PWM makes bright LEDs look dim by only turning them on some of the time and letting the human optical system average out the on and off periods to some perceived level of brightness.
I am wondering if the LEDs are really bright but only on for low duty cycle do they still have the ability to damage the eye? During their on period they will be as bright as they would be if on continuously, is this enough to cause damage? Or does the averaging mitigate any possibility of damage?
It seems you are not the first one who is concerned about this.
I did cover or remove some of the (bright blue) LEDs of some of my testing boards because I started to see some very persistent after images.
Was wondering that a few years back. 
I have neopixel projects that are very bright; they sit 20 feet across the room, what do they say about intensity if you double the distance ?
Let’s say you look at the sun directly for 1 hour 
Now let’s say you looked at the sun for a tenth of a second.
I think common sense would suggest short durations is okay.
Hum . . . say you are 70 years old, you look at the sun directly for 1/10 second throughout that 70 years, cataracts ? or let’s say you were a welder now with retina burns. 
Yeah... Interesting question!
If this was potentially dangerous you might be able to damage your eyes with a short-flash that you can't even see!
I know you can damage your eyes with a laser that's not in the visible spectrum.
I always have interesting "technical discussions" with my optometrist...
I saw her about a month ago and I mentioned how strange it is that by eyesight "blacks out" during the peripheral vision test... You look into this machine and one eye is tested at a time while the other sees blackness. It drives me nuts because it makes it hard to "pass" the test! I couldn't understand what was happening and it's my least-favorite thing about the visits.
She explained that my brain blending what both eyes are seeing and trying to make sense of it. I said, "Weird... That doesn't happen when I close one eye", and she said it's because my brain knows one eye is closed, or if you have an eye-patch on or something "normal" like that. So, next year I'm going to close one eye and maybe the test won't be so frustrating. I should have asked her if I could try it again!
I ask myself that when I'm driving and pull up behind some new cars at a stop light. Some of the diffusers don't work very well, and those lights are PWM'ed. Blue spots in my eyes for half a minute after.
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