As part of the Infragram project, folks across Public Lab are taking their own DIY multispectral photos to assess plant health. But this work is largely comparative -- since we're using modified consumer cameras, we don't usually know if they're more or less sensitive in one color channel vs. another.
That means we compare imagery, but don't assume that we have an "absolute" measure of NDVI that we can compare across cameras, or in very different lighting conditions.
Anyhow, I've seen some great work by @nedhorning trying to use a standard color reference calibration card to calculate the absolute NDVI, and enable comparisons across different cameras or lighting conditions. How does this work?