The human brain is usually quite good at estimating the relative depth or distance and size of objects. This is an essential skill, especially when driving a vehicle. However, imaging systems struggle with this because standard image sensors display a 3D scene with a 2D image. The use of two image sensors in a stereoscopic arrangement, which is similar to the human eye, makes it possible to derive depth data - albeit with limitations in terms of distance accuracy and dependence on ambient brightness.

The situation is different when using lidar technology: Lidar stands for 'Light Detection and Ranging' and is a method for measuring distance. It is based on a laser and measuring the time it takes for the laser beam to be reflected back from an object. Different wavelengths can be used depending on the application, but infrared (IR) is most commonly used. Depth data provided by lidar enables measurement regardless of lighting conditions and eliminates ambiguities in an image. This allows objects within a scene to be distinguished and interpreted. By combining a light pulse that is directed at an object and reflected by it and a precise time measurement, the distance of the object can be calculated.

Autonomous perception

Many applications in the automotive sector lend themselves to lidar, especially in semi-autonomous vehicles operating at SAE levels L3 to L5: from detecting objects in the vehicle's surroundings to better recognition over hundreds of meters on the highway. The technology is also frequently used in delivery robots and other applications that require autonomous perception.

In outdoor applications, lidar technology can be used to create 3D depth maps quickly and with high accuracy - a process that would take days using conventional surveying techniques. In agriculture, for example, lidar is used to survey fields or areas in order to create maps and assess the condition of crops. In this way, farmers can model and predict crop yields and select the appropriate pesticides or fertilizers. The fill level of grain in silos and liquids in tanks can also be measured immediately with Lidar. The system simply needs to be attached to the top of the silo or tank without the need for contact with the contents.

Environmental organizations use lidar to assess deforestation, measure coastal erosion or monitor glacier melt. In addition, mounting lidar on unmanned aerial vehicles (UAVs) and drones enables surveying in remote, inaccessible areas.

And last but not least, smart factory operators use lidar for automated guided vehicles (AGVs) that transport raw materials for processing and finished goods to the shipping area. Lidar helps with precision tasks, for example, and alerts robots to the presence of humans to ensure work safety.

Different types of lidar and their application

1. The principle behind the most common type of lidar, dToF (direct Time-of-Flight), is simple: the time it takes for a light pulse to travel to a target and back to the sensor is measured. As the speed of light is a known physical quantity, the distance between the transmitter/detector and the reflecting target can be easily calculated. A single short pulse is emitted from a light source (usually a laser) and a precise timer is activated at the same time. If the light pulse hits an object that is within range, it is reflected back to a highly sensitive light sensor located next to the laser. As soon as the return pulse is detected, the timer is stopped and the time taken to travel to the object and back is recorded.
   The dToF approach is fast and can measure multiple echoes, allowing multiple objects to be detected in the lidar's field of view. The technology can be used at close range and at long range (0.1 to 300 m).
2. Another lidar approach, iToF (in-direct Time-of-Flight), uses a continuous light wave, again from a laser. Here, the elapsed time/ToF is not measured directly, but is determined from the phase difference between the transmitted and received waveform.
   iToF is more suitable for applications with a relatively short range (<10 m), especially indoors where the lighting conditions are less demanding than outdoors where the contrast is often much greater. iToF is limited to the detection of individual objects as only the strongest echo can be detected.
3. The third type of lidar, FMCW (Frequency-Modulated Continuous Wave), is used for short and long range applications. This technique uses a tunable laser to generate a continuous wave of light that is mixed with the reflected light at the detector. This mixing creates a beat frequency between the local and reflected waveforms, which can be used to calculate the object's distance and directional velocity.
   While FMCW is capable of providing excellent distance measurement and also directional velocity information, the system costs for such a lidar system are quite high due to the tunable laser with polarization control and the short-wave infrared wavelengths required - as 'exotic' semiconductor components are needed for the laser and detector.

The debate about wavelength

One of the most controversial issues surrounding lidar is the question of wavelength. IR is preferred to visible light as it causes much less background noise and the resulting signal-to-noise ratio (SNR) is better, making it easier to detect the reflected light.