Light bulb query

[DetlefSchmitz]

Most LED's running on ac flicker at 50 or 100Hz, much more so than fluo's

It depends. The good ones have a smoothing capacitor. The cheap knockoffs leave the capacitor out for cost saving.

 

[AndyPRK]

Yeah bridge rectifier and cap I would have thought.

Can’t see flicker. Just checked.

 

[SUNRAY]

Yeah bridge rectifier and cap I would have thought.

Can’t see flicker. Just checked.

Watch something like the chuck of a drill get up to speed and slow down under LED light and compare it with incandescent.

 

[JohnW2]

Watch something like the chuck of a drill get up to speed and slow down under LED light and compare it with incandescent.

Sure, but stroboscopic effects are different from visible flicker, since in the former case what one is seeing is a static or slowly moving image - one can therefore see stroboscopic effects with light of a frequency far above the 'flicker fusion threshold' (the frequency above which the human eye cannot perceive flicker, because of 'persistence' of the eye's receptors).

Kind Regards, John

 

[ban-all-sheds]

And even 50Hz is above that threshold - how else would cinema and TV have worked.

 

[333rocky333]

We have to take pictures of our completed jobs and put in a lot of them 600mm sguare ceiling panels, its quite weird what you see through the lens, the whole room appears strobing.

 

[JohnW2]

Quite so - in fact, for most people, the threshold (dictated by the 'rod' receptors') is, I think, no more than about 15Hz - just as well given 24/25 fps TV and films. This leads me ti wonder what actually is the nature of '50Hz flickering' (let alone 100Hz flickering) we keep hearing about people seeing.

Kind Regards, John

 

[SUNRAY]

Sure, but stroboscopic effects are different from visible flicker, since in the former case what one is seeing is a static or slowly moving image - one can therefore see stroboscopic effects with light of a frequency far above the 'flicker fusion threshold' (the frequency above which the human eye cannot perceive flicker, because of 'persistence' of the eye's receptors).

Kind Regards, John

In that case why does the flicker of LED lights cause so much trouble to migraine sufferers?

If you turn quickly or run through an area lit by LED, static objects will flicker or even move.

 

[333rocky333]

So would Led be dangerous too use over Saws etc, like the old single fluorescent was, back in the day, I recall it was even in the then current regs book .

 

[SUNRAY]

So would Led be dangerous too use over Saws etc, like the old single fluorescent was, back in the day, I recall it was even in the then current regs book .

I would try to avoid doing it. But alternatives are getting harder.

 

[rsgaz]

I think, no more than about 15Hz - just as well given 24/25 fps TV and films.

I think the contrast has a lot to do with it. Yes, to perceive motion you don't need many frames per second, but alternating frames of black and white at 60fps is easily noticeable. (your PC and/or monitor may totally ruin this though! My laptop displays at 60Hz)

I guess that 60fps alternating frames should be called a '30Hz flicker', though. Left side of the screen is therefore 30Hz, right side is 15Hz. Make sure you're in full screen or manually set it to 1080p60.

PHOTOSENSITIVE EPILEPSY WARNING!!

 

[JohnW2]

In that case why does the flicker of LED lights cause so much trouble to migraine sufferers?

Dunno. As I wrote ...

...This leads me ti wonder what actually is the nature of '50Hz flickering' (let alone 100Hz flickering) we keep hearing about people seeing.

See also what I'm about to write in response to rsgaz.

If you turn quickly or run through an area lit by LED, static objects will flicker or even move.

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

 

[JohnW2]

I think the contrast has a lot to do with it. Yes, to perceive motion you don't need many frames per second, but alternating frames of black and white at 60fps is easily noticeable. (your PC and/or monitor may totally ruin this though! My laptop displays at 60Hz) ... I guess that 60fps alternating frames should be called a '30Hz flicker', though. Left side of the screen is therefore 30Hz, right side is 15Hz. Make sure you're in full screen or manually set it to 1080p60.

Yes, that is very easily 'noticeable'. but there are so many 'electronic issues' in terms of getting it onto my screen, that I really don't know what to make of it - other than literally a demonstration of what happens if one tries to display 60Hz (or 30 Hz) 'flicker on a computer screen and then view it.

I used to know a lot about this stuff, but that was decades ago and therefore most has now faded (or vanished) - I'll see if I can find some of my dusty notes! In the meantime, a few comments, mainly about movies and TV ....

It might be related to brightness, rather than contrast. Persistence of vision is a purely photochemical phenomenon. Once a receptor in the eye has been stimulated, there is a finite time, dictated by the 'chemical recovery processes' before it can respond to further light, and nothing c an change that. However, it's possible that, at some brightness levels, some receptors may respond some times and other receptors at other times.

It's important to understand the difference between 'frame rate' and 'refresh rate'. Ever since the start of movies with sound (the sound required film movement rate to be standardised), right up to today, Movie film (movies) have always used 24 frames per second. Traditionally, a 2-bladed shutter result in each frame being projected twice, hence a 'refresh rate' (as perceived by viewer) of 48 per second. I think that modern projectors can project each frame three times, hence a refresh rate of 72 per second. It is the refresh rate that determines how often the image being seen changes.

Even with the traditional 48 per sec movie refresh rate if one was looking at a movie of something unchanging (like a static caption) one was unaware of any significant 'flicker'.

Similarly with TV. Frame rate was tied to the local electricity frequency - hence 25 per second (half of 50Hz) in Europe and 30 per second (half of 60Hz) in US. However, since nearly all TV systems used 'interlaced' frames (each transmitted frame containing only alternate lines), there were twice this number of frames transmitted- hence 50 and 60 per second respectively. Traditionally, each transmitted frame was displayed only once, hence refresh rates of 50 and 60 per second. However, we are now seeing TVs with higher refresh rates (e.g. 120 Hz), so that each transmitted frame is displayed two or more times, hence increasing the 'frequency' of what the viewer sees.

As I said, I'll see what I can remind myself about what I once knew about visual perception!

Kind Regards, John

 

[EddieM]

Quite so - in fact, for most people, the threshold (dictated by the 'rod' receptors') is, I think, no more than about 15Hz - just as well given 24/25 fps TV and films. This leads me ti wonder what actually is the nature of '50Hz flickering' (let alone 100Hz flickering) we keep hearing about people seeing.

Kind Regards, John

Your frontal vision is much less affected by flicker, your peripheral vision much more so, if you "look" at an old crt TV for instance our of the corner of your eye you are more likely to see it flicker.

 

[JohnW2]

Your frontal vision is much less affected by flicker, your peripheral vision much more so, if you "look" at an old crt TV for instance our of the corner of your eye you are more likely to see it flicker.

I think you're right, but, to confuse things, that seems almost back to front - since central ('frontal') vision majors on 'cone' receptors, which have a much higher flicker fusion frequency (if I recall, around 40-50 Hz) than the 'rod' receptors which predominate in terms of peripheral vision (around 15 Hz, if I recall correctly).

Kind Regards, John