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From: Louis Boyd
Subject: Re: Increasing IR LED Output
Date: Sat, 05 Oct 2002 14:37:34 -0700
Organization: Fairborn Observatory
NNTP-Posting-Date: Sat, 5 Oct 2002 21:37:34 +0000 (UTC)
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.18-3 i686)
> I'm designing a small IR LED based illuminator to use with a B&W CCD
> security camera.
> I know you can increase the maximum current (and hence the emission) of IR
> LEDs by driving them with a small duty factor pulse waveform
> My question is, does this method give an average increase in brightness as
> perceived by a CCD camera?
> I've had a look at the LED FAQ at
> http://www.pioneernet.net/optoeng/LED_FAQ.html but it doesn't quite answer
> this specific question.
Only a very small improvement. Most CCD chips integrate arriving
photons for a large percentage of each field. You can pulse an led to
high peak output but the total integrated energy remains about the
same. You can get away with a single higher pulse on a single field and
use a frame grabber (field grabber), but that won't give smooth motion
images. Using a filtered xenon strobe rather than an LED does wonders
A better approach is to mathematically add adjacent frames to reduce
noise. For example, display the sum of the last 20 fields. (1/3 sec).
Some additional smearing will occur but isn't particularly annoying to
watch. Cooling the CCD with thermoelectrics will reduce noise allowing
higher gain. Some security cameras use these techniques.
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