What filter is needed to modify a Nikon D70s for NDVI imaging?
Asked 11/9/2012
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I want to modify a Nikon D70s for NDVI (Normalized Difference Vegetation Index) work. After removing the camera’s internal IR-cut filter, what external or replacement filter should I use to capture the red and near-infrared bands needed for NDVI? Also, can a single DSLR image provide separate red and NIR channels directly, or does that require additional processing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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I was in Valencia for the AgEng/CIGR 2012 conference where I saw a presentation about adapting a DSLR to do exactly this. The paper contains the details how to get the best possible Red/NIR colour space for NDVI and shows application of the results. You need to remove the ir cut filter and replace with a ~600nm long pass filter.
http://cigr.ageng2012.org/images/fotosg/tabla_137_C1122.pdf
Then you do linear combinations of the the 3 colour bands. Their work base the calculations on actual measured spectral response curves, which you might not have the gear for. The result depends on the white balance but I have tried this technique without actually doing all this on a Basler ACA640-90GC camera. I got good result incorporating it into the bayer demosaiqueing, separating the NIR channel into the blue colour and red into the red colour and nullifying the green. The hard part is balancing the intensity balance of the two bands, since the response to NIR in the cmos is about 25%. I also found that the result is very sensitive to the bayer conversion algorithm. I found that Basler's SDK actually use nearest neighbor, which makes very bad NDVI images.
Example image - before and after converting to NDVI index:
And an example when using the Basler bayer conversion:
Originally by user11455. Source · Licensed CC BY-SA 4.0
user11455
13y ago
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Generated from our catalog & community — verify before relying on it.
For NDVI with a converted DSLR, the usual approach is to remove the camera’s internal IR-cut filter and replace it with a long-pass filter around 600 nm. Published work cited in the community answers reports good results with a ~600 nm long-pass filter, then deriving red and NIR information by processing the camera’s color channels using measured or estimated spectral response data.
A stronger NIR-passing filter around 695–700 nm (such as filters often used for infrared photography) can capture NIR, but it will not give you clean, separate red and NIR bands directly from a standard Bayer-filter DSLR. A normal DSLR sensor cannot perfectly isolate visible red in one channel and NIR in another without changing the sensor’s color filter array itself.
So, for a practical DSLR NDVI conversion:
- remove the IR-cut filter
- use a ~600 nm long-pass filter for combined red/NIR capture
- extract NDVI bands later through calibration and channel mixing in software
Be aware that results depend on white balance, demosaicing, and the camera’s spectral response.
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