Interesting that you would refer to that.
Total hogwash, a $15 FleaBay IR filter for a commerical camera is NOT a precise scientific instrumental spectroscopic IR filter with a precise IR cutoff.
And the fact that you could see anything from that FleaBay IR cutoff IR LED Illuminator means that you are seeing Visible light since IR electromagnetic heat radiation is totally and completely invisible, ie no eye converts IR heat radiation into visible red light.
In addition, a 850nm or 940nm IR monochromatic laser would be invisible even with that crappy IR cutoff filter.You don’t get scientific results with cheap stuff, nice try but no cigar!
The Osram iR LED emits a broad spectrum of visible light and invisible electromagnetic radiation from 650nm visible spectrumto over 1000nm invisible spectrum, which is the reason you can see a red glow.
The 1947 study quoted is also BS, as is the two 500nm photons hitting the same photoreceptor cell to produce a 1,000nm wavelength response, that old ridiculous study was for observing retinal cells in a Petri dish using IR to pick up heat transfer in the cells for diagnostic disease purposes. Good for a laugh, but not anything else.
If you ever see anything at all under any circumstance, it is visible light, the rest of the entire electromagnetic wavelength spectrum is completely invisible and will always be that way., even if someone sees an alien or UFO day or night.