For how long must light be emitted for a human eye to perceive it?

A 2016 Nature study found the human eye can detect a single photon just above chance, while conscious sight needs about 5 to 9 photons.

My understanding of the physics is that there's no lower limit. People can detect even individual photons.

As long as the pulse has enough energy to change the state of the material in your retina, you will sense it. Intuitively, if an attosecond pulse can melt steel, you can bet you'll see "something" if you fire it at your eyes, even if it's the last thing you see.

A 2016 Nature study

To be more specific, this study in Nature Communications: https://www.nature.com/articles/ncomms12172

Edit: Ha! NoSpotOfGround beat me by 2 minutes.

How much light is a photon, anyway? Does is take thousands or trillions of photons interacting with the eye to register as light?

Our eyes are actually extremely sensitive. When adjusted for complete darkness, it takes just two or three individual photons hitting a light-sensitive cell in the eye within a short period to fire off the corresponding neurons and be detected as a light.

When you're trying to see in very low light, do you notice how your vision looks a bit grainy, almost like there's a subtle bit of static? That's individual light-sensitive cells going off in your eyes, one at a time, each one triggered by only a few photons each. When there are so few photons coming into your eyes, there aren't enough to hit and trigger every single cell, so you get individual cells flashing on and off, causing this grainy/static texture.

Would a person notice a flash of light that lasted 10 ms?

Given the way eyes work, though, I'm not sure you'll ever find a pulse brief enough to be imperceptible. Our eyes have kind of a built-in afterglow effect -- if there are enough photons hitting the retina cells, no matter how brief the pulse, the cells will be triggered, and you will perceive a flash of light. The perceived duration of the flash will probably be much, much longer than the actual flash, as your cells aren't able to turn off as quickly as the light source might be able to.

Does the intensity of the light matter?

When you're talking about just a handful of individual photons, 'intensity' becomes less meaningful. The only way to make those few photons more intense would be to increase their frequency to something more energetic ... but once you get into ultraviolet frequencies and beyond, they will be outside the range of human perception anyway.

Depends on how far away it is. 😌

Lol yeah it's like ten light-years away.

Whether a flash of light is visible comes down to a fundamental concept in visual neuroscience known as temporal summation or generally Bloch's Law.

Our eyes don't process light instantaneously, they integrate light over a short window of time (around 10 to 100 milliseconds, depending on whether you are looking through your central or peripheral vision).

Bloch's Law states that for very short durations, the perceived brightness of a flash depends on the total number of photons hitting the retina during that time window, not the duration itself.

So you can flash a standard light source like a normal LED very quickly, like at 1 microsecond or faster, it will be completely invisible. The flash was so short that the total number of photons entering your eye fell way below the minimum threshold required to trigger your photoreceptors and send a signal to your brain.

inversely you can increase the intensity of the light enough that it will always be seen regardless of the duration of the pulse, a high-powered laser pulsed for a femtosecond will remain visible.

You can think of lightning: It is only visible for some 100s of microseconds (<0.5ms) but it is very well visible. Granted, a lot of photons are emitted in this very short though.

- How much light is a photon, anyway? Does is take thousands or trillions of photons interacting with the eye to register as light?

Others mentioned there is a atudy showing it could be a single photon in an extreme controlled environment.

But in normal conditions, eyes chemically adjust to a reasonable sensitivity, our dynamic range is not limitless.
Eyes actually have the very quick contraction and expansion of the iris, and then that chemical tuning on top, which is why it can take some minutes to see well in strong darkness.

So taking something more realistic you would see in different cases: In a darkened hall you can make out a faint led at 40m distance, faint being about a μW, which is unusually dim for most leds you encounter. A μW might yield you 100μl (micro lumens).

xkcd puts a firefly as 600μl lumen, which is about a millionth of a dim light-bulb. That comes to the same brightness at 100m distance, which is reasonable for seeing fireflys well.

Both of these shine that light output on a sphere, which your eyes capture a small circle of, whose size is your maximally expanded pupil. Looking at wikipedia "the size of the pupil [varies] between 1.5 mm and 8 mm", which having to look at wikipedias source (thanks for nothing) tells me it is the diameter, so radius 4mm.
That equates to 2.5ppb of the total area for the 40m distant led, so 2.5 billionths. So of those 100μl, the eye receives 250fl (femto lumens).

Assuming the color averages to green, specifically 555nm green, that has 1/683 W/l. A photon has energy in Ws of c•h/555nm, which works out to 2.8 quintillion (555nm) photons per second per lumen.

250fl is 250000 quintillionths of a lumen, so we are talking 7000 photons a second.
Now in the dark you might notice your vision is quite blurry, and takes a while to pin stuff down, analogous to a longer exposure time. So to detect say blinking, you might need to wait 100 or 200ms. So maybe we can say you can comfortably see units 700 to 1400 photons in a sufficiently dark environment.