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TOF camera
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TOF is short for Time of Flight and literally translates to flight time. The so-called time-of-flight 3D imaging is to obtain the target distance by continuously transmitting light pulses to the target, then receiving the light returned from the object with the sensor, and detecting the flight (round-trip) time of the light pulse. This technique is basically similar to the principle of a 3D laser sensor, except that the 3D laser sensor is a point-by-point scan, while the TOF camera obtains the depth information of the entire image at the same time. 
The TOF camera is similar to the ordinary machine vision imaging process, and is composed of several units such as a light source, an optical component, a sensor, a control circuit, and a processing circuit. The TOF camera has a fundamentally different 3D imaging mechanism than a binocular measurement system that is very similar to non-invasive three-dimensional detection and application. The binocular stereo measurement is matched by the left and right stereo image pairs, and then the triangulation is performed by the triangulation method, and the TOF camera is obtained by the target distance obtained by the injecting and reflecting light detection.
The TOF technology adopts the active light detection method. Unlike the general illumination requirement, the purpose of the TOF illumination unit is not illumination, but the distance measurement between the incident light signal and the reflected light signal is used. Therefore, the TOF illumination unit is The light is modulated at a high frequency and then emitted, such as the pulsed light emitted by an LED or a laser diode as shown in the figure below, and the pulse can reach 100 MHz. Similar to a normal camera, the TOF camera chip front end needs a lens that collects light. However, unlike ordinary optical lenses, a bandpass filter is required to ensure that only the same wavelength as the illumination source can enter. At the same time, since the optical imaging system has a see-through effect, the scenes at different distances are concentric spheres of different diameters, rather than parallel planes, so in actual use, subsequent processing units are required to correct this error. As the core of the TOF camera, each pixel of the TOF chip records the phase of the incident light back and forth between the camera and the object. 
The sensor structure is similar to a normal image sensor but more complex than an image sensor. It contains 2 or more shutters to sample reflected light at different times. For this reason, the TOF chip pixel size is much larger than the general image sensor pixel size, generally about 100um. Both the illumination unit and the TOF sensor require high-speed signal control to achieve high depth measurement accuracy. For example, a 10 ps offset of the sync signal between the illumination light and the TOF sensor is equivalent to a displacement of 1.5 mm. The current CPU can reach 3GHz, and the corresponding clock period is 300ps, and the corresponding depth resolution is 45mm. The arithmetic unit mainly performs data correction and calculation work. By calculating the relative phase shift relationship between the incident light and the reflected light, the distance information can be obtained.
Advantages of TOF: Compared with stereo cameras or triangulation systems, TOF cameras are small in size and have almost the same size as a typical camera. They are ideal for applications that require a lightweight, compact camera. The TOF camera is capable of quickly calculating depth information in real time, reaching tens to 100 fps. The depth information of the TOF. Binocular stereo cameras require complex correlation algorithms and are slower to process. The depth calculation of TOF is not affected by the grayscale and features of the surface of the object, and it can perform three-dimensional detection very accurately. A binocular stereo camera requires a good feature change in the target, otherwise depth calculations will not be possible. The depth calculation accuracy of TOF does not change with distance, and can basically be stabilized at the cm level, which is very meaningful for some large-scale motion applications.
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