Re: Any thoughts /news on Foveon sensors?

In article <dque5j$n1n$1@xxxxxxxxxxxxxxx>, davem@xxxxxxxxx says...

> Focusing on subcomponents makes a great deal of sense if it helps you to
> understand what's going on.

Dave, in the professional world, the specifications for an image usually
are 300 dpi at a specific print size, not 300 dpi of luminance
resolution, 150 dpi of chrominance, 200 dpi of green channel etc. You
need one number which defines the resolution of the image.

> For example, the human eye's luminance resolution is about 60 cycles per
> degree. If you make a print that, at normal viewing distance, resolves
> 60 cycles/degree with good contrast, it's going to be indistinguishable
> from the original scene in detail. While if the print resolves 30
> cycles/degree or 20 cycles/degree (more typical), it will still look
> good but not equal to the original scene.

That's all fine, but the problem is that with a Bayer sensor you don't
have accurate luminance information at the single pixel level. In the
RGGB block, the two green pixels give you a somehow good approximation
of luminance (because green is in the middle of the visible spectrum),
but the red and blue pixels do not.

The red pixel for instance might be illuminated by yellow or green
light, and you will not be able to know if the red pixel received a
somewhat less bright red light or light of another colour.

You need to do some interpolation, which in practice means that you can
get a (probably) good enough luminance information, but not at the
*individual* pixel level. In other words, with real world images the
luminance resolution in a Bayer sensor is less than the luminance
resolution in a full colour sensor.

Of course if you use black and white test targets you don't see this
resolution loss, because there is no colour.

<snip>

> >Have you seen this:
> >http://clarkvision.com/imagedetail/relative-lens-sharpness/
>
> >(look at the coloured boxes on the right side and cry with me)
>
> Why cry? It looks like whenever you can resolve modulation in the
> black/white pattern, you can still resolve colour in the patches on the
> right. What more could you want?

Did you have a look at figure 2 for instance ? With six pixel wide
lines, you can still clearly see the individual lines in the
monochromatic patterns, but the lines with the same width in the
coloured patterns to the right are hopelessly merged (the three red-
blue-red lines become a homogeneous magenta block, just to make an
example). The blue and green lines are also merged, as are the red and
magenta lines.

> >In any case, it seems that we differ on the amount of resolution loss
> >there is in a Bayer sensor (compared to a full colour sensor). My
> >guesstimate is that on average a Bayer sensor has about 60-70% of the
> >nominal resolution (the emphasis being on "average" - the loss will
> >depend on the scene photographed). Not too terrible, but not negligible.
>
> What is "nominal"? Are you comparing against the Nyquist limit of 2
> pixels per line pair? No camera can achieve that without aliasing.
> Good B&W cameras, with no Bayer filter, don't do more than about 80% of
> Nyquist either.

I just mean that with real world images a Bayer CCD with 6MP should have
approx. 4MP of "real" resolution (= 60-70% of 6MP), i.e. the same
resolution of a full colour 4MP sensor (or let's say the same
information content). With an 8MP Bayer sensor you should have around
6MP of "real" pixels. But this is of course just a personal guesstimate.

In any case, since the matter is important I'm surprised there is so
little research on the topic, or at least I have seen very little of it.
--

Alfred Molon
------------------------------
Olympus 50X0, 7070, 8080, E300, E500 forum at
http://groups.yahoo.com/group/MyOlympus/
Olympus E500 resource - http://myolympus.org/E500/
.

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