Figure 1: A low pass (visible light blocking) filter, that allows infrared to pass thru it unblocked.
Normally, IR is invisible to the human eye. So, how could a person detect infrared radiation with the eye? It can be done indirectly. In figure 1 is the low pass visible light blocking filter known to all IR photographers. It blocks visible light (notice that it is jet black, even when held in front of a bright lamp).
The thing is, the human eye can detect IR already. It can detect even down to the level of upper level “far” IR!