That depends on how "near" they are.

The sensitivity to red light decreases quickly at wavelengths greater than 650 nm, but light can still be perceived if it is strong enough, up to around 780 nm.

Many so-called near-IR LEDs may actually be somewhere around 750 nm, so they are still visible on a dark background, even if they are perceived as extremely dim.

On the other hand, there are many near infrared LEDs around 900 nm and those are really invisible. Near-infrared LEDs around 1300 nm or around 1550 nm are also completely invisible.

An invisible near-infrared laser beam could become visible due to double-photon absorption, but if a beam of such intensity as to cause double-photon absorption hits your retina, there are more serious things to worry about.

I remember reading some people can perceive some near IR, but mostly that near-IR LEDs actually leak some red themselves due to imperfections in manufacturing or something?

https://evermorestud.io/retroreflective-chromakey-experiment...

You mean do strobe, strobe, strobe, strobe, pause, pause, pause, pause? I bet that's at least as bad as holding the source on for the first four intervals and then off for the latter four intervals.

In any case, if you actually have a scene bright for 1/24th of a second and then dark for 1/24th of a second, repeating, you're well within photosensitive epilepsy range. Don't do that to your actors unless you've discussed it with them and with your insurance company first.

No.

Incandescent lights flicker at twice your AC power frequency -- to a decent approximation, their power is proportional to V^2. But this is input power -- the cooling of the filament is slowish and the modulation depth is low. Most people aren't bothered by this.

Fluorescent lights with old or very crappy "magnetic" ballasts flicker at twice the mains frequency, with deep modulation. The effect on people varies from moderate to extremely unpleasant, and it's extra bad if anything is moving quickly (gyms, etc). There are even studies showing that office workers perform worse under such lighting even if they don't experience personally perceptible symptoms. The effect is so severe that people invented the "electronic ballast", which flickers at much, much higher frequency and avoids low-frequency components. Phew. (The light might still be a nasty color, but the temporal output is okay.)

"Driverless LEDs" are deeply modulated at twice the mains frequency. These are very nasty.

If you actually have a light that flickers at the AC power frequency (certain LED sources in a two-brightness diode-dimmed kitchen appliance fixture will do this, as will driverless LEDs with certain types of failures), then it's extra nasty.

There are plenty of people around who find (depending on the actual waveform) 60Hz flicker intolerable and 120Hz flicker extremely unpleasant. And there are plenty of people who can often perceive flicker under appropriate circumstances up to at least several hundred Hz and even into the low kHz with certain shapes of light sources. You can read up on IEEE 1789 to find a standard based on actual research on what lighting waveforms should look like.

The effect of 120 Hz flicker is bad enough that energy codes in some places (e.g. California) have started to require that LED sources minimize this flicker, but, sadly, it's poorly enforced.

phosphors and capacitors are a thing that mask that, so is high frequency switching way above this rate…

Anyway, an old HN submission I still use when buying light bulbs: https://news.ycombinator.com/item?id=14023196