Purple flare: Causes and remedies

Started Mar 1, 2013 | Discussions thread
kenw
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Re: Hmmm... CC experiment implies non-linear process...
In reply to Cani, Mar 2, 2013

Cani wrote:

My question was in relation to your perceptions of the changes in the picture, especially the colors. I was not sure what you and others observed, how you oserved it. I am still not sure except that the highlights kind of get "whiter", i.e., blue and red seem attenuated in the highlights only and enhanced in the rest of the picture. This matches Anders' speculation about the causes :attenuation of the incoming green light results in blue & red pixels (where flare shows) being struck with less light, thoughh the lower response.

I suppose it was obvious but...

Ah, sorry I missed the point of your question!

What I see:

First image (no filters of any kind) - There is obvious white flaring across the top of the image and there is also a rather obvious more localized purple flare.

Second image (polarizer) - The white flaring remains, but the purple flare is essentially gone. Most notably, the purple flare hasn't turned white - it has disappeared completely - that is to say if we made monochrome versions of the two images they would look different with the second image having no sign of the extra brightening in the region of the purple flare in the first image.

Third image (CC filters and WB adjust) - In this image the purple flare in the first image has now turned white, there is clearly additional flaring where the purple exists in the first image but now it is white instead of purple. This is distinct from the second image in which the flare associated with the purple regions has been completely eliminated.

To summarize what I see:

First image, general flare plus a purple "streak". Second image, general flare unchanged and purple "streak" completely gone. Third image, general flare unchanged and purple "streak" has turned white.

To summarize what I conclude from the tests:

First image: There is a strange purple streak, it is probably an optical reflection from some place (most flare is of course). Why is it purple though? Was the reflection itself purple (e.g. like we might see from some optical coatings) or is there something "funny" going on.

Second image: Demonstrates that the purple flare is from some reflection source because Anders could eliminate it with a polarizer (as opposed to a scattering source which wouldn't be polarized). Not really a surprise, but a useful illustration that there is a reflection someplace. Still left unanswered, is this reflection purple or is it white and then changed to purple by some other process further down the chain.

Third image: This is the image that proves the reflection itself is not purple. If it was truly purple then the CC+WB trick wouldn't make it become the same color as the rest of the originally white flare. This test strongly implies that the offending reflection is likely white (or close to it) and being changed to appearing purple in the final image because of the sensor reflections past the CFA from off axis light that Anders illustrated.  The implication is this part of the flare while actually white is originating from a point off the optical axis of the lens and that is why it turns purple when measured by the sensor.  The rest of the flare that appears white in the first image is coming from close to the lens axis and so is measured as white.

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Ken W
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