Who would have thought that pulses – an important concept in Physics would become the backbone of LiDAR technology? LiDAR has become one of the more popular remote sensing methods available but unlike its sister technologies i.e., radar which uses radio waves, LiDAR uses light – light pulses to be exact!
LiDAR systems can be used during the day or night since it provides its own energy conditions and can generate exceptionally accurate and precise data! But what does light waves have to do with this?
MORE ON PULSES…
LiDARs emit light in the form of short pulses, the pulse is then reflected from the ground or object it hits then bounces back to the LiDAR system. Clearly, it’s all about reflection! Now that we have established that LiDAR systems work with light pulses and how these light pulses reflect off surfaces and objects in its direct path, we also must keep in mind that these reflective surfaces can be above, below or at ground level depending on how the LiDAR system is being used.
When looking at LiDAR and how it works, it’s important to remember that the LiDAR system and light pulse do not have a 1-2-1 relationship – this means a single light pulse emitted from a LiDAR system can travel back to the system as one or many return pulses. Why many returns? In between the light pulse leaving the LiDAR system and hitting the ground – it could come across several different reflective surfaces in its direct path, because of these multiple reflective surfaces, the pulse is then split into many returns.
HOW DO LIDARS USE MULTIPLE PULSE RETURNS?
Let’s look at the Forestry industry as an example – LiDAR equipped drones/UAVs are used to survey dense forests and vegetation; this is done by sending out pulses.
How does this work? As the UAV flies over the tree canopy, it will send out pulses to the ground. These pulses can travel through things, i.e., the gaps between tree branches and leaves which could hit the ground directly or could also reflect off other parts of the forest before it hits the ground, i.e. trees, branches, foliage, and other objects.
What makes LiDAR so special? The light energy that passes through the canopy reflects off the branches and leaves within the canopy, this makes a LiDAR special as it can travel through and record information from either the upper, middle, lower tree canopy or even below the ground! With each object the pulse hits – a specific amount of the light is reflected, detected and recorded by the LiDAR system.
LiDAR systems can collect multiple returns from the same light pulse. Multiple returns from the canopy can provide information such as:
- Shape of the trees.
- Height of the trees.
- Density of the trees,
- Determines if there are shrubs on the forest floor below the trees.
This also means that when using a LiDAR sensor to get ground points, you need to be conscious of objects in the immediate range of the LiDAR sensor. As LiDAR does not “x-ray” through trees but “look through” the gaps in-between the leaves by emitting pulses which send multiple returns – thus gathering a series of data points – including a point of last return which may or may not represent the ground.
Return pulses in a nutshell:
- First return: The first pulse is considered to be the most significant return. This return correlates with the highest feature in the area, such as a rooftop of a building or a treetop. This pulse can also represent the ground, which means only one return will be detected by the LiDAR sensor.
- Multiple returns: Multiple returns are recorded when one laser pulse intercepts several surfaces, i.e., the top of a building, its windows and the ground or when it comes to trees – the top of a tree, branches and the ground.
- Last return: The last return does not always guarantee the position of the ground relative to a UAV . i.e., a last return point can simply be the furthest point to which a pulse is sent and reflected. It could be a pulse hitting a large rock on the ground and thus gives the measurement of this rock’s location relative to the UAV. But it does not tell the UAV how much further the ground is beneath the actual rock.
You and LightWare can be great partners!
Are you looking for accurate and precise microLiDAR solutions for your business? LightWare can help you find that perfect solution! We offer two types of LiDAR solutions; standard solutions that can easily add to your current business products or customized solutions. We understand that our customers have applications with specific needs and are happy to partner with you to provide and develop customised microLiDAR solutions.
At LightWare, we take great pride in the fact that all our microLiDAR devices are designed to a Class 1M laser safety classification. This means that our LiDAR sensors are safe for use under all conditions except when passed through magnifying optics. Our testing and compliance is in accordance with the international standard IEC 60825-1:2014 “Safety of laser products”. Furthermore, our sensors are safe for UGV and UAV positioning and provide accurate measurements over any texture, surface, or terrain!
Let LightWare help you find a microLiDAR solution best suited for your needs. Why not contact us today by email?