Christian Bräuer-Burchardt

3D shape measurement with phase correlation based fringe projection

Here we propose a method for 3D shape measurement by means of phase correlation based fringe projection in a stereo arrangement. The novelty in the approach is characterized by following features. Correlation between phase values of the images of two cameras is used for the co-ordinate calculation. This work stands in contrast to the sole usage of phase values (phasogrammetry) or classical triangulation (phase values and image co-ordinates - camera raster values) for the determination of the co-ordinates. The methods main advantage is the insensitivity of the 3D-coordinates from the absolute phase values. Thus it prevents errors in the determination of the co-ordinates and improves robustness in areas with interreflections artefacts and inhomogeneous regions of intensity. A technical advantage is the fact that the accuracy of the 3D co-ordinates does not depend on the projection resolution. Thus the achievable quality of the 3D co-ordinates can be selectively improved by the use of high quality camera lenses and can participate in improvements in modern camera technologies. The presented new solution of the stereo based fringe projection with phase correlation makes a flexible, errortolerant realization of measuring systems within different applications like quality control, rapid prototyping, design and CAD/CAM possible. In the paper the phase correlation method will be described in detail. Furthermore, different realizations will be shown, i.e. a mobile system for the measurement of large objects and an endoscopic like system for CAD/CAM in dental industry.

Phase unwrapping using geometric constraints for high-speed fringe projection based 3D measurements

A new methodology for 3D surface measurement based on phase shifted fringe pattern projection is presented which enables real-time measurement and high-speed performance of 3D measurements using stereo camera observation. The essence of the new technique is a drastic reduction of the fringe code which makes the process of pattern projection and image recording faster and real-time applicable. The new algorithm allows the complete omission of the typically used Gray code sequence or other additional code to the phase shifted sinusoidal fringe sequence. Its main concept is a special geometric design of the arrangement of the projection unit in relation to one of the involved cameras. The 3D point calculation is performed by triangulation between the two cameras with the help 3D point determination between the projection unit and one of the cameras. Whereas the second mode ensures the uniqueness, mode one realizes the accurate calculation. The realization of the uniqueness of the measurement is obtained by a corresponding arrangement of the parameters projected fringe width, the measurement volume size, the triangulation angle between the principal rays of first camera and the projection unit, and camera constants of second camera and projection unit. First experiments with a 3D measurement systems based on fringe projection technique show the robustness of the new method. Real time measurements can be performed which is the precondition of the supervision or quality check of several dynamic processes.

Intraoral 3D scanner

Here a new set-up of a 3D-scanning system for CAD/CAM in dental industry is proposed. The system is designed for direct scanning of the dental preparations within the mouth. The measuring process is based on phase correlation technique in combination with fast fringe projection in a stereo arrangement. The novelty in the approach is characterized by the following features: A phase correlation between the phase values of the images of two cameras is used for the co-ordinate calculation. This works contrary to the usage of only phase values (phasogrammetry) or classical triangulation (phase values and camera image co-ordinate values) for the determination of the co-ordinates. The main advantage of the method is that the absolute value of the phase at each point does not directly determine the coordinate. Thus errors in the determination of the co-ordinates are prevented. Furthermore, using the epipolar geometry of the stereo-like arrangement the phase unwrapping problem of fringe analysis can be solved. The endoscope like measurement system contains one projection and two camera channels for illumination and observation of the object, respectively. The new system has a measurement field of nearly 25mm × 15mm. The user can measure two or three teeth at one time. So the system can by used for scanning of single tooth up to bridges preparations. In the paper the first realization of the intraoral scanner is described.

Underwater 3D Surface Measurement Using Fringe Projection Based Scanning Devices.

In this work we show the principle of optical 3D surface measurements based on the fringe projection technique for underwater applications. The challenges of underwater use of this technique are shown and discussed in comparison with the classical application. We describe an extended camera model which takes refraction effects into account as well as a proposal of an effective, low-effort calibration procedure for underwater optical stereo scanners. This calibration technique combines a classical air calibration based on the pinhole model with ray-based modeling and requires only a few underwater recordings of an object of known length and a planar surface. We demonstrate a new underwater 3D scanning device based on the fringe projection technique. It has a weight of about 10 kg and the maximal water depth for application of the scanner is 40 m. It covers an underwater measurement volume of 250 mm × 200 mm × 120 mm. The surface of the measurement objects is captured with a lateral resolution of 150 μm in a third of a second. Calibration evaluation results are presented and examples of first underwater measurements are given.

Distance Dependent Lens Distortion Variation in 3D Measuring Systems Using Fringe Projection.

Measuring systems using fringe projection provide the possibility of very accurate touchless measurements. For the measurement of small objects compact devices are possible. However, in the case of very close distances between the optical system and the measuring object there is a considerable influence of the measuring distance to the lens distortion. This will lead to considerable measuring errors if neglected. In typical cases of our measuring systems the amount of the distortion may change by a factor greater than two in the range of a distance difference of a few centimetres. Results of the distance dependence will be given for the lenses of two devices.

On the accuracy of point correspondence methods in three-dimensional measurement systems using fringe projection

Abstract. Different concepts of correspondence findings in contactless optical three-dimensional (3-D) measurement systems using fringe projection are analyzed concerning the accuracy of the 3-D point calculation. These concepts are different concerning the kind of performance of the triangulation procedure in order to calculate the resulting 3-D points and the use of geometric constraints versus second projection sequence. Triangulation may be alternatively performed between camera pixels and the phase origin of the projection, between one camera pixel in the prior camera and the image of the corresponding measured phase value in the second camera, or between the image points of certain raster phase values in the two observation cameras. Additionally, triangulation procedures can be distinguished concerning the use of two perpendicular projection directions of the fringes versus the use of geometric constraints, i.e., epipolar geometry instead of the second projection direction. Advantages and disadvantages of the different techniques are discussed. In addition, a theoretical analysis of the application of synthetic data has been simulated as well as experiments performed on real measurement data. Both simulations and real data experiments confirm the theoretical assumptions concerning the magnitudes of the random errors occurring in 3-D point determination.

Fringe Projection Based High Speed 3D Sensor for Real-Time Measurements

A sensor based on fringe projection technique was developed which allows ultrafast measurements of the surface of flat measuring objects which realizes a data acquisition rate up to 8.9 million 3D points per second. The high measuring velocity was achieved by consequent fringe code reduction and parallel data processing. Fringe sequence length was reduced using geometric constraints of the sensor arrangement including epipolar geometry. Further reduction of the image sequence length was obtained by omission of the Gray code sequence by using the geometric constraints of the measuring objects. The sensor may be used e.g. for inspection of conductor boards.

Phase Unwrapping in Fringe Projection Systems Using Epipolar Geometry

A new method for phase unwrapping is introduced which realizes the unwrapping of phase images without binary codes produced by fringe projection systems using at least two cameras and one projector. The novelty of the method is the use of the epipolar geometry between the two cameras and the projector in order to achieve a unique point correspondence. The method is suited for systems which should realize a short recording time for the image sequence acquisition. It is very robust even at positions with abrupt change of depth.

Cordless hand-held optical 3D sensor

A new mobile optical 3D measurement system using phase correlation based fringe projection technique will be presented. The sensor consist of a digital projection unit and two cameras in a stereo arrangement, whereby both are battery powered. The data transfer to a base station will be done via WLAN. This gives the possibility to use the system in complicate, remote measurement situations, which are typical in archaeology and architecture. In the measurement procedure the sensor will be hand-held by the user, illuminating the object with a sequence of less than 10 fringe patterns, within a time below 200 ms. This short sequence duration was achieved by a new approach, which combines the epipolar constraint with robust phase correlation utilizing a pre-calibrated sensor head, containing two cameras and a digital fringe projector. Furthermore, the system can be utilized to acquire the all around shape of objects by using the phasogrammetric approach with virtual land marks introduced by the authors1, 2. This way no matching procedures or markers are necessary for the registration of multiple views, which makes the system very flexible in accomplishing different measurement tasks. The realized measurement field is approx. 100 mm up to 400 mm in diameter. The mobile character makes the measurement system useful for a wide range of applications in arts, architecture, archaeology and criminology, which will be shown in the paper.

Optical 3D sensor for large objects in industrial application

A new self calibrating optical 3D measurement system using fringe projection technique named “kolibri 1500” is presented. It can be utilised to acquire the all around shape of large objects. The basic measuring principle is the phasogrammetric approach introduced by the authors /1, 2/. The “kolibri 1500” consists of a stationary system with a translation unit for handling of objects. Automatic whole body measurement is achieved by using sensor head rotation and changeable object position, which can be done completely computer controlled. Multi-view measurement is realised by using the concept of virtual reference points. In this way no matching procedures or markers are necessary for the registration of the different images. This makes the system very flexible to realise different measurement tasks. Furthermore, due to self calibrating principle mechanical alterations are compensated. Typical parameters of the system are: the measurement volume extends from 400 mm up to 1500 mm max. length, the measurement time is between 2 min for 12 images up to 20 min for 36 images and the measurement accuracy is below 50μm.The flexibility makes the measurement system useful for a wide range of applications such as quality control, rapid prototyping, design and CAD/CAM which will be shown in the paper.