Structured-light 3D scanner

The stripe projection, which is sometimes also referred to as Streifenlichttopometrie comprising optical measuring methods are used in the image sequences for three-dimensional detection of surfaces.

• 5.1 Surveying the scene
• 5.2 Reverse Engineering
• 5.3 wound documentation

Principle

Construction of a sensor

A strip projection sensor is composed of at least a pattern projector, a slide similar to the principle, and at least one digital or analog video camera. In commercial systems, meanwhile structures with a projector, and one or two cameras have become established.

Taking a Measurement

The projector illuminates the object to be measured sequentially in time with the pattern of parallel light and dark stripes of different widths. The camera ( s ) to register the projected fringe pattern at a known angle to the projection. For each projection pattern a picture is taken with any camera. As a time sequence of different brightness values ​​for each pixel from all the cameras.

Calculation of the surface coordinates

The desired three-dimensional coordinates of the surface can be calculated in two steps.

Determination of Projektorkoordinate

At a given object point, the image coordinates of the camera image are known. The projector corresponds to a reverse camera. From the sequence of brightness values ​​measured from the image sequence for each camera pixel, the number of the strip can be calculated. In the simplest case of a binary code (eg, a Gray code) is done identifies the number of the strip as discrete coordinate in the projector. Higher accuracy can be achieved with the so-called phase-shift method, because it can not determine a discrete coordinate. It can be used either as a supplement or as a Gray code absolute measuring heterodyne.

Forward incision

The strip number in the projector corresponding to the image coordinates in the camera. The strip number specifies a light plane in space, the image coordinate a light beam. With known camera and projector position, the intersection of the plane and the straight line are calculated. Which is the desired three-dimensional coordinate of the object point in the coordinate system of the sensor. The geometric location of all image rays must be known accurately. The exact calculation of the radiation takes place with the forward section known from photogrammetry.

Calibration

Important for the calculation of the coordinates and the guaranteed accuracy of the results is a precise calibration of the imaging properties. All imaging properties of projectors and cameras are described using a mathematical model. The basis is a simple pinhole camera, where all image rays from the object point in three-dimensional space through a common point, the projection center, run and be displayed in the corresponding pixel on the sensor or film.

In addition to the non-ideal properties in this model of real lens systems, resulting in distortion of the image can be adjusted by a distortion.

The mentioned parameters of the pinhole camera and its position and orientation in space are determined from a series of calibration images with photogrammetric methods, in particular with a bundle adjustment computation.

Navigation

A single measurement with fringe projection sensor is limited in their entirety by the visibility of the object surface. So that a point on the surface can be detected, it must be illuminated by the projector and observed by the cameras. Points that are, for example, on the back of the object, must be recorded in a separate measurement under a different angle of the sensor.

For a complex object may be necessary for the complete detection of very many ( several hundred) individual measurements. So you can merge the results of all measurements in a common coordinate system, the following methods are used: the attachment of point markers on the object, the matching of object features, or to accurately measure the position sensor with an additional measuring system. This process is called navigation.

Accuracy

The achievable measurement accuracy is proportional to the square root of the measurement volume. Commercial systems, which are used in the field of reverse engineering, to achieve an accuracy of 0.003 mm to 0.3 mm depending on the technical requirements and the measurement volume. Systems with microscopic measurement fields below 1 cm ² can contribute to the assessment of micro- geometries, such as Radii are used to cutting edges or the assessment of microstructures and reach there measuring accuracies below 1 micron.

Applications

In addition to applications in medicine, dentistry and pathology fringe projection sensors are mainly used in industry, in the design process for new products ( reverse engineering ) and in the shape control of workpieces and tools ( target-actual comparison ) are used. With thousands of systems installed in Germany (estimated as of April 2005), they are very widely used in the automotive and aircraft industries and provide in many applications, a preferred alternative to mechanical coordinate measuring dar.

Surveying the scene

With the help of strip light scanners in surface cameras forensic scientists scan a scene from three-dimensional and so create a 3D image of the scene, which can be accurately analyzed and screened for traces out without the police crime scene group must enter the premises and might be altered or destroyed evidence. A partial application of these is the three-dimensional scanning of soil marks and footprints on the previous pouring with gypsum has the advantage that the crime scene is non-contact and the impression can be duplicated as many times.

Reverse Engineering

The example illustrates the reverse engineering process using the example of an historic race car. , The Silver Arrow W196, built in 1954 From Original ( 1) in 14 hours measuring time a cloud of points (2) to 98 million measurement points was generated. These have been reduced to paraxial cuts at a distance of two centimeters (3 ) on which a CAD model (4) is constructed in approximately 80 hours. Based on the CAD model, finally, a replica ( 5) in 1:1 scale was made, which is on display at the Mercedes- Benz Museum in Stuttgart -Untertürkheim today.

Wound documentation

The wound documentation using Streifenlichttopometrie evident to all imaging methods used in forensic practice as superior because it allows the spatial metric exact representation with realistic colors of individual relevant points of injury. The result is a "digital wounds man " and allows for the objectification of the outer inquest.

Credentials

• Klaus Körner, Ulrich Droste: Deep Scanning fringe projection ( DSFP ) University of Stuttgart

• Fringe 2005, The 5th International Workshop on Automatic Processing of Fringe Patterns Berlin: Springer, 2006 ISBN 3-540-26037-4 ISBN 978-3-540-26037-0.

• Christian Kohler, Klaus Körner: Streifentriangulation with spatial light modulators ( University of Stuttgart)

• Hof, C., Hopermann, H.: Comparison of Replica -and In Vivo Measurement of the Microtopography of Human Skin University of the Federal Armed Forces, Hamburg

• Frankowski, G., Chen, M., Huth, T.: Real-time 3D Shape Measurement with Digital Stripe Projection by Texas Instruments Micromirror Devices ( DMD ) Proc. Of SPIE Vol. 3958 (2000), pp. 90-106

• W. Wilke: segmentation and approximation of large point clouds ( Darmstadt University dissertation, 2000; PDF, 4.5 MB. )

• Frankowski, G., Chen, M., Huth, T.: Optical Measurement of the 3D Coordinates and the Combustion Chamber Volume of Engine Cylinder Heads Proc. Of " Fringe 2001 ', pp. 593-598

• G. Wiora: Optical 3D metrology precision shape measurement with an extended strip projection method (Dissertation Univ Heidelberg, 2001. )

• Peng, T., Gupta, S.K., Lau. K.: Algorithms for constructing 3 -D point clouds using multiple digital fringe projection patterns (PDF, 2.0 MB). CAD Conf., Bangkok, Thailand, June 2005

• Song Zhang, Peisen Huang: High-resolution, real -time 3 -D Shape Measurement ( PhD dissertation, Harvard University, 2005; PDF; 8.1 MB. )

• Elena Stoykova, Jana Harizanova, Venteslav Sainov: Pattern Projection Profilometry for 3D Coordinates Measurement of Dynamic Scenes. In: Three Dimensional Television, Springer, 2008, ISBN 978-3-540-72531-2

• Optical Metrology