Why do flashing things appear on and not off?

PoundSign2 · 2013-11-02 04:12

So I was watching a video on joule thief circuits since it's something I've never actually built, and I think my son would enjoy building one with me for the first time plus a great learning opportunity. While the basis behind this circuit is simple, it gains high voltage allowing sufficient current to flow in and out of the LED. This happens very quickly, so quickly in fact that it just appears as "On" all the time. We can say that the LED is also on an equal amount of time that it is off. And if On is the opposite of Off and vice verse, and things of equal and opposite value cancel out... why then does the LED appear on and not off?

My main hunch is that it has to do with something about the time it takes for the light to hit your eyes and something related to our max FPS or something. In any case I find this a fascinating question. What's your take?

Mark_T · 2013-11-02 04:42

You confuse two kinds of opposite - to humans on and off are "opposite" in a very sloppy sense - it
is either on or off, therefore they are opposites.

But mathematically the term for opposite is "inverse" and the additive inverse of 1 is -1, not 0.

Since an LED cannot absorb light the same way as is emits, the average of being on and off
is being half-on. When its on its emitting photons of light, when off its not emitting photons
of light, therefore if its on half the time it emits, time-averaged, photons at half the rate.

[ consider this: is the opposite of running standing still? Or is it running backwards? If you
spend 1 minute running, then 1 standing still, then 1 running, etc etc you are still going forward ]

doggiedoc · 2013-11-02 04:47

It appears on because of persistence of vision. The same thing that makes animation appear fluid.

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

skylight · 2013-11-02 07:52

It's a bit like is a glass of beer half empty or half full? I'm with doggiedoc about the persistance of vision , Think how many photons have hit the eye in even a short on period. also there is more brain response to the flash of light as opposed to an off led which would merge into the background.

whicker · 2013-11-02 10:48

I like Mark_T's explanation the best.
The LED would have to draw all the light it emitted back into itself.

potatohead · 2013-11-02 11:00

Your eyes respond to the accumulation of photons and you see light.

jones · 2013-11-02 11:51

That's a really interesting question. I would suspect it has to do with the difference between the way the eye-brain system perceives light vs. how it perceives an absence of light. The response to the onset of light stimulus must be faster than the response to the end of that stimulus. Photoreception/transduction is a *really* complicated system ( e.g. http://en.wikipedia.org/wiki/Photoreceptor_cell ).

I found a discussion of the details: http://webvision.med.utah.edu/book/part-viii-gabac-receptors/temporal-resolution/

erco · 2013-11-02 12:21

It's all about contrast. A half-lit, 50% PWM LED will be perceived as "plenty bright" in a dark room. But in a bright room, you'd be hard pressed to tell if it was on. Traffic signals are usually nice & bright and easily visible, but when the sun is setting behind it, you often can't tell.

mindrobots · 2013-11-02 13:40

Mark_T wrote: »

[ consider this: is the opposite of running standing still? Or is it running backwards? If you
spend 1 minute running, then 1 standing still, then 1 running, etc etc you are still going forward ]

This is starting to sound like the jet on the treadmill problem.

As for the LED blinking, I'm going for persistence of vision and some psychological bias our brains have.

skylight · 2013-11-02 14:06

Actualy erco's post made me rethink and it could be to do with contrast, consider a chase of leds if you flash one led in a sequence the one on it's own lit as "dot" becomes the focus but consider the chase again in "bar" ie only one led is off at any one time then that off led becomes the focus.

Heater. · 2013-11-02 14:44

Well, permanently on or not there is still energy coming out of that LED into your eye in the form of photons. What do you expect that energy to do? Nothing? Something?

PoundSign2 · 2013-11-02 20:55

Heater. wrote: »

Well, permanently on or not there is still energy coming out of that LED into your eye in the form of photons. What do you expect that energy to do? Nothing? Something?

The meaning of life is more accurate.

erco · 2013-11-02 21:06

skylight wrote: »

Actualy erco's post made me rethink and it could be to do with contrast, consider a chase of leds if you flash one led in a sequence the one on it's own lit as "dot" becomes the focus but consider the chase again in "bar" ie only one led is off at any one time then that off led becomes the focus.

Actually, skylight's post #4 mentioning beer made me rethink and down a frosty Corona. I no longer care about these errant photons.

Heater. · 2013-11-02 23:58

PoundSing2,

The meaning of life is more accurate.

Err...what? I did not understand that.

doggiedoc · 2013-11-03 03:55

Heater. wrote: »

PoundSing2,

Err...what? I did not understand that.

I suspect he forgot the add [noparse]</sarcasm>[/noparse] at the end.

PJAllen · 2013-11-03 05:18

Is this a contest?

ctwardell · 2013-11-03 05:59

If I only spill half a glass of water why does the floor still get wet?

C.W.

skylight · 2013-11-03 13:29

erco wrote: »

Actually, skylight's post #4 mentioning beer made me rethink and down a frosty Corona. I no longer care about these errant photons.

You must have consumed the half full portion as opposed to the half empty

Does that make you half cut?

PoundSign2 · 2013-11-03 14:24

Heater. wrote: »

PoundSing2,

Err...what? I did not understand that.

You asked what you wanted the energy to do for us. I wanted it to solve a riddle. Yes, it was sarcasm. :P

In any case I put some more thought into it and I think there is another variable we did not consider. The ambient light. Light that is also coming from other angles being absorbed and reflected by the red/green/blue/clear cap on the end. Light from other angles could hit the cap and come back to you, filling in that 'delay' of OFF/ON time that light is emitted from the LED. So if the the LED is on 50% of the time, the other time it is off the ambient light could fill in that 'gap' that it is off. IDK, just a thought. Credible or totally off the mark?

ElectricAye · 2013-11-03 15:33

PoundSign2 wrote: »

...
In any case I put some more thought into it and I think there is another variable we did not consider. The ambient light. Light that is also coming from other angles being absorbed and reflected by the red/green/blue/clear cap on the end. Light from other angles could hit the cap and come back to you, filling in that 'delay' of OFF/ON time that light is emitted from the LED. So if the the LED is on 50% of the time, the other time it is off the ambient light could fill in that 'gap' that it is off. IDK, just a thought. Credible or totally off the mark?

Considering the phenomenon can happen in a totally dark room, I doubt it.

I have another version of this question: considering the brains of 99% of the people are working only 1% of the time, why doesn't everyone think they're entitled to my opinion?

erco · 2013-11-03 16:59

I just ran a full marathon today, and a half full beer on a treadmill still looks good to me.

mindrobots · 2013-11-03 17:15

I probably would have picked the 1/2 marathon and the full beer.

erco · 2013-11-03 18:16

mindrobots wrote: »

I probably would have picked the 1/2 marathon and the full beer.

My ignorance is complete, m'lord abbot.

That's an obscure quote from "A Canticle for Leibowitz". But in a related story, Jesus stole all the marathon headlines today (maybe wine, but no beer):

http://news.yahoo.com/nyc-marathon-jesus-200033428.html

Why do flashing things appear on and not off?

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