straight at it.(B) Also, if I wave a pen back and forth in front of the screen, I see 7 static shadow forms of the pen. Why?
[quote]
A. Because you recieve a lower ‘frame rate’ from the cells at the sides of your eyes. The cells at the centre take in the most information and therefore your brain sends you (ie your mind) information at a greater speed for these areas. Essential for reading, for example. Elaboration below.
B. This is also due to the frame rate of the screen. The screen itself refreshes at a certain speed of frames per second. Therefore you pass your pen in front of the screen and the screen flicks on and off behind it. You therefore only see the pen standing out against the light screen when the screen refreshes, ie a strobe affect. If you are passing the pen across the screen in exactly 1 second, your screen must be refreshing at 7 fps (if you see 7 pens, though obviously you are actually doing it faster). You will see this happen on TV screens as well.
As for stars, see What happens when you “see stars”?. Simly put, this is due to your eye receptor cells recieving a shock of some kind, ie a sudden change in blood pressure, or being hit (Tom and jerry style). This can cause chemicals in the receptors to stimulate the production of an impulse in the nerve, so you get a ‘false’ signal to the brain making you think you saw light.
PS just previewed, Running with scissors has B pretty accurately.
Actually, I don’t think this is due to a “lower framerate” in the rods and cones on the outer part of your eyes. I read once (sorry, I can’t give you a cite just now) that your peripheral vision can detect movement with greater sinsitivity than when you look directly at something. In the reading I remember the author mentioning that people in the armed forces can use this ability to detect movement where they might not regularly be able to see any. I’m not an opthomalogist, but if that’s the case, it could explain how the cells on the periphery of your eyes are detecting the refresh rate of you monitor more easily than those on the front.
Abel, you may be right. Possibly the frame rate IS faster, but I think this is more of a reflex thing in that it is built in as an instinct for animals to quickly look towards any movement in their peripheral vision, and hence notice it more easily. For this you would not necessarly need a higher frame rate. It makes sense to have a higher framerate from the more used area of the retina, called the fovea. (the area has more concentrated cells, and, whats more, is closer to the optic nerve).
It may also be due to the way the rods are around the outside of the retina, so it is actually easier to see in the dark by looking just away from the thing you are trying to see. This is particularly noticeable when looking at dim stars or satellites (okay, the reflection of the sun on satellites)
This occasionally happens to me also when eating something that “crunches” and staring at something bright with sharply defined edges (ie text on a monitor screen). You can get approximately the same effect in some cases (though not as pronounced) by sticking a vibrating object in contact with your jaw bone or your skull near your temple.
As a WAG the vibration of the crunch is resonating through your skull and vibrating your eye sockets. This makes the images jump around. Only specific frequency ranges of a “crunch” have this effect.
Abel, you may be right. Possibly the frame rate IS faster, but I think this is more of a reflex thing in that it is built in as an instinct for animals to quickly look towards any movement in their peripheral vision, and hence notice it more easily. For this you would not necessarly need a higher frame rate. It makes sense to have a higher framerate from the more used area of the retina, called the fovea. (the area has more concentrated cells, and, whats more, is closer to the optic nerve).
It may also be due to the way the rods are around the outside of the retina, so it is actually easier to see in the dark by looking just away from the thing you are trying to see. This is particularly noticeable when looking at dim stars or satellites (okay, the reflection of the sun on satellites)
No answers yet to the most important part of the question, which appears to be a variation of this question. Which also didn’t get any replies. But at least its good to see others have noticed the same phenomenon. Also noticed the same thing with digital clocks–both the red ones and the blue ones on microwaves. Odd…
Actually, I emailed Cecil with a question very similar to the first question. My question was this: I realize that a low refresh rate on a monitor looks like a freakin’ strobe light to me and gives me one hell of a headache. Meanwhile, there are other people I know who can’t even see the screen flicker at all.
So who has the better eyesight - me, the guy who can catch a low refresh rate, or them, the people who can’t? And how come they can’t see it while I can?
Not that much of a mystery. A lot of it is training and being sentitized to the difference that higher and lower refresh rates can make to the overall picture. Most people can easily recognize a flickering/flickery screen due to a low refresh rate without any trouble at all if they are made aware of the difference between a high refresh rate and a low refresh rate and how this affects the screen.
Most people will not see anything wrong with a flickery screen simply because they don’t know any better and the flickery screen is seen as a “normal” monitor screen.
As others said aleady, the peripheral vision is very sensitive to movement but not good at resolving patterns. I hear this is because the primary purpose of peripheral vision is (was) to detect predators jumping at you. The center of the field of vision is specialized towards resolving complex patterns, and not very sensitive to movement.
The screen is not emitting light constantly - it emits brief flashes of light at a high frequency, usually 60 to 100 times a second. When you wave something in front of the screen you won’t see a big blurry shadow, you see many distinct shadows. Just like what happens at a dance, with strobe lights.
As I mentioned above, the image on the screen is not there all the time - it consists of brief flashes, and the rest of the time you are just “looking” at the afterimage. When the monitor moves (or your eyes move), the afterimage doesn’t move together. You won’t see the contents of the screen move until it flashes on again. So the image on the screen lags behind and you see it as flickering around.
The 7-segment LEDs work similarly - they flash on and off very fast. The difference is that the whole LED array doesn’t flash on together - the segments flash on one at a time. So if you shake the clock or your head, they appear to flicker around independently.
I have noticed this as well. If you want to see a dramatic demonstration, watch television with a vibrating massager of some kind placed at the base of your skull near where your jaw, skull and neck come together. The television screen will appear to have waves passing under it. I’m not sure any of the explanantions in this thread adequately address the phenomenon.
I really don’t know what causes this.