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I have converted my canon sx260 hs so it can capture NIR part of specter. Can anyone tell me how can i calculate NDVI now? My guess is that on the blue channel is now NIR value and Green channel=(TRUE_GREEN + NIR) And Red channel =(TRUE_RED + NIR) So NDVI would be: NDVI= (NIR-TRUE_RED)/(NIR+TRUE_RED) Am i on the right tracks? Any hints?

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If you converted a Canon camera to pseudo-NIR by removing the NIR filter in front of the sensor, then you will end up with a blue/green/nir sensor (vegetation appears white). Indeed, all channels will be affected with extra intensity from the NIR depending on their sensitivity to this wavelength, with effect on RED >> BLUE > GREEN. Red sensors are the most affected and the amount of NIR light is larger than visible light, so you don't have true red anymore. In this case you cannot compute NDVI because you are missing te Red and blue or green are not correlated with red to provide a substitute.

It is technically possible to create a Green/Red/NIR camera (vegetation appears red), but far more difficult (and I don't know how to do it). With such a camera, NDVI can be computed with your equation. Alternatively, you could try using two cameras (one with and one without IR cut filter) that you could register then compute your NDVI.

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