We present a measurement setup for the acquisition of topographic and 3-D point cloud data using the depth-scanning fringe projection technique (DSFP). We describe the signal generation, its processing using techniques known from short coherence interferometry and discuss a direct 3-D calibration method. Our measurement system delivers an absolute phase map of the scene under measurement. Calibration procedures for macroscopic measurement methods like fringe projection and / or photogrammetry consider the principal distance (that is to say the distance between the center of projection and the image plane) as a constant. This is feasible as long as no focusing and zooming are performed during measurement. Consequently the depth of the measurement volume is limited by the depth of sharpness of the imaging system. By focusing through the whole depth of the measurement volume, our system overcomes this problem, and offers a virtually unlimited measurement depth. However, we have to take the issue of focusing into consideration in order to calibrate our system. The well-known direct calibration method has been adapted to our DSFP setup in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. It has been completed to a set of three polynomial transformations, which allow to include the depth-scanning principle in the calibration of the system.
When conducting three-dimensional measurements with fringe projection, the quality of the grating applied for the generation of the fringes is very important. It has a direct influence on the achievable height resolution when phase-shifting algorithms are used. Hence, the created fringes should have an ideal sinusoidal intensity profile. In the past, Ronchi gratings, placed in a defocused position, or gratings written in nematic liquid crystal displays (LCDs) or generated with digital micromirror devices (DMDs) have been used. The latest developments in the field of ferroelectric liquid-crystal-on-silicon (F-LCOS) displays make these devices interesting as the fringe generating element. They offer both high speed operation and high flexibility. Unlike other devices,
F-LCOS displays can also be operated under oblique incidence, still generating sufficient fringe contrast. We report on the characterization of a F-LCOS display and its application in two different setups. A comparison to Ronchi gratings and gratings written in transmissive twisted nematic LCDs is given. The achievable measurement resolutions as well as the measurement times are discussed. Results of measurements conducted on technical and medical surfaces are presented.
We report on the depth-scanning fringe projection technique (DSFP) which is an innovative triangulation method for absolute 3-D profiling of macroscopic scenes. This measurement principle combines the confocal principle with the fringe projection and phase evaluation techniques known from white light interferometry. Scanning of the focal plane and additional lateral shifting allow the phase to be determined for any desired depth range of the measurement volume. Hence, the limitations of the depth of sharpness occurring with projected light techniques can be overcome. A 3-D calibration method has been implemented in order to deal with the problem of geometrical aberrations and to provide a 3-D point cloud. The well-known direct calibration method has been adapted to our DSFP setup. It has been completed to a set of three polynomial transformations which allow to include the depth-scanning principle in the calibration of the system.