Nada,
Hope you found some help in your project.
I found this thread: https://community.pix4d.com/t/5201-create-a-camera-model-for-NDVI#gsc.tab=0
with this article in it: https://support.pix4d.com/hc/en-us/articles/204894705
that explains the use of modified cameras. It is somewhat complicated for me, but i hope eventually doable, but I will need help from my lens manufacturer (IRPro) and possibly from Pix4D as I attempt to produce relative-NDVI maps.