LiDAR is a light-based simulation of sonar or radar, often described as “laser pointer detection and ranging” (Light Detection And Ranging) or “laser imaging, detection and ranging” (Light Imaging, Detection, And Ranging) . It is a distance measurement method based on the time of flight of light. A light source (usually a laser) scans objects in the field of view. Light is reflected directly back from the surface of the object and is detected by the sensor. Since the speed of the light is constant, the round-trip time can be used to determine the distance to the object, thereby establishing a 3D map of the field of view. Time of flight can be calculated directly, or pulsed or modulated light sources can be used for range-gated imaging, and the phase difference can be calculated through optical heterodyne detection.
In any application where a laser pointer is scattered at a certain distance, sensitivity is the most important. The higher the sensitivity, the better the accuracy and ability to detect low reflectivity objects. At the same time, it also helps to use weaker light sources, increasing safety and reducing power consumption. However, many LiDARs work in outdoor environments, so in the operating band, durable windows and optical systems with high transmission capabilities are necessary.
Varying optical coatings are extremely important for lidar. Maintaining low reflectance over a wide range of incident angles is the key to LiDAR. The figure shows the change in reflectivity of an anti-emissive layer with a center value of 905 nanometers formed on a polycarbonate substrate as a function of incident angle.
Depending on the design, narrow V-coats may be needed to help exclude ambient light from broadband AR coatings, allowing multiple wavelengths to be used. Laser pointers have a wavelength distribution from visible to near-infrared light that can match the specific needs of different applications and environments (from marine mapping with a wavelength of 532 nanometers to the eye-safe 1550 nanometer range).