Sensor Technology: 2D vs. 3D

As the pioneers of multi-sensor inspection (MSI) for advanced pipeline assessment, RedZone Robotics has a long-storied and rich history with robotics and sensor technology. Two of the most misunderstood concepts are 1) differentiation between LASER and LiDAR technologies and 2) collection and interpretation of 2D versus 3D data. These are two fundamentally different questions, yet few take the time to explain why they are different and how these technologies actually work. Today’s post will explore sensor technology options and discuss the best use case and methodology associated with each type of technology.

What is LASER?

Light Amplification by Simulated Emission of Radiation (LASER) technologies have been around since the 1960’s. A LASER is a device capable of converting light or electrical energy into a focused, high energy beam. Laser light is monochromatic (single light frequency – i.e., one color) and although the beam can travel long distances, it diverges and becomes less accurate. LASER is a device or technology, not a methodology.

With regards to advanced pipeline assessment, the term “laser” is often used synonymously with “ring laser”, “laser profiling” or “structured light” inspections. In essence, a laser light (beam, ring, etc.) is projected onto the interior wall of a pipe surface and a separate camera is used to record the image of the light ring.

What is LiDAR?

Having been in use for more than 30 years, LiDAR is a methodology that employs the use of lasers. Light Detection and Ranging (LiDAR) is a remote sensing methodology that measures distances to objects by illuminating the target with lasers, then analyzing the reflected light. As it pertains to MSI, LiDAR is functionally similar to sonar techniques in that the time of flight (TOF) or propagation time of the echo is the distance between the sensor and the target. LIDAR utilizes LASER technology, but vice versa is not true. Besides TOF, LiDAR sensors can be placed in Doppler (laser phase shifts) or Geiger (laser energy) modes to estimate ranges (distances).

You can read more about LiDAR here.

Which technology/ methodology is more accurate?

Accuracy depends on the size of the pipe diameter being measured. Ring lasers have stated inaccuracies that are a percentage of the distance being measured (e.g., +/- 0.5%), which is most often a function of the resolution of the camera capturing the laser image. LiDAR sensors have internal processors with stated inaccuracies that are fixed across the useful distance of the sensor (e.g., +/- 30mm). In essence, ring lasers are more accurate in small to medium sized pipes and LiDAR is more accurate in larger pipes.

2D vs. 3D Laser Inspection – What’s the Difference?

As the name implies, 2D sensors use a single plane of lasers to capture X and Y dimensions. This could be accomplished with a continuous ring of projected light or a single spinning laser beam. Either way, ring lasers and 2D LiDAR sensors collect the same type of X and Y dimensional data. The movement of the sensors down a pipe facilitate collection of successive slices of 2D data that are often presented in 3D formats. This can be misleading and should not be confused with true 3D LiDAR. 2D sensors are most suitable for performing detection and ranging tasks.

3D LiDAR sensors function like their 2D counterparts, but additional measurements are taken along the Z axis to collect real 3D data. The third axis data collection is most often accomplished with multiple lasers at varying angles or lines of vertical projection. Modern laser projection and wide-area scanning technologies allow high accuracy, high resolution 3D data to be collected without blind spots. However, this type of data collection comes with a cost: money and time. 3D sensors are significantly more expensive than their 2D counterparts. Also, traditional 3D LiDAR scanners need to be steady and stable (not moving) during data acquisition and data processing is inherently more complicated. Specifically, as it pertains to advanced pipeline assessment, 3D LiDAR will be more costly than 2D sensors due to the time required to collect and process the data. 3D LiDAR is most suitable for mapping and detailed analysis, such as bend radius, to be used for engineered designs.

What’s on the horizon?

Over the past decade, there have been significant advances in 3D sensor technologies and methodologies. From optical technologies that facilitate the integration of photogrammetry and augmented reality to new solid-state and flash LiDAR’s, modern technology is enabling faster scanning, higher resolution (accuracy), smaller sized sensors and cheaper prices. We see this everyday on our mobile phones or with Google maps. However, once these terrestrial sensors are placed in a corrosive, GPS-denied environment, such as a sewer, the time and algorithms required to produce meaningful information is inherently longer and more expensive. The simultaneous localization and mapping (SLAM) of our deteriorated buried infrastructure is a computational problem that has yet to be solved on scale.

Over the coming years, I would expect inspection sensors and technologies will become more standardized, easier to deploy, more accurate and more easily able to produce the same types of deliverables that we see from our above-ground counterparts.