Deflectometry
Deflectometry refers to the non-contact acquisition or measurement of reflective surfaces. Techniques from photometry or radiometry , photogrammetry , laser scanning or laser distance measurement are used here.
While diffusely reflective bodies can be recorded by analyzing the brightness distribution of reflected light sources ( shape from shading ), the mirror images of known patterns are analyzed in the case of flat or curved, highly reflective surfaces in order to determine the shape of the surface.
Technical importance
Reflective freeform surfaces occur e.g. B. with eyeglass lenses, glazing of vehicles or their polished, painted surfaces, etc. on. During production and the subsequent quality control, the surfaces often have to be recorded with accuracy down to the sub-micrometer range. Scanning or dulling the surface, for example with chalk spray, is usually not desirable, so that only an optical measurement remains.
Physical basics
The pure reflection of light rays can often be adequately described with the law of reflection in the context of geometric optics . However, this only leads to simple imaging properties for symmetrical surfaces ( flat, spherical , parabolic mirrors ). For surfaces with any curvature, the law of reflection only applies locally to the tangential planes of the surface. Light rays can therefore be tracked using ray tracing . If the 3D coordinates of an object point and its mirror image (e.g. by laser distance measurement ) are known, in principle the reflection point on the reflecting surface can be calculated from this ( shape from image-object conjugation ). In the photographic measurement of an object point, the distance information is generally missing, that is, only directional information is available, from which the surface shape can be derived using more complex mathematical methods or in combination with other physical effects. For this purpose, the deformed images of stripe or checkerboard patterns are often examined.
Other uses
If the exact shape of a reflective surface is known, objects reflected in it can be reconstructed from their distorted images or, conversely, the distorted images of these objects can be calculated. Applications range from criminology (e.g. reconstruction of a face from a mirror image in a reflective door handle) to architecture (e.g. simulations of a street scene with glazed, curved facades).
literature
- Thomas Luhmann: Close-range photogrammetry. Wichmann, Heidelberg 2003, ISBN 3-87907-398-8 .
- Boris Thielbeer: Brilliant views - reflection measurement technology in vehicle construction. In: Th. Luhmann, Ch. Müller (ed.): Photogrammetrie-Laserscanning, Optical 3D-Messtechnik, Contributions to the Oldenburg 3D-Days 2010. Wichmann Verlag, Heidelberg 2010, ISBN 978-3-87907-494-5 .
- Thomas Schramm, Carlos Acevedo Pardo: Reflections on mirrors. In: Th. Luhmann, Ch. Müller (ed.): Photogrammetrie-Laserscanning, Optical 3D-Messtechnik, Contributions to the Oldenburg 3D-Days 2010. Wichmann Verlag, Heidelberg 2010, ISBN 978-3-87907-494-5 .
- Markus Knauer: Measurement of reflective surfaces - a task of optical 3D sensors . In: Photonics . No. 4 , 2004, p. 62-64 ( PDF ).
- Jochen Bähr, Ulrich Krackhardt, Dennis Dietrich: Image-free measurement of reflective free-form surfaces . In: Photonics . No. 5 , 2007, p. 80-83 ( PDF ).