Vampire bats use modified capsaicin (chili pepper) receptors in their nose to detect infrared light:

https://www.nature.com/nature/journal/v476/n7358/full/nature...

They use this to find their warm-bodied prey, but of course it's only detection, not vision.

I learned this today, and now I'm happy I did.

I was about to ask if you work where I do, then I saw your username, then I remembered that you hired me through HN..

You guys seem to have amazingly interesting jobs. Please hire me, I'm your future-colleague.

Pit vipers too. The pits are used to 'see' infrared.

Thank you. Definitely the coolest thing I will learn today.

sci-hub link https://sci-hub.ac/https://www.nature.com/nature/journal/v47...

One thing is that water has absorption bands at 970nm, 1200nm, 1450nm, and 1950nm, and eyes have a lot of water in them. So they simply aren't very transparent in the near infrared.

Also, why would you evolve eyes tuned to wavelengths that don't penetrate water well? That seems maladaptive in an aquatic environment.

https://en.m.wikipedia.org/wiki/Electromagnetic_absorption_b...

Why would you evolve eyes that could see wavelengths that don't penetrate your environment?

Yeah, why would you?

I'm not sure if this exchange means that there is no good reason to do that, or if it means that the reason why one would want to do that is blindingly obvious to everyone except me. :)

The reason you want to see wavelengths that don't penetrate your environment is so that you can see your environment.

None of that light would ever reach your eyes for you to see it, though, right? Or am I missing something really obvious?

Maybe I should have elaborated a bit more. My original point was that it makes sense that the visible spectrum was colocated with the frequencies not readily absorbed by water.

Which is to say, eyes were made for seeing the light that was bouncing around in the sea (and not the frequencies that had already been absorbed).

So yes, the simplest answer you can imagine.

Also to the same point: eyes evolved to see the wavelengths where the Sun outputs the majority of its energy, which is a significant factor in its ability to penetrate water to any depth.

Right, that makes perfect sense. I'm mostly unsure of what tempestn said.

Yeah, if the water is the surroundings.

I was answering the broader question of not wanting light to penetrate "your surroundings" - ie the desk in front of me or walls around me. Would make life difficult if everything was transparent. Normally when one talks about "surroundings" it means that stuff, not the air (analogous to the water for marine life) both of which obviously need to be at least somewhat transparent for vision to be useful.

In that case, how well will this new lidar work in the rain?

Rain is actually very little water at any one path. A drop is falling at 10 meters per second and heavy rain is ~1 cm per hour. So to ballpark it's around 1 part in 3.5 million water on average and assuming 1mm drop size you will see ~3.6km on average before you encounter a drop.

Note this is instantaneous, human vision deals with after images which adds blur.

That's a good calculation. http://hypertextbook.com/facts/2007/EvanKaplan.shtml backs up your facts and claims that a raindrop is actually more like 6 mm, which makes the straight line view much longer.

However in the rain, on a highway, we have water falling, water splashed up from the ground, a mist formed by cars, high humidity, and a wet layer on the outside of the car. These factors reasonably could impact this device if the laser is emitting at water's absorption frequency.

I currently work on a self-driving lorry, we get around this issue by installing over a dozen lidars.

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?

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.

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.

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

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.

> 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.

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

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

It's worth pointing out that there are aircraft mounted LiDAR systems that already operate at 1550nm, such as [0]. Another company has an interesting paper about which wavelength is best for their activities[1].

http://www.teledyneoptech.com/index.php/product/titan/

[1] (PDF!) http://www.riegl.com/uploads/tx_pxpriegldownloads/Paper_ILMF...

Pit Vipers can sense IR with a secondary sight organ called a fossa. https://en.wikipedia.org/wiki/Pit_viper

>> He finds that adding the additional NIR band did not improve task performance

But did they try in low light? It seems that seeing in IR should help with detection of predators or prey at night.

IR near the visible range is associated with night vision because we use active night vision goggles that use an infrared spotlight at 850 or 940 nm wavelengths (700 nm is red). This is not particularly useful without an illuminator, you're basically just getting visible light amplification then.

Passive thermal vision that might pick out a warm object in the dark require sensing of ~10,000 nm/10 um infrared, a very different problem! And one that wouldn't​ be easy to solve biologically, as it basically requires being colder than the object you are sensing. Edit: but not impossible! https://en.m.wikipedia.org/wiki/Infrared_sensing_in_snakes

Are you referring to sensing body heat of other animals? That's a long wave IR emission, ~8000 nm and up. Some animals do indeed sense that and it is quite useful at night. This laser, and the paper I linked to, are operating with much shorter IR wavelengths (~800 to 1550 nm). This spectral region overwhelmingly comes from sunlight under natural conditions, and its intensity drops sharply at night like the ordinary visible range.

Nocturnal animals function by having a large proportion of rods. This was true of our anthropoid ancestors, resulting in humans having very limited high acuity color vision. We get round this by rapidly scanning and stitching the world together (the inability to do this is a sore problem for visual prosthesis). That the world seems sharp and rich in color is a testament to the feats of predictive inference our brains regularly perform.