The exposure limits for lasers in the 1500nm range are almost certainly set by the intensity at which they cause photochemical cataracts. The geometry just doesn't work out for the exposure limits to be set by anything else; a non-visible laser 1500x as powerful as a legal-limit visible-light laser would still only heat the cornea up a hundredth as much as the legal-limit visible-light laser would the retina (intensity at cornea 1/200000th that of intensity at retina, see my comment below [4]). Physical damage to the retina starts around 10 degrees C, so this means that an exposure-limit non-visible laser might change the temperature of your cornea by a tenth of a degree. The normal range for corneal temperature is thirty celsius wide [1, 2] because it's strongly affected by air temperature and air movement. Corneal temperature is controlled to stay below 33-35 Celsius in extremely hot environments (45 C). Studies on microwave burns support this, saying that you have to get rabbit corneas up to 41 degrees C for cataracts to even start to form, implying a reasonable safety factor [3]. Under normal conditions (still air at room temperature), corneal temperature hovers closer to 30 degrees C. A tenth or a hundredth of a degree just isn't going to do anything when the system normally has ten degrees of safety. The vitreous humor varies less but still varies pretty significantly. Conclusion, the exposure limits for 1500nm lasers are not set by thermal damage.

It's generally safe to assume that you're not going to think about this for ten seconds and discover a danger that has been missed by every single person to ever contribute to the exposure limits by thought or by case study.

1: https://iovs.arvojournals.org/article.aspx?articleid=2127035 2: http://iovs.arvojournals.org/data/journals/iovs/933602/596.p... 3: https://en.wikipedia.org/wiki/Microwave_burn#Eyes 4: https://news.ycombinator.com/item?id=18887393

Exactly, it's the same with unshielded microwaves, and it's astonishing that someone would think it's safe to crank the power on these lasers with that justification.

> When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce cataracts by this mechanism,[28] because the microwave heating denatures proteins in the crystalline lens of the eye (in the same way that heat turns egg whites white and opaque). The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat.

https://en.wikipedia.org/wiki/Microwave#Effects_on_health

But wouldn't that be true of all electromagnetic radiation that's absorbed by the fluid? I can't imagine this being more harmful than looking at a sunset from the pov of total energy being pumped into the eyeball. Or walking outside at mid-day among shiny buildings.

It is quite easy to believe that as-lased from lasers in the lidar units, the beams are generally safe. But could other optical mechanisms such as eye-glasses accidentally focus these dispersed beams in such a way that they could again be dangerous?