Depending on the blue filter used, photos from a Pi NoIR camera would capture (instead of R,G,B) NIR+R, NIR+G+B, B+NIR+G. If you use a good red filter that really blocks all blue and green, you could capture R+NIR, NIR+R, NIR. Some little CMOS sensors give poor NDVI results with a blue filter compared to a red filter, but I don't know about the Pi NoIR camera.
There are lots of DIY tests to learn what is being captured in each channel:
- Take a photo of an NIR LED (or remote control) that cannot be seen with the naked eye. Which channels record the LED? In which channel is it brightest?
- Take a photo of the spectrum of a compact fluorescent lamp created with a slit and piece of DVD. Which channels record the blue, green, and red peaks?
- Take a photo of healthy green foliage outside. In which channels does the foliage look brightest? That's the channel with the most NIR.
Do these tests with a normal Pi camera and Pi NoIR camera both with and without a blue filter and then red filter. You will know more than most people about what is going on.
Your professor should not be satisfied that "NDVI is really acceptable" until you do what @claytonb did: https://publiclab.org/notes/Claytonb/08-13-2016/plant-health-ndvi-white-balance. All you need is a Canon S100 and some Boom Chicka Pop (and an expensive filter).
Check out these notes (and comments) about Pi cameras and NDVI: