Quantitative comparison of Foveon and Bayer spectral response

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DavidWright2010 Senior Member • Posts: 1,483
Quantitative comparison of Foveon and Bayer spectral response
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In a previous discussion, I was trying to design a simple, reproducible, way to put numbers on the color precision (ability to distinguish colors that are very similar) of Foveon and Bayer sensors. (BTW, I did a brief literature search, and found thousands of papers on the design of the sensor, but nothing on the spectral response of the color filters. I probably needed to use different set of keywords.)

This is related to the claim that Bayer sensors cannot distinguish the subtle shades of green in foliage.

I started a new thread for this specific goal - quantitation. Not that I'm opposed to personal opinion, but an opinion can't be measured.

So my current test uses a prism, which I bought from Amazon for $10. I set the prism to intercept a beam of sunlight coming through a window, and display the resultant spectrum on a white surface. I then take a picture of the spectrum with each camera.

For the Bayer sensor I got:

And for the F0veon sensor (DP3M):

I then opened these image in ImageJ (thanks, Ted!), separated the colors, and plotted the spectral response.

Here Bayer is on the left, the Foveon on the right. There are labels on the individual plots, but if you can't read them, the plots have red at the top, blue at the bottom. The X-axis is just in pixels, as I haven't figured out a way to put wavelength values correctly.

Unfortunately for the question under consideration, the Bayer filter response seems perfect in the green region. Red and Blue both go to zero at pixel 200, which is the center of the green response. To me, that means that a Bayer sensor should be very sensitive to subtle changes in green hues.

That's not to say that the Foveon isn't. Both Red and Blue are changing rapidly at 240 pixels, which is the maximum response of the Foveon 'green' channel.

So I need to think about this a bit.

Comments and (gentle!) criticism welcomed.

David

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