Lidar Technology Overview - The role of diffuse targets in testing and calibrating Advanced Driver Assistance Systems (ADAS)

Lidar is a mature technology that is increasingly deployed in consumer products and driverless vehicles. LIDAR is an acronym for Light Detection And Ranging. Lidar systems have been in use for more than 50 years, but until recently, the cost of such systems prevented them from being widely used in the mass market.

While radar is widely used in autonomous vehicle technology, such as adaptive cruise control, LIDAR is considered the sensor of choice for driver-assisted vehicles because it can accurately map location and distance for detecting small objects and 3D imaging. It uses pulsed lasers with time-of-flight sensing and solid-state light to measure distance. Characterization of liDAR systems requires compensating the sensor's response to pulsed laser or solid-state light levels over a wide reflectivity dynamic range. For this purpose, a large area reflectance diffuse reflectance target plate with known and stable reflectance is required. Labsphere's Permaflect diffused coated target plates, ranging from 5% to 94% reflectivity, enable automotive manufacturers Oems and their suppliers to characterize and calibrate their LIDAR systems under a wide range of environmental conditions.

FIG. 1 The Permaflect coating of Labsphere (Blue Field Optics) on the target plate

Lidar technology

The most basic form of lidar is the laser rangefinder, which has been widely used in military applications since the 1980s. The laser rangefinder consists of a pulsed laser (transmitter) and a photodetector (receiver). The rangefinder is designed to accurately measure distance (so-called "ranging"), primarily measuring the time it takes for the laser pulse to be reflected and received by the detector (this is known as a "time of flight" measurement).

The rangefinder aims at the target and fires a laser pulse. The laser hits the target, is scattered, and a portion of the reflected light is measured by the detector. Because the speed of light is very precise, the distance between the rangefinder and the target can be measured very precisely. More advanced liDAR systems use the same principle, but use optics and moving or multiple detectors to map targets in two dimensions. These systems typically pulse thousands of times per second and can detect thousands of points per second. Analyzing the data from this point cloud can create an accurate map of the target region. Lidar works similarly to radar and sonar, which use radio waves and sound waves respectively. Data from radar and sonar can be used to map the surrounding environment in a similar way, but liDAR systems use shorter-wavelength infrared radiation rather than shorter-wavelength radio waves. Due to the shorter wavelengths used, LiDAR measurements are more accurate than radar.

Lidar systems deployed in self-driving cars typically use scanning laser beams and flash technology to measure 3D points in space relative to sensors. These liDAR systems typically fire thousands of laser pulses per second so that vehicles can react to obstacles such as pedestrians and other vehicles. Lidar allows autonomous vehicles to transmit and receive reflected light from objects and the surrounding environment with high precision, high resolution and long detection distances. More advanced AI (artificial intelligence) systems are being developed to predict vehicle and pedestrian paths and react accordingly. When you combine LIDAR data with location information (using GPS or similar information), you can fully map the vehicle's surroundings.

The performance of liDAR is largely dependent on the laser power and wavelength used. For safety reasons, there is an upper limit to the laser power that can be used. In the absence of higher laser power, you can use a detector with higher sensitivity, or use a laser whose wavelength extends further into the infrared (IR). Due to the technology maturity of existing lasers, wavelengths of 850nm, 905nm or 1550nm are commonly used. The 1550nm laser is safer than other options because infrared radiation beyond 1400nm no longer passes through the cornea of the eye, so it is not focused on the retina, but because water absorbs 1550nm light more strongly, 1550nm requires more power to compensate.

Lidar in consumer electronics and self-driving cars

Lidar is a key skill used in automation along with camera systems and other sensors. Lidar systems have been used commercially for many years in professional mapping and related applications. However, it is only in recent years that lidar has become more common, largely due to the need for smaller and cheaper devices for autonomous vehicle applications (driverless cars). Lidar has been used in semi-autonomous vehicles since the early 1990s as a basis for adaptive cruise control, and Lidar was first used in autonomous vehicles in 2005.

In the consumer electronics space, the latest generation Apple iPad Pro (and now the iPhone 12 Pro) has integrated LIDAR sensors into its camera array specifically for imaging and augmented reality (AR) applications. The LIDAR sensor allows the iPad to correctly parse the position of the real object relative to the AR object imaged by the camera array. AR is still in its infancy, so the adoption of LIDAR on smartphones and other consumer devices remains to be seen, but there is a great deal of interest in AR developed for professional applications where LIDAR can be a very useful enhancement. Professional AR has a variety of applications, from helping warehouse workers find the fastest and safest path to needed parts, to helping engineers understand the process of complex repairs. Lidar in these applications enables precise positioning and alignment, which is important for any application that requires high precision.

The role of diffuse reflection target plate in testing and calibration of LiDAR system

For many years, Pro-Lite and Labsphere (Blue Field Optics) have been supporting the development of LIDAR systems using diffuse reflectors for many years. Labsphere More compact Spectralon? Diffuse reflectance target panels are commonly used by the military to test laser rangefinders. Precisely calibrated spectral reflectance combined with near-Lambert (diffuse) reflectance means that for these applications, you have an accurate, repeatable diffuse target plate to test your system in the lab or in the field.

Lidar systems for larger scale mapping or autonomous vehicle applications require larger target areas. Since most natural objects diffuse light, Labsphere's diffuse reflective materials are a natural choice for users, providing quality assurance, field testing and comparison. Labsphere has developed Permaflect target plates to meet the need for large-area, durable and optically stable target plate materials. The large diffuse reflectance target plate size (up to 1.2m x 2.4m standard size), combined with calibrated spectral reflectance data, allows accurate measurement of the LIDAR range. At long test distances such as 100m, 200m, 300m, larger target boards are needed to reflect representative points on the target.

Permaflect is a spray-on diffuse reflective coating that can be applied to large areas or 3D shapes so that real-world objects can be simulated. Few objects in the real world are as flat as the target panel, so Permaflect-coated objects can achieve repeatable near-Lambertian reflectivity levels that can be applied to mannequins to simulate pedestrians, for example.

FIG. 2 Human model sprayed by Labsphere (Lanphy Optical) Permaflect

LIDAR diffuse reflection target panels are typically deployed outdoors, so some drift in calibrated reflectivity values can be expected over time when the surface of the diffuse reflection target panel is exposed to the atmosphere. Labsphere's diffuse reflective materials are easy to clean. To see if there is a decrease in reflectance, a calibrated reflectance meter (" reflectance meter ") can be used, which measures the reflectance of the diffuse target plate in situ and takes into account any changes in the infrared reflectance. The variation of the reflectance of the diffuse reflectance target plate will directly affect the measurement range. The figure below shows the effect of reflectance changes on the measurement range in the reflectance horizontal range of different diffuse reflectance target plates. Small changes in reflectance will have a large effect on the measurement range of the lower reflectance target plate. For example, if the reflectivity of the target plate is reduced from 5% to 4%, the original measurement range of 300 m will be reduced to 30 m. The way to see what's happening in real time is to measure the reflectivity of the target board and then modify your calculations based on this adjustment.

FIG. 3 Simulation sensitivity for distance measurement of object reflectance at 300nm wavelength

FIG. 4 Labsphere developed the Permaflect diffuse reflector target plate to meet the need for large-area, durable and optically stable diffuse reflector target plate materials.

Source: Lanfei Optics