# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 2 Basics of Lidar Imaging
### Imaging Strategies
#### 2.2.1 Scanners  
Currently, in the automotive lidar market, most of the proposed commercial systems rely on scanners of different types [41; 42; 43; 44]. In the most general approach, the scanner element is used to re-position the laser spot on the target by modifying the angular direction of the outgoing beam, in order to generate a point cloud of the scene. This poses questions related to the scanning method, its precision, its speed, its field of view, and its effect on the beam footprint on the target, which directly affects the spatial resolution of the images [45]. A large number of scanning strategies have been proposed for lidar and laser scanning, and are currently effective in commercial products. This includes e.g., galvanometric mirrors [46] or Risley prisms [47]. When it comes to its use in autonomous vehicles, three main categories may be found: mechanical scanners, as described, which use rotating mirrors and galvanometric or piezoelectric positioning of mirrors and prisms to perform the scanning; micro-electromechanical system (MEMS) scanners, which use micromirrors actuated using electromagnetic or piezoelectric actuators to scan the field of view, often supported by expanding optics; and optical phased arrays (OPAs), which perform pointing of the beam based on a multibeam interference principle from an array of optical antennas. Direct comparison of scanning strategies for particular detector configurations has been proposed [42]. Although we will focus on these three large families, it is worth noting that other approaches have been proposed based on alternative working principles such as liquid crystal waveguides [48], \begin{table}
\begin{tabular}{c l l l} \hline  & **Pulsed** & **AMCW** & **FMCW** \\ \hline Parameter measured & Intensity of emitted & Phase of modulated & Relative beat of modulated \\  & and received pulse & amplitude & frequency, and Doppler \\  & & & shift \\ \hline Measurement & Direct & Indirect & Indirect \\ \hline Detection & Incoherent & Incoherent & Coherent \\ \hline Use & Indoor/Outdoor & Only indoor & Indoor/Outdoor \\ \hline Main advantage & Simplicity of setup; & Established & Simultaneous speed and \\  & long ambiguity range & commercially & range measures \\ \hline Main limitation & Low SNR of returned & Short ambiguity & Coherence length/Stability \\  & pulse & distance & in operating conditions \\  & & & (e.g., thermal) \\ \hline Depth resolution (typ) & 1 cm & 1 cm & 0.1 cm \\ \hline \end{tabular} Note: maximum attainable range has been avoided as it needs the definition of several other parameters (instantaneous FOV, reflectivity of target, eye safety level, etc.)  
\end{table} 
Table 1: Summary of working principles. electrowetting [49], groups of microlens arrays [50] and even holographic diffraction gratings [51]. (a) Mechanical scanners  
Lidar imaging systems based on mechanical scanners use high-grade optics and some kind of rotating or galvanometric assembly, usually with mirrors or prisms attached to mechanical actuators, to cover a wide field of view. In the case of lidar, units with sources and detectors jointly rotate around a single axis. This may be done by sequentially pointing the beam across the target in 2D, as depicted in Figure 6, or by rotating the optical configuration around a mechanical axis, in which case a number of detectors may be placed in parallel along the spinning axis. In this latter case 360 deg FOVs of the sensor may be achieved, covering the whole surroundings of the vehicle. The mirrors or prisms used may be either rotating or oscillating, or may be polygon mirrors. This is the most popular scanning solution for many commercial lidar sensors, as it provides straight and parallel scan lines with a uniform scanning speed over a vast FOV [52; 53], or angularly equispaced concentric data lines. In rotating mirrors, the second dimension is usually obtained by adding more sources and/or detectors to measure different angular directions simultaneously.