If it's really infrared or not it depends on what definition of visible light is adopted. Some authors states that visible light ends at 700 nm, others around 800 nm. Regardless if it is NIR or not, it's very amasing to see that things can look different at extreme red (e.g., plants get very reflective). I had the opportunity of seeing in this frequency range myself when looking at the grass using a filter that allowed almost only the IR radiation to pass (I don't remeber the cuttof waveleght, it was around 800 nm). Given that with this filter it took around 1 minute for my eye to get adapted to the darkness, I guess that the peak response of my eye+filter was around 0.001% - 0.0001% of eye sensitivity around 570 nm. Peak wavelenght probably was between 700nm - 850nm.