Fixed Pattern Noise
Fixed Pattern Noise (FPN), as the name suggests; is a form of noise the affects a sensor in the same way (spatially) for every image.
When FPN is visible in an image it can be quite annoying and difficult to remove.
In sensor analysis FPN is often removed by subtracting a bias frame, a black frame taken under the same conditions as the image frame.
Every Sigma camera that I have data for does FPN removal by subtracting a bias frame.
The good news is that there will be no FPN in Sigma images.
The bad news is that this makes long exposures even longer.
I don't think this wasn't done simply to improve Image Quality (IQ).
I suspect that the Foveon sensor is subject to strong FPN and it would have been unacceptable not to do FPN removal.
Bill (visit me at http://home.comcast.net/~NikonD70/ )