What is ToF

TOF principle

ToF is the abbreviation of Time of Flight, which is literally translated as flight time. By continuously sending light pulses to the target, and then receiving the light returned from the object with the sensor, the distance of the target is obtained by detecting the flight (round trip) time of these emitted and received light pulses.

Industrial-grade LiDAR used in military and driverless cars also uses ToF technology, which uses laser beams to detect target position, velocity and other characteristic quantities. It combines laser, global positioning system GPS and inertial measurement devices ( Inertial Measurement Unit (IMU) performs scanning point by point to obtain the depth information of the entire detected object.

Our main concern here is the miniaturized ToF camera that can be integrated into consumer electronics. ToF cameras are similar to ordinary cameras in imaging process, and are mainly composed of several key units such as light source, photosensitive chip, lens, sensor, drive control circuit and processing circuit. The ToF camera includes two core modules, a transmitting lighting module and a photosensitive receiving module, and generates depth information based on the correlation between the two core modules. The ToF camera's sensor chip is also divided into single-point and area-array sensor chips according to the number of pixel units. In order to measure the position and depth information of the entire three-dimensional object surface, a single-point ToF camera can be used to obtain the three-dimensional geometric structure of the detected object by scanning point by point. You can also use the array ToF camera to take a scene picture to obtain the surface geometry information of the entire scene in real time. The array ToF camera is more vulnerable to the construction of consumer electronics systems, and the technical difficulty is also greater.

TOF's irradiation unit emits high-frequency pulses after modulating light. Generally, LEDs or lasers (including laser diodes and VCSELs) are used to emit high-performance pulsed light. The pulses can reach about 100 MHz, and infrared light is mainly used. Most of the existing ToF camera technologies currently on the market are based on continuous wave (continuous wave) intensity modulation methods, and some are based on optical shutter methods, with slightly different principles.。

The principle of the optical shutter-based method is very simple. A pulsed light wave is emitted, and the time difference t of the light wave reflected back to the three-dimensional object is quickly and accurately obtained through the optical shutter. Since the speed of light c is known, as long as the time difference between the irradiated and received light The distance back and forth can be shown by publicizing d = t / 2 · c. The principle of this method seems very simple, but in practice, it still has great challenges to achieve high accuracy. For example, the clock that controls the optical shutter switch requires very high accuracy, but also can produce high precision and high repeatability. Ultra-short pulses, irradiation units and TOF sensors require high-speed signal control to achieve high depth measurement accuracy. If the clock signal between the illuminated light and the ToF sensor is shifted by 10ps, it is equivalent to a displacement error of 1.5mm.

Another existing ToF working principle based on continuous wave intensity modulation isA beam of illumination light is emitted, and a distance measurement is performed using a phase change of a transmitted light wave signal and a reflected light wave signal. Among them, the wavelength of the lighting module is generally in the infrared band, and high frequency modulation is required. The ToF sensor module is similar to the ordinary mobile phone camera module. It consists of chips, lenses, circuit boards and other components. Each pixel of the ToF sensor chip records the specific phase between the round-trip camera and the object that emits light waves, and processes the data. The unit extracts the phase difference and calculates the depth information from the formula. The sensor structure of this chip is similar to the CMOS image sensor used by ordinary mobile phone camera modules, but it is more complicated. It contains a modulation control unit, A / D conversion unit, data processing unit, etc. Therefore, the pixel size of the ToF chip is larger than that of ordinary image sensors It is much larger, usually about 20um. A lens that collects light is also required, but unlike ordinary optical lenses, an infrared band-pass filter is required to ensure that only light with the same wavelength as the illumination source can enter. Since the scenes of the optical imaging system at different distances are concentric spheres of different diameters, rather than parallel planes, in actual use, subsequent data processing units need to correct this error. ToF camera calibration is the most important process indispensable in the production process. Without the calibration process, the ToF camera cannot work properly.

TOF Advantages

1、Small size and high accuracy

The TOF camera requires that the receiving end and the transmitting end are as close as possible. The closer they are, the smaller the errors caused by different transmitting and receiving paths are, which has a natural advantage from the perspective of compact size;

2、Output depth information directly

TOF can directly output depth information, and does not require similar calculations such as binocular stereo vision or structured light to obtain depth information.

3、Strong anti-interference ability

TOF is not affected by surface grayscale and features. Since sunlight is not modulated, TOF is also resistant to strong light. The accuracy of TOF does not change with the change of distance, and can basically be stabilized at the cm level.

TOF disadvantages

1、Low resolution and high power consumption，The power consumption needs to be improved.

2、Solution is not mature enough