I recently purchased a Filter Kit and installed the included #2007 blue filter on a new ELPH 130IS. Yesterday I attached the camera to my quadcopter and collected ~600 images covering 60 acres of mixed agricultural land. Lacking anything other than a random solid blue Mead folder that was lying around, I set the white balance on the camera with the frame filled with the folder. The images mostly came out great and processed flawlessly in Photoscan, but there seemed to be an awful lot of light coming into the Green channel. In fact, the green histogram is nearly identical to the red channel of every image. I assume this is an issue with how I set the white balance.
Does anyone have any specific recommendations with what medium is currently preferred to correctly minimize the green channel of the starting image? I've seen suggestions of test paper and led lights, but I'm not sure what the best option is for most field settings.
I'll post some of the full NIR 3D models when I get everything processed.
I see what you mean about the green channel being almost as bright as the red channel. The Rosco 2007 filter blocks almost as much green as red, so both of those channels are going to be dominated by NIR. A different white balance setting could darken the green channel some, but it is still going to include mostly information about NIR. The green channel does not have to enter into computations of NDVI, so it might not be so important.
A more important issue might be that the blue channel is very dark. That provides very good separation between the VIS (blue) and NIR (red) channels so NDVI values will be plenty high. But the blue channel is so dark that most plant pixels and many other things have a similar brightness. This might be causing poor distinction between plants and non-plants in the NDVI values.
Many non-plant things (like the round bales and deck) have higher NDVI values than they should. Some of this is due to the nature of the blue filter which uses blue light for VIS and therefore does not allow healthy foliage to be distinguished from some dead things (including wood and dead foliage). Some of the problem might also be due to the particular white balance setting -- the red and blue channels probably don't have to be so well separated.
You could try another white balance with another blue paper (in the shade under a blue sky) and see how it works. Or you could try a red filter which will generally provide better distinction between healthy foliage and things like wood and dead leaves. There are some advantages to using a blue filter, but only if the cutoffs are very steep on either side of red. The Rosco 2007 does not have very steep cutoffs.
Chris
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