All about automotive lidar

Recorded: Dec. 3, 2025, 3:04 a.m.

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All about automotive lidar

August 29, 2025
By Daniel Lawrence Lu

1 What lidar does2 Measuring distance2.1 Direct time of flight pulsed lidar2.1.1 Photodiodes used in pulsed lidar2.1.2 SPAD macropixels2.1.3 Multi-shot ranging2.1.4 Silicon photomultipliers2.2 Amplitude modulated lidar2.3 Frequency modulated lidar2.4 Parallax lidar3 Discerning bearing3.1 Arrays for discerning bearing3.1.1 Discrete arrays3.1.2 Solid state arrays3.2 Scanning and beam steering methods3.2.1 Spinning3.2.2 Spinning mirror3.2.3 Oscillating mirrors/galvos3.2.4 MEMS mirror3.2.5 Optical phased arrays3.2.6 Baraja SpectrumScan3.2.7 Risley prisms3.3 Combining two 1D methods
Here I'll provide a comprehensive overview of automotive lidar technology.
Lidar is used for autonomous vehicles and robotics because it's a cool technology.
FIGURE 1 Waymo Jaguar I-Pace with several lidars.
FIGURE 2 A Chrysler Pacifica Hybrid with 8 Ouster lidars.
1 What lidar does
A lidar is a sensor which operates by bouncing light off surrounding surfaces.
Lidars typically quantify:
distance, by measuring how much time it takes for light to bounce backbearing, by shining the light or pointing the detector in a particular directionreflectivity, by measuring how much light has bounced backspeed, by measuring the Doppler shift in the reflected light.ambient, by measuring the amount of light in the environment in a particular direction
FIGURE 3 Ambient, intensity, and range channels of a really old Ouster OS1-64.
In general, we are most interested in distance and bearing.
Surface reflectivity is also valuable, as it allows detection of road lines in the automotive case.
By measuring distance in many directions, an autonomous vehicle can perceive its environment.
Each measurement corresponds to a discrete 3D point in space.
Through a decade of steady research, engineers designed algorithms capable of leveraging this 3D point cloud to unlock spatial understanding. Obstacle avoidance and precise positioning are just two direct results of this technology.
Distance and bearing measurements can be converted into 3D Cartesian points. For example, given range and bearing , , the 3D point is:
1
In
contrast, a camera only measures bearing and ambient light intensity.
Each pixel of a photo is a measurement of how much light there is in
that particular direction.
But generally, a camera has much higher bearing resolution than a lidar.
FIGURE 4 A point cloud accumulated from an Ouster OS1-64 lidar.
2 Measuring distance
Measuring distance is also known as ranging.
Basically, it just measures how close something is.
There are in general two ways of doing this:
Measuring the time somehow, exploiting the fact that light travels at a constant speed (the speed of light)Parallax
For measuring the time, there are again two ways:
Direct time of flight, where we directly measure the timeModulated lidar, where we modulate some attribute of the outgoing light, e.g. amplitude, frequency, or polarization
2.1 Direct time of flight pulsed lidar
Direct detection pulsed lidar fires one or more laser pulses.
Then, we simply measure the time to see the reflection from the pulse.
2
where is the speed of light ( m/s). The division by 2 is because the range is half of the round trip distance.
Imagine if we have a stopwatch that measures in, say, a nanosecond resolution.
If we measure 1000 nanoseconds, then it means the round trip distance was 300 m, which means that the range is 150 m.
This
involves measuring the time series data of how much light is seen at
any point in time.
Since electronics typically run at 1 GHz or so, the time series is
discretized on the order of 1 ns, which corresponds to a range of 15 cm.
To further improve the ranging accuracy, an interpolation filter is a standard technique in signal processing.
Typically ranging accuracy at the centimeter level is possible.
After getting the time series data, the peaks in the series are found, and these correspond to the range.
FIGURE 5 A simplified time series plot of number of photons vs time.
Usually,
it is better to have stronger, shorter pulses.
Diode lasers can produce pulses on the order of a couple of nanoseconds,
and fiber lasers can produce even shorter pulses with much higher peak
energy.
In practice, the laser pulse has some finite duration and
shape (rather than being an infinitely short impulse function), so the
peak is found in the cross correlation
of the outgoing pulse’s shape with the return data, rather than the raw
time series data itself.
It is possible to send a randomly shaped pulse (or sequence of pulses),
and cross correlate the return data against that. This provides much
greater resistance against noise, interference, and crosstalk, and is
known as a matched filter.
We
should note, however, that the shape of the return pulse could be
distorted or “smeared out”. This can be due to, for example, hitting a
very slanted surface.
One strategy to overcome this is to try correlating it with a bunch of
different pulse shapes. This technique may be called template matching,
dictionary matching, matched filter bank, or model-based detection.
2.1.1 Photodiodes used in pulsed lidar
In
order to get a time series of the amount of light per unit time at a
super high rate, we need a really fast sensor that can operate at 1 GHz.
Usually one of these two types of sensors is used:
Linear-mode avalanche photodiodes (APD)Geiger-mode avalanche photodiodes, also known as single-photon avalanche photodiodes (SPAD)
Other types of sensors such as CCD sensors are not fast enough for this application.
A photodiode is a diode that also has the photoelectric effect.
A
diode is like a one-way valve for electricity.
Just like a one way valve for water, if you try to force things
sufficiently in the opposite direction, it will break down, resulting in
a huge gush of water.
Likewise, if you apply a strong voltage in the reverse direction, it’s
called a reverse bias, and a sufficiently strong voltage will cause a
sudden spike in electrical current.
This is called avalanche breakdown.
Meanwhile, some metals produce an electric current when shining light on it, in an effect known as the photoelectric effect.
Avalanche photodiodes have a reverse bias, meaning that a voltage is applied in the opposite direction of the one-way valve.
If the reverse voltage exceeds a certain amount known as the breakdown voltage, it stops acting like a diode.
Suddenly, a large current can flow through the device.
Linear-mode
APDs have a reverse bias slightly below the breakdown voltage.
Here, the current is linearly related to the voltage, but the gain is
very high, so that even changing a small voltage results in a large
change in the current.
Hence, it is a very sensitive way of measuring light intensity.
Geiger-
mode avalanche photodiodes (GMAPDs) or single-photon avalanche diodes
(SPADs) have such a strong reverse bias that even getting hit by a
single photon can make them break down, resulting in a large current
spike.
The output of a SPAD can be directly connected to a voltage
discriminator so that the spike becomes a digital signal from logic 0 to
1.
FIGURE 6 I-V diagram of avalanche photodiodes
In the above I-V diagram, we see the relationship between the voltage (V) and the current (I).
The breakdown voltage is labelled.
As you can see, where the linear-mode APD operates, the current is linearly proportional to the voltage.
The Geiger-mode APD operates where the slope is effectively infinitely steep.
Note on terminology:
Typically the word avalanche photodiode (APD) refers to linear-mode
APDs. Meanwhile, GMAPDs and SPADs operate in the same way but the term
SPAD often refers to silicon devices sensitive to near infrared (850 nm
to 940 nm) and GMAPD often refers to InGaAs devices sensitive to longer
wavelengths (1064 nm to 1550 nm).
SPADs have the following advantages:
CMOS compatibility:
Silicon SPADs can be made with the complementary metal-oxide-semiconductor (CMOS)
process, the same way as computer CPUs and the such.
Since they output digital signals, you can fabricate them on the same
chip that is used to process the signals.
Hence the whole detection pipeline can be made cheaply on a silicon
application-specific integrated circuit (ASIC).
In contrast, the output of an APD is an analog signal, so a high-speed
analog-to-digital converter (ADC) is required.
This is very expensive and introduces extra noise.
Silicon SPADs also benefit from the immense scaling potentials of the
CMOS process, allowing very large arrays to be fabricated at a very fine
manufacturing node.
Hence, SPADs can be used to make very high resolution, dense arrays, as
opposed to APDs which are relatively large and expensive discrete
components.Higher gain: SPADs have a higher gain than linear mode APDs.
In fact, the gain of a SPAD is essentially infinite, allowing it to detect even a single photon.Lower temperature dependence:
SPADs are less sensitive to temperature than APDs, for which different
temperatures can change the sensitivity of the sensor and also affect
the dark current.Better timing jitter: SPADs output such a sharp spike that you can measure the timing very accurately and reliably.
Meanwhile, APDs have these advantages:
No dead time and quenching:
A linear mode APD essentially continually outputs an analog signal, so
there is no need to recharge. In contrast, after a SPAD fires, it takes a
while to recharge.
During a SPAD avalanche event, it can be destroyed by its own huge
current, so the current must be quenched with a resistor to discharge
it.
After quenching, it needs to recover to its original biasing condition.
The reverse bias voltage is typically supplied by a capacitor, which
needs to take time to charge back up again.
Hence, there is a dead time ranging from around a few nanoseconds
(silicon SPADs) to a microsecond (GMAPDs).
By avoiding all this, APDs can have simpler circuitry.Dynamic range per detector:
APDs output a continuous analog output that you can digitize however
finely you want, gives better dynamic range per detector (meanwhile a
single SPAD has a dynamic range of only 1 bit, it’s either 0 or 1).No range walk: Linear mode APDs avoid intensity-dependent range walk and saturation issues, which I discuss in more detail below.
If the return signal from a pulse is very strong, a SPAD array can be saturated at the very beginning of the pulse.
If the pulse length is long, ranging may be biased when measuring the range of retroreflective materials.
This is also known as range walk.
SPADs are so sensitive that they can be triggered by single photons, but this also makes them sensitive to ambient illumination.
Therefore saturation is a concern.
In contrast, the continuous signal from an APD can be digitized with many bits.
To
prevent SPADs from being drowned out by ambient light, the probability
of detection of any single SPAD must be kept very low. Some techniques
include:
SPADs are usually made really smalla tight band-pass filter can reject most ambient lightsometimes
an attenuating filter (e.g. a neutral density filter, which attenuates
all wavelengths equally) is needed to attenuate the signal even further
2.1.2 SPAD macropixels
FIGURE 7 The Sony IMX479
SPAD sensor is physically a 105×1,568 pixel array, with a total of
approximately 164,000 pixels, but it combines many pixels into
macropixels, so the final output is only 520 macropixels. This allows it
to have amazing dynamic range and produce this beautiful image. Note
that the lower image is the raw ambient image output from the lidar
rather than a separate photo taken by a camera.
Instead of a single SPAD per pixel, several SPADs can be combined into a single “macropixel”.
This trade-off results in lower spatial resolution, but the benefit is that it mitigates most of the drawbacks of SPADs.
Dynamic range increases from 1 bit to as many bits as you have SPADs in the macropixel.Dead
time of any individual SPAD is mitigated since it is unlikely that all
the SPADs will fire at once, meaning that there are always available
ones.
Of course, in some circumstances (such as retroreflectors) it is still
possible for all the SPADs in a macropixel to be saturated.SPADs
can be made individually smaller, making it unlikely for all of them to
fire at once, resulting in better resilience against saturation.
2.1.3 Multi-shot ranging
Even
with a macropixel, ranging with SPADs can be noisy as there may only be
as many photons measured as there are SPADs in the macropixel.
To increase signal strength, the lidar can fire many shots and aggregate
the time series data from each shot. This is known as multi-shot
ranging.
As an additional bonus, making multiple low-energy shots
is somewhat safer than a single high-energy shot as the peak laser
energy is less.
The tradeoff is that it takes a longer time to make a measurement, during which you could suffer from motion blur.
2.1.4 Silicon photomultipliers
Silicon photomultipliers are a group of SPADs whose outputs are combined into a single analog signal.
This has some advantages:
Just like the SPAD macropixel, by combining many SPADs, the dead time of any individual SPAD is a less big concern.It can be more sensitive than regular linear mode APDs.Without the need for digital logic, the chip is simpler and possibly denser than a digital SPAD macropixel.
However, an ADC is still required to digitize the signal.
2.2 Amplitude modulated lidar
Instead
of firing pulses, an amplitude modulated lidar continually modulates
the laser amplitude at some radio frequency, say, 1 GHz.
In other words, it is just a fast blinking light that turns on and off
rapidly.
Meanwhile, there are two detectors that turn on and off at the same rate but are out of phase.
That is, when detector 1 is on, detector 2 is off, and vice versa.
The
range can be estimated by checking the ratio of the light falling in
two detectors, for ranges up to a multiple of the modulation wavelength
For example, at 1 GHz, the wavelength is 15 cm.
To resolve the
range absolutely, the sensor changes the modulation frequency slightly,
say, to 1.05 GHz, giving a range estimate modulo a different wavelength.
The unknown multiples can then be found as a least common multiple
problem.
The advantage of this type of amplitude-modulated lidar is that it is very cheap.
There is no need for high-speed timing electronics to count photons at a high speed.
Instead, a simple oscillator is sufficient to make the lights and detectors blink at 1 GHz.
Since
the detectors just need to measure intensity rather than timing
information, they do not need to be very fast, and basic CMOS or CCD
sensors will suffice.
This type of lidar is used in RGBD sensors
such as the Kinect V2.
However, the ranging accuracy is much poorer than needed for automotive
purposes, so this type of lidar is not typically used for automotive.
2.3 Frequency modulated lidar
A frequency modulated lidar has a laser that can change in frequency rapidly.
Now, the laser beam goes through a beam splitter, and part of it is sent out, where it hits something, and bounces back.
Then, you can combine the part that didn’t go out with the part that bounced back.
When you combine two waves of similar but slightly different frequency, you’ll end up with something called beat.
When the waves line up, they will double their strength, and when they
are out of phase, they cancel each other out.
Then, you can use a photodiode to measure the time series of the
combined wave in order to determine the beat frequency, which in turn
tells you the range.
3
Here’s a plot that shows this effect.
The main thing is that the beat frequency is proportional to the difference in frequency, so you can measure it relatively easily with a photodiode.
FIGURE 8
When combining two frequencies, you get a wave of the sum of the two
frequencies multiplied by the difference of the two frequencies. Source:
Ansgar Hellwig on Wikimedia Commons
Frequency modulated lidar is known as frequency modulated continuous wave (FMCW) since the laser beam is always on (a continuous wave) that doesn’t turn off.
The principle of using the beat to determine the range is known as optical heterodyne detection.
Here, “heterodyne” means comparing two slightly different frequencies
(as opposed to “homodyne”, where you have the same frequency).
With FMCW lidar, you can also measure the speed of things by measuring the Doppler shift.
The
main tradeoff is that you’ll need an expensive fiber laser that can do
frequency modulation with highly linear chirps, increasing the overall
cost.
2.4 Parallax lidar
A parallax lidar works by triangulation, that is, similar to coincidence rangefinding.
This does not use any timing information at all.
A linear photodetector is placed physically offset from the laser.
The detector measures the incident angle of the reflected light and obtains the range by triangulation.
FIGURE 9 Parallax rangefinding, figure from “Low cost laser distance sensor” by K. Konolige et al.
This
is rarely or never used in automotive applications but is instead found
in robotic vacuum cleaners and other low-speed, low-cost applications.
A famous example is the “Low cost laser distance sensor” by Kurt Konolige et al.
Many robotic vacuum cleaner sensors are based on this.
A structured light
depth camera, also known as active stereo, is a special case of
parallax rangefinding.
Instead of a single laser beam, it projects a bunch of different dots at
once, and instead of a 1D line scan sensor, it has a regular 2D sensor.
But the depth measurement is again based on triangulation.
Structured light depth cameras are used in the early versions of the
Kinect as well as many Intel Realsense cameras.
With parallax rangefinding, it measures disparity, which is the inverse of range, so the uncertainty in range is quite high and grows quadratically with range.
As such, it is less suitable for advanced robotics and autonomous cars.
3 Discerning bearing
As
mentioned in our introduction, lidar sensors combine distance readings
with bearing to produce 3D points. Now that we’ve covered distance, we
are ready to discuss how to figure out the directions (bearing) of
things.
Lidars can either:
discern bearing for both tx (the outgoing laser beam) and rx (the detector), ordiscern bearing only for rx but not tx (i.e. a flash lidar), ordiscern
bearing only for tx but not rx (for example, some optical phased array
lidars only steer the laser beam, but have a “staring” detector that
doesn’t distinguish angle).
Discerning bearing is also known as “imaging”. People may describe a system as having both imaged rx and tx, for example.
Generally,
having imaged rx and tx is vastly better, since you are only pointing
your laser beam where you’re looking, so you get more range and
efficiency, and meanwhile the imaged receiver rejects off-angle
background light.
There are two main approaches for discerning bearing:
an array of elements already pointing in different directionsbeam steering, by pointing either your detector or laser in various directions
FIGURE 10 Animation showing arrays vs steering
As with the methods for measuring distance, each method has advantages and disadvantages.
The
advantages of arrays are that they don't have any moving parts, each
array element can be a lot cheaper, potentially leading to overall
cheaper cost, and that it can produce a much greater quantity of points.
The advantage of beam steering is that it works with high quality but
expensive laser sources such as fiber lasers, the scan pattern may be
configurable.
Being able to use high quality lasers also unlocks the ability to use
ranging modalities unavailable to array-based lidars, such as FMCW.
Note
that if you rely on steering very few (even one) lasers, the number of
points per second is limited by the speed of light.
It takes light a microsecond to travel 300 m round trip, meaning that a
single beam lidar is limited to about a million points per second at a
range of 150 m.
Meanwhile, array-based lidars can easily pump out several million points
per second.
3.1 Arrays for discerning bearing
The simplest way to determine direction is to just have an array of elements pointed in various directions.
Basically, you’ll need cheap and small array elements in order to have an array.
Laser typePerformanceCostArray?VCSELLowLowSolid state 2D arrays of hundreds of lasers are possibleEdge-emitting diodesMidMidDiscrete 1D arrays of dozens of lasers are typicalFiberHighHighNo, typically used as single laser + beam steeringTable 1 Comparison of lasers
Sensor typeSizeCostArray?SPADSmallLowSolid state 2D arrays of even millions of SPADs are possibleAPDMidMidDiscrete 1D arrays of dozens of APDs are typicalTable 2 Comparison of receivers
3.1.1 Discrete arrays
With
discrete arrays, you have discrete components like edge-emitting laser
diodes and avalanche photodiodes that are pointed in different
directions.
Some early lidars, like the Velodyne VLP 16, literally have 16 circuit
boards, each with one laser diode on them, and another 16 circuit
boards, each with one APD on them.
Then, these 32 circuit boards are glued into place.
FIGURE 11 The inside of the Velodyne VLP 16. Source: xtech
FIGURE 12 Detail of the array of 16 PCBs in the Velodyne VLP 16. Source: xtech
The
reason for doing that is because, due to the simple design of the lens,
it was necessary to arrange the lasers and detectors along a curved
arc.
Interestingly, a Google (now Waymo) patent US8836922B1 describes using a flexible substrate to achieve the curve.
FIGURE 13 Curved detector array in the receive block in US8836922B1.
3.1.2 Solid state arrays
Solid
state arrays put lasers or detectors on a single chip. The obvious
benefit is vastly simpler manufacturing and consistency.
High performance edge-emitting laser diodes shoot lasers to the sides so
you can’t just put a bunch of them in an array on a chip, so you’ll
have to make do with lower power VCSELs.
FIGURE 14 The Ouster L3 chip, a SPAD array. Each square is a macropixel with many SPADs. Source: Ouster blog post
FIGURE 15
Instead of 32 circuit boards, there are just two in this Ouster lidar:
one containing the chip full of lasers, and one containing the chip full
of detectors. Source: How Ouster Digital Lidar Works
Since the laser array or detector array is now flat, the optical design will be somewhat more complex.
You’ll need the lens to be image space telecentric since your flat array of lasers all produces parallel beams.
FIGURE 16 Ouster lidars have relatively complex multi-element lenses compared to single-element lenses on early Velodyne lidars. Source: How Ouster Digital Lidar Works
For
lasers, only VCSELs are compatible with this method.
As for detectors, SPADs are also vastly more amenable to solid state
arrays, although APD arrays are also available (but with fewer
elements).
This is because, as discussed earlier, SPADs are compatible with typical
chipmaking technologies and they output digital signals rather than
analog ones, so you can fabricate them on a single chip, whereas APDs
would typically require discrete components.
With large arrays, a lidar could sequentially fire small parts of an array instead of all of it at once.
This is called electronically scanning.
In effect, it is similar to scanning, except there are fixed elements
already pointed in different directions rather than the same element
being made to point in different directions.
Electronically scanning has the advantage of less pixel
crosstalk/blooming (more on this later) as well as being able to output
more power per beam without running into thermal or safety limits.
3.2 Scanning and beam steering methods
3.2.1 Spinning
Perhaps
the most straightforward way to do beam steering is to just spin the
whole lidar, which gives you 1D angular discernment.
The first advantage is that this gives you 360 degree field of view.
This also has the advantage of being highly compatible with arrays, so
you can have a vertical array while spinning horizontally.
Spinning lidars have basically only one moving part.
FIGURE 17 Size comparison between some spinning lidars.
An encoder is used to measure the angle of the turret.
The challenges of spinning are that:
You
need to send power and data between the spinning turret and the
stationary base somehow. The earliest spinning multi-beam lidar, the
Velodyne HDL-64E, used a mercury-wetted slip ring (Mercotac 305). This
is very efficient, but expensive, fragile, and somewhat environmentally
unfriendly. Later spinning lidars transmit power wirelessly through a
transformer, and data wirelessly through an optical link.The
cylindrical window can degrade optical performance. Some lidars have
compensator optics to suppress aberrations from the cylindrical window.
The Quanergy M8 had a variant with an octagonal window instead. Some
lidars spin externally (such as the Waymo Laser Bear Honeycomb, Velodyne
HDL-64E, and Velodyne HDL 32). But spinning externally makes it less
robust against the environment.Thermal dissipation. The
spinning turret houses most of the energy-intensive lasers but it has no
direct contact with the outside except through the bearing.Despite
having only one moving part, the spinning turret is a rather large and
heavy part, and some early spinning lidars like Velodynes tended to fail
a lot when the bearing was damaged or wore out. This is an especially
big problem if the turret isn’t well-balanced.
3.2.2 Spinning mirror
Using
a spinning polygonal mirror is one of the oldest and most reliable ways
to scan a laser beam, which is again a 1D scanning method.
This is used in, for example, laser printers.
FIGURE 18 Animation of a laser beam being reflected by a rotating hexagonal mirror.
As with spinning lidars, an encoder is used to measure the angle of the polygonal mirror.
Compared
to spinning the whole turret, this has the main drawback of having a
much narrower field of view (about 120 degrees is typical, as opposed to
360 degrees).
However, it has the advantage of having a lighter moving part without
having to deal with power transmission and heat dissipation and stuff.
3.2.3 Oscillating mirrors/galvos
This is a flat mirror that oscillates in angle to steer the beam, which can be either 1D or 2D.
FIGURE 19 Animation of a laser beam being reflected by an oscillating mirror.
Typically, a lightweight mirror is connected to a galvanometer in what’s called a mirror galvanometer
(galvo).
A galvanometer is one of the most basic ways to measure electrical
current: it consists of a spring, a magnet, and a solenoid.
When a current passes through the solenoid, it creates a magnetic field,
which causes a torque to be applied as it tries to align itself with
the magnet.
The spring resists this force, so the amount it ends up turning is
dependent on the current.
Nowadays, fast galvos are incredibly
good and are used in all sorts of applications, like laser light shows,
engraving, and so on.
Compared to spinning mirrors, this is
somewhat less reliable, since reciprocating motion is typically less
reliable than constant rotation.
Unlike spinning mirrors, you can
have a single mirror that’s actuated in two axes (a 2D galvo) that
allows you to steer your beam in both directions with a single mirror.
3.2.4 MEMS mirror
A MEMS (micro-electromechanical system) mirror is simply a mirror that is really small, typically an oscillating mirror.
Because it is so small, it is typically considered “solid state” even if it is physically a moving part.
Like macroscopic oscillating mirrors, MEMS scanner may be either 1D or 2D.
The
primary advantage of MEMS is low cost and relatively better
reliability.
After all, the rate at which your moving part wears out is strongly
dependent on the mass and moments of inertia of that moving part, so
keeping it as light as possible makes it more resilient.
There are, however, a couple of drawbacks:
The
optical aperture may be limited by the tiny size of the mirror. With
lasers, the bigger the aperture, the more it stays collimated (and hence
the less it spreads out), so you want the aperture to be as large as
possible usually.Cooling a tiny mirror may be hard. Your entire
laser output is bouncing off a tiny surface with tiny thermal pathways.
Hence, the mirror should be kept as reflective as possible.
3.2.5 Optical phased arrays
A phased array
has many array elements whose phase is slightly offset. As the
contributions from each element interfere, a beam is formed where they
interfere constructively, and everywhere else, destructive interference
causes it to cancel out.
FIGURE 20 Phased array animation. Source: Chetvorno on Wikimedia Commons.
Phased
arrays are common in radar. However, the fundamental physical problem
of phased arrays is that the element size must be close to the size of
the wavelength, and the wavelength of light (about a micron) is way
smaller than the wavelength of radio waves (ranging from millimeters to
many meters).
If your array spacing is too big, your beam would have very poor
collimation and tons of side lobes.
You can use phased arrays for both the transmitter and receiver.
For the receiver, you would have an array of optical antennae which are tiny nanophotonic detectors that can each measure the phase and amplitude.
So far, due to the physical challenges with phased arrays, there have been no commercial successes. The Quanergy S3 and an Israeli startup called Oryx Vision were two well-known entrants to attempt optical antennae.
3.2.6 Baraja SpectrumScan
This
uses a frequency sweep laser with a fixed prism.
Prisms have dispersion, which means that the index of refraction changes
with wavelength (hence turning sunlight into a rainbow), so by changing
the wavelength of the laser, the angle is changed.
This allows it to scan in 1D.
Baraja uses a MEMS mirror for the other axis.
FIGURE 21 Baraja lidar.
This requires using a high quality fiber laser or tunable diodes that can do large frequency sweeps, which can be costly.
3.2.7 Risley prisms
A prism is a triangular piece of glass that can bend light.
Risley prisms are a pair of two prisms that can rotate along the optical axis.
When the prisms are lined up, they both bend light the same way, and the beam gets bent a lot.
When they are opposite of each other, they cancel each other out, and the beam goes through straight without bending.
FIGURE 22 Animation of how Risley prisms work.
FIGURE 23 Simplified diagram showing how Risley prisms work.
Basically, when you have two prisms, one with angle and one with angle , the direction of the beam is proportional to:
4

Speed ratio between prisms: -0.743

The Livox lidars are notable for using Risley prisms.
You can make other scan patterns by varying the speed of the prisms, and
by putting an array of multiple lasers (e.g. the Livox Horizon’s 6
lasers) instead of one laser.
FIGURE 24 Livox Mid-70. Source: Livox website.
The
advantage of Risley prisms is that, like polygonal mirrors, it’s cheap
and robust to have things spinning at a constant speed.
However, the disadvantage is very narrow field of view, and a weird
scanning pattern.
For some applications, the scan pattern can be an advantage, for example
surveying applications where the lidar can be stationary for long
periods of time, gradually covering a dense area.
3.3 Combining two 1D methods
Many lidars combine two 1D methods, e.g.:
Horizontally spinning turret + vertical array (e.g. Velodyne pucks, Ouster OS series, Hesai Pandar)Horizontally spinning turret + vertically spinning polygonal mirror (e.g. Leica BLK360)Horizontal spinning mirror + vertical spinning or oscillating mirror (e.g. Luminar Iris, Seyond Falcon)Horizontal spinning mirror + array (e.g. Hesai AT512)
4 Choice of wavelength and eye safety
For lidars, two choices of wavelength are popular:
near infrared, e.g. 850 nm, 865 nm, 905 nm, 940 nm1550 nm
The
main advantage of near IR is that silicon is sensitive in that region,
allowing much cheaper, more sensitive silicon detectors, as well as
cheap laser sources.
In contrast, with 1550 nm, you would need InGaAs semiconductors for your
detectors, which are less sensitive and very expensive.
Meanwhile,
the main advantage of 1550 nm is that eye safety regulations allow
devices to output vastly more power at 1550 nm than in the near IR
regime.
As a result, 1550 nm lidars tend to have longer range in general.
FIGURE 25 Maximum permissible exposure according to IEC60825. Source: Hankwang from English Wikipedia
You
can see in the above chart that you are allowed to output hundreds of
times more power in the steady state scenario (red curve) at 1550 nm
compared to, say, 905 nm. The reason is that the human eyeball focuses
near IR light to small spots on the retina, so intense light may damage
the retina. On the other hand, 1550 nm light is not focused and is
attenuated by water, but at high enough intensities, it will damage the
cornea instead.
In practice, manufacturers carefully tune the
power of the lasers to be just below the eye safety threshold for both
1550 nm lidars and near IR lidars.
That is to say, 1550 nm lidars do in fact output up to 1,000,000 times
more pulse energy than 905 nm ones.
Paradoxically, 1550 nm lidars may be more dangerous overall, because of the following reasons:
If
you have many lidars around, the beams from each 905 nm lidar will be
focused to a different spot on your retina, and you are no worse off
than if there was a single lidar. But if there are many 1550 nm lidars
around, their beams will have a cumulative effect at heating up your
cornea, potentially exceeding the safety threshold.1550 nm
lidars more often rely on beam steering since it is impractical to have
an array of expensive fiber lasers. However, if the beam steering were
to fail, the laser beam may be fixed in one direction. This can cause
laser energy levels thousands of times stronger than the safety
threshold in a particular direction, even when the lidar would be under
the threshold when it’s scanning properly.1550 nm lidars are known to damage cameras. For example, both AEye lidars and Luminar lidars on the Volvo EX90
are known to destroy cameras. This is an especially worrisome problem
with pulsed 1550 nm lidar, but FMCW lidars have a continuous wave with
lower peak power and may be slightly safer.
Eye safety
aside, 1550 nm is also somewhat more attenuated by both water and water
vapor, so they are likely to perform worse in poor weather.
In fog, Mie scattering
of the water droplets may also impact 1550 nm lidar more, as fog
droplets are about 1.5 microns, and the scattering is more as the size
of the sphere approaches the wavelength.
That said, 1550 nm lidars do have better range to begin with, thanks to
outputting a lot more power, so even with attenuation, they are still
competitive in rainy situations.
5 Laser sources
There are basically three commonly used types of lasers:
Vertical cavity surface emitting laser (VCSEL)Edge-emitting laser diodesFiber lasers
Laser typeTypical wavelength(s)Beam quality (M²)Coherence / FMCW-readyPower per elementCostArray?VCSEL850–940 nmVery good, circularLow–mid linewidthLow (mW-tens mW)LowExcellent: monolithic 2D arrays (10²–10⁵ emitters), fine pitch, easy eye-safetyEdge-emitting diodes905 nm, 1350-1550 nmGood (often elliptical)Mid–highMid (100 mW–W class with bars)MidGood: 1D bars/arrays (dozens–hundreds)Fiber/ECDL1550 nmExcellentHigh (kHz–100 kHz LW) best for FMCWHigh (W class via fiber amps)HighPoor as dense arrays; usually single source + split/steerTable 3 Summary of laser types
Vertical
cavity surface-emitting lasers (VCSELs) are very cheap and you can make
a bunch of them on a chip in a chip-scale solid state array.
They are called “vertical cavity” because the beam comes out
perpendicular to the chip.
You make them by depositing several layers of material on the chip.
The main drawback is that they are low peak power.
Edge-emitting laser diodes are a mature technology and are cheap enough to be 1D arrays.
Fiber
lasers produce high quality light that is highly coherent. But they are
quite expensive so you can probably just afford one or two per lidar.
Some lidars split one laser between many lidar heads, as in the case of
Baraja’s lidar.
Not only are fiber lasers more coherent, they can output millions of
times greater power than edge-emitting diode lasers and VCSELs as well
as much shorter pulses.
Having shorter pulses is very advantageous for pulsed lidar as it
improves the range resolution.
Some fiber lasers can also vary the wavelength in highly linear chirps,
allowing use in FMCW lidars.
The development of these lasers is
highly driven by the telecommunications industry where they are used in
fiber optics, so the lidar industry sort of profits from that for free.
6 Common lidar problems
6.1 Beam angle calibration
Most
spinning + array lidars need a calibrated list of angles, one per beam.
Some manufacturers, like Ouster, provide a JSON metadata file containing
the elevation and azimuth angles of each of the 128 beams, which is
calibrated per lidar.
Some manufacturers simply give a nominal set of beam angles for a lidar
model that is assumed to be the same for each individual lidar, but in
practice, each lidar varies slightly due to manufacturing tolerances.
Early Velodynes had very bad beam angles as each of the many circuit
boards was individually glued in place and manually aligned.
Here are some ways lidar measurements could have bad beam angles:
Accidentally forgetting to use the lidar-specific beam angles, or the manufacturer doesn’t provide themAll
the beams are offset by some angle even with lidar-specific beam
angles, e.g. bad factory calibration, the lidar got bumped, or due to
thermal expansion
This would typically manifest as the
ground curving slightly, or the trajectory of the robot curving up or
down even when it is expected to be flat.
The well-known KITTI
dataset is known to have bad beam angles, and some publications have to
manually calibrate them in order to achieve good results. For example,
in IMLS-SLAM by J. E. Deschaud:
The
drift we get on the KITTI benchmark is not as good as the results we
obtained with the Velodyne HDL32. This is due to three facts. First, we
found a distortion of the scan point clouds because of a bad intrinsic
calibration (we did a calibration of the intrinsic vertical angle of all
laser beams of 0.22 degrees using the training data). Second, we found
big errors in GPS data (used as ground truth) with, for example, more
than 5 m in the beginning of sequence 8.
6.2 Range offsets
Lidars
sometimes have different range offsets for each laser.
This can happen when using discrete arrays where each laser-detector
pair are separate components that need to be individually calibrated.
FIGURE 26 Two views of a flat wall in 2011_09_26/2011_09_26_drive_0084_extract/velodyne_points/data/0000000035.txt from the KITTI dataset.
Due to uncalibrated range offsets for some of the lasers of the
Velodyne HDL-64E used, points from certain beams are offset by several
centimeters.
6.3 Pixel crosstalk/blooming
Blooming affects many lidars.
Think of pointing a camera at the sun. There would be huge lens flare and brightness all around the sun.
In effect, the light from the sun is “smeared” out onto neighboring pixels.
Likewise, when there’s a strong lidar return, there could be spurious returns next to the shiny object.
With
array lidars, neighboring detectors sometimes pick up on the return
meant for a different detector. This is called crosstalk.
However, even single beam lidars can suffer from blooming just due to
the fact that the beam has some divergence and that the optics are
imperfect.
FIGURE 27
An early prototype of the now-defunct Argo lidar illuminates the scene a
whole column at a time, making it susceptible to blooming in the form
of vertical columns. Source: Argo AI on YouTube
FIGURE 28
Spurious bloom returns around a retroreflector for an early Ouster OS1
prototype from 2022. Note that later firmware upgrades mitigated the
issue. Source: Ouster marketing data.
This effect typically can’t be easily calibrated away, and is usually handled in lidar firmware.
6.4 Intensity-dependent range bias
This typically affects SPAD lidars like early Ouster lidars and the now-defunct Argo (formerly Princeton Lightwave) lidar.
The reason is that when the return is very strong, all the SPADS get saturated at the beginning of the pulse.
FIGURE 29 For specular reflections, there’s a spike in the point cloud from this early prototype of the Argo lidar. Source: Argo AI on YouTube
FIGURE 30
Subtle range bias on highly reflective painted stripes of a pedestrian
crossing for an early Ouster OS1 prototype from 2022. Source: Ouster marketing data.
Typically, a pulse is a few nanoseconds long, which means up to a few meters in physical length of the light pulse.
Even a slight saturation effect can cause the the peak of the time series to be biased significantly.
Very advanced signal processing techniques are needed to compensate for this.
6.5 Encoder hysteresis
Hysteresis
in an encoder would typically manifest as some kind of lag, e.g. if
it’s rotating clockwise, it could output measurements with slightly
different offset than when it’s at the same angle but rotating counter
clockwise.
Some lidars, such as the Luminar Iris, use encoders for an oscillating
beam scanner for vertical beam scanning.
It also has a mode where part of the point cloud is an “up-scan” and the
other part is a “down-scan”, and the two are superimposed.
Often, the point cloud of the up- and down-scans do not align well, even
when the vehicle is stationary, suggesting that there may be hysteresis
in the encoder.
This may manifest as double-layer point clouds in the ground.
6.6 Encoder physical offset
The encoder used in many spinning lidars is a circular ring with a bunch of ticks engraved on it at regular intervals.
However,
it is possible that the encoder is physically offset to the side,
because the ring is often just glued in place by humans.
This results in a sinusoidal error.
5
FIGURE 31 Diagram of offset encoder and plot of measured angle vs true angle.
This effect can cause a straight corridor to appear consistently curved to one side.
6.7 Multiple lidars in a box
Some lidars are packaged in such a way that there are two or more separate lidars in a box.
For example, the Livox Mid 100 comprises three Mid-40s arranged side by side.
FIGURE 32 Livox Mid 40 and 100. Source: Livox website.
FIGURE 33 Luminar Hydra has two separate lidars side by side. Source: Luminar via Venturebeat.
Sometimes, physically jostling the lidar can cause the multiple separate lidars to become misaligned.
It would then be necessary to treat them as separate lidars and calibrate their orientations accurately.
7 FAQ
7.1 LiDAR vs lidar
Should it be capitalized as “LiDAR” instead of “lidar”?
No!
We should use the lowercase because it’s a commonly used word just like
radar.
When radar was some sort of highly exotic military technology, it made
sense to use all caps for the acronym “radio detection and ranging”, but
by now it is so common that we should use lowercase.
Many other words started out capitalized when new and exotic, but became
lowercase once commonplace:
laser, Light Amplification by Stimulated Emission of Radiationscuba, Self-Contained Underwater Breathing Apparatustaser, Thomas A. Swift’s Electric Rifle
Now that most phones and some cars are equipped with lidar, it’s a good time to just use lowercase.
Perhaps the main barrier to doing so is Apple’s autocorrect.
7.2 Are rectangular lidars solid state?
No, not necessarily.
Whether or not something is solid state is based on whether it has macroscopic moving parts in it, not based on its shape.
Livox
lidars are often mistakenly assumed to be solid state, but they are in
fact mechanically scanning with some using Risley prisms and some using
mirrors.
FIGURE 34 They may be rectangular, but they aren’t solid state.
Likewise,
Luminar lidars are often assumed to be solid state, but they are not.
The Luminar Hydra uses galvos and the Luminar Iris uses polygonal
mirrors.
Solid state lidars are often perceived to be more durable and reliable.
Lidar manufacturers have taken note of this market bias in customers, and marketed accordingly.
For example, the Velodyne HDL-64 was marketed as solid state (even though it is externally spinning) in their 2016 press release announcing the VLP-32A.
Based
on his experience during this challenge, Hall recognized the
limitations of stereovision and developed the HDL-64 Solid-State Hybrid
LiDAR sensor.
As justification, however, one might
consider that it has an array of 64 lasers to distinguish vertical
bearing, so perhaps it could be called 50% solid state as mechanical
scanning is used only for the horizontal direction! In contrast, Luminar
and Livox lidars use mechanical scanning for both directions, despite
being a single non-spinning box.
FIGURE 35 Velodyne booth at CES 2020. In the back corner you can see David Hall.
FIGURE 36 Velodyne formerly marketed these as “Solid-State Hybrid”.

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All about automotive lidar

August 29, 2025
By Daniel Lawrence Lu

Here I'll provide a comprehensive overview of automotive lidar technology.

Lidar is used for autonomous vehicles and robotics because it’s a cool technology.

FIGURE 1 Waymo Jaguar I-Pace with several lidars.

FIGURE 2 A Chrysler Pacifica Hybrid with 8 Ouster lidars.

1 What lidar does

A lidar is a sensor which operates by bouncing light off surrounding surfaces.

Lidars typically quantify:

distance, by measuring how much time it takes for light to bounce back

bearing, by shining the light or pointing the detector in a particular direction

reflectivity, by measuring how much light has bounced back

speed, by measuring the Doppler shift in the reflected light

ambient, by measuring the amount of light in the environment in a particular direction

FIGURE 3 Ambient, intensity, and range channels of a really old Ouster OS1-64.

In general, we are most interested in distance and bearing.

Surface reflectivity is also valuable, as it allows detection of road lines in the automotive case.

By measuring distance in many directions, an autonomous vehicle can perceive its environment.

Each measurement corresponds to a discrete 3D point in space.

Through a decade of steady research, engineers designed algorithms capable of leveraging this 3D point cloud to unlock spatial understanding. Obstacle avoidance and precise positioning are just two direct results of this technology.

Distance and bearing measurements can be converted into 3D Cartesian points. For example, given range and bearing , the 3D point is:

x = range * cos(bearing)

y = range * sin(bearing)

z = 0

In contrast, a camera only measures bearing and ambient light intensity.

Each pixel of a photo is a measurement of how much light there is in that particular direction.

But generally, a camera has much higher bearing resolution than a lidar.

FIGURE 4 A point cloud accumulated from an Ouster OS1-64 lidar.

2 Measuring distance

Measuring distance is also known as ranging.

Basically, it just measures how close something is.

There are in general two ways of doing this:

Measuring the time somehow, exploiting the fact that light travels at a constant speed (the speed of light)

Parallax

For measuring the time, there are again two ways:

Direct time of flight, where we directly measure the time

Modulated lidar, where we modulate some attribute of the outgoing light, e.g. amplitude, frequency, or polarization

2.1 Direct time of flight pulsed lidar

Direct detection pulsed lidar fires one or more laser pulses.

Then, we simply measure the time to see the reflection from the pulse.

Let t be the time taken for light to travel at the speed of light (c = approximately 300,000,000 m/s), so:

Distance = c * t

Imagine if we have a stopwatch that measures in, say, a nanosecond resolution.

If we measure 1000 nanoseconds, then it means the round trip distance was 300 m, which means that the range is 150 m.

This involves measuring the time series data of how much light is seen at any point in time.

Since electronics typically run at 1 GHz or so, the time series is discretized on the order of 1 ns, which corresponds to a range of 15 cm.

To further improve the ranging accuracy, an interpolation filter is a standard technique in signal processing.

Typically ranging accuracy at the centimeter level is possible.

After getting the time series data, the peaks in the series are found, and these correspond to the range.

FIGURE 5 A simplified time series plot of number of photons vs time.

Usually, it is better to have stronger, shorter pulses.

Diode lasers can produce pulses on the order of a couple of nanoseconds,

and fiber lasers can produce even shorter pulses with much higher peak energy.

In practice, the laser pulse has some finite duration and shape (rather than being an infinitely short impulse function).

so the peak is found in the cross correlation of the outgoing pulse’s shape with the return data, rather than the raw time series data itself.

It is possible to send a randomly shaped pulse (or sequence of pulses), and cross correlate the return data against that. This provides much greater resistance against noise, interference, and crosstalk, and is known as a matched filter.

We should note, however, that the shape of the return pulse could be distorted or “smeared out”. This can be due to, for example, hitting a very slanted surface.

One strategy to overcome this is to try correlating it with a bunch of different pulse shapes. This technique may be called template matching, dictionary matching, matched filter bank, model-based detection.

2.1.1 Photodiodes used in pulsed lidar

In order to get a time series of the amount of light per unit time at a super high rate, we need a really fast sensor that can operate at 1 GHz.

Usually one of these two types of sensors is used:

Linear-mode avalanche photodiodes (APD)

Geiger-mode avalanche photodiodes, also known as single-photon avalanche photodiodes (SPAD)

Other types of sensors such as CCD sensors are not fast enough for this application.

A photodiode is a diode that also has the photoelectric effect.

A diode is like a one-way valve for electricity.

Just like a one way valve for water, if you try to force things sufficiently in the opposite direction, it will break down resulting in a huge gush of water.

Likewise, if you apply a strong voltage in the reverse direction, it’s called a reverse bias, and a sufficiently strong voltage will cause a sudden spike in electrical current.

This is called avalanche breakdown.

Meanwhile, some metals produce an electric current when shining light on it, in an effect known as the photoelectric effect.

Avalanche photodiodes have a reverse bias, meaning that a voltage is applied in the opposite direction of the one-way valve.

If the reverse voltage exceeds a certain amount known as the breakdown voltage, it stops acting like a diode.

Suddenly, a large current can flow through the device.

Linear-mode

APDs have a reverse bias slightly below the breakdown voltage.

Here, the current is linearly related to the voltage, but the gain is very high, so that even changing a small voltage results in a large change in the current,

Hence, it is a very sensitive way of measuring light intensity.

Geiger-mode

Avalanche photodiodes (GMAPDs) or single-photon avalanche diodes (SPADs) have such a strong reverse bias that even getting hit by a single photon can make them break down, resulting in a large current spike.

The output of a SPAD can be directly connected to a voltage discriminator so that the spike becomes a digital signal from logic 0 to 1.

FIGURE 6 I-V diagram of avalanche photodiodes

In the above I-V diagram, we see the relationship between the voltage (V) and the current (I).

The breakdown voltage is labelled.

As you can see, where the linear-mode APD operates, the current is linearly proportional to the voltage.

Note on terminology:

Typically the word avalanche photodiode (APD) refers to linear-mode APDs. Meanwhile, GMAPDs and SPADs operate in the same way but the term SPAD often refers to silicon devices sensitive to near infrared (850 nm to 940 nm) and GMAPD often refers to InGaAs devices sensitive to longer wavelengths (1064 nm to 1550 nm).

SPADs have the following advantages:

CMOS compatibility:

Silicon SPADs can be made with the complementary metal-oxide-semiconductor (CMOS) process, the same way as computer CPUs and the such.

Since they output digital signals, you can fabricate them on a single chip that is used to process the signals.

Hence the whole detection pipeline can be made cheaply on a silicon application-specific integrated circuit (ASIC).

Meanwhile, the output of an APD is an analog signal, so a high-speed analog-to-digital converter (ADC) is required.

This is very expensive and introduces extra noise.

Better timing jitter:

SPADs output such a sharp spike that you can measure the timing very accurately and reliably.

No dead time and quenching:

A linear mode APD essentially continually outputs an analog signal, so there is no need to recharge. In contrast, after a SPAD fires, it takes a while to recharge, so during a SPAD avalanche event, the current must be quenched with a resistor to discharge it.

During a SPAD avalanche event, it can be destroyed by its own huge current, so the current must be quenched with a resistor to discharge it.

After quenching, it needs to recover to its original biasing condition.

The reverse bias voltage is typically supplied by a capacitor, which needs to take time to charge back up again.

Hence, there is a dead time ranging from around a few nanoseconds (silicon SPADs) to a microsecond (GMAPDs).

Hence, there is no dead time and quenching:

After quenching, it needs to recover to its original biasing condition.

The reverse bias voltage is typically supplied by a capacitor, which needs to take time to charge back up again.

In summary, SPADs have a higher gain than linear mode APDs.

Lower temperature dependence:

SPADs are less sensitive to temperature than APDs, for which different temperatures can change the sensitivity of the sensor and also affect the dark current.

Better timing jitter:

SPADs output such a sharp spike that you can measure the timing very accurately and reliably.

Meanwhile, APDs have these advantages:

No dead time and quenching:

During a SPAD avalanche event, it can be destroyed by its own huge current, so the current must be quenched with a resistor to discharge it.

After quenching, it needs to recover to its original biasing condition.

The reverse bias voltage is typically supplied by a capacitor, which needs to take time to charge back up again.

If the return signal from a pulse is very strong, a SPAD array can be saturated at the beginning of the pulse.

If the pulse length is long, ranging may be biased when measuring the range of retroreflective materials.

This is also known as range walk.

SPADs are so sensitive that they can be triggered by single photons, but this makes them sensitive to ambient illumination.

Therefore saturation is a concern.

In contrast, the continuous signal from an APD can be digitized with many bits.

To prevent SPADs from being drowned out by ambient light, the probability of detection of any single SPAD must be kept very low. Some techniques include:

SPADs are usually made really small a tight band-pass filter can reject most ambient lightsometimes

an attenuating filter (e.g. a neutral density filter, which attenuates all wavelengths equally) is needed to attenuate the signal even further

2.1.2 SPAD macropixels

FIGURE 7 The Sony IMX479

SPAD sensor is physically a 105×1,568 pixel array, with a total of approximately 164,000 pixels, but it combines many pixels into macropixels, so the final output is only 520 macropixels. This allows it to have amazing dynamic range and produce this beautiful image. Note that the lower image is the raw ambient image output from the lidar rather than a separate photo taken by a camera.

Instead of a single SPAD, we can have many SPADs in the same macropixel, so we can have macropixels that are much bigger!

2.1.3 Multi-shot ranging

FIGURE 8 A simplified time series plot of number of photons vs time.

Usually, it is better to have stronger, shorter pulses.

Diode lasers can produce pulses on the order of a couple of nanoseconds,

and fiber lasers can produce even shorter pulses with much higher peak energy.

In practice, the laser pulse has some finite duration and shape (rather than being an infinitely short impulse function).

so the peak is found in the cross correlation of the outgoing pulse’s shape with the return data, rather than the raw time series data itself.

We should note, however, that the shape of the return pulse could be distorted or “smeared out”. This can be due to, for example, hitting a very slanted surface.

3 Discerning bearing

As mentioned in our introduction, lidar sensors can either:

discern bearing for both tx (the outgoing laser beam) and rx (the detector), or

discern bearing only for rx but not tx (for example, some optical phased arrays

Generally, having imaged rx and tx is vastly better, since it allows you to point your laser beam where you’re looking, so it has more range and efficiency, and meanwhile the imaged receiver rejects off-angle background light.

There are two main approaches for discerning bearing:

Arrays for discerning bearing

An array of elements already pointing in different directions

beam steering, by pointing either your detector or laser in various directions

3.1 Arrays for discerning bearing

The simplest way to determine direction is to just have an array of elements pointed in different directions.

Basically, you’ll need cheap and small array elements in order to have an array.

Laser typeTypical wavelength(s)Beam quality (M²)Coherence / FMCW-readyPower per elementCostArray?

VCSEL850–940 nmVery good, circularLow–midlinewidthLow (mW-tens mW)LowExcellent: monolithic 2D arrays (10²–10⁵ emitters), fine pitch, easy eye-safety

Edge-emitting diodes905 nm, 1350-1550 nmGood (often elliptical)Mid–highMid (100 mW–W class with bars)High: 1D bars/arrays (dozens–hundreds)

Fiber/ECLD1064 nm-1550 nmExcellentHigh (kHz–100 kHz LW)High: single source + split/steer

Table 3 Summary of laser types

3.2 Scanning and beam steering methods

Spinning

Perhaps the most straightforward way to scan a laser beam is to just spin the whole lidar, which gives you 1D angular discernment.

The main advantage is that it has a 360 degree field of view.

This also has the advantage of being highly compatible with arrays, so you can have a vertical array while spinning horizontally.

Spinning lidars have basically only one moving part.

FIGURE 17 Size comparison between some spinning lidars.

A spinning mirror

Using a spinning polygonal mirror is one of the oldest methods for scanning a laser beam, which is also known as beam steering.

FIGURE 18 Animation showing a spinning mirror.

Oscillating mirrors/galvos

An oscillating mirror is a mirror that oscillates in angle, which is also known as beam steering.

FIGURE 19 Diagram of oscillating mirror.

MEMS mirror

A MEMS mirror is simply a mirror that has microscopic moving parts.

MEMS mirrors are often used for beam steering.

4 Parallax lidar

Parallax lidar works by using a prism.

Prisms bend light, so by using a prism, we can achieve beam steering.

Risley prisms

Risley prisms are a pair of prisms that can rotate along the optical axis.

When the prisms are aligned, they both bend light the same way, and the beam gets bent a lot.

FIGURE 22 Diagram of Risley prisms.

Combining two 1D methods

It’s possible to combine two 1D methods for beam steering, such as spinning a mirror and oscillating a mirror.

5 Points to remember

It’s possible to have two 1D methods for beam steering simultaneously.

6 Common lidar problems

6.1 Beam angle calibration

Most

spinning + array lidars need a calibrated list of angles, one per beam.

Some manufacturers, like Ouster, provide a JSON metadata file containing the elevation and azimuth angles of each of the 128 beams, which is calibrated per lidar.

Some manufacturers simply give a nominal set of beam angles for a lidar model that is assumed to be the same for each individual lidar, but in practice, each lidar varies slightly due to manufacturing tolerances.

Here are some ways lidar measurements could have bad beam angles:

Accidentally forgetting to use the lidar-specific beam angles, or the manufacturer doesn’t provide them

All the beams are offset by some angle even with lidar-specific beam angles, so it can cause the beams to overlap, or create ghosting effects.

6.2 Range offsets

Range offsets can cause problems when measuring range.

6.3 Intensity-dependent range bias

It’s possible to have intensity-dependent range bias, which is caused when the laser signal is saturated at the beginning of the pulse.

The main issue is that the intensity can lead to range bias, and cause range confusion.

6.4 Multi-shot compensation

To measure a 3D point cloud, we need to measure range and bearing.

7 FAQ

7.1 LiDAR vs lidar
Should it be capitalized as “LiDAR” instead of “lidar”?
No!
Typically the word avalanche photodiode (APD) refers to linear-mode APDs. Meanwhile, GMAPDs and SPADs operate in the same way but the term SPAD often refers to silicon devices sensitive to near infrared (850 nm to 940 nm) and GMAPD often refers to InGaAs devices sensitive to longer wavelengths (1064 nm to 1550 nm).