3D machine vision. This is such a magical combination of words. There are dozens of different solutions on the market, but they are typically not universal enough or they are so universal that they are not sufficient for your application. In this blog, I will introduce different approaches for 3D technology and review what principle that will be the best for future usage.
Bonus: I created a poll asking professionals what 3D vision technology they believe is best and I’ve shared the results.
Triangulation
One of the most used technologies in the 3D camera world is triangulation, which provides simple distance measurement by angular calculation. The reflected light falls incident onto a receiving element at a certain angle depending on the distance. This standard method relies on a combination of the projector and camera. There are two basic variants of the projections — models with single-line structure and 2-dimensional geometric pattern.
A single projected line is used in applications where the object is moving under the camera. If you have a static object, then you can use multiple parallel lines that allow the evaluation of the complete scene/surface. This is done with a laser light shaped into a two-dimensional geometric pattern (“structured light”) typically using a diffractive optical element (DOE). The most common patterns are dot matrices, line grids, multiple parallel lines, and circles.
Structured light
Another common principle of 3D camera technology is the structured light technique. System contains at least one camera (it is most common to use two cameras) and a projector. The projector creates a narrow band of light (patterns of parallel stripes are widely used), which illuminate the captured object. Cameras from different angles observe the various curved lines from the projector.
Projecting also depends on the technology which is used to create the pattern. Currently, the three most widespread digital projection technologies are:
- transmissive liquid crystal,
- reflective liquid crystal on silicon (LCOS)
- digital light processing (DLP)
Reflective and transparent surfaces create challenges.
Time of Flight (ToF)
For this principle, the camera contains a high-power LED which emits light that is reflected from the object and then returns to the image sensor. The distance from the camera to the object is calculated based on the time delay between transmitted and received light.
This is really simple principle which is used for 3D applications. The most common wavelength used is around 850nm. This is called near infrared range, which is invisible for human and eye safety.
This is an especially great use since the camera can standardly provide 2D as well as 3D picture in the same time.
An image sensor and LED emitter are used as an all-in-one product making it simple to integrate and easy to use. However, a negative point is that the maximum resolution is VGA (640 x 480) and for Z resolution expect +/- 1cm. On the other hand, it is an inexpensive solution with modest dimensions.
Likely applications include:
- mobile robotics
- door controls
- localization of the objects
- mobile phones
- gaming consoles (XBOX and Kinect camera) or industrial version Azure Kinect.
Stereo vision
The 3D camera by stereo vision is a quite common method that typically includes two area scan sensors (cameras). As with human vision, 3D information is obtained by comparing images taken from two locations.
The principle, sometimes called stereoscopic vision, captures the same scene from different angles. The depth information is then calculated from the image pixel disparities (difference in lateral position).
The matching process, finding the same information with the right and left cameras, is critical to data accuracy and density.
Likely applications include:
- Navigation
- Bin-picking
- Depalletization
- Robotic guidance
- Autonomous Guiding Vehicles
- Quality control and product classification
I asked my friends, colleagues, professionals, as well as competitors, on LinkedIn what is the best 3D technology and which technology will be used in the future. You can see the result here.
As you see, over 50% of the people believe that there is no one principle which can solve each task in 3D machine vision world. And maybe that’s why machine vision is such a beautiful technology. Many approaches, solutions and smart people can bring solutions from different perspectives and accesses.
