This is bollocks. Humans can't see IR. This just means that the IR filter he is using lets through enough red light to see by.

>>After all, if the infrared light was EXTREMELY BRIGHT, such as the IR of a sunny day, human eyes might still detect it. (And remember, if 30KHz ultrasonic sound is loud enough, you will hear it. Same basic idea.)<<

Pure GIGO. The only reason he thinks he is seeing infrared if it is "bright" is that any transmission source of radiation will emit a spread of radiation across a part of the spectrum and some of that may be in the visible range. A glowing tungsten coil, for example, emits IR as heat, but the dull red color isn't IR at all -- it's just the visible part of the full spectrum of radiation being emitted. This is a prime example of why "seat of your pants" intuition turns out to be wrong so often -- it is based on flawed or even false assumptions about science instead of being based on known facts.

maf: Many humans can see some IR. Maybe not the level that he is claiming, but I've done it. Here's how to try it.

Take a working IR remote. Go into a windowless dark room, or some other place of complete darkness. Give your eyes a few minutes to adjust. Stick the remote's IR LED up to your face, and start pressing buttons. If you are like many people, you will be able to see a faint glow when you press the buttons.

Human vision doesn't have a sharp cut-off where light less than X nm is perfectly visible, and light greater than X nm is completely invisible. There's a gradual weakening of the response as the wavelength increases, so the exact dividing line between "infrared" and "visible" depends on what you use as the threshold of "minimally visible".

So this technique lets you see light of wavelengths that are only barely perceptible, such that normally it's swamped by the light nearer the middle of the perceptible spectrum.

Whether you want to call that light "infrared" or not depends on exactly where you draw the line between "visible" and "not visible". It's light that normally doesn't affect your perception of your surroundings, but in the absence of any other light, it's (barely) visible.

I believe there are anecdotal cases of people with brain damage who are able to see areas of the spectrum that are usually invisible to humans. I think there is a case in 'An Anthropologist on Mars'...