Naked Science Forum
General Science => General Science => Topic started by: i_have_no_idea on 14/02/2006 23:48:12
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Do our eyes see at a certain frequency or refresh what we see hundreds of times a second like some cameras (taking pictures every second) or do we just somehow constantly see things?
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"" A critical part of understanding these visual perception phenomena is that the eye is not a video camera: there is no "frame rate" or "scan rate" in the eye: instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience.""
http://en.wikipedia.org/wiki/Persistence_of_vision
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Thanks Robert...I was wondering about the answer myself because that was a good question Phil (I_have_no_idea)...
Men are the same as women.... just inside out !!
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When we stare at an object our eyes are actually making rapid small movements in order to refresh the receptor cells on the retina. Is that true?
Dr B
Istanbul
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quote:
Originally posted by Dr B
When we stare at an object our eyes are actually making rapid small movements in order to refresh the receptor cells on the retina. Is that true?
I don't have a definitive answer for this, but there are a few things that I am aware of that may be relevant.
Most importantly, we do not have an even acuity throughout our field of view. Our vision is more acute near the centre of the field of view, and less so towards the periphery of our vision, excepting we have a blind spot exactly where the optic never comes into the eye. Thus it would make sense that the eye would shift the centre of vision from place to place in order to pick up specific detain about different parts of the field of vision.
It may also be that the eyes will try ant fix on an object to judge its range more accurately, and in doing so the eyes will converge upon that object (since, with both eyes looking along exactly parallel tracks would mean that the point of convergence would be at infinity).
George
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Your eyes are more sensitive to high speed movements in your peripheral vision than in the centre, this is why monitors sometimes flicker at the edge of your vision, but if you look directly at the monitor it looks fine.
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quote:
Originally posted by Dr B
When we stare at an object our eyes are actually making rapid small movements in order to refresh the receptor cells on the retina. Is that true?
Dr B
Istanbul
I believe you are referring to "Saccidic Movements".
As Another Someone said, only the central part of the field of view is sharp: the sharp area is not large enough to image (say) an entire face at once, so these small Saccidic movements scan the sharp area over the face to bulid up a sharp image (in the brain).
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Do light bulbs flash at a cretin frequency?
"Ideas are more powerful than guns. We would not let our enemies have guns, why should we let them have ideas." -Joseph Stalin
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quote:
Originally posted by i_have_no_idea
Do light bulbs flash at a cretin frequency?
Incandescent lightbulbs (typical 60/100Watt types) flicker at mains frequency :
50Hz (50 times per second) in UK, 60Hz in USA.
Flickering / flashing lights can induce seizures in epileptics:-
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11593256&query_hl=1&itool=pubmed_docsum
I'm not sure how flashing lights would affect "a cretin" [:)]
http://www.thefreedictionary.com/cretin
http://www.answers.com/topic/cretinism
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Your eyes are more sensitive to high speed movements in your peripheral vision than in the centre, this is why monitors sometimes flicker at the edge of your vision, but if you look directly at the monitor it looks fine.
So that's it! I live in a country where most people believe that ghosts exist. My work desk is located next to a window and sometimes I saw a flash movement in my peripheral vision though mostly I am able to convince myself that I was just seeing things. One day, I noticed that the flash movement seen from the window was just a reflection on said window of a flying fly on the opposite direction of the window. Sometimes I wonder how did a small fly resulting in a flash movement I thought has the same size as me, until I saw the above post which confirmed that it was just due to the high sensitivity of the peripheral vision.
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Incandescent lightbulbs (typical 60/100Watt types) flicker at mains frequency :
50Hz (50 times per second) in UK, 60Hz in USA.
A slight adjustment to this:
- Incandescent light bulbs are brighter when the amplitude of the voltage is higher.
- Being mostly resistive, the current is also higher when the voltage is higher
- Due to thermal inertia, the peak temperature and peak brightness is slightly after the peak voltage.
- The mains voltage is approximately a sine wave, with two voltage peaks per cycle
Conclusion: Incandescent lightbulbs flicker at twice the mains frequency: 100Hz in UK, 120Hz in USA.
Since the lamp filament doesn't entirely cool down, the light varies between a minimum and maximum intensity. For most people, this is enough to prevent this flickering from being bothersome.
See: https://en.wikipedia.org/wiki/Flicker_fusion_threshold
In countries using ripple control, incandescent lamps with phase-controlled brightness can flicker at visible rates when the ripple control signals are active.
See: https://en.wikipedia.org/wiki/Load_management#Ripple_control
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My experience is different.
The voltage and current of an incandescent bulb flickers, but not the heat or the intensity. A florescent bulb's intensity does flicker with voltage. It strobes. Incandescents do not.
A LED flickers with ac voltage at 60 Hz, which we can not discern due to the persistence of our vision. Everyone should have a strobe light and play with it. You can beat/mix/sync with them.
All EM emission flickers, but EM emission at IR and above is usually a flux, i.e.....many emitters.
AN AM broadcast is one flicker. If you could see it. Our radio detectors(wave absorber) react/reset, during the off period, allowing us to believe that the emission is continuous. It's a process of absorption. And continuous sound FIELD emission re-enforces this apparency.
But light is different. It's intermittent and discreet. Disguised in a flux.
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The voltage and current of an incandescent bulb flickers, but not the heat or the intensity.
Oh yes it does. It's easy to detect the flicker of a low wattage bulb running on high AC voltage (say 240V 20W) because the filament is very thin and heats and cools quickly. This is why workshop equipment like lathes use low voltage (12 or 24 V) lamps with high current, thick filaments, to minimise strobing. It's tempting to use a LED nowadays but you need to supply it with smooth DC for the same reason.