Reducing Moire in Imaging [was: Dot Simulation]
Reducing Moire in Imaging [was: Dot Simulation]
- Subject: Reducing Moire in Imaging [was: Dot Simulation]
- From: "Fleisher, Ken" <email@hidden>
- Date: Thu, 15 Jan 2009 16:12:52 -0500
- Thread-topic: Reducing Moire in Imaging [was: Dot Simulation]
On 1/15/09 1:38 PM, "Robin Myers" <email@hidden> wrote:
> If it is possible, changing the magnification of the subject relative to the
> image plane (move toward or away from the subject, changing the angle to the
> subject) is a simple, quick and effective way to remove moiré without
> sacrificing image quality.
>
> Robin Myers
This is an excellent suggestion. Another suggestion that might not be as
obvious which I have used with success is stopping down the lens (higher
f-number), when possible. This has the effect of making the cutoff frequency
of the system be the lens and not the detector (i.e. you are preventing the
high frequencies from reaching the detector) thereby making it impossible
for aliasing, and the resulting moire pattern, to occur in the digital
system.
The problem with aliasing is that you get modulation of the spatial
frequency below the detector nyquist frequency from spatial frequency
content above the cutoff. By setting the optical cutoff at the nyquist
frequency of the detector, no aliasing can occur. Now you can make your
images a bit sharper by setting the lens cutoff above the detector nyquist
frequency (by stopping down, i.e. using a higher f-number) and use the low
contrast of the pattern to effectively render it gone.
Ken Fleisher
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