Question: How to calibrate the white-balance for NDVI using the BLUE filter?

cagiva asked on January 10, 2019 20:50
95 | 1 answers | #18097


I purchased the Blue DIY filter pack; which unfortunately arrived without the blue card used to do the white-balancing calibration (PublicLab is sending me one though). In the meantime, I used this grey card for the calibration and tried to process some pictures on the Infragram sandbox. Unfortunately, I'm not able to obtain the same vibrant results shown in this video. Could it be possible that the grey card is not suitable for the Blue filter or do I need a higher resolution camera; instead of my Canon PowerShot A3000IS?

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1 Comments

@cfastie, You were absolutely correct. The Blue card from PublicLab arrived today so I redid the white-balancing calibration with it and I got much vibrant results. Thanks!

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1 Answers

Performing a custom white balance on a blue-filtered NIR camera fools the camera into exaggerating the brightness of the red channel. The red channel is used for information about NIR light. Although modified consumer cameras are sensitive to NIR light, they are much more sensitive to blue light. To get a measurement of the proportion of NIR to blue in the sunlight reflected from foliage, you need equally sensitive devices measuring each, or you can arbitrarily exaggerate one so the proportion is closer to what it should be. That is what the custom white balance does and it must be done in a way that fools the camera in a certain way. A gray card will not do that, but any very blue surface could work. The surface must reflect a lot of blue light, or you can use some other source of blue light: https://publiclab.org/notes/cfastie/11-22-2013/white-balance-for-all

Chris


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