Comprehensive Overview of Depth Cameras

What Is a Depth Camera?

There are different types of 3D sensors, that can differ between different methods to acquire world data or how that data is processed in order to represent it in a convenient way. These sensors can vary in many ways such as acquisition method, resolution, range, etc. This page entails a brief overview of the different 3D sensor types and a small survey of what sensors are currently available.

João Alves, Aivero

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The Aivero team and Azure Kinect
The Aivero team and Azure Kinect
Zivid Two mounted on a collaborative robot
Zivid Two mounted on a collaborative robot

What are depth cameras used for?

Depth-sensing cameras enable machines to perceive the environment in three dimensions. Because of the richer information the depth cameras provide, they have gained importance in applications such as vision-guided robotics, inspection and monitoring. 3D data is less susceptible to environmental disturbances such as changing light conditions. Thus, machines become more reliable and precise when performing tasks, such as picking up an object and placing it elsewhere.

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Depth Camera Sensor Types

Depth Cameras typically detect depth using Stereo Sensors, Time-of-Flight calculations, Structured Light, or LiDAR.

Stereo sensors

Stereo sensors try to mimic human vision by using two cameras facing the scene with some distance between them (baseline, see Figure 1). The images from these cameras are acquired and then used to perform visual feature (relevant visual information) extraction and matching to obtain a disparity map between the cameras’ views.

The disparity information is inversely proportional to depth and can easily be used to obtain the depth map. This also means that some sensors of this type usually only work in feature-rich environments and encounter problems in featureless environments (e.g. a white wall). However, this type of sensor usually allows for greater depth image resolutions than other cameras.

These feature detection and matching computations are done onboard the camera, meaning they usually require more processing power than other methods. Also, the stereo working distance is limited by the baseline between the cameras as the error increases quadratically with objects’ distance to the sensor. Usual applications entail both indoor and outdoor, feature-rich environments.

Figure 1: Stereo Vision

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Time-of-Flight

Time of Flight (ToF) sensors flood the scene with light and calculate depth using each photon’s time to return to the sensor. This means that each pixel relates to one beam of light projected by the device, which provides more data density, narrower shadows behind objects and more straightforward calibration (no stereo matching).

Since they use light projection, these sensors are sensitive to different types of surfaces, such as very reflective or very dark ones. In these cases, invalid data usually appears in the image. However, they are much more robust to low lighting or dim conditions than stereo sensors which depend on the scene lighting. These sensors can be used both indoors and outdoors.

Structured Light

Structured light (SL) sensors, on the other hand, use a known pattern projected by the IF sensor onto the scene. The way the pattern deforms is used to construct the depth map.

This type of sensor does not require an external light source and is mainly used in indoor environments due to its low robustness to sunlight since interference may occur with the projected light pattern.

Figure 2: Structured Light versus Time of Flight illustration
Figure 3: LiDAR Sensor Operation

Light Detection Ranging

Similar to ToF but with scanning, light detection and ranging (LiDAR) sensors are more accurate than ToF since they use multiple IR pulses, whereas ToF sensors usually use one light pulse to obtain their image. By mapping the environment point-per-point instead of a burst of points (as in ToF), the overall noise for each point is reduced, producing more accurate results. Furthermore, LiDAR can usually obtain measurements at a greater distance than traditional ToF or Stereo sensors.

However, LiDAR sensors are usually more expensive than other 3D sensors and, since they perform scanning, can be affected by highly dynamic environments (motion blur).

List of Depth Cameras

List of Depth Cameras

Depth cameras are specialized devices that determine the distance between themselves and a target or between two entities. They play a pivotal role in various applications, from autonomous vehicles to facial recognition, enhancing accuracy and safety.

Depth-sensing cameras provide machines with a three-dimensional view of their surroundings. This 3D perspective allows machines to understand spatial relationships, detect obstacles, and navigate complex environments with precision

The main types of depth cameras include stereo sensors, Time-of-Flight (ToF) sensors, and Structured Light sensors. Each type operates on different principles and is suitable for specific applications.

Stereo sensors in depth cameras use two or more lenses to capture different views of a scene. By comparing these views, the camera can calculate depth information based on the disparity between the images.

ToF sensors measure depth by emitting a light pulse and calculating the time it takes for the light to bounce back after hitting an object. The time difference provides accurate depth information.

Structured Light sensors project a known pattern of light onto a scene. By analyzing the distortions in this pattern when it reflects off objects, the camera can deduce depth information.

LiDAR, or Light Detection and Ranging, uses laser beams to measure distances. It’s a form of depth sensing technology that’s especially popular in autonomous vehicles due to its high accuracy and long-range capabilities.

Modern depth cameras are equipped with infrared sensors and advanced algorithms that allow them to function effectively in various lighting conditions, from bright sunlight to complete darkness.

In robotics, depth cameras are used for navigation, obstacle detection, and object recognition. In inspection, they can be used to assess the quality of products, detect defects, and ensure accurate measurements.

Depth cameras provide machines with a detailed 3D perspective, enabling them to make more informed decisions, avoid obstacles, and interact safely with their environment.