Rethinking the Four “Rs” of LiDAR: Rate, Resolution, Returns and Range
Extending Conventional LiDAR Metrics to Better Evaluate Advanced Sensor Systems
By Blair LaCorte, Luis Dussan, Allan Steinhardt, and Barry Behnken
As the autonomous vehicle market matures, sensor and perception engineers have become increasingly sophisticated in how they evaluate system efficiency, reliability, and performance. Many industry leaders have recognized that conventional metrics for LiDAR data collection (such as frame rate, full frame resolution, points per second, and detection range) no longer adequately measure the effectiveness of sensors to solve real-world use cases that underlie autonomous driving.
First generation LiDAR sensors passively search a scene and detect objects using background patterns that are fixed in both time (no ability to enhance with a faster revisit) and in space (no ability to apply extra resolution to high interest areas like the road surface or pedestrians). A new class of solid-state, high-performance, active LiDAR sensors enable intelligent information capture that expands their capabilities — moving from “passive search” or detection of objects, to “active search,” and in many cases, to the actual acquisition of classification attributes of objects in real time.
Because early generation LiDARs use passive fixed raster scans, the industry adopted very simplistic performance metrics that don’t capture all the nuances of the sensor requirements needed to enable AVs. In response, AEye is proposing the consideration of four new corresponding metrics for extending LiDAR evaluation. Specifically: extending the metric of frame rate to include object revisit rate; extending the metric of resolution to capture instantaneous resolution; extending points per second to signify the overall more useful quality returns per second; and extending detection range to reflect the more critically important object classification range.
We are proposing that these new metrics be used in conjunction with existing measurements of basic camera, radar, and passive LiDAR performance. These extended metrics measure a sensor’s ability to intelligently enhance perception and create a more complete evaluation of a sensor system’s efficacy in improving the safety and performance of autonomous vehicles in real-world scenarios.
Our industry has leveraged proven frameworks from advanced robotic vision research and applied them to LiDAR-specific product architectures. One framework, “Search, Acquire [or classify], and Act,” has proven to be both versatile and instructive relative to object identification.
- Search is the ability to detect any and all objects without the risk of missing anything.
- Acquire is defined as the ability to take a search detection and enhance the understanding of an object’s attributes to accelerate classification and determine possible intent (this could be done by classifying object type or by calculating velocity).
- Act defines an appropriate sensor response as trained, or as recommended, by the vehicle’s perception system or domain controller. Responses can largely fall into four categories:
- Continue scan for new objects with no enhanced information required;
- Continue scan and interrogate the object further, gathering more information on an acquired object’s attributes to enable classification;
- Continue scan and track an object classified as non-threatening;
- Continue scan and instruct the control system to take evasive action.
Within this framework, performance specifications and system effectiveness need to be assessed with an “eye” firmly on the ultimate objective: completely safe operation of the vehicle. However, as most LiDAR systems today are passive, they are only capable of basic search. Therefore, conventional metrics used for evaluating these systems’ performance relate to basic object detection capabilities – frame rate, resolution, points per second, and detection range. If safety is the ultimate goal, then search needs to be more intelligent, and acquisition (and classification) done more quickly and accurately so that the sensor or the vehicle can determine how to act immediately.
Rethinking the Metrics
Makers of automotive LiDAR systems are frequently asked about their frame rate, and whether or not their technology has the ability to detect objects with 10% reflectivity at some range (often 230 meters). We believe these benchmarks are required, but insufficient as they don’t capture critical details, such as the size of the target, the speed at which it needs to be detected and recognized, or the cost of collecting that information.
We believe it would be productive for the industry to adopt a more holistic approach when it comes to assessing LiDAR systems for automotive use. We argue that we must look at metrics as they relate to a perception system in general, rather than as an individual point sensor, and ask ourselves: “What information would enable a perception system to make better, faster decisions?” In this white paper, we outline the four conventional LiDAR metrics with recommendations on how to extend them.
Conventional Metric #1: Frame Rate of 10Hz – 20Hz
Extended Metric: Object Revisit Rate
The time between two shots at the same point or set of points
Defining single point detection range alone is insufficient because a single interrogation point (shot) rarely delivers sufficient confidence – it is only suggestive. Therefore, passive LiDAR systems need either multiple interrogations/detects at the same location or multiple interrogations/detects on the same object to validate an object or scene. In passive LiDAR systems, the time it takes to detect an object is dependent on many variables, such as distance, interrogation pattern, resolution, reflectivity, the shape of the object, and the scan rate.
A key factor missing from the conventional metric is a finer definition of time. Thus, we propose that object revisit rate become a new, more refined metric for automotive LiDAR because a high-performance, active LiDAR, such as AEye’s iDAR™, has the ability to revisit an object within the same frame. The time between the first and second measurement of an object is critical, as shorter object revisit times keep processing times low for advanced algorithms that correlate multiple moving objects in a scene. The best algorithms used to associate/correlate multiple moving objects can be confused when time elapsed between samples is high. This lengthy combined processing time, or latency, is a primary issue for the industry.
The active iDAR platform accelerates revisit rate by allowing for intelligent shot scheduling within a frame. Not only can iDAR interrogate a position or object multiple times within a conventional frame, it can maintain a background search pattern while simultaneously overlaying additional intelligent shots. For example, an iDAR sensor can schedule two repeated shots on an object of interest in quick succession (30μsec). These multiple interrogations can be contextually integrated with the needs of the user (either human or computer) to increase confidence, reduce latency, or extend ranging performance.
These additional interrogations can also be data dependent. For example, an object can be revisited if a low confidence detection occurs, and it is desirable to quickly validate or reject it, enabled with secondary data and measurement, as seen in Figure 1. A typical frame rate for conventional passive sensors is 10Hz. For conventional passive sensors, this is the object revisit rate. With AEye’s active iDAR technology, the object revisit rate is now different from the frame rate, and it can be as low as tens of microseconds between revisits to key points/objects – easily 100x to 1000x faster than conventional passive sensors.
What this means is that a perception engineering team using dynamic object revisit capabilities can create a perception system that is at least an order of magnitude faster than what can be delivered by conventional passive LiDAR without disrupting the background scan patterns. We believe this capability is invaluable for delivering level 4/5 autonomy as the vehicle will need to handle complex edge cases, such as identifying a pedestrian in front of oncoming headlights or a flatbed semi-trailer laterally crossing the path of the vehicle.
Figure 1. Advanced active LiDAR sensors utilize intelligent scan patterns that enable an Object Revisit Interval, such as the random scan pattern of AEye’s iDAR (B). This is compared to the Revisit Interval on a passive, fixed pattern LiDAR (A). For example, in this instance, iDAR is able to get eight detects on a vehicle, while passive, fixed pattern LiDAR can only achieve one.
Within the “Search, Acquire, and Act” framework, an accelerated object revisit rate, therefore, allows for faster acquisition because it can identify and automatically revisit an object, painting a more complete picture of it within the context of the scene. Ultimately, this allows for collection of object classification attributes in the sensor, as well as efficient and effective interrogation and tracking of a potential threat.
Use Case: Head-On Detection
When you’re driving, the world can change dramatically in a tenth of a second. In fact, two cars traveling towards each other at 100 kph are 5.5 meters closer after 0.1 seconds. By having an accelerated revisit rate, we increase the likelihood of hitting the same target with a subsequent shot due to the decreased likelihood that the target has moved significantly in the time between shots. This helps the user solve the “Correspondence Problem,” determining which parts of one “snapshot” of a dynamic scene correspond to which parts of another snapshot of the same scene. It does this while simultaneously enabling the user to quickly build statistical measures of confidence and generate aggregate information that downstream processors might require, such as object velocity and acceleration. The ability to selectively increase revisit rate on objects of interest while lowering the revisit rate in sparse areas, like the sky, can significantly aid higher level inferencing algorithms, allowing perception and path planning systems to more quickly determine optimum autonomous decision making.
Use Case: Lateral Detection
A vehicle entering a scene laterally is the most difficult to track. Even Doppler Radar has a difficult time with this scenario. However, selectively allocating shots to extract velocity and acceleration when detections have occurred as part of the acquisition chain vastly reduces the required number of shots per frame. Adding a second detection, via iDAR, to build a velocity estimate on each object detection increases the overall number of shots by only 1%. Whereas, obtaining velocity everywhere with a fixed scan system doubles the required number of shots. This speed and shot saliency makes autonomous driving much safer because it eliminates ambiguity and allows for more efficient use of processing resources.
The AEye Advantage
Whereas other LiDAR systems are limited by the physics of fixed laser pulse energy, fixed dwell time, and fixed scan patterns, iDAR is a software-configurable system that allows perception and motion planning modules to dynamically customize their data collection strategies to best suit their information processing needs at design time and/or run time.
iDAR’s unique bore-sighted design eliminates parallax between the camera and the LiDAR, bringing it extremely close to solving the “Correspondence Problem.” The achievable object revisit rate of AEye’s iDAR system for points of interest (not merely the exact point just visited) is microseconds to a few milliseconds — which can be up to 3000x faster, compared to conventional LiDAR systems that typically require hundreds of milliseconds between revisits. This gives the unprecedented ability to calculate valuable attributes such as object velocity (both lateral and radial) faster than any other system, allowing the vehicle to act more readily to immediate threats and track them through time and space more accurately.
This ability to define the new metric, object revisit rate, which is decoupled from the traditional “frame rate,” is important also for the next metric we introduce. This second metric helps to distinguish “search” algorithms from “acquisition” algorithms. Separation of these two types of algorithms provides insight into the heart of iDAR, which is the principle of information quality (as opposed to data quantity): “more information, less data.”
Conventional Metric #2: Fixed Resolution Over a Fixed Field-of-View
Extended Metric: Instantaneous Resolution
The degree to which a LiDAR sensor can apply additional resolution
to key areas within a frame
Resolution as a conventional metric assumes that the Field-of-View will be scanned with a constant pattern and with uniform power. This makes perfect sense for less intelligent, passive sensors that have a limited ability to adapt their collection capabilities. Additionally, the conventional metric assumes that salient information within the scene is uniform in space and time, which we know is not true. This is especially apparent for a moving vehicle. However, because of these assumptions, conventional LiDAR systems indiscriminately collect gigabytes of data from a vehicle’s surroundings, sending those inputs to the CPU for decimation and interpretation.
An estimated 75% to 95% of this data is found to be useless or redundant and thrown out. In addition, these systems apply the same level of power everywhere, such that the sky is scanned at the same power as an object directly in the path of the vehicle. It’s an incredibly inefficient process.
As humans, we don’t “take in” everything around us equally. Rather, our visual cortex filters out irrelevant information, such as an airplane flying overhead, while simultaneously (not serially) focusing our eyes on a particular point of interest. Focusing on a point of interest allows other, less important objects to be pushed to the periphery. This is called foveation, where the target of our gaze is allotted a higher concentration of retinal cones, thus allowing it to be seen more vividly.
iDAR uses biomimicry (see the AEye white paper, The Future of Autonomous Vehicles: Think Like a Robot, Perceive Like a Human) to apply and expand upon the capabilities of the human visual cortex for artificial perception. Whereas humans typically only foveate on one area, iDAR can foveate on multiple areas simultaneously (and in multiple ways), while also maintaining a background scan to ensure it never misses new objects. We describe this feature as a Region of Interest (ROI). Furthermore, since humans rely entirely on light from the sun, moon, or artificial lighting, human foveation is “receive only,” i.e., passive. iDAR, by contrast, foveates on both transmit (regions that the laser light chooses to “paint”) and receive (where/when the processing chooses to focus).
An example of this follows.
Figure 2 shows two systems, System A and System B. Both systems have a similar number of shot points on the same scene (left). System A represents a uniform scan pattern, typical of conventional, passive LiDAR sensors. These fixed scan patterns produce a fixed frame rate with no concept of an ROI. System B shows an adjusted, active scan pattern. The shots in System B are gathered more densely within and around the ROI (the small box) within the square. In addition, the background scan continues to search to ensure no new objects are missed, while focusing additional resolution on a fixed area to aid in acquisition. In essence, it is using intelligence to optimize the use of power and shots.
Looking at the graphs (right) associated with Systems A and B, we see that the active scan pattern of System B can revisit an ROI within a much shorter interval than the fixed scan pattern of System A. System B not only can complete one ROI revisit interval, but multiple ROIs within a single frame. Whereas, System A cannot revisit. iDAR does what conventional, passive LiDAR cannot: it enables dynamic perception, allowing the system to focus on, and gather more comprehensive data about, a particular Region of Interest at unprecedented speed.
Figure 2. Region of Interest (ROI) and foveation of iDAR (B) compared to conventional scan patterns (A).
Within the “Search, Acquire, and Act” framework, instantaneous resolution allows the iDAR system to search an entire scene and acquire multiple targets, capturing additional information about them. iDAR also allows for the creation of multiple simultaneous ROIs within a scene, allowing the system to focus and gather more comprehensive data about specific objects, enabling it to interrogate them more completely and track them more effectively.
Use Case: Object Interrogation
When objects of interest have been identified, iDAR can “foveate” its scanning to gather more useful information about them and acquire additional classification attributes. For example, let’s say the system encounters a jaywalking pedestrian directly in the path of the vehicle. Because iDAR enables a dynamic change in both temporal and spatial sampling density within a Region of Interest, what we call instantaneous resolution, the system can focus more of its attention on this jaywalker, and less on irrelevant information, such as parked vehicles along the side of the road. Regions of Interest allow iDAR to quickly, efficiently, and accurately identify critical information about the jaywalker, such as speed and direction. The iDAR system provides the most useful, actionable data to the domain controller to help determine the most timely course of action.
We see instantaneous resolution being utilized in three primary ways to address different use cases:
1. Fixed Region of Interest (ROI): Today, passive systems can only allocate more scan lines at the horizon – a very simple foveation technique limited by their fixed resolution. With second generation intelligent systems, like iDAR, that enable instantaneous resolution, an OEM or Tier 1 will be able to utilize advanced simulation programs to test hundreds (or even thousands) of shot patterns—varying speed, power, and other constraints – to identify an optimal pattern that integrates a fixed ROI with higher instantaneous resolution to achieve their desired results.
For example, a fixed ROI could be used to optimize the shot pattern of a unit behind a windshield with varying rakes. Additionally, a fixed ROI could be used in urban environments, where threats are more likely to come from the side of the road – such as car doors opening, pedestrians, and cross traffic – or in the immediate path of the vehicle. An ROI is defined by applying additional resolution to a fixed region that covers both sides of the road and the road surface immediately in front of the vehicle (see Figure 3B). This instantly provides superior resolution (both vertical and horizontal) in the area of greatest concern. Once a pattern is approved, it can be fixed for functional safety.
2. Triggered ROI: A Triggered ROI requires a software-configurable system that can be programmed to accept a trigger. The perception software team may determine that when certain conditions are met, an ROI is generated within the existing scan pattern. For example, a mapping or navigation system might signal that you are approaching an intersection, which generates an appropriately targeted ROI on key areas of the scene with greater detail (see Figure 3C).
3. Dynamic ROI: A Dynamic ROI requires the highest level of intelligence and utilizes the same techniques and methodology deployed by Automatic Targeting Systems (ATS) in fighter jets to continuously interrogate objects of high interest over time. As these objects move closer or further away, the size and density of the ROI varies. For example, pedestrians, cyclists, vehicles, or other objects moving in the scene can be detected and a Dynamic ROI automatically applied to track their movements (see Figure 3D).
Figure 3. Figure 3A shows a scene as a vehicle approaches an intersection. Figure 3B shows a Fixed Region of Interest (ROI) covering the sides of the road and the area immediately in front of the vehicle. Figure 3C shows a Triggered ROI where the navigation system triggers specific ROIs as the vehicle approaches the intersection. Figure 3D shows a Dynamic ROI where several objects of interest are detected and tracked as they move through the scene.
The AEye Advantage
A major advantage of iDAR is that it is active in nature, meaning it can adjust its scan patterns in real time, and therefore, can take advantage of concepts like time multiplexing. This means it can simultaneously trade off temporal sampling resolution, spatial sampling resolution, and even range, at multiple points in the “frame.” This allows the system to dynamically change the scan density over the entire Field-of-View, enabling the robust collection of useful, actionable information.
In a conventional LiDAR system, there is (i) a fixed Field-of-View, (ii) a fixed uniform or patterned sampling density, and (iii) a fixed laser shot schedule. AEye’s technology allows for these three parameters to vary almost independently. This leads to an endless stream of potential innovations and will be the topic of a later paper.
Instantaneous resolution conveys that resolution, as a metric, is not something dictated by physical constraints alone, such as beam divergence, or number of points per second (the next metric). Rather, it starts with a faster, more efficient active LiDAR and then intelligently optimizes resources. The ability to instantaneously increase resolution is a critical enabler in the fourth metric we introduce.
Conventional Metric #3: Points per Second
Extended Metric: Quality Returns per Second
A high confidence, confirmatory return from an object on a single frame basis
Traditionally, the industry has favored achieving the highest number of points per second. In theory, a higher number of laser shots would mean that the sensor system would receive a higher number of returns. However, a high number of shots does not guarantee a high number of returns, nor does it necessarily mean that the data being returned is useful in any way to help safely and efficiently guide an autonomous vehicle. As mentioned earlier, passive conventional LiDAR systems simply gather data about the environment indiscriminately and without discretion, sending those inputs to the CPU, wherein, 75% to 95% of this data is thrown out. This creates a huge strain on interrogation times, bandwidth, and processing. Therefore, a conventional system that is purporting to deliver a high quantity of shots (i.e., high rate of shots per second) will suffer a latency penalty because it cannot separate the valuable information from the invaluable (or redundant) in a timely manner.
As safety is the ultimate goal of these systems, then having full scene coverage without missing anything, while simultaneously increasing probability of detection (i.e., knowing something is there) and reducing false positives, is a fundamental requirement. AEye proposes replacing the metric of points per second with the more meaningful quality returns per second. Measuring quality returns per second is significantly more beneficial to the development of automotive LiDAR systems because it quantifies the crux of the information actually needed to enable accurate and efficient perception. While points per second gives little to no indication of the value of the information received, quality returns per second does.
Because there is no agreed upon standard in the industry for measuring returns per second, AEye defines quality returns per second as: high probability of detection, low false positive rate, often non-isolated, returns from an object on a single frame basis (i.e., “quality” returns). And, in valuing the efficiency of LiDAR sensor systems (and thus, the safety of the autonomous vehicle and its passengers) above all else, we urge the rest of the LiDAR community to do the same.
A high probability of detection, low false positive rate return will deliver actionable data to the vehicle’s perception system. AEye is able to define quality returns per second in this way because of our bistatic architecture (the subject of a future paper). As mentioned earlier, iDAR can foveate and isolate on both transmit (regions that the laser light chooses to “paint”) and receive (where/when the processing chooses to focus). Our patented bistatic architecture keeps the transmit and receive channels separate, allowing optimization of both paths. As each laser pulse is transmitted, the receiver is told where and when to look for its return. No other LiDAR sensor system on the market can do this. Our system is agile and active enough to enable it to select where to scan, making the returns that much more efficient in capturing the most salient, actionable data.
Within the “Search, Acquire, and Act” framework, our bistatic architecture allows the iDAR system to search the entire scene without missing anything, but focus on what matters most in a vehicle’s surroundings, actively favoring the swift acquisition and tracking of real, actionable data for smarter, more accurate decision making (action) and safer vehicle autonomy.
Use Case: Brick in the Road on the Highway
Being able to swiftly and accurately acquire small objects on the road can be the key to preventing fatal high-speed accidents. Imagine a vehicle with highway autopilot driving at fast speeds on the highway. The vehicle detects an object close to the road. Is it a false positive? It is tumbleweed or is it a brick? Being able to acquire the object and the overridability with absolute certainty (high probability of detection, low false positive rate, and multiple returns) as quickly as possible is critical in this scenario. By favoring quality returns per second (as opposed to straining interrogation times and bandwidth on the collection of irrelevant data to receive the highest number of returns possible), low probability of detection cues the system to instantaneously interrogate further (see Figure 4).
Figure 4. What is that object on the road ahead? AEye’s active, intelligent iDAR is able to acquire the object as quickly as possible. By favoring quality returns per second, low pro