Part 4: 3D Imaging

3D imaging is a very popular topic today but where and when to use 3D imaging for inspection is not always understood. Here Multipix Imaging aims to explain when 3D is most applicable and what 3D imaging techniques are available for Machine Vision.

Selecting the correct 3D Imaging Method

There are several questions about the application that need careful consideration. The answers to these will assist in selecting the correct 3D method;
Question Solution
What type of inspection is required? Shape, position, orientation, alignment, presence…
Which degree of accuracy is required? Micrometer, millimeter, centimeters…
What are the features of the objects? Size, reflectivity, transparency…
What are the general conditions? Illumination, in motion, processing time, budget…
Using the answers to these questions, you can select the best method for your application. Now let’s explore in more detail 3D inspection methods.

Advanced Structured Light Technique

A conventional structured light technique is derived from stereo vision. In the stereo vision (inspired by a nature of a human vision), one uses a pair of cameras to capture an object from two different perspectives. On the other hand, in the advanced structured light approach, one of the cameras is interchanged by a pattern projector, which emits a well-defined structured illumination. The camera and the pattern projector are a calibrated unit and are focused towards the scanning area (Object). The pattern projector emits a set of coding patterns projected in a succession, one after another. These coding patterns encodes a spatial information. The camera captures the scanning area once per every projected pattern. The algorithm computes an exact 3D position of the object point. A big advantage of this technique is that it works independently of the object’s texture and results in high 3D reconstruction quality. It also offer a good speed of acquisition compared to say standard stereo-vision methods. Photoneo PhoXi Scan is an ideal example of a 3D camera system employing advanced structure lighting and uses a unique custom-designed projection unit.     Read more about Photoneo PhoXi 3D scanners

Laser Triangulation (Sheet of light)

With this method of 3D imaging, there is one camera which has a known physical relationship with a laser line projector. (Fig.3a) The laser line is projected across the surface of an object which creates a profile of that instance, in this example a connection rod is scanned. (Fig.3b) The deformation in the laser line, as being captured by the camera, determines the depth of an object at that point on the laser line. Many profiles are taken as the object moves under the camera-laser combo and these are used to recreate the surface of the object in software such as HALCON. (Fig.3c) Check out the SmartRay 3D ECCO Series, pre-calibrated all-in-one 3D laser triangulation cameras.

Time of Flight (ToF)

3D ToF cameras use either a pulsed or continuous wave Near Infra-Red light to illuminate the object/scene. When using pulsed light, the time it takes for the light to be reflected back is measured and used to calculate distance. Whereas with continuous wave, it is the phase shift between emitted and reflected light that is measured to determine distance.

The importance of selecting the correct 3D imaging method

As can be seen on the chart below, if you wanted to carry out a robot pick and place on individual frozen peas, then a ToF camera would not be the most appropriate technique but now consider if it was pumpkins ….ToF could be a great option as the accuracy is likely to be sufficient! Aside from accuracy, object surface/material is also important as it can influence how the light is reflected and captured (or not) by the sensor. If the object is moving or stationary is also a factor to consider when choosing 3D imaging technology.
3D Technology Hardware Requirements Object Size Accuracy
Binocular Stereo Two Cameras Calibration Object 50mm – 2.5M mm’s
Multi-View Stereo Multiple Cameras Calibration Object 50mm – 2.5M mm’s
Sheet of Light / 3DLT Camera Laser Line Projector Unit to move the object, Calibration Object Object must fit onto the moving unit Single microns
Photometric Stereo Telecentric Camera At least three Telecentric light sources Restricted by view of telecentric lens 0.3mm to 5mm
3D Cameras Depth from focus i.e Calibrated ToF Telecentric camera Hardware to vary the focus position 50cm – 13m approx. <2cm +/-1cm Single microns

Read more on our Imaging Guide in Part 5: Imaging Software options explained Part 3: Machine Vision Camera Technology Part 5: Imaging Processing Software