From: Godfrey DiGiorgi
A Fovean sensor stacks three photosites in each pixel location on the sensor, with a color filter between them. I don't know precisely how they order the stack, but it would make sense to order it blue, green, red since short wavelengths are more easily absorbed than longer wavelengths. But what this means is that the light energy hitting the bottom layer is inevitably reduced compared to what hits the top layer. They compensate to some degree by making the three layers differentially sensitive, either by adjusting the signal amplification or by staggering the photosite sizings, I don't know.
If memory serves blue, green, red was the order of the emulsion layers on film.
There's a cross section of the Foveon X3 buried down in the article showing a blue sensor layer on top, a green sensor layer that is twice as thick in the middle and a bottom red sensor layer that is twice the thickness of the green layer on the bottom.
I don't know if the thickness differences are real or just an illustration to show how the X3 amplifies the light signal in the lower layers to achieve color balance.
So, I guess the problem with high ISO performance might be inherent in the sensor.
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