Why is the Foveon X3 called a “direct” image sensor?
Asked 5/31/2015
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I understand that the Foveon X3 uses stacked layers so each pixel location records color information without Bayer demosaicing. What does the word “direct” mean in the term “direct image sensor,” and is it a technical distinction or mainly marketing language?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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It's meaningless and actually misleading marketing speak:
A direct image sensor is an image sensor that directly captures red, green, and blue light at each point in an image during a single exposure.
The sensor doesn't capture blue, green and red neatly in each layer as applied by the diagram, the top layer is sensitive to red, green and blue (it's effectively unfiltered), the middle layer is sensitive mainly to red and green (but is still partially sensitive to blue), and the bottom layer is mostly sensitive to red light (but is still partially sensitive to green and to a lesser extent also blue light).
If you were being very generous you could call it a direct white, yellow, red sensor, but seeing as nobody wants a camera that shoots WYR images it has to be converted into RGB. This requires a lot of complex calculations to be applied to try to separate the colours. I say "try to" because there are some cases when it can't, leading to errors.
In addition to this, due to depth diffusion varying degrees of sharpening and other processing are required for each layer. This is one of the reasons processing Foveon RAW images is very difficult, and support from major RAW editing software is completely missing (there's only two converters I know apart from Sigma's own who have even attempted it).
So the idea there's anything "direct" about their approach is I'm afraid completely false.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
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Generated from our catalog & community — verify before relying on it.
“Direct” refers to the idea that a Foveon sensor records full color information at each pixel location in a single exposure, rather than using a Bayer pattern and then interpolating missing colors with demosaicing.
So in marketing terms, it is called “direct” because RGB information is derived for every pixel position without the usual Bayer-style interpolation step.
That said, the term is somewhat marketing-driven and can be misleading if taken too literally. A Foveon sensor’s stacked layers do not each capture perfectly pure red, green, and blue independently. The layers have overlapping spectral sensitivities, so the camera still has to do color separation and processing to derive the final RGB values. In that sense, it is not “direct” in the simplistic diagrammatic way often shown.
So the short answer is: it’s called “direct” because it captures color at every pixel location in one shot, but the term is more promotional than a strict technical category.
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