4 ms·

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.

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

p1esk 9y ago
But even though I can't see it, I can definitely feel it.
- hwillis 9y ago
  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.
  - deleted 9y ago
    [deleted]
  - p1esk 9y ago
    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.
    - hwillis 9y ago
      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 https://en.wikipedia.org/wiki/File:IEC60825_MPE_W_s.png
  - thaumasiotes 9y ago
    > 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.
    - hwillis 9y ago
      The near-1550 nm wavelengths do not cause cancer or blind people.
  - fsilva 9y ago
    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://www.rp-photonics.com/laser_safety.html https://spie.org/Documents/Publications/00%20STEP%20Module%2002.pdf 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_images/fso-wp-figure-10.jpg 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 9y ago

p1esk 9y ago

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

hwillis 9y ago
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.
- deleted 9y ago
  [deleted]
- p1esk 9y ago
  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.
  - hwillis 9y ago
    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 https://en.wikipedia.org/wiki/File:IEC60825_MPE_W_s.png
- thaumasiotes 9y ago
  > 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.
  - hwillis 9y ago
    The near-1550 nm wavelengths do not cause cancer or blind people.
- fsilva 9y ago
  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://www.rp-photonics.com/laser_safety.html https://spie.org/Documents/Publications/00%20STEP%20Module%2002.pdf 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_images/fso-wp-figure-10.jpg 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 9y ago