# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 2 Basics of Lidar Imaging
### Optical Phased Arrays
#### 2.2.2 Detector Arrays
The divergence of the illuminating area and the backscattering at the target significantly reduce the amount of optical power available, so very high peak illumination power and very sensitive detectors are required in comparison to single-pixel scanners. Detectors are usually SPADs (single photon avalanche diodes), discussed in Section 3.2.2. This has caused the present flash setups to keep being concentrated in sensing at medium or short-range applications in autonomous vehicles, where they take advantage of their lack of moving elements, and they have acceptable costs in mass production due to the simplicity of the setup [52]. Eye-safety considerations are significant in these flood illumination approaches [52]. However, the main disadvantage of flash imagers comes from the type of response of the detectors used, which are used in Geiger mode and thus produce a flood of electrons at each detection. The presence of retro-reflectors in the real-world environment, designed to reflect most of the light and backscatter very little to be visible at night when illuminated by car headlamps, is a significant problem to these cameras. In roads and highways, for instance, retro-reflectors are routinely used in traffic signs and license plates. In practice, retro-reflectors flood the SPAD detector with photons, saturating it, and blinding the entire sensor for some frames, rendering it useless. Some schemes based on interference have been proposed to avoid such problems [79]. Issues related to mutual interference of adjacent lidars, where one lidar detects the illumination pattern of the other, are also expected to be a hard problem to solve in flash imagers. On the positive side, since flash lidars capture the entire scene in a single image, the data capture rate can be very fast, so the method is very resilient to vibration effects and movement artifacts, which otherwise could distort the image. Hence, flash lidars have proven to be useful in specific applications such as tactical military imaging scenarios where both the sensor platform and the target move during the image capture, a situation also common in vehicles. Other advantages include the elimination of scanning optics and moving elements and potential for creating a miniaturized system [80; 81]. This has resulted in the existence of systems based on flash lidars effectively being commercialized at present in the automotive market [82]. (b) AMCW cameras  
A second family of lidar imaging systems that use detector arrays are the TOF cameras based on the AMCW measuring principle. As described in Section 2.1.2, these devices modulate the intensity of the source and then measure the phase difference between the emitter and the detected signal at each pixel on the detector. This is done by sampling the returned signal at least four times. Detectors for these cameras are manufactured using standard CMOS technology, so they are small and low-cost, based on well-known technology and principles, and capable of short-distance measurements (from a few centimeters to several meters) before they enter into redundancy issues due to the periodicity of the modulation. Typical AMCW lidars with a range of 10 m may be modulated at around 15 MHz, thus requiring that the signal at each pixel to be sampled at around 60 MHz. Most commercially available TOF cameras operate by modulating the light intensity in the near-infrared (NIR) and use arrays of detectors where each pixel or group of pixels incorporates its own phase meter electronics. This in practice poses a maximum value on the spatial resolution of the scenes to the available capabilities of lithographic patterning, being most of them limited to some tenths of thousands of points per image. It is a limit comparable to that of the flash array detectors just mentioned, also due to manufacturing limitations. However, TOF cameras perform usually in the NIR region, so their detectors are based on silicon, while flash imagers are more often working at 1.55 \(\upmu\)m for eye-safety considerations.