Time-of-flight camera: Wikis

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Encyclopedia

A time-of-flight camera (TOF camera) is a camera system that creates distance data with help of the time-of-flight (TOF) principle which is different from time-of-flight mass spectrometry. The principle is similar to that of LIDAR scanners with the advantage that whole scene is captured at the same time.

Time-of-flight cameras are relatively new devices, as the semiconductor processes have only recently become fast enough for such devices. The systems cover ranges of a few meters up to about 60 m. The distance resolution is about 1 cm. The lateral resolution of time-of-flight cameras is generally low compared to standard video cameras, at 320 × 240 pixels or less.[1][2][3][4][5] Only one camera reports 484 x 648 pixels of resolution using a standard CCD sensor.[6] The biggest advantage of the cameras may be that they provide up to 100 images per second.

Types of devices

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

Pulsed light source with digital time counters

There are devices with a pulsed laser and a custom imaging integrated circuit with a fast counter behind every pixel. These devices produce depth values for each pixel on every frame. Typical image sizes are 128 x 128 pixels. Ranges up to 22,000 feet with an eye-safe narrow beam have been achieved. Detectors are typically InGaAs (indium-gallium-arsenide) devices.[7]

RF-modulated light sources with phase detectors

Photonic Mixer Devices (PMD) [8] and the Swiss Ranger works by modulating the outgoing beam with an RF carrier, then measuring the phase shift of that carrier on the receive side. This is a compact, short-range device. This approach has a modular error challenge; ranges are mod the maximum range, which is the RF carrier wavelength. With phase unwrapping algorithms, the maximum uniqueness range can be increased. The Swiss Ranger has ranges of 5 or 10 meters, with 176 x 144 pixels. The PMD can provide ranges up to 60m. Illumination is pulsed LEDs, rather than a laser. The demodulation is usually achieved by gating the sensor in synchrony with the light source modulation, so in essence they are range gated imagers.[9]

Range gated imagers

This is the most promising technology, invented by Antonio Medina. The phase detector is the gate or shutter in the camera. The gate allows collection of portions S2 and S1 of the received light pulse S. The portions are dependent of the time of arrival, and range is derived from them according to Medina's equation, z =R(S2-S1)/2S+R/2 for an ideal camera. R is the camera range, determined by the round trip of the light pulse.[6][10]

The Z-cam,[11] and Canesta 3D cameras are range-gated systems.

Similar principle is used in the ToF camera line developed by Fraunhofer Institute of Microelectronic Circuits and Systems and TriDiCam. These cameras employ photodetectors with fast electronic shutter used as a gated integrator and pulsed lasers.

There are also "range gated imagers", which are not 3D cameras. The gate is open to collect the totality of the reflected pulse and nothing else. Anything outside a specified distance range can be suppressed. Those are useful for seeing through fog. A pulsed laser provides illumination, and an optical gate allows light to reach the imager only during the desired time period.[1] [12]

Components

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

• Illumination unit: It illuminates the scene. As the light has to be modulated with high speeds up to 100 MHz, only LEDs or laser diodes are feasible. The illumination normally uses infrared light to make the illumination unobtrusive.
• Optics: A lens gathers the reflected light and images the environment onto the image sensor. An optical band pass filter only passes the light with the same wavelength as the illumination unit. This helps suppress background light.
• Image sensor: This is the heart of the TOF camera. Each pixel measures the time the light has taken to travel from the illumination unit to the object and back. Several different approaches are used for timing; see types of devices above.
• Driver electronics: Both the illumination unit and the image sensor have to be controlled by high speed signals. These signals have to be very accurate to obtain a high resolution. For example, if the signals between the illumination unit and the sensor shift by only 10 picoseconds, the distance changes by 1.5 mm. For comparison: current CPUs reach frequencies of up to 3 GHz, corresponding to clock cycles of about 300 ps - the corresponding 'resolution' is only 45 mm.
• Computation/Interface: The distance is calculated directly in the camera. To obtain good performance, some calibration data is also used. The camera then provides a distance image over a USB or Ethernet interface.

Principle

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. The illumination is switched on for a very short time, the resulting light pulse illuminates the scene and is reflected by the objects. The camera lens gathers the reflected light and images it onto the sensor plane. Depending on the distance, the incoming light experiences a delay. As light has a speed of c = 300,000,000 meters per second, this delay is very short: an object 2.5 m away will delay the light by:

$t_D = 2 \cdot \frac D c = 2 \cdot \frac {2.5\;\mathrm{m}} {300\;000\;000\;\frac{\mathrm{m}}{\mathrm{s}}} = 0.000\;000\;016\;66\;\mathrm{s} = 16.66 \;\mathrm{ns}$[13]

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

$D_\mathrm{max} = \frac{1}{2} \cdot c \cdot t_0 = \frac{1}{2} \cdot 300\;000\;000\;\frac{\mathrm{m}}{\mathrm{s}} \cdot 0.000\;000\;05\;\mathrm{s} =\!\ 7.5\;\mathrm{m}$

These short times show that the illumination unit is a critical part of the system. Only with some 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, running at several gigahertz, is connected to each photodetector pixel and stops counting when light is sensed.

In the diagram of an 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.[14] Because only small amounts of light hit the sensor within 50 ns, not only one but several thousands 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 Medina's formula:

$D = \frac{1}{2} \cdot c \cdot t_0 \cdot \frac {S2} {S1 + S2}$

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

$D = 7.5\;\mathrm{m} \cdot \frac {0.33} {0.33 + 0.66} = 2.5\;\mathrm{m}$

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.

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 very easy to extract the distance information out of the output signals of the TOF sensor, therefore this task uses only a small amount of processing power, again in contrast to stereo vision, where complex correlation algorithms have to be implemented. After the distance data has been extracted, object detection, for example, is also easy to carry out because the algorithms are not disturbed by patterns on the object.

Speed

Time-of-flight cameras are able to measure the distances within a complete scene with one shot. As the cameras reach up to 100 frames per second, they are ideally suited to be used in real-time applications.

Background light

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 today's TOF cameras can provide an illumination level of about 1 watt. The Sun has an illumination power of about 50 watts per square meter after the optical bandpass 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.

Interference

If several time-of-flight cameras are running at the same time, the cameras may disturb each others' measurements. There exist several possibilities for dealing with this problem:

• Time multiplexing: A control system starts the measurement of the individual cameras consecutively, so that only one illumination unit is active at a time.
• Different modulation frequencies: If the cameras modulate their light with different modulation frequencies, their light is collected in the other systems only as background illumination but does not disturb the distance measurement.

Multiple reflections

In contrast to laser scanning systems, where only a single point is illuminated at once, the time-of-flight cameras illuminate a whole scene. Due to multiple reflections, the light may reach the objects along several paths and therefore, the measured distance may be greater than the true distance.

Applications

Range image of a human face captured with a time-of-flight camera

Automotive applications

Time-of-flight cameras are also 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.[15][16] [17]

Human-machine interfaces / 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.[18]

Measurement / machine vision

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 objects and help robots find the items, for instance 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.

Brands

• CanestaVision - TOF modules and software by Canesta
• FOTONIC-B70 - TOF cameras and software by Fotonic powered by Canesta CMOS chip
• PMD[vision] - TOF imager, modules, cameras and software by PMDTechnologies
• SwissRanger - an industrial TOF-only camera line originally by the Centre Suisse d'Electronique et Microtechnique (CSEM), now developed by the spin out company Mesa Imaging.
• 3D MLI Sensor - TOF imager, modules, cameras, and software by IEE (International Electronics & Engineering), based on modulated light intensity (MLI)
• ZCam - TOF camera products by 3DV Systems, integrating full-color video with depth information
• Optrima - TOF cameras and modules[19]
• TriDiCam - TOF modules and software, the TOF imager original developed by Fraunhofer Institute of Microelectronic Circuits and Systems, now developed by the spin out company TriDiCam

References

1. ^ Schuon, Sebastian; Theobalt, Christian; Davis, James; Thrun, Sebastian (2008-07-15), "High-quality scanning using time-of-flight depth superresolution", written at Anchorage, Alaska, IEEE Computer Society Conference on Computer Vision and Pattern Recognition Workshops, 2008, Institute of Electrical and Electronics Engineers, pp. 1–7, doi:10.1109/CVPRW.2008.4563171, ISBN 978-1-4244-2339-2, retrieved 2009-07-31, "The Z-cam can measure full frame depth at video rate and at a resolution of 320×240 pixels."
2. ^ PMD[vision] CamCube 2.0 Datasheet (No. 20090601 ed.), Siegen, Germany: PMDTechnologies, 2009-06-01, p. 5, retrieved 2009-07-31, "Type of Sensor: PhotonICs PMD 41k-S (204 x 204)"
3. ^ SR4000 Data Sheet (Rev 2.6 ed.), Zürich, Switzerland: Mesa Imaging, August 2009, p. 1, "176 x 144 pixel array (QCIF)"
4. ^ "Canesta 101: Introduction to 3D Vision in CMOS" (Portable Document Format). Sunnyvale, California: Canesta. March 2008. p. 16. Retrieved 31 July 2009. "Our current sensor features an array of 160 x 120 pixels. […] We are considering a 320x240 sensor."
5. ^ PMD[vision] S3 Datasheet (No. 20090601 ed.), Siegen, Germany: PMDTechnologies, 2009-06-01, p. 4, retrieved 2009-07-31, "Type of sensor—PhotonICs 3k-S2, resolution: 64[V] x 48[H] pixels"
6. ^ a b Medina A, Gayá F, and Pozo F. "Compact laser radar and three-dimensional camera". 23 (2006). J. Opt. Soc. Am. A. pp. 800–805 http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-23-4-800.
7. ^ Stettner, Roger and Bailey, Howard. Eye-safe laser radar 3D imaging. Advanced Scientific Concepts.
8. ^ Christoph Heckenkamp: Das magische Auge - Grundlagen der Bildverarbeitung: Das PMD Prinzip. In: Inspect. Nr. 1, 2008, S. 25–28.
9. ^ "Mesa Imaging - Products". August 17, 2009.
10. ^ Medina, Antonio. "Three Dimensional Camera and Rangefinder". January 1992. United States Patent 5081530.
11. ^ Iddan, G. J. and G. Yahav, G.. "3D Imaging in the studio (and elsewhere)". 4298. SPIE. p. 48.
12. ^
13. ^ "CCD/CMOS Lock-In Pixel for Range Imaging: Challenges, Limitations and State-of-the-Art" - CSEM
14. ^ Gokturk, Salih Burak; Yalcin, Hakan; Bamji, Cyrus (2005-01-24), "A Time-Of-Flight Depth Sensor - System Description, Issues and Solutions", IEEE Computer Society Conference on Computer Vision and Pattern Recognition Workshops, 2004, Institute of Electrical and Electronics Engineers, pp. 35–45, doi:10.1109/CVPR.2004.17, retrieved 2009-07-31, "The differential structure accumulates photo-generated charges in two collection nodes using two modulated gates. The gate modulation signals are synchronized with the light source, and hence depending on the phase of incoming light, one node collects more charges than the other. At the end of integration, the voltage difference between the two nodes is read out as a measure of the phase of the reflected light."
15. ^ Hsu, Stephen; Acharya, Sunil; Rafii, Abbas; New, Richard (2006-04-25), "Performance of a Time-of-Flight Range Camera for Intelligent Vehicle Safety Applications", Advanced Microsystems for Automotive Applications 2006, Springer, pp. 205–219, doi:10.1007/3-540-33410-6_16, ISBN 978-3-540-33410-1, retrieved 2009-09-36
16. ^ Free space determination for parking slots using a 3D PMD sensor - Scheunert, U. Fardi, B. Mattern, N. Wanielik, G. Keppeler, N. - Intelligent Vehicles Symposium, 2007 IEEE, Istanbul, 13–15 June 2007
17. ^ Elkhalili, O., Schrey, O., Ulfig, W., Brockherde, W., Hosticka, B. J., "A 64x8 Pixel 3-D CMOS TOF Image Sensor for Car Safety Applications," Proc. European Solid-States Conference, 2006, pp. 568-571
18. ^ Captain, Sean (2008-05-01). "Out of Control Gaming". PopSci.com. Popular Science. Retrieved 2009-06-15.
19. ^ http://www.wired.com/epicenter/2010/02/televison-will-soon-watch-you-for-instructions/