For autonomous vehicles to drive safely in complex highway or urban traffic conditions, the forward field of view is critical. When a vehicle travels at 120 km/h, emergency braking and system response require a depth perception range of over 100 meters. As the core sensor for autonomous driving, the detection range of LiDAR directly determines the safety redundancy of the system. So, what factors affect the detection range of LiDAR?
01. How Can the Transmitter Break Through the Limitations of Light Energy?
LiDAR measures distance using the Time of Flight (ToF) method. The system emits extremely short laser pulses, records the time difference for the beam to hit the target and return, and then calculates the distance based on the speed of light. From an energy perspective, the higher the peak power of the emitted laser pulse, the more photons will travel to distant objects and successfully return.
However, increasing the transmission power faces strict physical and regulatory limitations, the most critical of which is the eye safety standard. Currently, mainstream ToF LiDARs primarily use two wavelengths: 905 nm and 1550 nm. 905 nm lasers utilize Edge-Emitting Lasers (EEL) or Vertical-Cavity Surface-Emitting Lasers (VCSEL), featuring mature technology, lower costs, and the ability to directly use inexpensive silicon-based photodetectors. However, light at this wavelength can penetrate the human eye's lens and focus on the retina.
To avoid causing permanent burns to the human eye, the International Electrotechnical Commission (IEC) has set an upper limit on the transmission power of 905 nm lasers, which directly restricts its maximum detection range. In pursuit of a longer detection range, high-specification LiDARs have begun to shift to the 1550 nm wavelength. Infrared light at this wavelength is absorbed by the lens and vitreous body of the eye before reaching the retina, making it impossible to focus on the retina.
Therefore, 1550 nm LiDARs can allow the use of pulse power tens or even hundreds of times higher than that of 905 nm systems. Coupled with expensive fiber lasers, the beam can be easily sent to greater distances, achieving ultra-long-range perception of over 200 meters.
02. How Do Target Objects Affect Light Recovery?
After laser pulses travel through the air and hit an obstacle, they scatter in all directions. The number of photons that ultimately return along the original path to the radar's receiver depends on the physical surface characteristics of the target object. In radar technology, we use reflectivity to quantify this characteristic. Objects with different materials, colors, and roughness levels vary drastically in their ability to absorb and reflect laser light. Highly reflective targets, such as aluminum traffic signs or white car bodies, can reflect the vast majority of the laser energy back.
Under such ideal conditions, the detection range of LiDAR can reach extremely high levels. However, on actual roads, low-reflectivity objects are the norm. For instance, a black sedan painted entirely with dark, light-absorbing car paint, or a pedestrian wearing dark clothing, may have a surface reflectivity of only 10% or even lower. Most of the laser energy is absorbed the moment it hits the object, resulting in very few returning photons.
For this reason, when evaluating LiDAR performance, looking solely at an absolute distance figure is meaningless; its performance metrics must be tied to reflectivity. Typically, the industry uses the distance at which the system can clearly detect objects at a 10% reflectivity as a hard standard for measuring core detection capabilities. Only LiDARs that can maintain a detection range of around 200 meters under low-reflectivity conditions can ensure that autonomous vehicles promptly detect dangerous dark obstacles at night.
03. How Does the Receiver Capture Weak Echoes?
When the attenuated, weak photon flow returns to the radar, the sensitivity of the receiver sensor becomes another critical factor determining the detection range. The receiver chip must not only detect photons from chaotic signals but also have the capability to amplify weak signals to a level sufficient for digital processing. Early LiDARs primarily relied on Avalanche Photodiodes (APD), which require hundreds or thousands of photons to strike simultaneously to generate an electrical signal of sufficient strength.
This means that if the distance is too far and the returning echo photons are too sparse, the system will miss detections. To break through this bottleneck, new LiDARs have introduced photon detection technology, adopting Single-Photon Avalanche Diodes (SPAD) or Silicon Photomultipliers (SiPM). These sensors possess extremely high gain capabilities; even if triggered by a single photon, they can generate a distinct electrical signal, enabling the radar to identify weak echoes returned from extremely long distances.
Beyond the receiver chip itself, the optical design of the optoelectronic system also plays a vital role. The larger the effective aperture of the receiving lens, the larger the funnel for collecting photons in space, meaning a greater total number of photons can be captured. Meanwhile, the industry is developing Frequency-Modulated Continuous Wave (FMCW) technology. Unlike traditional pulse technology, this approach allows the transmitted and received light to undergo coherent constructive interference within the chip.
Through this amplification effect of the local optical signal, FMCW radar can achieve ultra-long detection ranges even at extremely low transmission power, while also possessing strong anti-interference capabilities.
04. How Does Environmental Noise Compress the Detection Limit?
During actual vehicle operation, LiDAR does not work in a clean, pure physical environment; it must combat ubiquitous environmental background noise, with sunlight being the largest source of interference. The solar spectrum contains a massive amount of infrared bands. Under the scorching sun in midsummer, dense solar photons continuously flood into the radar's receiving lens. If these useless solar photons trigger the receiving sensor, they will form background noise.
When the effective photons reflected from distant obstacles are submerged in the surging sea of solar photons, the optoelectronic signal-to-noise ratio at the receiving end drops sharply. If the signal cannot be separated from the noise, the radar cannot confirm the target's distance. To solve this problem, radars are equipped with ultra-narrow bandpass filters internally to precisely filter out the vast majority of sunlight, allowing only lasers of specific wavelengths to pass through, thereby maintaining the original detection range as much as possible in strong light environments.
The distribution of media in the air is another environmental variable that cannot be ignored. In clear weather, the absorption and scattering effects of air on laser light are minimal. However, in heavy fog, rainstorms, dense smoke, or sandstorms, the air is filled with high-density water droplets or particles. During propagation, the laser continuously undergoes Mie scattering with these particles, causing the light energy to be rapidly weakened and scattered. This topic was also specifically discussed previously by Autonomous Driving Frontier: Why does autonomous driving LiDAR perform poorly in rain and fog? This atmospheric attenuation causes the laser energy to decrease exponentially, meaning a radar originally designed to detect 200 meters might only be able to see tens of meters in dense fog. This is also a common physical limitation currently faced by the entire autonomous driving industry in severe weather conditions.