3D imaging techniques for industry

The four main 3D imaging techniques are laser triangulation, stereo vision and light stripe projection which all use geometric procedures, and time of flight, which is time dependent.

Laser triangulation

In laser triangulation the object to be measured is passed through a precise line of light produced by a laser line. A camera is positioned at a known angle to the laser to record the profile of the laser line, whose shape changes as the object passes through it.

A number of profiles are generated during this process from which a three-dimensional image is created. The displacement from the original line for each point along the profile is used for height information which is then colour-coded in a so-called 2.5D range map using the corresponding grey values. The 2.5D range maps can then be converted to real 3D point clouds to allow compensation for deviations in position and rotation of the objects in all six degrees of freedom. Some 3D cameras calculate this information internally, thus saving processing on the host computer, whereas other systems convert the range maps using a PC. This method means it is not necessary to precisely position or feed the objects mechanically. The prime requisite for laser triangulation is that the object moves relative to the camera and laser line, making it suited to production line applications. One potential problem is shadowing, where the view of the laser line is blocked by higher regions of the object so that data from the structures behind cannot be obtained. One solution is the use of several cameras which track the laser line from different angles and then merge the different data sets to a single height profile using sophisticated software tools. Using this method, object data will only be missing if it is absent from all of the input data sets.

Stereo vision

The stereo vision technique is also based on using two cameras. Similar to the pair of eyes humans possess, two cameras are used to record 2D images of an object. Employing the triangulation technique it is then possible to calculate a three-dimensional image from the two 2D images. This also allows for movement of the objects to be measured during recording.

However, in order to clearly assign each object point of the inspected object to a pixel in the two 2D images, reference markings or random patterns on the object are essential when using stereo vision. It is therefore not really suitable for a production environment. It is, however, often encountered in the measurement of coordinates, the 3D measurement of objects, in working areas with industrial, service or mobile robot applications, as well as for 3D visualisation of working areas which are hazardous or inaccessible for humans.

Light stripe projection

In contrast to laser triangulation or stereo vision, 3D image processing based on light stripe projection requires static objects.

Here, structured light in the form of fringes is projected in stripes onto the object, and the resulting light pattern is recorded by a camera set at a known angle. The large number of points acquired simultaneously gives improved height resolution compared to laser scanners by a factor of 2. Light stripe projection is suitable for position and completeness checks as well as for target/actual comparisons, especially of complex free-shaped objects.

Time of flight

Time of flight (ToF) cameras are 3D camera systems which measure the time taken for a light pulse to reach the object and return for each image point. This time is directly proportional to the distance of each image point, thus providing height information at each point. This technology covers distances from a few metres up to around 40m with up to 100 images per second, with a distance resolution of roughly 1cm and lateral resolution of approximately 200 x 200 pixels.

One of the best known applications of time of flight technology is the identification of truck classes at toll bridges above roads. Here, a fast scanning laser helps to create a 3D profile of the trucks and from that, the number of axes can be provided. Due to their relatively low resolution ToF systems have limited applications in an industrial environment.

Components for 3D imaging

Components for industrial 3D imaging, including laser line sources, lenses, 3D image processing software and 3D cameras, have increased significantly in performance while decreasing in price, making real-time 3D production line inspection an affordable option. 3D matching tools have been introduced that enable 3D models to be compared to a known 3D or ‘golden’ template. Also, a new range of 3D ‘smart’ cameras, which feature an integrated laser source and optics and provide on-board processing of 3D data, allowing the direct output of the measured results. With a comprehensive portfolio of 3D imaging components as well as sophisticated 3D software tools within its own Common Vision Blox imaging toolkit, Stemmer Imaging has considerable experience in 3D imaging.