Focus variation

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The focus variation , also called shape-from- (de) focus , is a method for obtaining an image with full depth of field and 3D information of the object in optics with limited depth of field . This can be used for surface topography measurements. The method is also combined with active lighting in order to also measure poorly structured, for example reflective, surfaces.

algorithm

The algorithm includes the following steps:

  1. First, pictures are taken with different focus.
  2. Then the sharpness on the individual images is determined for a group of adjacent pixels.
  3. The point with the best sharpness determines the sharply depicted texture. The depth information can also be determined with the information on the distance at which this point was sharply imaged.

Areas of application

This technology can be used for optics with low magnification ( macro lens ) up to optics with very high magnification ( microscope with 5000 ×). In particular, with this method the topography of the surface of a sample can be measured so precisely that measurements of roughness and geometry are possible. When measuring the topography by means of focus variation, the sample can not only be illuminated coaxially, but also with a ring light. This means that the maximum possible inclination of the sample is not limited by the numerical aperture of the objective .

Normalization

The focus variation is described, among other things, in the standard DIN EN ISO 25178-2 as a possible method for measuring surface roughness. The draft standard DIN EN ISO 25178-606 also describes metrological features of microscopes with focus variation for measurement for the creation of images of the surface topography.

See also

literature

  1. ↑ Microscopic Shape from Focus Using Active Illumination, M. Noguchi and SK Nayar, IAPR International Conference on Pattern Recognition (ICPR), Vol. A, pp. 147-152, Oct. 1994.
  2. ↑ Standard DIN EN ISO 25178-2: 2012: Geometrical product specifications (GPS) - Surface quality: Areal - Part 2: Terms and surface parameters
  3. ↑ Draft standard DIN EN ISO 25178-606: 2013: Geometrical product specifications (GPS) - Surface quality: Areal - Part 606: Features of non-contact measuring devices (focus variation)
  • R. Danzl, Franz Helmli, S. Scherer: Focus variation - a new technology for high resolution optical 3d surface metrology. The 10th International Conference of the Slovenian Society for Non-Destructive Testing, 2009, Ljubljana, Slovenia.

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