LiDAR, or light detection and ranging, is a technology that uses laser light pulses to collect information from surfaces. LiDAR systems throw out thousands of laser pulses per second, and each one reflects back to the system sensor after hitting a surface — this could be a road surface, building, car or an object like a tree or street sign.
LiDAR sensors measure the time taken for laser pulses to complete that journey, and can therefore calculate the distance each pulse travels. Using that information, LiDAR systems calculate millions of “points” (xyz coordinates) that together form a point cloud. When processed with CAD software and point cloud processing software, point clouds are brought to life in the form of a 3D digital model and valuable information can be extracted from the data.
But for LiDAR systems to accurately and safely collect valuable point cloud data, the correct LiDAR wavelengths need to be used. In this article, we’ll explore the different LiDAR wavelengths and outline when certain wavelengths should be used and why.
A Bit About Hardware: What Type of Lasers Do LiDAR Systems Use?
Before we compare LiDAR wavelengths, it’s important to understand the type of laser light used by LiDAR systems: semiconductor diode lasers. This type of laser works similarly to LED light in that light is emitted when an electric current flows to a p-n junction — an interface sandwiched between p-type and n-type layers of semiconductor material. Electrons then pass through the n-type layer (a layer with an abundance of electrons) and drop into a “hole” in the p-type layer (p-type layers are rich in “holes” because they lack electrons). Excess energy from electrons is then converted into photons — light waves.
Typically, semiconductor diode lasers can produce a wide range of different wavelengths. This is because diode lasers have more applications than just LiDAR systems. They can be found in supermarket barcode scanners, laser printers and they’re also used in telecommunications — just to name a few uses of semiconductor diode lasers.
What Wavelengths Do LiDAR Systems Use?
Most LiDAR systems use near-infrared wavelengths of either 905 or 1550 nanometers, although other wavelengths are occasionally used. Some LiDAR systems can also use multiple wavelengths.
The Safety of LiDAR Wavelengths
It used to be that 905nm was the standard for LiDAR systems, as they are more affordable than 1550nm lasers, and can be more accurate. However, wavelengths of 905nm can be dangerous to people’s eyes as the eye transmits this visible light to the retina.
Wavelengths of 1550nm are often considered safer, as this invisible light doesn’t reach the retina. With longer wavelengths like 1550nm, it’s also possible to safely use a higher output power. But it’s worth noting that 1550nm can still cause damage to the eye lens.
So whichever wavelength is used, precautions need to be taken and LiDAR systems need to be specifically designed to meet eye-safety standards.
Cost Efficiency and Accuracy
The trouble with 1550nm is that these longer wavelengths can suffer more scattering from atmospheric moisture. 1550nm has a high water absorption rate, so adverse weather conditions can cause issues with accuracy and data resolution. 905nm wavelengths absorb less water, and this can result in more accurate scans.
To compensate for water absorption and its impact on accuracy, 1550nm systems often send out more laser pulses. However, this means systems that use 1550nm use more power and often need to be larger in size, and as a result, they’re more expensive. 1550nm systems are also more complex and costly to create compared to 905nm systems, which use widely available, lower-cost technology.
Another factor to consider when comparing these two wavelengths is that longer wavelengths have shorter frequency and lower energy, so they may not be able to travel as far or gather information as quickly as shorter wavelengths. So airborne LiDAR mapping systems will usually use shorter wavelengths, such as 905nm.