3 ms·
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
by Scarblac 9y ago
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.
- ethbro 9y agoAlso, 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_by_water#/media/File%3AAbsorption_spectrum_of_liquid_water.png https://en.m.wikipedia.org/wiki/Electromagnetic_absorption_b...
- JshWright 9y agoWhy would you evolve eyes that could see wavelengths that don't penetrate your environment?
- linkregister 9y agoYeah, why would you?
- gknoy 9y agoI'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. :)
- tempestn 9y agoThe reason you want to see wavelengths that don't penetrate your environment is so that you can see your environment.
- edaemon 9y agoNone of that light would ever reach your eyes for you to see it, though, right? Or am I missing something really obvious?
- tempestn 9y agoYeah, 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.
- ethbro 9y agoMaybe 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.
- btilly 9y agoIn that case, how well will this new lidar work in the rain?
- Retric 9y agoRain 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.
- btilly 9y agoThat's a good calculation. http://hypertextbook.com/facts/2007/EvanKaplan.shtml 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.
- qwerrtytest 9y agoI currently work on a self-driving lorry, we get around this issue by installing over a dozen lidars.