# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 2 Basics of Lidar Imaging
### Microelectromechanical scanners  
Microelectromechanical systems (MEMS)-based lidar scanners enable programmable control of laser beam position using tiny mirrors with only a few mm in diameter, whose tilt angle varies when applying a stimulus, so the angular direction of the incident beam is modified and the light beam is directed to a specific point in the scene. This may be done bidimensionally (in 2D) so a complete area of the target is scanned. Various actuation technologies are developed including electrostatic, magnetic, thermal and piezoelectric. Depending on the applications and the required performance (regarding scanning angle, scanning speed, power dissipation or packaging compatibility), one or another technology is chosen. The most common stimulus in lidar applications based on MEMS scanners is voltage; the mirrors are steered by drive voltages generated from a digital representation of the scan pattern stored in a memory. Then, digital numbers are mapped to analog voltages with a digital-to-analog converter. However, electromagnetic and piezoelectric actuation has also been successfully reported for lidar applications [52; 57; 58; 59]. Thus, MEMS scanners substitute macroscopic mechanical-scanning hardware with an electromechanical equivalent reduced in size. A reduced FOV is obtained compared to the previously described rotary scanners because they have no rotating mechanical components. However, using multiple channels and fusing their data allows us to create FOVs and point cloud densities able to rival or improve mechanical lidar scanners [60; 61]. Further, MEMS scanners have typically resonance frequencies well above those of the vehicle, enhancing maintenance and robustness aspects. MEMS scanning mirrors are categorized into two classes according to their operating mechanical mode: resonant and non-resonant. On one hand, non-resonant MEMS mirrors (also called quasi-static MEMS mirrors) provide a large degree of freedom in the trajectory design. Although a rather complex controller is required to keep the scan quality, desirable scanning trajectories with constant scan speed at large scan ranges can be generated by an appropriate controller design. Unfortunately, one key spec, such as the scanning angle, is quite limited in this family compared to resonant MEMS mirrors. Typically, additional optomechanics are required in order to enlarge the scan angle, adding optical aberrations like distortion to the scanning angle. On the other hand, resonant MEMS mirrors provide a large scan angle at a high frequency and a relatively simple control design. However, the scan trajectory is sinusoidal, i.e., the scan speed is not uniform. However, their design needs to strike a balance where the combination of scan angle, resonance frequency, and mirror size is combined for the desired resolution, while still keeping the mirror optically flat to avoid additional image distortions which may affect the accuracy of the scan pattern [52; 62; 63]. Laser power handling at the surface of the mirror is also an issue that needs be carefully taken into account to avoid mirror damage, especially in long-range units. A 2D laser spot projector can be implemented either with a single mirror with two oscillation axes or using two separate, orthogonal mirrors oscillating each along one axis. Single-axis scanners are simpler to design and fabricate, and are also far more robust to vibration and shock; however, dual-axis scanners provide important optical and packaging advantages, essentially related to the simplicity of the optical arrangement and accuracy required in the relative alignment of the two single-axis mirrors. One crucial difficulty with dual-axis scanners is the crosstalk between the two axes. With the increasing requirements of point cloud resolution, it becomes harder to keep the crosstalk to acceptable levels, which would be a reason to choose the bulkier and more complex two-mirror structure.