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Comment by RcouF1uZ4gsC | Hacker News Reader

What are the safety implications of this? If something goes wrong, will people be blinded? What effect do these lasers have long term on the retina, the lens, and the vitreous humor of the eye?

I did not see any part of the article address these issues.

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Given that the lasers are light shining in a straight line, and that they're presumably calibrated to intensities the eye can deal with (otherwise we couldn't see them/they'd be less sharp) there won't be any more long term effects than just normal eye use.

Our eyes are designed to let light in. The power level of lasers that can run all day on a 350mAh battery they share with some electronics is going to be miniscule, like maybe a fraction of a milliwatt of power. A common laser diode available right now that outputs 1mW continuously consumes about 36 mA of power to do that. Three lasers outputting 1mW would consume roughly 363 or 108mA, so a 350mA battery would only power those three lasers for about edit 3.4 hours, with nothing left for the electronics to use.

Less than 1mW is not much, even for a laser, and at visible wavelengths it's not going to transfer enough heat to even measurably change the temperature of the human eye, much less damage it.

But don't lasers need to scan in order to draw a full image (vs just a dot)? Doesn't that necessitate making the actual laser beam brighter than other light entering your eye? Are there any negative consequences to having a bright light scan over your retina quickly vs a dimmer light shining on your entire retina at once? What happens if the scanning mechanism fails and the laser ends up shining on one tiny point in the center of your eye for an extended period of time?

Lots of interesting safety questions unique to this particular display technology.

Remember that A) The laser is close to your eye and shining directly in and B) It's coherent, so it's focused on a single small spot. It doesn't have to be so much brighter than all the light around you given those things.

But it isn't focused on a single small spot; it has to scan across multiple small spots in order to form a recognisable 2d image. If it were just focused on a single small spot the entire time, all you'd see is a dot.

That means one of two things must be true. Either a) the beam is way brighter than other light entering your eye, such that it delivers in a single focused beam the same amount of energy that would otherwise be distributed evenly across the surface of your retina if you were viewing natural light. (Thus the questions I posed in my previous comment.) Or b) the beam is the same brightness as other light entering your eye, meaning the amount of energy in any individual pixel is far _less_ than what your eye receives from natural light. (I imagine this would make the image appear very dim.)

Depends how the circuit fails. I suspect the pump will fail to lase if it receives too much current, physically capping the laser power sounds like a good design decision with something going right into your eyeballs (unlike CD lasers).

What if someone hacks it to intentionally blind you?

Just look away. It's just a display device, so at most it's going to display confusing pictures.

It's not possible to "hack" more output power into lasers with software changes. Would that it was. You can change the duration of the beam, but you can't pulse the beam without a Q switch in a way that changes the instantaneous power.

In the normal operations, the laser will scan across the retina without long dwell times at a single spot. Software is likely able to cause the laser to track a single point on the retina (I expect that the device needs some sort of an eye tracker and thus a camera aimed at the eye). I don't know if that can produce a harmful power density.

Given that it doesn't harm the retina when scanning, I'd say "full white" is the best it could do, which could be surprising and a bit uncomfortable, but not actually damage the eye.

Picture your monitor going all white... bright for a sec, but that's about it.

Given that it's scanning the duty cycle as seen by any part of the eye is very small. If the perceived brightness was a function of power density averaged over time, then it would very obviously have to be able to be much brighter than "full white" to create the full white experience[^].

[^] in reality the perceived brightness is somewhat higher than the mean (i.e. a light source that's twice as bright with twice smaller duty cycle appears brighter). I'm not sure how large an effect that is, and whether it has anything to do with pupil size adjustment.

> Picture your monitor going all white... bright for a sec, but that's about it

I remember when a CRT monitor would mess up and stop scanning right the white point in the middle was much brighter than the normal brightness, I was always worried it would burn in and would shut off the monitor right away

There's going to be a hard limit to the amount of power you can direct through this laser. The actual hardware is going to be responsible for safety in this dimension, not software.

Then you get some marketing logo etched into your retina :)

I'm wondering how is the laser scanned over the retina (probably some MEMS chip). What happens if the beam scanning is suddenly stopped in place, wouldn't you get the whole laser power concentrated in a small dot. And it happens so quickly that you won't have time to react.

There may also be some phantom image effects like we used to have on those old TV monitors. Or like we can have when fixating a picture.

What's probably more insidious would be projecting some very lightly superimposed structure. You probably can induce some unconscious cognitive load or nausea. By decreasing the discomfort for example when the user is looking at an ad, you will increase the effectiveness of the ad.

I guess maxing out its power would be comparable to looking at a full-white LCD screen. If the device has an ambient light sensor to adjust its brightness, then circumventing it and setting to full power could be comparable to switching on a bright light in previously completely dark room.

Laser is just a means to make light. You stare at the sun and you will go blind, but the majority of people spend their entire life looking at light for all of their waking hours and die at an old age with eyesight. But to answer your question, yes absolutely. The power will have to be low enough to prevent damage.

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