I'm very curious what the effect would be over the course of decades of having even very weak lasers hitting your retina directly.
(However... since I often see this sorta misinterpreted in the wild on the internet, note that is an if-then statement. If the antecedent is false, I make no claim.)
The primary safety concern I have would be met by designing the lasers such that if they are overdriven for any reason, they will physically burn out before outputting enough light to be dangerous. Per the classic Therac-25 [1] case study though, that is one safety feature I absolutely want in hardware. There is no amount of software I would accept to implement that.
I would also additionally stick some fuses into the system, tuned below the threshold where the power will burn out the laser, along with of course building the whole battery system to not be able to deliver enough power to power the lasers to a dangerous level. However, I really want excess power to physically burn out the lasers. (I wouldn't want to find out the hard way that an EMP of some sort can overdrive the lasers.)
For all that I'm laying out safety systems here, I am quite confident that it could be done safely. We trust our lives to much more dangerous systems all the time. I will say that I can't explain to you how you'd audit that safety, though.
[1]: https://www.bowdoin.edu/~allen/courses/cs260/readings/therac...
That part is not likely. If you concentrate the light over just a few retina cells, dangerous levels are very low.
True, but we usually get a pretty wide spread of light energies. These are likely going to be very specific frequencies, hitting similar areas over and over. I wonder if the retina can get fatigued of specific frequencies.
You can use the effect to generate colors that are actually impossible in the real world (and are only perceived):
Since we only see in three color dimensions, it's hard for us to notice day-to-day, but for instance, some fluorescent bulbs are just 5 spikes in particular frequencies. It looks fairly "white" to us, but it's far from normal light.
(I am interpreting your comment as being fatigued of/damaged by very specific frequencies in a way that it would not be fatigued/damaged for the same amount of energy spread out over a wider range still within the given cone's sensitivity range.)
Yep, that's my concern. I don't know if anyone's done long-term studies about low-level light of identical wavelength.
I mean, on the one hand, people used to be afraid of fast-moving vehicles, convinced that it was impossible for a human body to survive going faster than 40 miles per hour.
But on the other hand, people used to strap radium to their faces because they thought it was a cure-all, too.
I thought they kinda did. By being very similar wavelength and power, compared to normal light which has all sorts of wave lengths and power. This difference can mean certain rods/cones being stressed more than average and not triggering the sort of fatigue that normal light would cause.
Perhaps the best example of a similar concept, though not with lasers, is looking at a total solar eclipse right before or after the sun is fully eclipsed. There is a small period of time where extremely bright light makes it into our eyes, but not he frequencies that cause pain normally associated with looking at the sun. This means that our default defenses against looking at the sun don't kick in and doing permanent eye damage is extremely easy without feeling any pain as the damage is done.
The consequence of being a point light source is that lenses can refocus the beam, parallel or not, back into a point. Doing so concentrates a lot of energy on a tiny surface. And if that surface is your retina, then that when it becomes dangerous, literally burning a tiny hole into it.
It can never happen with, say, a regular light bulb, or even the sun. It the light source is spread out, the image on your retina, or anywhere else, will be spread out, limiting the energy density. This is an indirect consequence of the second law of thermodynamics called the conservation of etendue.