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PostPosted: Tue Feb 13, 2007 03:20 
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I'm a bit "over the hill" in these matters, but! In "my" day, LED's were "pulsed" to reduce the average power consumption in the device (so as not to destroy them) whilst giving "visually" a higher light output.....

methinks that things haven't changed.....Higher powered LED's ARE available. But for a constant light output, they cost a whole lot more!.....


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PostPosted: Tue Feb 13, 2007 04:37 
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I still haven't seen any strobey LEDs on cars. What am I looking out for?
I've seen some cars and buses which have LEDs as rear lights, especially some BMWs, but they don't appear to strobe or draw me towards them.

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PostPosted: Tue Feb 13, 2007 10:00 
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Then you are most fortunate!


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PostPosted: Tue Feb 13, 2007 10:18 
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Draco wrote:
I'm a bit "over the hill" in these matters, but! In "my" day, LED's were "pulsed" to reduce the average power consumption in the device (so as not to destroy them) whilst giving "visually" a higher light output.....

methinks that things haven't changed.....Higher powered LED's ARE available. But for a constant light output, they cost a whole lot more!.....

This applies to lasers. Perhaps this also applies to really old LEDs, I don’t know because I don’t have any and I’m not that old :)

Diode lasers need a decent amount of current passed through them (the voltage across them remaining relatively constant) before they even start lasing (I have a 2W infra-red laser, it needs 500mA before it does anything, except get warm – which is a problem if used without adequate heatsinking as the heat will break the aperture mirror). Once lasing, the output intensity rises more or less linearly with the additional current/power (dpss green lasers are especially bad, their output are hugely non-linear as the crystals need a decent amount of laser power before they do anything). Hence it is better to rapidly pulse a laser to make it more efficient.

LEDs have an output intensity more or less directly proportional to the current (again the voltage across them remaining relatively constant). Hence they need not be pulsed for efficiency (but it is better to pulse them to avoid wavelength shift at low currents).


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PostPosted: Tue Feb 13, 2007 10:28 
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Ziltro wrote:
I still haven't seen any strobey LEDs on cars. What am I looking out for?
I've seen some cars and buses which have LEDs as rear lights, especially some BMWs, but they don't appear to strobe or draw me towards them.

You won’t see any effect if you look steadily straight at them. The visual effect occurs as you sweep your eyes across them and they cut through your field of vision. As you are driving, your eyes will invariably move as you scan the environment.

If sweeping your eyes left to right (or the other way) at a non-pulsing light source you will see this pattern temporarily ‘etched’ on to your retina:

___________________________________



If sweeping your eyes in the same way at a pulsing light source you will instead see:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _



The lines would be vertical if sweeping up and down, I won’t draw that with text but I hope you understand it.


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PostPosted: Tue Feb 13, 2007 16:52 
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smeggy wrote:
You won’t see any effect if you look steadily straight at them. The visual effect occurs as you sweep your eyes across them and they cut through your field of vision. As you are driving, your eyes will invariably move as you scan the environment.


Actually, some of us can see the pulsing even when looking directly at the thing. Personally I find the effect just forces my eyes to lock upon the LED.


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PostPosted: Thu Feb 15, 2007 00:45 
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Yes but smeggy, are the manufacturers skimping and using a lesser wattage LED.........pulsed to reduce the internal dissipation whilst maximising brightness. Rather than use a more expensive higher rated device on a constant DC current, do you think?


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PostPosted: Thu Feb 15, 2007 01:02 
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Lum wrote:
Actually, some of us can see the pulsing even when looking directly at the thing. Personally I find the effect just forces my eyes to lock upon the LED.

That would be impressive given their typical repetition rate (brake lights pulse at 100-200hz by my estimate [comparing against streetlamps]).....

Unless some of us are just plain weird :bighand: :)


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PostPosted: Thu Feb 15, 2007 01:04 
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Draco wrote:
Yes but smeggy, are the manufacturers skimping and using a lesser wattage LED.........pulsed to reduce the internal dissipation whilst maximising brightness. Rather than use a more expensive higher rated device on a constant DC current, do you think?

It doesn't work with (modern) LEDs, I've tried.


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PostPosted: Thu Feb 15, 2007 01:37 
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smeggy wrote:
Lum wrote:
Actually, some of us can see the pulsing even when looking directly at the thing. Personally I find the effect just forces my eyes to lock upon the LED.

That would be impressive given their typical repetition rate (brake lights pulse at 100-200hz by my estimate [comparing against streetlamps]).....

Unless some of us are just plain weird :bighand: :)


I thought it had been established that the rate was more like 60Hz, which would make sense as anything below 72Hz is problematic for me (and 72 itself is borderline, I prefer 75 and above where possible). My GF suffers from it far worse than I do and we have to have a 100Hz TV as a result.


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