https://en.wikipedia.org/wiki/Infrared_photography#Color_infrared_film :
Color infrared transparency films have three sensitized layers that, because of the way the dyes are coupled to these layers, reproduce infrared as red, red as green, and green as blue. All three layers are sensitive to blue so the film must be used with a yellow filter, since this will block blue light but allow the remaining colors to reach the film.
Pour les appareils numériques, apparemment ça consiste à enlever le filtre IR si j'ai bien compris :
http://www.naturfotograf.com/UV_IR_rev00.html#top_page
Mais Pen² pourra sûrement expliquer tout ça mieux que moi
Au passage, il y a un article sur la photographie Brunniesque :
https://en.wikipedia.org/wiki/Full-spectrum_photographyen.wikipedia.org