> -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of M. Adam Davis > Sent: 08 February 2010 15:27 > To: Microcontroller discussion list - Public. > Subject: Re: [EE] : Lumens what are we talking about here ? > > On Fri, Feb 5, 2010 at 5:44 PM, Chris McSweeny > wrote: > > So the perceived brightness for the same power > > input isn't actually better. > > Ah, earlier when I was asserting that pulsing a LED can make it appear > brighter I was not talking about efficiency. I was talking about > significantly overdriving an LED during the pulse. > > For instance, if an LED is rated for 10mA you may be able to drive it > with 2A pulses at a 5 or 10% duty cycle, and it will appear brighter > than simply running it steady state at 10mA. I realise these number were somewhat arbitrary, but I suspect they are way off the mark. An increase from 10mA to 2A would increase the I^2 losses by a factor of 40,000, coupled with the IV losses which would be more than 200 times higher since Vf would be significantly increase being overdriven by that amount. To keep the average power levels within safe levels, the duty cycle is likely to be very small in this case, much lower than 5% and probably a tiny fraction of 1%. The efficiency discussion is completely relevant in this case, because driving an LED harder does not give you proportionally more light output above some threshold. As you increase peak current and reduce duty cycle to maintain safe power dissipation you will actually be getting less average light out of the device. As others have mentioned, for frequencies above the persistence of vision, the eye tends to integrate light rather than respond to the peak so it's the average light output you are interested in. Hence pulsing the LED is not likely to give an improvement in brightness. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist