I'm starting this thread with the only purpose of finding such an explanation. Other discussions in this thread should be avoided in order to keep momentum in the direction of an explanation.
Let's first look at the cleanest example possible: images taken with a non-shiny opaque white sheet of plastic in front of a wide angle lens. Both presented here are from the 21/2.8 Elmarit ASPH at f/16. The first one is not coded and the second one is coded as an 21/2.8 Elmarit ASPH. The images are taken at 160 ISO and with an exposure compensation of +2 stops, lens distance ring turned to infinity. No postpressing other than loading in LightRoom 3.4.1. Light from overcast clouded sky.
First you can see the excellent job this firmware (1.138) does in correcting the italian flag phenomenon and it takes care of some (fortunately not all) vignetting.
Secondly, you can see that the italian flag phenomenon is not red-white-green from left to right, but from left lower corner to right upper corner.
Thirdly the true discolouration is not red, but magenta (lack of green) lower left corner and cyan/green (lack of red) upper right corner.
Now what we know is the following (looking along the path of the light rays):
1] Any colour change from the optics (lens only) should be rotational symmetric around the optical axis, so although the lens produces the angle of incidence on the sensor, it does not produce the change in colour we see in the italian flag phenomenon.
2] There is a cover glass on the sensor that takes care of IR filtering, which is quite thick (1 mm). This cover glass disperses a white ray of light consisting of different wavelengths (colours) to slightly different positions on the sensor. This effect increases at an increasing obliqueness of the rays. However, this effect also is rotational symmetric around the optical axis. Moreover, rays that are neighbours of the rays we look at simply add up to the full spectrum of white light. So the chromatic dispersion of the cover glass on its own (!) can not be responsible for the observed discolouration.
3] There are microlenses on top of the pixels of the sensor that are shifted (increasingly towards the edge of the sensor) relative to the pixels in the direction of the center of the sensor, to be able to catch sufficient lightrays from wide angle lenses. When it is assumed that this shift is rotational symmetric around the optical axis, this shift can also not be responsible for the oberved phenomenon. This is even true when these microlenses themselves have chromatic aberration.
4] The sensor consists of pixels that are sensitive to three different overlapping bands of the visible spectrum: red, green and blue. These pixels are arranged in a Bayer pattern in which there are two times as many green pixels than either blue or red sensitive pixels. In principle when you have an even number of pixels along the length of a sensor, the pattern is asymmetric, since you may start with a red/green pair at the left edge and you end up with a green/blue pair at the right edge of the sensor. But this asymmetry extends only over 1 only one pixel and so the changes of colour over larger distances can not be explained with it. Moreover, the algorithm that calculates the rgb components of one image pixel in the produced image uses a weighted combination of red, green and blue sensor pixels.
So we are left without an explanation. The effect may be produced by a combination of properties of the cover glass, micro lenses, sensor and algorithm. But how exactly?
So let us look at some observations:
a] According to some observers, the italian flag phenomenon is known from sensors without micro lenses, so perhaps we can exclude them from an explanation.
b] There is a relation with focal length and not with the place on the sensor, so the angle of incidence of the lightrays is certainly a necessary condition of the phenomenon.
Some observations I would like to see filled in:
i] If you take the area covered by the M8 sensor out of an M9 image, using the same lens, and you use no coding, do you get the same amount of colour change? I have no M8, so can not do the test. I'm asking this to see what the effect is of the thickness of the cover glass.
ii] What are clear examples of italian flag phenomenon in other digital sensors than M9 and M8. What do these sensors have in common?
Michael J. Hußmann (user mjh) has repeatedly stated that no explanation has been given. So can we do better?
Edited by Lindolfi, 05 June 2011 - 12:56.