Time-of-flight camera

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Time of flight of a light pulse reflecting off a target 20200501 Time of flight.svg
Time of flight of a light pulse reflecting off a target

A time-of-flight camera (ToF camera), also known as time-of-flight sensor (ToF sensor), is a range imaging camera system for measuring distances between the camera and the subject for each point of the image based on time-of-flight, the round trip time of an artificial light signal, as provided by a laser or an LED. Laser-based time-of-flight cameras are part of a broader class of scannerless LIDAR, in which the entire scene is captured with each laser pulse, as opposed to point-by-point with a laser beam such as in scanning LIDAR systems. [1] Time-of-flight camera products for civil applications began to emerge around 2000, [2] as the semiconductor processes allowed the production of components fast enough for such devices. The systems cover ranges of a few centimeters up to several kilometers.

Contents

Types of devices

Several different technologies for time-of-flight cameras have been developed.

RF-modulated light sources with phase detectors

Photonic Mixer Devices (PMD), [3] the Swiss Ranger, and CanestaVision [4] work by modulating the outgoing beam with an RF carrier, then measuring the phase shift of that carrier on the receiver side. This approach has a modular error challenge: measured ranges are modulo the RF carrier wavelength. The Swiss Ranger is a compact, short-range device, with ranges of 5 or 10 meters and a resolution of 176 x 144 pixels. With phase unwrapping algorithms, the maximum uniqueness range can be increased. The PMD can provide ranges up to 60 m. Illumination is pulsed LEDs rather than a laser. [5] CanestaVision developer Canesta was purchased by Microsoft in 2010. The Kinect2 for Xbox One was based on ToF technology from Canesta.

Range gated imagers

These devices have a built-in shutter in the image sensor that opens and closes at the same rate as the light pulses are sent out. Most time-of-flight 3D sensors are based on this principle invented by Medina. [6] Because part of every returning pulse is blocked by the shutter according to its time of arrival, the amount of light received relates to the distance the pulse has traveled. The distance can be calculated using the equation, z = R (S2S1) / 2(S1 + S2) + R / 2 for an ideal camera. R is the camera range, determined by the round trip of the light pulse, S1 the amount of the light pulse that is received, and S2 the amount of the light pulse that is blocked. [6] [7]

The ZCam by 3DV Systems [1] is a range-gated system. Microsoft purchased 3DV in 2009. Microsoft's second-generation Kinect sensor was developed using knowledge gained from Canesta and 3DV Systems. [8]

Similar principles are used in the ToF camera line developed by the Fraunhofer Institute of Microelectronic Circuits and Systems and TriDiCam. These cameras employ photodetectors with a fast electronic shutter.

The depth resolution of ToF cameras can be improved with ultra-fast gating intensified CCD cameras. These cameras provide gating times down to 200ps and enable ToF setup with sub-millimeter depth resolution. [9]

Range gated imagers can also be used in 2D imaging to suppress anything outside a specified distance range, such as to see through fog. A pulsed laser provides illumination, and an optical gate allows light to reach the imager only during the desired time period. [10]

Direct Time-of-Flight imagers

These devices measure the direct time-of-flight required for a single laser pulse to leave the camera and reflect back onto the focal plane array. Also known as "trigger mode", the 3D images captured using this methodology image complete spatial and temporal data, recording full 3D scenes with single laser pulse. This allows rapid acquisition and rapid real-time processing of scene information. For time-sensitive autonomous operations, this approach has been demonstrated for autonomous space testing [11] and operation such as used on the OSIRIS-REx Bennu asteroid sample and return mission [12] and autonomous helicopter landing. [13] [14]

Advanced Scientific Concepts, Inc. provides application specific (e.g. aerial, automotive, space) Direct TOF vision systems [15] known as 3D Flash LIDAR cameras. Their approach utilizes InGaAs Avalanche Photo Diode (APD) or PIN photodetector arrays capable of imaging laser pulse in the 980 nm to 1600 nm wavelengths.

Components

A time-of-flight camera consists of the following components:

Principle

Principle of operation of a time-of-flight camera:

In the pulsed method (1), the distance, d =
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c t/2
q2/q1 + q2 , where c is the speed of light, t is the length of the pulse, q1 is the accumulated charge in the pixel when light is emitted and q2 is the accumulated charge when it is not.

In the continuous-wave method (2), d =
c t/2p arctan
q3 - q4/q1 - q2 . Time of flight camera principle.svg
Principle of operation of a time-of-flight camera:

In the pulsed method (1), the distance, d = c t/2q2/q1 + q2 , where c is the speed of light, t is the length of the pulse, q1 is the accumulated charge in the pixel when light is emitted and q2 is the accumulated charge when it is not.

In the continuous-wave method (2), d = c t/2π arctan q3 - q4/q1 - q2 .
Diagrams illustrating the principle of a time-of-flight camera with analog timing TOF-camera-principle.jpg
Diagrams illustrating the principle of a time-of-flight camera with analog timing

The simplest version of a time-of-flight camera uses light pulses or a single light pulse. The illumination is switched on for a very short time, the resulting light pulse illuminates the scene and is reflected by the objects in the field of view. The camera lens gathers the reflected light and images it onto the sensor or focal plane array. Depending upon the distance, the incoming light experiences a delay. As light has a speed of approximately c = 300,000,000 meters per second, this delay is very short: an object 2.5 m away will delay the light by: [17]

For amplitude modulated arrays, the pulse width of the illumination determines the maximum range the camera can handle. With a pulse width of e.g. 50 ns, the range is limited to

These short times show that the illumination unit is a critical part of the system. Only with special LEDs or lasers is it possible to generate such short pulses.

The single pixel consists of a photo sensitive element (e.g. a photo diode). It converts the incoming light into a current. In analog timing imagers, connected to the photo diode are fast switches, which direct the current to one of two (or several) memory elements (e.g. a capacitor) that act as summation elements. In digital timing imagers, a time counter, that can be running at several gigahertz, is connected to each photodetector pixel and stops counting when light is sensed.

In the diagram of an amplitude modulated array analog timer, the pixel uses two switches (G1 and G2) and two memory elements (S1 and S2). The switches are controlled by a pulse with the same length as the light pulse, where the control signal of switch G2 is delayed by exactly the pulse width. Depending on the delay, only part of the light pulse is sampled through G1 in S1, the other part is stored in S2. Depending on the distance, the ratio between S1 and S2 changes as depicted in the drawing. [4] Because only small amounts of light hit the sensor within 50 ns, not only one but several thousand pulses are sent out (repetition rate tR) and gathered, thus increasing the signal-to-noise ratio.

After the exposure, the pixel is read out and the following stages measure the signals S1 and S2. As the length of the light pulse is defined, the distance can be calculated with the formula:

In the example, the signals have the following values: S1 = 0.66 and S2 = 0.33. The distance is therefore:

In the presence of background light, the memory elements receive an additional part of the signal. This would disturb the distance measurement. To eliminate the background part of the signal, the whole measurement can be performed a second time with the illumination switched off. If the objects are further away than the distance range, the result is also wrong. Here, a second measurement with the control signals delayed by an additional pulse width helps to suppress such objects. Other systems work with a sinusoidally modulated light source instead of the pulse source.

For direct TOF imagers, such as 3D Flash LIDAR, a single short pulse from 5 to 10 ns is emitted by the laser. The T-zero event (the time the pulse leaves the camera) is established by capturing the pulse directly and routing this timing onto the focal plane array. T-zero is used to compare the return time of the returning reflected pulse on the various pixels of the focal plane array. By comparing T-zero and the captured returned pulse and comparing the time difference, each pixel accurately outputs a direct time-of-flight measurement. The round trip of a single pulse for 100 meters is 660 ns. With a 10 ns pulse, the scene is illuminated and the range and intensity captured in less than 1 microsecond.

Advantages

Simplicity

In contrast to stereo vision or triangulation systems, the whole system is very compact: the illumination is placed just next to the lens, whereas the other systems need a certain minimum base line. In contrast to laser scanning systems, no mechanical moving parts are needed.

Efficient distance algorithm

It is a direct process to extract the distance information out of the output signals of the TOF sensor. As a result, this task uses only a small amount of processing power, again in contrast to stereo vision, where complex correlation algorithms are implemented. After the distance data has been extracted, object detection, for example, is also a straightforward process to carry out because the algorithms are not disturbed by patterns on the object. The accuracy is usually estimated at 1 % of the measured distance. [18] [19]

Speed

Time-of-flight cameras are able to measure the distances within a complete scene with a single shot. As the cameras reach up to 160 frames per second, they are ideally suited to be used in real-time applications.

Disadvantages

Background light

When using CMOS or other integrating detectors or sensors that use visible or near infra-red light (400 nm - 700 nm), although most of the background light coming from artificial lighting or the sun is suppressed, the pixel still has to provide a high dynamic range. The background light also generates electrons, which have to be stored. For example, the illumination units in many of today's TOF cameras can provide an illumination level of about 1 watt. The Sun has an illumination power of about 1050 watts per square meter, and 50 watts after the optical band-pass filter. Therefore, if the illuminated scene has a size of 1 square meter, the light from the sun is 50 times stronger than the modulated signal. For non-integrating TOF sensors that do not integrate light over time and are using near-infrared detectors (InGaAs) to capture the short laser pulse, direct viewing of the sun is a non-issue because the image is not integrated over time, rather captured within a short acquisition cycle typically less than 1 microsecond. Such TOF sensors are used in space applications [12] and in consideration for automotive applications. [20]

Interference

In certain types of TOF devices (but not all of them), if several time-of-flight cameras are running at the same time, the TOF cameras may disturb each other's measurements. There exist several possibilities for dealing with this problem:

For Direct TOF type cameras that use a single laser pulse for illumination, because the single laser pulse is short (e.g. 10 nanoseconds), the round trip TOF to and from the objects in the field of view is correspondingly short (e.g. 100 meters = 660 ns TOF round trip). For an imager capturing at 30 Hz, the probability of an interfering interaction is the time that the camera acquisition gate is open divided by the time between laser pulses or approximately 1 in 50,000 (0.66 μs divided by 33 ms).

Multiple reflections

In contrast to laser scanning systems where a single point is illuminated, the time-of-flight cameras illuminate a whole scene. For a phase difference device (amplitude modulated array), due to multiple reflections, the light may reach the objects along several paths. Therefore, the measured distance may be greater than the true distance. Direct TOF imagers are vulnerable if the light is reflecting from a specular surface. There are published papers available that outline the strengths and weaknesses of the various TOF devices and approaches. [21]

Applications

Range image of a human face captured with a time-of-flight camera (artist's depiction) TOF Kamera 3D Gesicht.jpg
Range image of a human face captured with a time-of-flight camera (artist’s depiction)

Automotive applications

Time-of-flight cameras are used in assistance and safety functions for advanced automotive applications such as active pedestrian safety, precrash detection and indoor applications like out-of-position (OOP) detection. [22] [23]

Human-machine interfaces and gaming

As time-of-flight cameras provide distance images in real time, it is easy to track movements of humans. This allows new interactions with consumer devices such as televisions. Another topic is to use this type of cameras to interact with games on video game consoles. [24] The second-generation Kinect sensor originally included with the Xbox One console used a time-of-flight camera for its range imaging, [25] enabling natural user interfaces and gaming applications using computer vision and gesture recognition techniques. Creative and Intel also provide a similar type of interactive gesture time-of-flight camera for gaming, the Senz3D based on the DepthSense 325 camera of Softkinetic. [26] Infineon and PMD Technologies enable tiny integrated 3D depth cameras for close-range gesture control of consumer devices like all-in-one PCs and laptops (Picco flexx and Picco monstar cameras). [27]

Smartphone cameras

The Samsung Galaxy S20 Ultra features three rear-facing camera lenses and a ToF camera. Ruckseite Galaxy S20 Ultra 20200305.jpg
The Samsung Galaxy S20 Ultra features three rear-facing camera lenses and a ToF camera.

Several smartphones include time-of-flight cameras. These are mainly used to improve the quality of photos by providing the camera software with information about foreground and background. [28]

The first mobile phone released with such technology was the LG G3, from early 2014. [29] The BlackBerry Passport and the LG G Flex 2 were also launched with a ToF sensor. [30]

Measurement and machine vision

Range image with height measurements TOF Kamera Boxen.jpg
Range image with height measurements

Other applications are measurement tasks, e.g. for the fill height in silos. In industrial machine vision, the time-of-flight camera helps to classify and locate objects for use by robots, such as items passing by on a conveyor. Door controls can distinguish easily between animals and humans reaching the door.

Robotics

Another use of these cameras is the field of robotics: Mobile robots can build up a map of their surroundings very quickly, enabling them to avoid obstacles or follow a leading person. As the distance calculation is simple, only little computational power is used. Since these cameras can also be used to measure distance, teams for FIRST Robotics Competition have been known to use the devices for autonomous routines.

Earth topography

ToF cameras have been used to obtain digital elevation models of the Earth's surface topography, [31] for studies in geomorphology.

Brands

Active brands (as of 2011)

Defunct brands

See also

Related Research Articles

<span class="mw-page-title-main">Lidar</span> Method of spatial measurement using laser

Lidar is a method for determining ranges by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. Lidar may operate in a fixed direction or it may scan multiple directions, in which case it is known as lidar scanning or 3D laser scanning, a special combination of 3-D scanning and laser scanning. Lidar has terrestrial, airborne, and mobile applications.

<span class="mw-page-title-main">Photonics</span> Technical applications of optics

Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Photonics is closely related to quantum electronics, where quantum electronics deals with the theoretical part of it while photonics deal with its engineering applications. Though covering all light's technical applications over the whole spectrum, most photonic applications are in the range of visible and near-infrared light. The term photonics developed as an outgrowth of the first practical semiconductor light emitters invented in the early 1960s and optical fibers developed in the 1970s.

<span class="mw-page-title-main">Time of flight</span> Timing of substance within a medium

Time of flight (ToF) is the measurement of the time taken by an object, particle or wave to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a way to learn about the particle or medium's properties. The traveling object may be detected directly or indirectly. Time of flight technology has found valuable applications in the monitoring and characterization of material and biomaterials, hydrogels included.

Liquid crystal on silicon is a miniaturized reflective active-matrix liquid-crystal display or "microdisplay" using a liquid crystal layer on top of a silicon backplane. It is also known as a spatial light modulator. LCoS initially was developed for projection televisions, but has since found additional uses in wavelength selective switching, structured illumination, near-eye displays and optical pulse shaping.

<span class="mw-page-title-main">Imaging radar</span> Application of radar which is used to create two-dimensional images

Imaging radar is an application of radar which is used to create two-dimensional images, typically of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths. It uses an antenna and digital computer storage to record its images. In a radar image, one can see only the energy that was reflected back towards the radar antenna. The radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so.

<span class="mw-page-title-main">Laser rangefinder</span> Range finding device that uses a laser beam to determine the distance to an object

A laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used. It is a type of scannerless lidar.

<span class="mw-page-title-main">3D scanning</span> Scanning of an object or environment to collect data on its shape

3D scanning is the process of analyzing a real-world object or environment to collect three dimensional data of its shape and possibly its appearance. The collected data can then be used to construct digital 3D models.

<span class="mw-page-title-main">Light field camera</span> Type of camera that can also capture the direction of travel of light rays

A light field camera, also known as a plenoptic camera, is a camera that captures information about the light field emanating from a scene; that is, the intensity of light in a scene, and also the precise direction that the light rays are traveling in space. This contrasts with conventional cameras, which record only light intensity at various wavelengths.

The following are common definitions related to the machine vision field.

<span class="mw-page-title-main">Structured light</span>

Structured light is the process of projecting a known pattern on to a scene. The way that these deform when striking surfaces allows vision systems to calculate the depth and surface information of the objects in the scene, as used in structured light 3D scanners.

Range imaging is the name for a collection of techniques that are used to produce a 2D image showing the distance to points in a scene from a specific point, normally associated with some type of sensor device.

A structured-light 3D scanner is a 3D scanning device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.

ZCam is a brand of time-of-flight camera products for video applications by Israeli developer 3DV Systems. The ZCam supplements full-color video camera imaging with real-time range imaging information, allowing for the capture of video in 3D.

Optical heterodyne detection is a method of extracting information encoded as modulation of the phase, frequency or both of electromagnetic radiation in the wavelength band of visible or infrared light. The light signal is compared with standard or reference light from a "local oscillator" (LO) that would have a fixed offset in frequency and phase from the signal if the latter carried null information. "Heterodyne" signifies more than one frequency, in contrast to the single frequency employed in homodyne detection.

Canesta was a fabless semiconductor company that was founded in April, 1999, by Cyrus Bamji, Abbas Rafii, and Nazim Kareemi.

<span class="mw-page-title-main">Neptec Design Group</span> Canadian vision systems company

Neptec Design Group is an Ottawa-based Canadian vision systems company that provides machine vision solutions for space, industrial, and military applications. Privately owned and founded in 1990, Neptec supplies operational systems to NASA's Space Shuttle and International Space Station programs as one of their prime contractors. In 2000, Neptec expanded its technology to include active 3D imaging systems and 3D processing software. This led to the development of the Laser Camera System, an operational system used by NASA to inspect a shuttle's external surfaces during flight. Neptec also used this system to develop the TriDAR, a 3D imaging and tracking system designed for automated on-orbit rendezvous, inspection, and docking. It combines the LCS with a long range LIDAR sensor into the same optical path.

<span class="mw-page-title-main">MESA Imaging</span> Swiss time-of-flight camera company

MESA Imaging is a time-of-flight camera company. As privately financed organization, MESA was founded in July 2006 as a spin out from the Swiss Center for Electronics and Microtechnology (CSEM) to commercialize its time-of-flight camera technologies. As of 2009, its primary product line, the SwissRanger, is in its fourth generation with the SwissRanger SR4000. In 2014 MESA was bought by Heptagon

<span class="mw-page-title-main">Depth map</span> Image also containing data on distances of objects from the camera

In 3D computer graphics and computer vision, a depth map is an image or image channel that contains information relating to the distance of the surfaces of scene objects from a viewpoint. The term is related to depth buffer, Z-buffer, Z-buffering, and Z-depth. The "Z" in these latter terms relates to a convention that the central axis of view of a camera is in the direction of the camera's Z axis, and not to the absolute Z axis of a scene.

Nanophotonic coherent imagers (NCI) are image sensors that determine both the appearance and distance of an imaged scene at each pixel. It uses an array of LIDARs to gather this information about size and distance, using an optical concept called coherence.

<span class="mw-page-title-main">PMD Technologies</span> German company using photonic mixer devices

PMD Technologies is a developer of CMOS semiconductor 3D time-of-flight (ToF) components and a provider of engineering support in the field of digital 3D imaging. The company is named after the Photonic Mixer Device (PMD) technology used in its products to detect 3D data in real time. The corporate headquarters of the company is located in Siegen, Germany.

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Further reading