why are debayered DSLR images green?

[mikeoday]

Hi guys

The common answer I have seen to this question is that it comes from the fact that the CFA matrix has 2 green pixels.

However, the documentation for the SuperPixel debayer method effectively says that the one pixel is formed from the RGG'B square such that R1 = R,  B1=B and G1 = (G+G')/2 .  That is, the two green pixels are being averaged so I still don't understand why the image should still  be green using this method.

Does anyone know the answer?

Thanks
Mike

[mikeoday]

Ok, I think have found the answer ...

From   http://www.odelama.com/photo/Developing-a-RAW-Photo-by-hand/

"Different Photosite Sensitivities

In a photo taken to a white object (e.g. a sheet of paper) while using white light (e.g. outdoor, directly illuminated by the sun in a clear day), we would expected the RGB values in the raw image file would have the same coordinate values R = G = B. After all, "it uses the RGB model" where the neutral colors have as RGB color components the same value. Well, that really doesn't happens at all.
...
The red, green and blue sensor photosites have very different relative sensitivity,... The most sensitive is the green photosite, then the blue and finally the red. This behaviour is like if the camera would have a greenish-cyan lens filter and by looking through it, a white object looks greenish and the red-wine colors has equal RGB values in raw data.
...
This is not a matter of sensor quality. If we look at the top of the camera sensors ranking at DxO Mark, we will find this characteristic pretty much alike in all sensor brands.

The following table shows data from that site, showing the multipliers required to apply to the red and blue raw values in order to get the level of the green ones for pixels corresponding to a white object under a CIE illuminant D50.

Brand    Model    Red raw    Blue raw
Nikon    D800E        2.02        1.49
Sony    A7R        2.52        1.59
Phase One    IQ180        1.46        1.98
Pentax    K-5 IIs        2.10        1.72
Canon    EOS 1Dx        1.97        1.50
Nikon    D7000        2.11        1.50

This characteristic is the cause of color shifting when there is highlight clipping —and also when there is shadow clipping— in the raw data as we will see below. "

[cbobchin]

I have a Pentax K-1 and have the same issue with the image being greenish once debayered. How do I apply these corrections in PI?

[sharkmelley]

I do it in PixelMath but it may not be the best way

Untick the box "Use a single RGB/K expression" and use formulae similar to these, with the white balance multipliers for your own camera:
R/K:   $T[0]*2.07
G:      $T[1]
B:      $T[2]*1.54

Mark