[fsilva]

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!)

[hwillis]

>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.