driving them like this also means you can pump more current through them thus making them brighter (maybe this is the reason rather than saving battery life?)
driving them like this also means you can pump more current through them thus making them brighter (maybe this is the reason rather than saving battery life?)
Sorry, but this is a very common misconception. They can produce excellent high brightness narrow beams of light, which a filament lamp can't. This can be an advantage in some applications like spotlamps so the beam might look brighter than a filament lamp. However, if you take something like a 10º beam angle, which is 78 square degrees, and divide by the 41253 square degrees in the sphere which a filament lamp lights, this shows that the LED is only lighting up 0.2% of the area which the filament lamp did. That's why they appear so efficient, but in fact they aren't.
Nobody can currently get 30 lumens/watt from a white LED, which is what you get from highest efficiency halogen lamp. Nichia are predicting 60 lumens/watt for a product they expect to ship in
2005, which is still less than a fluorescent lamp. The efficiency improvement manufacturers have managed over the last couple of years is very disappointing compared with what went before, so it may be that some type of limit has now been reached without a significant change in technology.
The mean brightness is still the same, since the mean current must be the same, because you can't leave the LED on at the current used when on or it will burn out. However driving them switch mode allows the circuit to run more efficiently which IS why they are pulsed.
Steve
Mary: I agree. I don't think I can even speleologically spell that anyway! Terry.
In message , snipped-for-privacy@isbd.co.uk writes
You can certainly buy cycle lamps with what are described as halogen bulbs - btw, has the law been updated yet to allow cyclists to use non-filament bulbs?
"Dave Plowman" wrote | Mike Barnes wrote: | > Well, yes, but they'd only need to be replaced every few decades. The | > lamps, that is. | I'm not so sure. Once you start driving LEDs hard as a light source rather | than just an indicator, their life span reduces dramatically.
I noticed last time I was passing through Waverley that the solari-style LEDboards had some characters missing.
The main concourse ones were advertising trains calling at Newcas and Leices and the ticket hall ones were welcoming passengers to the First ass Lounge.
Owain
"Steve" wrote | >>My curiosity knows no bounds, it appears. When I bought one of | >>those LED cycle lights, I hooked up a sillyscope to it and | >>discovered that on the "steady" setting, it actually spent 3/4 of | >>the time switched off. 100 Hz square wave mark to space 1:3. | The mean brightness is still the same, since the mean current must be | the same, because you can't leave the LED on at the current used when on | or it will burn out. However driving them switch mode allows the circuit | to run more efficiently which IS why they are pulsed.
Is there a certain frequency above which this pulsed light is considered a continuous light and therefore acceptable as the continuous light required under the Road Traffic Act or whatever legislation says that flashing lights on bicycles are verboten?
Owain
Well LED rear lights have had BS markings for a while and now the new LED front ones do as well so presumably they're legal as they conform to the required British Standard.
They're common as centre stop lights on cars. And as all the various tail lights and indicators on buses. Oh - and the taillights on my BMW. And of course there are HID headlamps (xenon) which aren't filament lamps either.
Actually LEDs get slightly more efficient at higher current, so the mean brightness is higher.
Furthermore, it appears that eyes don't perceive mean brightness, but perceive something which is between the mean and peak brightness, so a flashing light will appear brighter than a steady light of the same mean brightness (over some frequency range).
Neither of these effects is very large though. I've seen conflicting reports of which of these effects first resulted in pulsed operation of visible LEDs, although the high pulse current techneque was used to drive infra-red LEDs before visible LEDs were invented.
The police are probably just happy that a bike actually has lights
That certainly used to be true, but again for the Luxeon jobbies we use, it ain't anymore.
Is this a medical fact ? In recent discussion on sci.elect.design, this effect was poo-poohed by several of the engineers in there whose opinions I would trust.
Steve
I've seen it stated on a number of occasions. I have not personally tested it but it sounded plausable. BTW, the frequency range over which it works is not wide. It needs to be above the optical fusion frequency (varies from one person to another, but never higher than
70Hz), and below the frequency response of the receptors and optic nerve (which I believe is always over 100Hz, well over in some people). One person I spoke to about it a while back didn't think the frequency response of the optic nerve was the upper limit for the effect to work (thought it was higher). These frequences vary a little across the field of vision too, and with other factors.
Early LEDs did have a light output that increased more than the current increase, so it certainly made sense to operate LEDs in a high current, low duty cycle pulsed mode. Modern LEDs are more linear, so the only reason for pulsing would be to make the brightness adjustable (or constant, as the battery goes down), or maybe to resolve differences between the dc supply and the LED's fwd voltage drop (but that would need an associated inductor/transformer).
I have also seen the opinion on another newsgroup (c/w research references) that pulsed LEDs can appear brighter in conditions of low ambient light levels (ie, if our eyes are dark-adapted).
The only restrictions relate to steady lights and red to rear.
Is the Act enforced?
Mary
But they're not pedal cycles ...
I have my own Toy Boy, thank you :-)
Mary
I'm really glad I asked this question.
I think.
Mary
It's T H A T, Terry ...
Mary
Lumileds has some impressive 5w suckers with heatsinks on the back but it's Nichia's 50mW blue or violet lasers that I covet!
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