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Comment by prestonbriggs | Hacker News Reader

I believe that I can't see that wavelength, but the energy has to go somewhere (similarly, I can't see microwave radiation, but don't want to put my head in a running microwave, a la Infinite Jest). I think the better question is: Will it blind you/me/animals?

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Indeed, I find it mildly painful to look at IR emitters, for example, on a "night vision" security camera. I don't believe it is dangerous or will blind anyone, or anything, but it is plausible to me that there could be an unanticipated effect.

EDIT: It appears that this is given a passing mention in the article:

. . . it can receive at a wavelength of 1550 nanometers, deep in the infrared part of the spectrum. That makes this wavelength much safer for human eyes than today’s standard wavelength, 905 nm.

The laser in the article is on the order of tens of milliwatts. Even directly into your eye would not damage it. It would be painful, especially if it was in a visible wavelength, but the laser spot from a lidar is in one location for less than a microsecond.

In terms of direct heating, tens of milliwatts isn't even enough to really make a human-detectable rise.

> is in one location for less than a microsecond

When it is working properly.

And self-diagnostics will know when it isn't.

Same with elevators, microwaves, airbags, and in fact literally anything that plugs into the wall. 120 volts can kill you if you contact it.

The laser is eye-safe, meaning you can look directly at it continuously and be fine. Though, if you did, you would deserve injury. Kind of like sticking your hand in a blender.

The device will not manage to malfunction and miraculously shoot you in the eye from meters away.

Would it even be painful at that wavelength?

No, although lasers can sometimes be dangerous without being painful. Even if they aren't, by the time it hurts it is too late.

>similarly, I can't see microwave radiation, but don't want to put my head in a running microwave, a la Infinite Jest

Yet wireless access points spew microwave radiation at you all day.

Not at 1,000W.

However, you do receive a couple hundred watts of near infrared radiation (ie the 1550 nm in the laser) just by being in direct sunlight:

https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe...

But even though I can't see it, I can definitely feel it.

The premise of the parent comment is that even invisible lasers could be harmful. You pointed out that microwaves may not be as harmful at lower levels of power. I pointed out that the output of a lidar beam is far below the level that you get from sunlight, which doesn't even cause cancer and certainly doesn't blind people.

Just because I like running numbers, here's the comparison: the laser in this lidar has a power of ~40 mW, ~1000x less than sunlight. Lidar pulses are on the order of 10 nanoseconds, 100,000,000x less than a second. The energy on your eyes is less than 100 billion times less than a second in bright sunlight. There is certainly no mechanism for thermal damage to occur.

40mW would be dangerous for a constant laser (class 3b), but I'm not sure about a short pulsed one. I think you should look at the amount of energy per pulse.

It would be class 3r, for which direct retina exposure is "low risk". In general for pulsed lasers you can use the duty cycle. I did more analysis in another comment:

Using a maximum permissible exposure chart gives a better sense of danger. Lidar pulses are on the order of 10 nanoseconds, or 1/100th the smallest division on this chart[2]. Even so, you can see that the maximum safe power for 1550 nm is somewhere between 8 and 10,000 watts, at least 800x more than the lidar emits.

1550 nm doesn't chemically damage biological receptors. The damage would have to be thermal, but the laser is pulsed for an incredibly short time and even if it wasn't it's well under what you can be exposed to indefinitely.

[2] https://en.wikipedia.org/wiki/File:IEC60825_MPE_W_s.png

> the output of a lidar beam is far below the level that you get from sunlight, which doesn't even cause cancer and certainly doesn't blind people

Sunlight certainly does cause cancer, although we generally attribute that to the UV in sunlight. Sunlight is also well-known for blinding people.

The near-1550 nm wavelengths do not cause cancer or blind people.

Just to add to your comment, even so that the energy in each pulse is less than 100 billion times less than the energy from bright sunlight emitted in a second, it is still significant, but the number of laser pulses per second is key to knowing if the (peak) intensity of the laser is dangerous to humans or not, as they accumulate over said second. If you only had one pulse per second at 40 mW you would be looking at 30 mJ of energy per pulse, capable of making a nice plasma in air if focused, or machining metal. However they are probably working at 100s of kHz in order to be able to measure an image fast enough, so the energy per pulse is much lower. Key here is the fact that our eye's lens doesnt transmit 1550nm so well, so the interaction with the eye doesnt involve focused light on the retina, hence the interaction intensity is much lower as the light is spread over most of the surface of the eye. Hence they can use much more power and still be eye safe/class 1 or 2, because whatever light gets to the eyes never gets focused down to a tiny spot in the retina, unlike visible lasers.

Also, sunlight is specified in intensity (power/area), and here we only have laser power, not laser spot size, so it is hard to make a fair comparison. nevertheless, sunlight is roughly ~1kW/m^2, so if the laser had a spot size of 2 mm (required to be this big for diffraction over hundreds of meters of propagation to not reduce its intensity too much) we can calculate the power of sunlight in that area: 3 mW. So the laser would be actually ~10x more intense. (if the laser would be 7.2 mm wide then the intensity would be the same as sunlight) But one is comparing totally different wavelengths, so the laser safety rules are different. Two intros to laser safety for whomever is interested: https://www.rp-photonics.com/laser_safety.html https://spie.org/Documents/Publications/00%20STEP%20Module%2...

About thermal damage - funnily enough there is lots of time in nanoseconds to transmit heat and cause thermal damage. Heat transfer is actually reasonably fast at the submicroscale. As a curiosity see https://www.semrock.com/Data/Sites/1/semrockimages/technote_... for the difference between punching a hole with a femtosecond laser (no time for thermal diffusion to happen) and a nanosecond pulsed laser. The area around the laser just completely melts in the nanosecond case. Nevertheless this was done at much higher energies per pulse than the lidar lasers (thankfully!)

>If you only had one pulse per second at 40 mW you would be looking at 30 mJ of energy per pulse

40 mW peak power, not average power. Average power somewhat less, maybe 20 mW.

>Also, sunlight is specified in intensity (power/area), and here we only have laser power, not laser spot size, so it is hard to make a fair comparison.

With sunlight and with a laser scanner, the entire body will be illuminated with that power, very roughly. If you stand directly in front of the laser scanner that 40 mW will fall over your entire body, and a similar fraction of the energy will go into your eyes as with sunlight.

It depends on the power density. I have an IR laser that is invisible and it would burn your eyes out of your head.

~40 mW peak, ~10 nS pulses

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