Gunther Notni

Digital fringe projection in 3D shape measurement: an error analysis

Optical 3D measurement systems are used in a lot of applications, for instance for quality control and reverse engineering. Active optical 3D measurement systems are very often based on the fringe projection technique. The key element of such a shape measurement system is the projection unit, where nowadays digital projection units based on LCD, LCoS, or DMD technology are used. Despite of a lot of opportunities these displays reveal some disadvantages for the use in fringe projection, which can reduce the phase measurement accuracy. In contrast to slide-like structures, these displays are separated in small picture elements -- so called active pixels -- with dimensions of about 8x8μm2 up to 20x20μm2. For this reason the cos function which has to be projected is spatially quantized. Therefore this function is not continuous. Another problem is the reproduction of the correct intensities given by the cos function in the projection. Usually, every projection engine shows some kind of gamma correction function or nonlinearity. This is either caused by the micro display itself or (in the case of video beamers) it is implemented into the beamer’s electronic to project proper visual images. All these effects will distort the intensity to be projected. All the mentioned effects cause phase measurement errors of different strength. Here we discuss different approaches to determine and compensate these influences of the projection engine to the 3D measurement. If the compensation is well accomplished, the accuracy of the measurement results may increase by up to one magnitude.

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.

High-speed 3D shape measurement using array projection

Measuring the three-dimensional (3D) surface shape of objects in real time has become an important task e.g. in industrial quality management or medical sciences. Stereo vision-based arrangements in connection with pattern projection offer high data acquisition speed and low computation time. However, these coded-light techniques are limited by the projection speed which is conventionally in the range of 200. . .250Hz. In this contribution, we present the concepts and a realized setup of a so-called 3D array projector. It is ultra-slim, but nonetheless able to project fixed patterns with high brightness and depth of focus. Furthermore, frame rates up to the 100 kHz range are achievable without any need of mechanically moving parts since the projection speed is limited mainly by the switching frequency of the used LEDs. According to the measurement requirements, type and structure of the patterns can be chosen almost freely: linear or sinusoidal fringes, binary codes such as the Gray code, square, hexagonal or random patterns and many more. First investigations on the functionality of such a 3D array projector were conducted using a prototype with a combination of Gray codes and phase-shifted sinusoidal fringes. Our contribution proves the high brightness of the proposed projector, its sharpness and the good Michelson contrast of the fringe patterns. We deal with the patterns’ homogeneity and the accuracy of the phase shift between the sinusoidal patterns. Furthermore, we present first measurement results and outline future research which is, inter alia, addressed to the use of other structured light techniques with the help of new purpose-built 3D array projector prototypes.

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.

InN as THz emitter excited at 1060 nm and 800 nm

InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.

Optimum lens aperture in phase-shifting speckle interferometric setups for maximum accuracy of phase measurement.

Negative exponentially distributed intensities of speckle fields seem unfavorable in terms of precision metrology, if interferometric setups are involved with a saturable photodetector and an analog-to-digital converter that imposes a finite resolution. By spatial integration, extended detector apertures modify the intensity distribution toward a less awkward function. However, because the detector aperture also integrates over points of rapidly changing speckle phases, this is done at the expense of a lower modulation of measured intensity during phase shift. An optimum set of parameters is calculated here, consisting of values for the lens aperture, the mean speckle intensity, and the beam ratio. The remaining phase-measurement error assumes its minimum of 10.6 mrad when the space-bandwidth product of the lens-detector system (thus concerning the lens aperture) is 0.31, the mean speckle intensity is 1/11 of the saturation intensity, and the reference intensity is four times higher than the mean speckle intensity. The 90 degrees phase-shift algorithms with either three, four, or five frames turned out to be quite powerful, even with interference signals of rather poor modulation. Not needing a very small lens aperture is interesting, because stopping down the lens is a trade-off with the limited power of the laser.

Three-coordinate measuring system with structured light

The measurement of three-dimensional object shapes plays an important role in machine vision, solid modeling and other industrial applications. Well known optical methods for 3D- shape inspection are moire-techniques and contouring. Unfortunately, as a rule they are lacking because they measure only one coordinate, i.e., the height above a reference surface. The other coordinates in the reference plane have to be determined using the pixel distance of the observing CCD-camera or by other means. Here, we demonstrate a real 3D-coordinate measuring system using fringe projection techniques where the scale of coordinates is given by the illumination-structures. This method has the advantage that the aberration of the observing system and the depth-dependent imaging scale have no influence on the measuring accuracy. Moreover, the measurements are independent of the position of the camera with respect to the object under test.

Experimental comparison of phase-shifting fringe projection and statistical pattern projection for active triangulation systems

Active triangulation systems are widely used for precise and fast measurements. Many different coding strategies have been invented to solve the correspondence problem. The quality of the measurement results depends on the accuracy of the pixel assignments. The most established method uses phase shifted-patterns projected on the scene. This is compared to a method using statistical patterns. In both coding strategies, the number and the spatial frequency of the projected patterns is varied. The measurements and calculations for all presented results were done with exactly the same measurement setup in a narrow time window to avoid any changes and to guarantee identical technical preconditions as well as comparability.

Phasogrammetric optical 3D sensor for the measurement of large objects

In the following, optical 3D-measurement systems based on fringe projection techniques according to the principle of phasogrammetry are introduced. These self-calibrating measuring systems allow the automated measurement of complex objects. In combination with adequate software tools to evaluate the data, a variety of different tasks can be performed in a productive environment.