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From: Louis Boyd
Subject: Re: Increasing IR LED Output
Date: Sat, 05 Oct 2002 20:28:46 -0700
Organization: Fairborn Observatory
References: <3D9F5B9E.70DB3C27@apt0.sao.arizona.edu> <3D9F8489.9FE65295@webaccess.net>
NNTP-Posting-Date: Sun, 6 Oct 2002 03:28:47 +0000 (UTC)
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.18-3 i686)
Chuck Simmons wrote:
> Louis Boyd wrote:
> > 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.
> Perhaps I am a bit off of the wavelength you are on but mathematically
> adding frames is not as good for reducing noise as integrating on the
> CCD itself. If the CCD is capable of slow scanning (most are), it would
> be better to scan and grab one frame every 1/3 second than to grab the
> sum of 20 frames (this assumes industrial sync with 60 frames per second
> rather than TV which is 30). Moreover, this technique only reduces noise
> in the stationary background. Moving objects are not as much improved.
> Have I missed something?
I agree that you usually get a better image by integrating on the chip,
at least if thermal defects aren't dominant. However, for human viewing
having the image change every 1/3 second or so is quite annoying. The
method of adding new frames and dropping old one is still changing 30
times a second so the eye tends to smooth the changes. Yes, it looks a
bit "wormy" because the noise patterns have persistance. With memory
(required for chip integration and adding multiple frames) it's also
practical to remove any static defects, such as hot pixels and "clocking
noise" which can help a lot.
Moving objects, if viewed with a "strobed" LED will give a series of
overlapping clear images rather than a smear. That can work with either
on chip integration or memory integration and makes easier to see what
the object was and estimate it's velocity and direction. None of these
approaches beats having adequate lighting and continous frames faster
than the eye's flicker response rate.
Another way (perhaps obvious) to perk up the sensitivity of a CCD camera
is to use a faster lens. The penalty is cost and reduction of depth of
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