Compared with structured light and binocular stereo vision, what are the advantages of ToF technology?

Phone withTof function

Time of Flight (ToF), Structured Light (Structured Light), and Stereo Vision (Stereo Vision) together form the current three mainstream technology routes for 3D imaging and sensing. Among them, the 3D structured light solution is the most mature and has been widely used in industrial 3D vision. ToF has become a more promising solution on the mobile side by virtue of its own advantages.

What are the advantages of ToF?

One of the technical means available to realize 3D solutions at present is to imitate human eyes. Therefore, the development trend of mobile phones is from monocular to more and more cameras. Mobile phone monoculars cannot meet the development of some of our applications, so mobile phone developers will use more The camera is used to simulate the depth scene, and the biggest change we see is the change from single camera to dual camera, that is, simulating the eyes of a person and realizing the extraction of depth information.

Binocular Stereo Vision technology began in the mid-1960s. It is based on the parallax principle and uses imaging equipment to obtain two images of the measured object from different positions. By calculating the position deviation between the corresponding points of the image, To obtain the three-dimensional geometric information of the object. After decades of development, stereo vision has become more and more widely used in the fields of robot vision, aerial surveying and mapping, reverse engineering, military applications, medical imaging and industrial inspection.

The way to realize binocular stereo vision is relatively simple and close to human perception. But it has a big disadvantage, that is, it cannot extract the depth data very accurately. For example, we have no way of knowing how many meters or centimeters an object is from us through our eyes. Binocular stereo imaging systems often suffer from performance degradation when the scene lacks features, so they have not been used in smartphone imaging. For example, in the case of facing a flat and smooth surface of a wall, the 3D information captured by a stereo imaging system is usually incomplete or inaccurate.

This introduces another 3D technology, namely 3D structured light. This technology uses a near-infrared laser to project light with certain structural characteristics to the object, and a special infrared camera collects and obtains the three-dimensional structure of the object, and then performs in-depth processing and imaging of the information through calculations. In terms of structured light contrast with binoculars, accurate distance information can be obtained, and depth information can be obtained with only one imaging. It has the advantages of low energy consumption and high imaging resolution, and can achieve high security in security, so it is widely used Used in scenes such as face recognition and face payment. However, the recognition distance of structured light technology is relatively short, and it is only suitable for short distances, between 0.2 meters and 1.2 meters, so that its application is limited to the front camera of mobile phones.

TOF (Time of Flight) technology is a 3D imaging technology that was only applied to mobile phone cameras in 2018. It transmits continuous infrared light pulses of a specific wavelength to the target, and then a specific sensor receives the light signal from the object to be measured, and calculates The flight time or phase difference of the light round-trip, so as to obtain the depth information of the target object. TOF lens is mainly composed of light-emitting unit, optical lens and image sensor. Its recognition distance can reach 0.4 meters to 5 meters. TOF technology has the characteristics of strong anti-interference and higher FPS refresh rate, so it can perform better in dynamic scenes. In addition, TOF technology has a small amount of depth information calculation, and the corresponding CPU/ASIC calculation amount is also low, so the requirements for the algorithm are lower.

Although ToF has some technical shortcomings, such as relatively low 3D imaging accuracy and depth map resolution, high power consumption, etc., compared with structured light and binocular technology, it has a wider range of adaptation, covering from short distances to farther distances. Long distance, and it can also extract very accurate depth information.

Li Jia, engineer from Utmel Electronic, concluded that ToF has the following technical advantages:

High quantum efficiency CCD scheme @940nm light wavelength, with good outdoor performance
Can solve the mutual interference problem of multiple machines in the same scene
High resolution
Low power consumption
Narrow pulse, high speed, global exposure, low noise interference
Long detection distance

Conclusion

ToF sensor is superior to structured light and stereo vision. First of all, ToF has only two components, structured light and stereo vision have 3 to 4. In addition, ToF does not require a baseline, ensuring a compact shape and no baseline calibration, while also improving durability and reducing production costs. Finally, ToF does not need to use complex image recognition algorithms, saving a lot of power consumption. In short, compared with other 3D technologies, ToF technology is smaller, cheaper and more durable.

In the future, most smartphones will be equipped with one or two 3D cameras, 3D face authentication will become the de facto standard for secure mobile payments, and realistic augmented reality technology will push mobile games to a new stage.

Not only that, ToF technology will also be applied in smart factories and smart homes, such as household appliances, which require sweepers with clear obstacle images. In addition, the technology can also be used in access control systems for buildings and even private residences.