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Comment by BuildTheRobots | Hacker News Reader

If it can do that to a camera, what's it doing to eyes?

edit: > "Cameras are up to 1000x more sensitive to lasers than eyeballs," Dussan wrote. "Occasionally, this can cause thermal damage to a camera's focal plane array." - I'm still not entirely sold.

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The article actually has the answer:

> Other lidar makers use lasers with a wavelength of 1550nm. This tends to be more expensive because sensors have to be made out of exotic materials like indium-gallium arsenide rather than silicon. But it also has a big advantage: the fluid in the human eye is opaque to 1550nm light, so the light can't reach the retina at the back of the eye. This means lasers can operate at much higher power levels without posing an eye safety risk.

> the fluid in the human eye is opaque to 1550nm light

sure, but does this hold true for most land-based animals and birds? How much range does this give us? reflections can shift by half a wavelength - is that still opaque too?

I'm not saying this isn't safe, but suddenly having a tonne of high power laser sources pointing everywhere at all times might actually have some consequences still...

Mammals, lizards, amphibians, and birds all have water-based eyes, but I wonder what this would do to insects.

If I am reading the chart labelled "The visible and UV spectra of liquid water" at http://www1.lsbu.ac.uk/water/water_vibrational_spectrum.html (about 2/3rds down the page) correctly, water is fairly opaque at 1550nm. If my hurried wikipedia education on the topic is correct, that chart is saying it's quite opaque at that frequency, even at bat eyeball scales. Corrections welcomed from people with more training in this field.

(I'm not sure I'm not getting some crossed units when I tried to resolve that into numbers normals like me would understand; is that really 1/e^1000 transmitted per centimeter travelled? e^1000 is a big number. There's even bigger ones on that chart. Then again, if a two-atom-thick layer of gold is enough to make something look like gold and completely obscure what's underneath, I guess that might make sense and my intuition is just off, because when I convert that into this sort of scheme I get big numbers there, too.)

If memory serves me well, water absorbs that wavelength, so it would basically function sort of like a microwave oven, but with such a much lower energy level that you can basically ignore it.

How will LIDAR work under the rain or snow then?

The same way regular vision works under snow. Snowflakes are opaque to visible light too, so it'd work the same way human eyesight works for drivers currently.

so maybe we don't need Lidar, because the cameras can see like the human eye in the snow, too. Depth can be given by doing like insects and adding many triangulation points.

Ok, this suggests a solution: use a similar fluid in front of cameras.

the fluid in the human eye is opaque to 1550nm light, so the light can't reach the retina at the back of the eye.

So...it'll just fry the front of the eye?

Water absorbs that wavelength if memory serves me well, so it's more like boil than fry -- kind of like what happens in a microwave oven. But the energy levels involved are so small it shouldn't be a concern.

The eye focuses light. Formally, this transforms incoming photons so that their position on the retina is a smooth function of their direction. Diffraction and atmosphere mean a laser will be distributed somewhat in position, so the incoming photons may be spread over a 2mm spot on the front of the eye. However, all of those photons have almost identical direction and end up being concentrated in a single spot on the retina, up to the limits of the eye's optics. 20/20 human vision can resolve down to about .02 of a degree so a point source should be very similar to a .02 degree circle, the visual nerve blind spot is 7.5 degrees wide and is caused by a 1.5mm obstruction on the retina, so a point source like a laser should land in a spot on the retina about .005mm across. 2mm diameter / .005mm diameter is 400 and area is proportional to the square, so this would be something like 160,000x decrease in area with equivalent increase in intensity. Result: Not even enough to warm your skin up but instantly boils a bit of your retina.

(I'm rather happy with how well my estimate corresponds to the figure from Wikipedia: "The eye focuses visible and near-infrared light onto the retina. A laser beam can be focused to an intensity on the retina which may be up to 200,000 times higher than at the point where the laser beam enters the eye.")

I asked a neuroscientist specializing in eyes I know on Twitter. His opinion is that it's not quite so simple as "it's safe".

https://twitter.com/BWJones/status/1083847128663769088

> That energy longer than 1400nm is generally absorbed by the cornea and lens, but it is still energy, and it is not a hard bandpass filter per se. Safety is relative at higher wattages.

Wonderful, the eye boiling future I have to look forward to..

The fluid is opaque so all the energy is dumped in the fluid, heating it up?

Also why should I listen to some rando salesman named Dussan? Is he an ophthalmologist?

From the article "Cameras are up to 1000x more sensitive to lasers than eyeballs," Dussan wrote. "Occasionally, this can cause thermal damage to a camera's focal plane array."

Another site claims "However, there are no MPEs for sensors such as CMOS or CCD chips. This means a show may be perfectly safe for eyes, but could possibly damage a camera sensor. One reason is that camera lenses may gather more laser light, and concentrate it to a finer point. Another reason is that CMOS or CCD sensors are more easily damaged than the eye."[1]

[1] http://www.laserpointersafety.com/ilda-camera-info.html

I'd love to see some independent results from someone who doesn't have skin in the game. Whilst damage might be more obvious in a camera CCD, even small amounts of cumulative damage over time could be doing serious harm without it being immediately obvious.

I've seen f/1.6 lenses, where as the eyes are apparently f/8.3 - f/2.2, so yes the eyes take in less light, but it's substantially harder than an EF-S mount to swapout.

edit: also, what about animals with good night vision? they are more sensitive and take in more light - are they at danger?

They address in the article that the frequency they're using can't damage the retina because the fluid in the eye is opaque to light at this frequency.

A camera is not the human eye. Just because they are both technically optical sensors does not mean they are equivalent really at all.

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