Light bulb query

[JohnW2]

Maybe I'll ask my sis, she's is an optician after all. (a real one not a dispensing one before anyone asks)

Yes, she ought to know, probably better than I ever did all those decades ago (in the context of neurophysiology), before I forgot!

Kind Regards, John

 

[SUNRAY]

Dunno. As I wrote ... See also what I'm about to write in response to rsgaz.
You're again essentially talking about stroboscopic effects - which, as I said, have got nothing to do with 'flicker', 'flicker fusion thresholds' or 'persistence of vision'.

Kind Regards, John

In my understanding of the situation it has everything to do with flicker, if my 'tests' are made under ac LED, or to a lesser extent fluo, the results are totally noticeable. However if made under flicker free light such as incandescent or dc LED the strobe effect will be non existent.
So yes I am, with reservations, referring to strobe effect but caused solely by flickering light,

 

[JohnW2]

In my understanding of the situation it has everything to do with flicker, if my 'tests' are made under ac LED, or to a lesser extent fluo, the results are totally noticeable. However if made under flicker free light such as incandescent or dc LED the strobe effect will be non existent.

Yes, of course, but I think there is probably a terminological misunderstanding here. 'Flicker' usually refers (and this is the sense in which I've used the word) to flicker which is visible due to the fluctuating nature (sinusoidal, pulsatile or whatever) of the light source - and, although we can debate the actual figures, that is something that can only happen with relatively low frequencies of fluctuation (say up to 60 Hz or so, if that).

Stroboscopic (e.g. when one is illuminating a rotating object) effects are different, because they are due to an interaction between the frequency of variation of the light and the speed of rotation/movement of the illuminated object. If you illuminated an object which was rotating at very high speed with, say, an LED fed with 1000 Hz electricity (far too high for any visible 'true flicker' of that source), you could see 'flickering' of any frequency, maybe only a few Hz (or even 'zero', which is the way that stroboscopes work), depending on the speed of rotation of the object in relation to the frequency of variation of the light.

Kind Regards, John

 

[SUNRAY]

Yes, of course, but I think there is probably a terminological misunderstanding here. 'Flicker' usually refers (and this is the sense in which I've used the word) to flicker which is visible due to the fluctuating nature (sinusoidal, pulsatile or whatever) of the light source - and, although we can debate the actual figures, that is something that can only happen with relatively low frequencies of fluctuation (say up to 60 Hz or so, if that).

Stroboscopic (e.g. when one is illuminating a rotating object) effects are different, because they are due to an interaction between the frequency of variation of the light and the speed of rotation/movement of the illuminated object. If you illuminated an object which was rotating at very high speed with, say, an LED fed with 1000 Hz electricity (far too high for any visible 'true flicker' of that source), you could see 'flickering' of any frequency, maybe only a few Hz (or even 'zero', which is the way that stroboscopes work), depending on the speed of rotation of the object in relation to the frequency of variation of the light.

Kind Regards, John

OK in that case let's use the word flash or flashing if flicker is inappropriate. Whatever word is used I am referring to a LED going on and off at 50Hz which is very noticeable to some people and troublesome to some health sufferers.

 

[JohnW2]

OK in that case let's use the word flash or flashing if flicker is inappropriate. Whatever word is used I am referring to a LED going on and off at 50Hz which is very noticeable to some people and troublesome to some health sufferers.

No, what you are talking about there really is 'flicker' (it was the introduction of mention of rotating tools and moving heads which shifted the discussion onto essentially stroboscopic effects).

The debate has been about the extent to which people ought to be able to see such 50 Hz flicker, since it seems that it is at (or probably beyond for many people) the high end of what they should be able to perceive.

Assuming you don't have one of these new-fangled high-refresh-rate TVs, if you pause playback of a video recording, so that you are looking at an unchanging image, does it seem to you to be flickering?

Kind Regards, John

 

[EFLImpudence]

Would a rotating object not still appear stationary - if rotating at the appropriate speed - even though the flicker is not actually noticable?

 

[JohnW2]

Would a rotating object not still appear stationary - if rotating at the appropriate speed - even though the flicker is not actually noticable?

That was exactly the point I was making, wasn't it?

As I said, the rotating object would appear stationary if the speed was just right in relation to the frequency of fluctuation of the light. As the speed changed relative to the light frequency, one would start seeing movement (which could be called 'flickering') gradually increasing in frequency as the speed moved further from the 'strobe speed'. As I also said, that remains true even if the frequency of the light source fluctuation is far too high (e.g. 1000 Hz) for the source itself to be perceived as flickering.

Kind Regards, John

 

[SUNRAY]

No, what you are talking about there really is 'flicker' (it was the introduction of mention of rotating tools and moving heads which shifted the discussion onto essentially stroboscopic effects).

The debate has been about the extent to which people ought to be able to see such 50 Hz flicker, since it seems that it is at (or probably beyond for many people) the high end of what they should be able to perceive.

Assuming you don't have one of these new-fangled high-refresh-rate TVs, if you pause playback of a video recording, so that you are looking at an unchanging image, does it seem to you to be flickering?

Kind Regards, John

Ah there is a problem, the 'flicker' rate on video picture is much higher than 50Hz. The whole frame refreshes at that speed but in the days of 625 line analogue signals each line is refreshed at 625x50Hz and then the image stays there by the persistence of the CRT, I know it's a very simplistic description which is riddled with error but the principle is there.
If you are familiar with an oscilloscope, imagine turning the speed down low - say 1s/cm, turn the brightness up high. Now it may be possible to see the displayed line until after a sweep has finished, ie may not have disappeared before being refreshed. Transposing that back to TV, each pixel will decay and not flash on and off.
A LED can flash on and off well within 10ms, in fact LED's can very easily be used to illuminate a fibre optic for the transmission of audio and beyond. Depending on the on/off ratio, I believe the 50Hz can become quite visible.

 

[EFLImpudence]

That was exactly the point I was making, wasn't it?

Ah sorry. I haven't read all of it.

I was just curious if it did as the light doesn't appear to be flickering.

 

[JohnW2]

Ah there is a problem, the 'flicker' rate on video picture is much higher than 50Hz. The whole frame refreshes at that speed but in the days of 625 line analogue signals each line is refreshed at 625x50Hz and then the image stays there by the persistence of the CRT ...

I'm getting confused, since you are now talking about CRT TVs in which, as you say, persistence of the CRT's phosphor went a long way to preventing flicker.

However, with an LED or LCD TV (and probably also plasma ones, but I know little about them, there is (just as with your LED lights), essentially no persistence/ as you go on to say ...

A LED can flash on and off well within 10ms, in fact LED's can very easily be used to illuminate a fibre optic for the transmission of audio and beyond.

Far 'beyond', I would have said. So no persistence in a modern TV, just as with LED lights. I therefore am a bit confused about what you are saying (unless you're trying to tell me that you still have a CRT TV ).

Kind Regards, John

 

[JohnW2]

Ah sorry. I haven't read all of it.

I seem to get told off if I produce quotes of what I've previously written, but if you look at post #48, I think you'll probably find that I was saying the same as you subsequently did.

Kind Regards, John

 

[SUNRAY]

I'm getting confused, since you are now talking about CRT TVs in which, as you say, persistence of the CRT's phosphor went a long way to preventing flicker.

However, with an LED or LCD TV (and probably also plasma ones, but I know little about them, there is (just as with your LED lights), essentially no persistence/ as you go on to say ... Far 'beyond', I would have said. So no persistence in a modern TV, just as with LED lights. I therefore am a bit confused about what you are saying (unless you're trying to tell me that you still have a CRT TV ).

Kind Regards, John

I'm not too well versed in the operation of LED/LCD TV's either, from the little I have picked up I believe the persistence is electronically generated, not least of which to maintain brightness/contrast. I'm very happy to be corrected/updated on this.

 

[EFLImpudence]

I seem to get told off if I produce quotes of what I've previously written, but if you look at post #48, I think you'll probably find that I was saying the same as you subsequently did.

I didn't say anything.

I was asking because I wasn't sure if the effect was the same as a strobe when the light didn't appear to be flickering.

 

[ban-all-sheds]

So no persistence in a modern TV, just as with LED lights.

White LEDs use phosphorescence - do they have any persistence?

 

[rsgaz]

I believe the persistence is electronically generated

This is a fascinating video...(from 1:45)

It's amazing just how much of a CRT screen isn't on! And from 4:28, an LCD TV has all pixels illuminated continuously.