On Tue, 24 Sep 2002 21:09:45 +0100 Bernard Boudet writes: > > *However*, the main point of the HP app note was that the important > thing is not to exceed the maximum junction temperature. At d.c. > this > is proportional to power dissipation (plus ambient temperature), but > with pulsed operation the junction gets to cool down between pulses, > also there is a certain rise time (thermal response) during the on > periods. > > So following HP's own data, it's possible to run an LED above it's > equivalent d.c. power rating (which has gotta mean more light, human > perception or not). But to do it properly you do need to know > various > parameters, which would be quoted on the data sheet for that LED > (e.g. > max. junction temp., thermal resistance, etc.) > This does seem strange to me. If, as I expect, the limiting factor is junction temperature, it seems that pure DC would give you maximum output. Going to pulses, the junction temperature will have "ripple." The magnitude of the ripple (assuming a perfect heat sink) would be proportional to peak input power and thermal resistance to the heat sink. It would be inversely proportional to the heat capacity of the device. With relatively high frequency pulses, the heat capacity keeps the junction from heating or cooling much, resulting in low ripple and a relatively stable junction temperature (similar to driving the LED with DC, in which case there'd be no ripple). Driving with a lower frequency would result in a larger ripple with the same average temperature, but a higher peak temperature, which seems like it could be a problem. It is good that you point out that increased current also causes increased voltage across the LED, so power dissipation increases more than just in proportion to the current. So, I'd expect pure DC on an LED would allow the LED to be driven with more power because the peak and average temperatures would be the same. The original question on perceived brightness is interesting. I'd like to see any research on that. I believe that as a sort of data reduction technique, our brain tends to notice flashing lights more than steady lights. When something is steady, we don't refresh the image in our brain, since it's just the same data anyway. If the image is changing (as in a flashing LED), it needs to be refreshed, so we notice the LED more than we'd notice a steady LED. I think this is especially true with peripheral vision, where we are especially only looking for changes. I would think, however, that the flash rate would have to be low enough that we perceive it as flashing. This is all just a guess, though. Seeing some experimental results would be interesting. Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com Reach broadcasters, engineers, manufacturers, compliance labs, and attorneys. Advertise at http://www.hallikainen.com/FccRules/ . ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.