
How Multi-Beam Flash Lidar Works - derek_frome
https://www.ouster.io/blog-posts/2018/11/8/how-multi-beam-flash-lidar-works
======
jpm_sd
This is a pretty incredible achievement. Be sure to follow the #3 "High
quality ambient imagery" link too, if you haven't seen it before:

[https://www.ouster.io/blog-posts/2018/8/31/the-camera-is-
in-...](https://www.ouster.io/blog-posts/2018/8/31/the-camera-is-in-the-lidar)

~~~
dllu
Relevant HN discussion about that post:
[https://news.ycombinator.com/item?id=17896942](https://news.ycombinator.com/item?id=17896942)

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dev_dull
> _And since we’re asked about it occasionally, the lasers we use are well
> under the eye safety limit and have been certified as Class 1 eye-safe
> lasers by third party labs._

Oh geeze. I’ve never even considered the fact that there’s a possibility
looking at these cars might cause vision issues!

~~~
edoo
It might be fun to come up with the probability of a worse case scenario, like
parked on the crest of a hill in bumper to bumper traffic.

Although probably not practical to worry about in reality, there is some
probability that all 200 cars you could see shine their lasers directly into
your eyes at the exact same time. There is some probability of accidentally
receiving a momentary higher than safe combined pulse from multiple vehicles.

~~~
DenisM
There are wavelength that are invisible to human eye, those are pretty safe
unless the energy levels are high enough to burn a hole in your retina.

Then there are wavelength that aren’t focuse by the eyes, the only way to get
hurt with these if the energy reaches weapon-grade levels.

~~~
salty_biscuits
Actually, some non visible wavelengths are dangerous because they don't make
you blink.

~~~
godelski
A lot of infrared lasers are dangerous because of this. Blink response is part
of the reason visible lasers are classified differently than invisible lasers.

------
randyrand
A big accuracy problem with flash approaches is multi path interference.

[https://www.microsoft.com/en-us/research/wp-
content/uploads/...](https://www.microsoft.com/en-us/research/wp-
content/uploads/2016/02/chr_mpi_cvpr_15.pdf)

I wonder if Ouster is susceptible to this. There's no mention of it, but I
imagine it will be a bigger problem as they add more lines of resolution.

~~~
rsp1984
This.

Through my work (shameless plug: www.dotproduct3d.com) I've assessed a fair
number of "Time of Flight" 3D sensors (which also use flashes of IR light --
but aren't really time-of-flight but that's another story) and, while
generally pretty cool, all of them so far have suffered more or less severe
multi-path effects. Which is why stereo / structured light or single-point
LiDAR are still better for a lot of use cases. I can't wait for these MPI
problems to be finally solved.

~~~
randyrand
It should be a smaller problem than with the Kinect. The Kinect is not true
time of flight like you said, whereas ouster is. If you know when photons
actually arrived you know the first light pulse you receive is the one you
care about, and the later longer paths you can ignore. The Kinect conflates
these.

So, I don't think it will be a problem for them but it is something they need
to consider when designing their DSP.

~~~
nomel
This makes me wonder how any of these systems handle the reflective surfaces
of cars.

So you hit a chrome bumper or maybe a glossy black point. The later pulses
could arrive at a much higher amplitude.

~~~
CamperBob2
They don't care about the amplitude, just the timing.

~~~
nomel
They're measuring the time to some amplitude at the sensor. There must be some
amplitude threshold, otherwise would trigger on noise.

~~~
CamperBob2
The reality is somewhat more complicated than that. Signal processing at this
level is statistical in nature, so the threshold itself is effectively timing-
driven.

Hard to explain in a few words, but the principle of the lock-in amplifier is
a good place to start.

~~~
nomel
I still don't understand how there is no threshold based on amplitude
(correlation or not). Something like mist would show up as a noise floor, a
speck of dust (that hangs around for long enough) may be one peak, a mirror
another, and an object seen in the mirror will be another. You will have
multiple peaks that may stay around longer than others and, but there will
still be peaks, one of them being the "true" surface.

I don't see how there is no threshold.

------
amelius
What happens when multiple lidar devices start shooting laser beams at a
scene? Will there be interference?

~~~
dllu
Compared to other sensors, lidars are not that prone to interference because:

* it only takes 1 microsecond to make a ranging measurement up to 150 m, so your detector is on for a short time

* lasers only illuminate a small spot, and the detector is also looking at a similarly small spot, so it is unlikely for two lidars to point in the same spot

Now, even if it does interfere, you may see a stream of random points pointed
towards the interference source. This may happen if, say, you point a lidar
directly at the sun, or if you have multiple lidars mounted on the same
vehicle. Such random points are easily rejected as outliers and do not affect
the vast majority of the scene. Most self driving cars (I hope) should have
outlier rejection schemes that deal with outliers caused by this and other
sources, such as snow, smoke, and so on.

For this reason we see many self driving cars bristling with a bunch of lidars
next to each other with no problems. For example the Cruise/GM ones have five
Velodynes on top.

~~~
Animats
There's a case for adding some jitter, maybe 10us, to the laser timing. That
prevents any attacker from synchronizing a jammer. Common technique in
military radars. If you can't synch, you can't present an illusion of
something being closer than it is, and any return from a real obstacle will
come in before a jamming signal from a further away transponder or mirror.

Active systems like radars have a "burn-through" short range - at some short
distance, the sensing system overpowers a jammer. So if you're seeing junk at
distance, but good signal at short range, you know you're being jammed and
have to slow down.

Active jamming is not that effective against things that receive
directionally, as people using car "radar jammers" near military bases
sometimes discover. They show up on military radars as hostile targets. That's
even filtering down to police LIDAR guns.

------
mmmBacon
I don’t think 850nm is a good choice for LIDAR for several reasons. Firstly,
the article talks about absorption but that’s not the only phenomenon that
they need to be concerned about. The other is scattering and 850nm will
scatter from particulate matter in the air (like water vapor in fog) and
suffer reduced range.

The 2nd reason why 850nm sources are not a good choice are VCSELs themselves.
VCSELs are low power devices and always will be due to the narrow current
aperture in their construction, you simply can’t send much current through it
because even at low currents the current density is already very high. The
850nm wavelength is also limited in how much output power you can get and
still have an eye safe device. Additionally, VCSELs do not work well at high
temperatures. Their LIV curve bends over meaning that the combination of
reduced slope efficiency and self heating of the junction cause the output
power to decrease as you increase current beyond a certain point.

Lastly, VCSELs are relatively noisy devices. VCSELs are not single spatial
mode devices and there is mode partition noise caused by the sloshing around
of power between the various modes. When combined with RIN, this places an
Upper bound on the SNR of the system. Finally, if you are already doing photon
counting, you’re pretty much at the limit of this tech already.

To my mind, longer wavelength tech is more promising because it can use higher
powers and still be eye safe, has better immunity to scattering in various
atmospheric conditions which gives it longer range. Longer wavelengths can
leverage technologies like silicon photonics to produce more advanced
detectors (coherent detectors which will be more sensitive and improve range).
Generally the lasers at these wavelengths are less noisy as they’ve been
optimized for decades for use in long distance communication.

~~~
rsp1984
As far as I know detection SNR goes way down with longer wavelengths. I think
luminartech.com claims that they have specific IP to make this work, but
apparently it's a hard problem, otherwise everybody would be doing it already.

------
karmicthreat
Really wish 3d lidar costs would come down. Everyone is designing for car
performance and price points it seems. I have a ton of applications I could
throw one of these on at 1k$.

~~~
bmc7505
If you don't need 3D, 2D LIDAR is pretty cheap right now:
[https://www.adafruit.com/product/4010](https://www.adafruit.com/product/4010)

~~~
dllu
That one only has 12 m range. The 2D lidars from Hokuyo, SICK, etc with
similar range to the Ouster OS-1 (which has 140 m range) cost thousands of
dollars.

------
Animats
Nice. Not really here yet, but the semiconductor physics sounds promising. The
device they're actually selling is yet another spinning scanner, not a true
flash unit.

~~~
Judgmentality
What are your thoughts on Geiger LIDAR, ie Princeton Lightwave (which was
acquired by Ford/Argo)?

~~~
DoctorOetker
SPADs (single photon avalanche photodiodes) _are_ APD's operated in Geiger
mode...

------
bobbygoodlatte
I'm curious how these systems would function against a malicious entity. For
example, if someone flooded the sensor with laser light.

That sort of behavior might be made illegal (if it isn't already), but I'd be
curious to know if self-driving systems have ways of dealing with it, or if
they just shut down

~~~
dllu
It's pretty illegal to point lasers at vehicles. Many states and countries
have laws against that. For example, in California,

California Penal Code Section 417.27

(c) No person shall direct the beam from a laser pointer directly or
indirectly into the eye or eyes of another person or into a moving vehicle
with the intent to harass or annoy the other person or the occupants of the
moving vehicle.

~~~
dsl
It is illegal for a _laser pointer_ , where most people defer to the FDA
definition of "... handheld lasers that are promoted for pointing out objects
or locations."

There is already established case law around this, where larger hobby lasers
intended for popping balloons and theatrical lasers are exempt.

You could build a large array of laser emitters intended to obstruct self
driving vehicles (say you wanted to create a fake wall to prevent a car from
driving the wrong way), and safely ignore these laws.

~~~
dragonwriter
> It is illegal for a laser pointer, where most people defer to the FDA
> definition of "... handheld lasers that are promoted for pointing out
> objects or locations."

No the law in question defines the term, it doesn't defer to the FDA; Cal.
Penal Code § 417.27(f): « _As used in this section, “laser pointer” has the
same meaning as set forth in subdivision (c) of Section 417.25_ »

Cal. Penal Code § 417.25(c): « _As used in this section, “laser pointer” means
any hand held laser beam device or demonstration laser product that emits a
single point of light amplified by the stimulated emission of radiation that
is visible to the human eye._ »

~~~
dsl
If you read the definition you just quoted at me, it is basically the same.

Any device that emits more than a single point of light, or is not hand held
is not covered.

------
hobolord
super neat stuff, I visited their offices recently and was really impressed by
what they were doing. From what I can tell, everything was done in house

------
exabrial
Is something like phased array lidar useful? The wavelength is so small I
could see how the interference patterns might not be useful

------
21
How dangerous are these lasers?

If hundreds of cars will have them, your eyes will be scanned multiple times
per day.

~~~
snops
Laser safety is a well known issue, IEC60825 offers mathematical models to
calculate the safety category (e.g class 1/2) of a laser device, which are
based on physics models of the eye from first principles and empirical data.
There is nothing particularly special about LIDAR lasers that makes these
models not valid as far as I know. Generally, there is a tradeoff between
exposure time and intensity, so LIDAR systems have redundant hardware methods
to ensure the pulse time is not exceeded.

Any laser product generally has to be tested to IEC60825 to check which
category it fits within, under the safety rules of the jurisdiction (e.g. CE
for EU, FDA for USA). This LIDAR system would have to pass that too, and it
should be reasonably easy for them to check if it would when developing it.

~~~
mtreiber
The laser isn't tested to IEC60825. Instead the manufacturer (Oester in this
case) would submitted information about the laser scanning system design to
the FDA to get approved as a Class 1 device.

