What is the difference between a low cost, hobby grade LiDAR and a much more expensive survey grade LiDAR? The short answer is that there are lots of differences. So the challenge becomes figuring out how to use the latest technology to achieve survey grade performance in a small, low cost package.
Ideally, a LiDAR should have perfect performance so that exact distances and angles can be measured instantaneously. This would result in precise 3D point clouds that can be used in conjunction with photogrammetry or directly for inspection and mapping. In reality, there are limitations to the performance of any product and in the case of LiDAR these limitations include:
- The size of the laser footprint on the ground
- The maximum measuring range achievable
- The absolute distance measurement accuracy
- The update rate of new measurements
- The precision of the aiming direction
- Knowledge of the position of the LiDAR itself
Each of these performance parameters can be very difficult to improve on and this is one of the reasons why high performance LiDARs cost several orders of magnitude more than those that might appear to do the same job.
Up to now, we (LightWare dev team) have been concentrating our LiDAR development program on making high quality, low cost LiDAR systems that are lightweight enough to use on small drones. Our emphasis has been on measuring the height above ground and locating obstacles in front of the drone since these applications have more relaxed specifications when compared to precision mapping. However, we have a longer term program to produce very precise mapping LiDAR at low cost and I would like to share with you our first steps in this direction.
The LW21 prototype pictured above is a variant of the ultra-small LW20 LiDAR that was released earlier this year. The LW20 configuration was developed with high performance applications in mind which is why it includes professional grade features such as an IP67 rated housing made from aluminum, first and last signal detection, long range and so on. In the LW21 we have added new optics and modified the laser beam in order to reduce the size of the ground footprint.
A small ground footprint is critical to obtaining precise measurements. You can think of this as the "pixel" size of the LiDAR and making the pixels smaller means that you know with greater certainty which point on the target surface is providing the return signals. This improves to the angular precision of the measurements.
The difficulties in making a small footprint relate to the beam divergence of the laser. The pulsed lasers used in time-of-flight LiDAR have a large chip size resulting in a much bigger emitting area than you would find in a regular, visible laser pointer. This larger chip makes it difficult to design a simple and small system of lenses that produce good collimation (a parallel beam). Instead, light from the edges of the laser chip travel in a slightly different direction from light at the center, making the edges of the laser beam diverge.
Replacing the laser with a smaller chip means that the transmission power will be reduced and therefore the signal strength and measuring range decrease. However, in the LW21 we have managed to introduce a smaller laser chip but still maintain the range. This has reduced the area of the ground footprint by a factor of 4.
The next limitation relates to the small physical size of the lenses. Small diameter lenses with short focal lengths have poorer optical characteristics than larger lenses, introducing distortions and a wide field of view. They also have a smaller collecting area so that return signals appear to be weaker. So this is why survey instruments that use lasers are quite large.
There was no way to economically overcome these fundamental, optical limits so we increased the focal length of the lenses thereby reducing spherical aberration and reducing the beam divergence by another factor of two on each optical axis.
The net result of these modifications is that the LW21 prototype gives a beam divergence of around 1.7mRad producing a spot size just 12cm across at a range of 50m. By comparison, the Velodyne Puck has a beam divergence of 3.0mRad producing a spot with almost four times the area of the LW21.
So what's next? We are currently working on a high precision aiming mechanism that will provide very accurate information about the direction that the LW21 is aiming. I'll keep you posted ;).