Martin Schaffer

High-speed pattern projection for three-dimensional shape measurement using laser speckles

We propose a high-speed projection system that is able to project statistical speckle patterns at a rate of 500Hz. Its purpose is to generate structured light for a real-time photogrammetry stereo vision setup. As conventional digital light projector (DLP) projection setups are limited in their maximum projection rate to 250Hz for gray-value patterns, stripe projection systems are usually applied for real-time three-dimensional (3D) measurements. However, these techniques can only be used on steady surfaces as phase unwrapping has to be done. In contrast, the proposed setup is able to measure the shape of multiple spatially separated objects at once. We compare the speckle setup with a system using a DLP projector and with other fast 3D shape measurement setups, like the widely used stripe projection methods, qualitatively and quantitatively.

High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection.

Many three-dimensional (3D) shape measurement techniques in stereophotogrammetry with temporal coded structured illumination are limited to static scenes because the time for measurement is too long in comparison to the object speed. The measurement of moving objects result in erroneous reconstructions. This is apparent to reduce measurement time to overcome this limitation, which is often done by increasing the projection rate for illumination while shrinking the amount of images taken for reconstruction. The projection rate limits most applications in its speed because digital light processing (DLP) projectors, which are widely used, bring a limited projection rate along. Our approach, in contrast, does not take a DLP. Instead we use laser speckles as projected patterns which are switched using an acousto-optical deflector. The projection rate is 10× higher than what the fastest stripe projection systems to our knowledge achieve. Hence, we present this uncommon but potential approach for highspeed (≈250 3Dfps= [3D measurements per second]), dense, and accurate 3D measurements of spatially separated objects and show the media that emphasizes the ability of accurate measurements while the objects under testing move.

Fast data acquisition for three-dimensional shape measurement using fixed-pattern projection and temporal coding

The field of image-based shape measurements using structured illumination has been an active research area for several decades now. A lot of different methods are widely used e.g. for industrial inspection, surveillance as well as multimedia purposes. Usually, short measurement times and high correspondence accuracy are mutually exclusive properties, due to the limitations of the used projection technology. Using a standard stereo-photogrammetry approach, as well as a simple, yet powerful structured light projection concept based on a slide-projector, we demonstrate an acquisition rate of more than 700 dense and accurate 3D measurements per second. C ©2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3646100] Subject terms: photogrammetry; optical metrology; structured light; image processing.. Paper 110814LR received Jul. 12, 2011; revised manuscript received Sep. 6, 2011; accepted for publication Sep. 14, 2011; published online Oct. 11, 2011. There are two distinct approaches to optical shape measurement systems employing structured light — profilometry1, 2 and photogrammetry.3, 4 The former methods exploit the known pattern structure and rely on a combined projectorcamera calibration. The latter methods only induce a desired illumination structure onto the object and use multi-camera calibration as well as image correlation algorithms. Furthermore, both approaches can be subdivided into techniques using spatial or temporal image information,1, 2 where spatial techniques require only one image per view for threedimensional (3D) reconstruction and temporal techniques need a sequence of images with varying illumination for 3D reconstruction. Although spatial techniques are fast, they are comparably inaccurate and usually lead to sparse 3D reconstructions.2 Single-image Fourier-transform fringe projection methods deliver dense reconstructions, but the phase unwrapping process, which is necessary to resolve ambiguous phases, still limits the complexity of scenes (height jumps larger than stripe pitch, steep slopes).5, 6 To conclude, temporal coding methods can provide dense and accurate reconstruction of complex scenes, although Refs. 1 and 2 state that due to the temporal length of the data acquisition these techniques are limited to static scenes. Several groups showed 0091-3286/2011/

Coherent two-beam interference fringe projection for highspeed three-dimensional shape measurements

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3D shape measurement of static and moving objects with adaptive spatiotemporal correlation

Two-beam interference is a fundamental and well-understood approach to create Fizeaus interference fringes. With a Mach-Zehnder interferometer, we utilize these two-beam interference Fizeau fringes for three-dimensional (3D) shape measurements. By introducing an acousto-optical deflector the phase of the interference fringes can be shifted with a rate of up to 200,000 Hz. When used in conjunction with highspeed cameras, this stereo-photogrammetric approach performs well for highspeed applications in comparison with the commonly used digital light processing projectors for stripe projection. Maximum speed and the achievable accuracy are discussed. Experiments and media substantiate the suitability, accuracy, and speed of this technique for very fast 3D shape measurements.

Outdoor three-dimensional shape measurements using laser-based structured illumination

Common correlation-based photogrammetric 3D shape measurement techniques evaluate either temporal or spatial features. Temporal approaches achieve high accuracies but are limited to the measurement of static objects. Spatial techniques can deal with moving objects but provide relatively inaccurate results. Our goal is to combine these methods in order to measure dynamic scenes that contain static and moving objects. Therefore, we present a spatiotemporal correlation that adapts its temporal and spatial support locally to the motion of the measured objects. In addition, our technique compensates motion by warping the correlated image regions temporally. Our approach is based on structured illumination of random patterns, which are well suited for dynamic scenes due to high possible frame rates. The proposed technique is tested with simulated data and real measurements.

Statistical patterns: an approach for high-speed and high-accuracy shape measurements

Abstract. Three-dimensional shape measurements using structured illumination can achieve accurate shape representation of arbitrary object scenes. High-speed approaches have been recently developed and are already on the way into industry. Structured illumination for outdoor applications is rarely used as it is difficult to achieve a high-pattern contrast under bright light conditions. Hence, laser scanning or photogrammetry without active illumination is usually preferred; often, measurements are carried out at night. A shape measurement approach is suggested, one that is suitable to quickly and accurately acquire object shapes, even under difficult light circumstances. This approach is based on laser speckle projection and spectral filtering. The results made with this setup are presented, along with media that underlines its capabilities.

Camera calibration using time-coded planar patterns

Abstract. Statistical patterns have been used for structured illumination within a stereo-photogrammetry setup to precisely measure the shape of nearly arbitrary objects in a short time. This contribution gives an overview of recently developed projection setups based on such statistical patterns. Coherent and incoherent approaches as well as the applied reconstruction algorithm are explained. The results show the suitability of the statistical pattern projection approach to replace the commonly used slow digital light processing (DLP) projectors of three-dimensional shape sensors and facilitate measurements in an ultrashort time frame (microsecond range), e.g., to track moving objects.

Coherent Pattern Projection for Highspeed 3D Shape Measurements

In this paper a novel pattern design for camera calibration using planar patterns with the Zhang calibration method is presented. In contrast to other work related to this calibration technique, which deals with the design and extraction of spatial image features, our focus lies on the precise and fully automated extraction of corresponding points by temporal image features using a sequence of planar patterns, which are displayed by a flat screen. The extraction of correspondences in our approach does not utilize areal properties of the images and therefore is hardly influenced by projective distortion, image distortion, or inhomogeneous illumination. Furthermore, not the whole pattern but only parts of it need to be visible in an individual view. In addition, the planarity as well as the physical precision of the control points of the pattern are ensured by the very nature of the flat screen. The overall calibration time may be reduced, as no human interaction is necessary. We compare the results gained from this novel temporal pattern design with those gained from commonly used checkerboard patterns. The proposed approach resulted in an increased precision for the calibration.

Influence of the structured illumination frequency content on the correspondence assignment precision instereophotogrammetry

Since many years laser triangulation is a key technology for metrological measurements. Manifold sensors exist that measure single spot distances or distance profiles via triangulation between the illumination source (e.g. laser diode) and an imaging system incorporating typically a digital detector. The physical limits have been investigated and derived by several researchers [1]. It has been stated that the main limitation for laser triangulation is subjective laser speckle noise that occurs when the laser spot or line is imaged onto the detector.