From: John Popelish
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Subject: Re: LED Pulsing - Apparent Brightness?
Date: Tue, 07 Jan 2003 14:47:37 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 09:47:37 EST
Mark Klinger wrote:
> I am experimenting with wireless communication via LEDs, and have a
> simple question of apparent brightness vs. actual light output.
> Keeping in mind that my receiver will be a photo detector (Fairchild
> L14N2) I'm trying to understand brightness.
> At DC, 20mA is the max drive for my LEDs and they are extremely
> bright. If, however, I put them in my circuit and pulse them with a
> width of around 1mS, they are barely even illuminated.
> Will the distance of the transmission suffer since they are
> "apparently" not as bright, or are they actually the same brightness
> as at DC to the photodetector, but my eyes just aren't seeing that?
> Ultimately what I am wondering is if what the photodetector sees is
> effected by the short pulse duration, or is it equally as bright at
> 1mS as it is at DC?
To a first approximation, the light output of an LED is proportional
to the current. for on off operation with times shorter than the
persistence of vision the brightness is roughly proportional to the
duty cycle (percent on time) If your 1 ms pulses look dim, either you
are not supplying them with the current you think you are, or there is
a lot more than a millisecond of off time between them.