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Ara Pacis
2010-Sep-17, 11:09 PM
I've noticed this recently that when I'm driving or just moving my head, I get two different effects from motion blur. On some lights I see a blur that looks like a swipe. On other lights, I see a line of that light repeated along the line of motion. Why is that?

At one time I wondered if it was an artifact from the 60 hertz AC power, but I wouldn't think that those lights cycled that fast. Moreover, some of these lights are DC power on cars, so I wouldn't expect them to cycle at all.

Or is it something about the human nervous system that makes certain light frequencies stutter while others swipe across the visual field?

Hornblower
2010-Sep-18, 02:30 AM
I've noticed this recently that when I'm driving or just moving my head, I get two different effects from motion blur. On some lights I see a blur that looks like a swipe. On other lights, I see a line of that light repeated along the line of motion. Why is that?

At one time I wondered if it was an artifact from the 60 hertz AC power, but I wouldn't think that those lights cycled that fast. Moreover, some of these lights are DC power on cars, so I wouldn't expect them to cycle at all.

Or is it something about the human nervous system that makes certain light frequencies stutter while others swipe across the visual field?

I have seen that flicker in the tail lights on some new cars. It is what I would expect of LEDs on AC or otherwise pulsed electricity. I do not know why it is not plain DC in a car. The wavelength or frequency of the light should have nothing to do with it.

Ara Pacis
2010-Sep-18, 04:04 AM
I've seen the LED tail lights react both ways. I've seen street lights react both ways too. I'm not sure what those street lights were, sodium vapor?

Antice
2010-Sep-18, 05:18 AM
The visual cortex runs on a 40Hz to 60Hz resonance. but in order to observe the effect the afterglow time on the light source has to be shorter than 2 full wavelengths. This should not be the case on any propperly functioning light. since this kind of resonance is known for causing epilepsy like seizures in many individuals. (some are more sensitive than others)

When there is a distinct position displacement like when it is motion blured there is also another effect going on that has nothing to do with the oscilation frequency of the light itself. it has to do with the update frequency of your eye. the update frequency of the human eye is fairly low. it can be as slow as 15 fps or maybe as fast as 25fps depending on a host of factors like age and visual cortex resonant frequenzy.
The effect is dependent on the relative motion of the light source. the after glow is caused by over saturation of the photoreceptor cells in the eye. casuing a distinct afterglow in the visual field. while the visual copying of the lightsource is caused by the relative motion being high enough that some of the motion data is missing. this is the same effect as when a wheel spins fast enough to apear to spin backwards.

Interestingly enough. the Biological eye seems to work in a manner very similar to a digital camera.

TrAI
2010-Sep-18, 12:38 PM
I've noticed this recently that when I'm driving or just moving my head, I get two different effects from motion blur. On some lights I see a blur that looks like a swipe. On other lights, I see a line of that light repeated along the line of motion. Why is that?

At one time I wondered if it was an artifact from the 60 hertz AC power, but I wouldn't think that those lights cycled that fast. Moreover, some of these lights are DC power on cars, so I wouldn't expect them to cycle at all.

Or is it something about the human nervous system that makes certain light frequencies stutter while others swipe across the visual field?

I assume you are thinking about the tail lights? These days, it is pretty common to combine the function of tail light and break light, so that you have a dim glow while driving, and a bright one when breaking. In older cars this might be done with dual filament bulbs, and such lamps would create a solid line. Newer cars often use LED lamps, and in some of these, in stead of having some LEDs off, the manufacturers choose to use PWM to dim the light. This means that you basically turn the LEDs on and off at a relatively high frequency, and by changing the amount of on time versus off time, you can change the average power of the lamps.

PWM is preferred over current limiting, since it is more efficient(you do not have to burn of energy in a resistor/regulator) and it allows you to drive the LEDs at full current, or even at a higher current than you could if driven continuously. So basically, the lamps are flashing many times a second, and if a low frequency is used, this flashing may be noticeable when moving your eyes.

Ara Pacis
2010-Sep-19, 05:09 AM
Ah, thank you. I now have some ideas to salve my sanity. I was wondering if my eyes or brain was playing tricks with me. I have to go see a neurosurgon as it is and was wondering if I should mention it.

TrAI
2010-Sep-19, 10:35 AM
Ah, thank you. I now have some ideas to salve my sanity. I was wondering if my eyes or brain was playing tricks with me. I have to go see a neurosurgon as it is and was wondering if I should mention it.

Well, I do not think you need worry about it, there are quite a few people out there that have noticed this. I am not really sure why anyone would design a low frequency PWM system for this application, I suppose that it may just be some cheap control system, or some idea of making the light more noticeable in the peripheral vision, but it seems to me that these things may actually pose a danger in traffic due to their distracting effect.

Antice
2010-Sep-19, 07:57 PM
Well, I do not think you need worry about it, there are quite a few people out there that have noticed this. I am not really sure why anyone would design a low frequency PWM system for this application, I suppose that it may just be some cheap control system, or some idea of making the light more noticeable in the peripheral vision, but it seems to me that these things may actually pose a danger in traffic due to their distracting effect.

Try being a person who has visually triggered epilepsy. They are not just noticeable, but downright annoying. but in general the intensity is not high enough to cause seizures right away. There are other unstable light sources that pose a greater seizure risk than some poorly designed rear lights. Personally i find worn archlights and incandescants to be particularly painfull. sometimes leading into Big seizures if i cannot get away fast enough.

HenrikOlsen
2010-Sep-20, 08:05 AM
Well, I do not think you need worry about it, there are quite a few people out there that have noticed this. I am not really sure why anyone would design a low frequency PWM system for this application, I suppose that it may just be some cheap control system, or some idea of making the light more noticeable in the peripheral vision, but it seems to me that these things may actually pose a danger in traffic due to their distracting effect.
It doesn't have to be really low frequency, the point of the OP's question was that the flickering didn't get noticeable until there was movement, which caused the light to be seen as a dotted line. This effect is visible at well over 100Hz.

PWM dimming isn't the only reason for pulse driving LED's, they have a highly non-linear power to light curve that means driving them with short pulses of higher current will get a higher average light output vs. average power that you'll get from driving them with DC.

That said, I've seen a badly designed system that had bright white LED's flickering at something like 25Hz, distinctly and horribly annoying.

TrAI
2010-Sep-20, 07:04 PM
It doesn't have to be really low frequency, the point of the OP's question was that the flickering didn't get noticeable until there was movement, which caused the light to be seen as a dotted line. This effect is visible at well over 100Hz.

Well, I expect the more noticeable lights use low frequencies, and 100Hz is still pretty low in my opinion, seeing as micro-controllers can easily supply PWM signals at several tens of kHz. Though many manufacturers of lights based on PWM seem to think anything that does not flicker when you watch it without moving is good enough...


PWM dimming isn't the only reason for pulse driving LED's, they have a highly non-linear power to light curve that means driving them with short pulses of higher current will get a higher average light output vs. average power that you'll get from driving them with DC.

Sure, but overdriving LEDs on a limited duty cycle is not the same as PWM, though you may combine the two to some extent. Anyway, I did note that you could use PWM to allow higher driving currents. But really, though the subject is interesting, I didn't think a detailed discussion on the uses of PWM technology was really needed, the OP only asked the reason for the effect some lights showed, so a rather basic description seemed to answer.


That said, I've seen a badly designed system that had bright white LED's flickering at something like 25Hz, distinctly and horribly annoying.

Indeed, there are some rather bad examples out there...

Ara Pacis
2010-Sep-21, 09:49 PM
You can talk about it, if you like. The subject interests me.

jj_0001
2010-Sep-23, 12:12 AM
Well, I wouldn't say the optical cortex "resonates", but rather that it has a maximum response time around the 60Hz range, which is controlled not only by the cortex but also by the speed at which the cones in the eye can repolarize and reset.

Interestingly, rods are about twice as fast. If you look at something out of the side of your eye, flicker will be much more evident than it will be straight ahead in the foeva.

In this case, we're talking about motion effects. If you see a set of lights along the path, that's some kind of light that is in fact time-modulated, and you're seeing the brightest parts as the spots. If you see a blur, it's a lkight that is noit being modulated.

It would seem silly for a PWM that drives a brake light to switch below 1000Hz, frankly, but apparently some do.

Flourescent lamps (some of them, those without persistant phosphors), mercury vapor lamps, lucalox, etc, can all show substantial 60Hz or 120Hz modulation, hwich can create the same sort of effect sometimes.

Ara Pacis
2010-Sep-23, 04:27 AM
Perhaps related is this: When I am at an intersection during the day and staring off into the distance, the cars passing laterally across my visual field seem to leave wakes in the air that seem lighter in brightness than before. If I were someone else, I might think I was seeing an "aura" but I'm guessing it is an illusion, similar to the black squares that causes the white intersections to look dark.

Antice
2010-Sep-24, 10:31 AM
Well, I wouldn't say the optical cortex "resonates", but rather that it has a maximum response time around the 60Hz range, which is controlled not only by the cortex but also by the speed at which the cones in the eye can repolarize and reset.

Interestingly, rods are about twice as fast. If you look at something out of the side of your eye, flicker will be much more evident than it will be straight ahead in the foeva.

In this case, we're talking about motion effects. If you see a set of lights along the path, that's some kind of light that is in fact time-modulated, and you're seeing the brightest parts as the spots. If you see a blur, it's a lkight that is noit being modulated.

It would seem silly for a PWM that drives a brake light to switch below 1000Hz, frankly, but apparently some do.

Flourescent lamps (some of them, those without persistant phosphors), mercury vapor lamps, lucalox, etc, can all show substantial 60Hz or 120Hz modulation, hwich can create the same sort of effect sometimes.

I beg to differ on there being resonance effects when the light is flickering at or close to the optical cortex response time or one of the other resonant frequencies. I'm most susceptible at around 50 to 60Hz and 110 to 130Hz It's the primary trigging cause of my diagnosis. There are neural feedback signals going back to the eye that control lens focus and pupil dilation motion. These both can and will sync with light inputs. and in some cases like mine it can enter into a feedback loop resulting in neural signals overloading their synapses and leaking into neighboring non connected synaptic receptors. If the effect is not stopped trough removal of all visual inputs in time the result is an electrical cascade effect that will rampage throughout my brain with all the not so funny effects of a grand epilepsy seizure.

According to my neurologist this feedback cycle is a normal part of the brain. the issue with my synapses is that they fire abnormally fast. thereby not allowing the natural dampening on the feedback cycle to compensate. I.E. my iris response is faster than average as well my eye being more sensitive to light in general. My Night vision is actually very excellent.

There is also the issue of light amplitude having similar effects but they work in a different manner. for instance begin subjected to a few slow but very high amplitude flashes or intense constant light/very high contrast will cause overloading of the visual cortex directly with a somewhat different progression of symptoms before I end up on the floor/ground.

The thing is. with a high and intense enough input of this kind everyone will get a seizure. having your brain drenched in sleep hormones due to being tired is known to make people more sensitive as well.

This is important. An fully normal healthy individual can develop epilepsy like symptoms from flashing strobe like lights. it's a well known effect and it is one that is being exploited in flash bang grenades to stun people. the flash and the stunningly loud bang act together to overload the brain with both a flash of light as well as a deafening sound. a single flash will only cause disruption for a second or so until the brain get's rid of the scrambling neural pulse. but a 10x repeat at short intervals both can and often will incapacitate the brain fully for several minutes afterwards.
This is the working method of the active denial system that uses loud sound pulses and flashing lights to incapacitate people.


@Ara Pacis: Your symptoms sound familiar to me. Not saying that you are developing any form of epilepsy here, but your latest symptom does sound a lot like what we "epileptics" do call an aura. yes it's named aura because it well.. what you are seeing when it happens resembles the mythical auras of lore. this is a very fragile state to be in and is often the last warning before larger seizures take over. The fact that it is your peripheral vision that is affected the most is also a poor indicator. you really need to talk to your neurosurgeon about these effects. They are important clues to your condition. Especially if you have a prior history of seizures or partial seizures. (sometimes a partial seizure can manifest in feeling drunken or dizzy, or just plain confusion. As if your mind is responding sluggishly, making it harder to think.)

Ara Pacis
2010-Sep-25, 06:48 AM
@Ara Pacis: Your symptoms sound familiar to me. Not saying that you are developing any form of epilepsy here, but your latest symptom does sound a lot like what we "epileptics" do call an aura. yes it's named aura because it well.. what you are seeing when it happens resembles the mythical auras of lore. this is a very fragile state to be in and is often the last warning before larger seizures take over. The fact that it is your peripheral vision that is affected the most is also a poor indicator. you really need to talk to your neurosurgeon about these effects. They are important clues to your condition. Especially if you have a prior history of seizures or partial seizures. (sometimes a partial seizure can manifest in feeling drunken or dizzy, or just plain confusion. As if your mind is responding sluggishly, making it harder to think.)

I'll mention it when I see him in a month or so. My current symptoms started after a fall that resulted in a head injury, but I've seen that "aura" thing for years.

Is any of this related to synesthesia? I sometimes see flashes of light when I hear a loud noise.

Antice
2010-Sep-25, 08:04 AM
I'll mention it when I see him in a month or so. My current symptoms started after a fall that resulted in a head injury, but I've seen that "aura" thing for years.

Is any of this related to synesthesia? I sometimes see flashes of light when I hear a loud noise.

Not directly afaik. people heavily afflicted with synesthesia seem to not have any seizures in most cases. it's more like their auditory system has been plugged into their visual cortex by some kind of direct neural bypass. But I have heard from people with epilepsy that have audio triggers that the sound sometimes do cause visual disturbances when they have an aura.
It's important to note that epilepsy is not a true diagnosis. it's more of a placeholder for a whole host of known and unknown failures of the mind. many epilepsy symptoms are also related to migraine symptoms.
If seing things in a ghostly light or other slight distortion in your pheirpheral vision is normal for you then that may just be a quirk of your particular neural pattern. Despite all the progress in neuroscience there are gaps in the knowledge base big enough to fit a whole universe trough. A head injury may exaberate a weakness in your neural makeup when the brain tries to compensate and ends up using pathways that it normally do not use for this task. this is most often temporary but sometimes it is permanent as well.
It has happened that people with fairly severe epilepsy have had a head injury that have "fixed" their epilepsy. but this is exceedingly rare. that it's fixed that is. not head injuries in general... seizures can be tough on the noggin since they tend to happen no matter what you are doing at the time. falling down stairs hurts a lot once you wake up :(

TrAI
2010-Sep-25, 09:15 AM
You can talk about it, if you like. The subject interests me.


I'll mention it when I see him in a month or so. My current symptoms started after a fall that resulted in a head injury, but I've seen that "aura" thing for years.

Is any of this related to synesthesia? I sometimes see flashes of light when I hear a loud noise.

The effect you describe sounds more like an afterimage illusion than the aura, as an aura is a sign of an incipient episode(a migraine attack or epileptic seizure, for instance), not a constant thing. An afterimage illusion may be things like a trail that seems to follow a moving light or object, or the negative of some scene you have been looking intently at.

Your experience may just be the normal afterimage trails, or perhaps it may just be some sort of afterimage caused by the bright outside lights and the darker areas of the cars bleaching the photoreceptors at differing rates combined with the fading effect caused by lack of eye movement. This fading is like in those illusions where you look at some image intently, and it starts to fade away, but the moment your eyes move it becomes visible again. However in the case you describe the movement is the car moving through your field of vision.

However Antice is right, you should talk to your neurologist about it. There is apparently a condition where a person is more prone to these effects(afterimages form quicker and may be stronger than normal, even to the level of being annoying) that is called palinopsia and this is often connected to some other neurological problem. You seem to be, if not worried by these effects, at least to some extent weary of them, and so it may be better to your peace of mind to discuss it with a competent person, and it is probably better to give too much than too little information to him/her.

Ara Pacis
2010-Sep-26, 05:09 AM
I was assuming it was the photorecepters reacting the way you described, TrAI.

BTW, I recall that the neurologist did an EEG after the concussion and it had strobe lights over my eyes for part of it. If I had some abnormal rhythms, I would assume he'd have mentioned it. Oddly enough, the technician said that I was asleep even though I was awake and lucid the whole time.

Antice
2010-Sep-26, 07:31 AM
I was assuming it was the photorecepters reacting the way you described, TrAI.

BTW, I recall that the neurologist did an EEG after the concussion and it had strobe lights over my eyes for part of it. If I had some abnormal rhythms, I would assume he'd have mentioned it. Oddly enough, the technician said that I was asleep even though I was awake and lucid the whole time.

That is the light test i mentioned. if that one didn't trigger anything then you can be pretty certain that your visual cortex is working close to normal. that strobe test almost threw me out of the bed when they did it to me. And among people with my diagnosis I am not that badly afflicted. I can do without drugs for instance as long as i am careful about exposure to certain stimuli. (I use polarized glasses to dampen contrast a lot. especially when driving).

That being asleep when awake thing is something I also do. apparently it has to do with one's ability to rest without actually shutting ones consciousness off. they do register it as a slumber even when you are capable of answering questions. I think everyone is capable of doing this by simply relaxing and not thinking about anything in particular. (daydreaming is a good word for it no?)

Ara Pacis
2010-Sep-27, 03:26 AM
Yeah, I was daydreaming about riding a roller coaster or flying, anything to make myself fee light and weightless and to fit with the disorientation.

holmes4
2010-Sep-27, 10:30 PM
When you do see your neursurgon please do ask as I'd love to hear his answer.

As I understand his it just depends on how fast you move your head.

Mark

Ara Pacis
2010-Sep-28, 06:26 AM
Moving my head can be a problem. The Physical Therapist says that I appear to have a problem with my VOR (Vestibulo-ocular Reflex) from the injury and I need to retrain it. I wonder if this is part of the stuttering visual that I sense too.

publiusr
2010-Sep-28, 08:54 PM
I can't stand driving in open country with the sun shining through narrow trees--the shadows they cast on the highway while passing through the light dark patches dazzles me.

Antice
2010-Oct-01, 06:05 PM
I can't stand driving in open country with the sun shining through narrow trees--the shadows they cast on the highway while passing through the light dark patches dazzles me.
This is perfectly normal. it takes a moment for your iris to respond to changes in the light levels. until it does the light will overload your retina and cause the dazzling sensation.
Try driving with polarized sunglases on. it dampens the difference between the bright and dark patches. Be glad you are not me. that dazzle effect can be more of a hazzard than most people think... :(

grapes
2010-Oct-01, 06:19 PM
Newer cars often use LED lamps, and in some of these, in stead of having some LEDs off, the manufacturers choose to use PWM to dim the light. This means that you basically turn the LEDs on and off at a relatively high frequency, and by changing the amount of on time versus off time, you can change the average power of the lamps.Pulse Width Modulation :)

Yeah, I had to google it...