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ISO 25178: Geometric Product Specifications (GPS) – Surface texture: areal is an "International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal "surface texture.

Contents

Structure of the standard[edit]

Documents constituting the standard :

Other documents might be proposed in the future but the structure is now almost defined. Part 600 will replace the common part found in all other parts. When revised, parts 60x will be reduced to only contain descriptions specific to the instrument technology.

New features[edit]

It is the first international standard taking into account the specification and measurement of 3D surface texture. In particular, the standard defines 3D surface texture parameters and the associated specification operators. It also describes the applicable measurement technologies, "calibration methods, together with the physical calibration standards and calibration software that are required.

A major new feature incorporated into the standard is coverage of non-contact measurement methods, already commonly used by industry, but up until now lacking a standard to support quality audits within the framework of "ISO 9000. For the first time, the standard brings 3D surface "metrology methods into the official domain, following 2D "profilometric methods that have been subject to standards for over 30 years. The same thing applies to measurement technologies that are not restricted to contact measurement (with a diamond point "stylus), but can also be optical, such as chromatic confocal gauges and "interferometric microscopes.

New definitions[edit]

The ISO 25178 standard is considered by TC213 as first and foremost providing a redefinition of the foundations of surface texture, based upon the principle that nature is intrinsically 3D. It is anticipated that future work will extend these new concepts into the domain of 2D profilometric surface texture analysis, requiring a total revision of all current surface texture standards (ISO 4287, ISO 4288, ISO 1302, ISO 11562, ISO 12085, ISO 13565, etc.)

A new vocabulary is imposed:

The new available filters are described in the series of technical specifications included in "ISO 16610. These filters include: the Gaussian filter, the spline filter, robust filters, morphological filters, wavelet filters, cascading filters, etc.

Parameters[edit]

Generalities[edit]

3D areal surface texture parameters are written with the capital letter S (or V) followed by a suffix of one or two small letters. They are calculated on the entire surface and no more by averaging estimations calculated on a number of base lengths, as is the case for 2D parameters. In contrast with 2D naming conventions, the name of a 3D parameter does not reflect the filtering context. For example, Sa always appears regardless of the surface, whereas in 2D there is Pa, Ra or Wa depending on whether the profile is a primary, roughness or waviness profile.

Height parameters[edit]

These parameters involve only the statistical distribution of height values along the z axis.

Parameter Description
Sq Root mean square height of the surface
Ssk Skewness of height distribution
Sku Kurtosis of height distribution
Sp Maximum height of peaks
Sv Maximum height of valleys
Sz Maximum height of the surface
Sa Arithmetical mean height of the surface

Spatial parameters[edit]

These parameters involve the spatial periodicity of the data, specifically its direction.

Parameter Description
Sal Fastest decay auto-correlation rate
Str Texture aspect ratio of the surface
Std Texture direction of the surface

Hybrid parameters[edit]

These parameters relate to the spatial shape of the data.

Parameter Description
Sdq Root mean square gradient of the surface
Sdr Developed area ratio

Functions and related parameters[edit]

These parameters are calculated from the material ratio curve ("Abbott-Firestone curve).

Parameter Description
Smr Surface bearing area ratio
Sdc Height of surface bearing area ratio
Sxp Peak extreme height
Vm Material volume at a given height
Vv Void volume at a given height
Vmp Material volume of peaks
Vmc Material volume of the core
Vvc Void volume of the core
Vvv Void volume of the valleys

Parameters related to segmentation[edit]

These parameters are derived from a "segmentation of the surface into "motifs (dales and hills). Segmentation is carried out using a "watershed method.

Parameter Description
Spd Density of peaks
Spc Arithmetic mean peak curvature
S10z 10 point height
S5p 5 point peak height
S5v 5 point valley height
Sda Closed dales area
Sha Closed hills area
Sdv Closed dales volume
Shv Closed hills volume

Software[edit]

At the moment a consortium of several company are working on a free implementation of 3D surface texture measurement software. This is called OpenGPS [1]. Several commercial packages provide part or all of the parameters defined in ISO 25178, such as "MountainsMap from Digital Surf, SPIP from Image Metrology[2] as well as the open source Gwyddion.

Instruments[edit]

Part 6 of the standard divides the usable technologies for 3D surface texture measurement into three families:

  1. "Topographical instruments: contact and non-contact 3D profilometers, interferometric and confocal microscopes, structured light projectors, stereoscopic microscopes, etc.
  2. Profilometric instruments: contact and non-contact 2D profilometers, line triangulation lasers, etc.
  3. Instruments functioning by integration: "pneumatic measurement, "capacitive, by optical diffusion, etc.

and defines each of these technologies.

Next, the standard explores a number of these technologies in detail and dedicates two documents to each of them:

Contact profilometer[edit]

Parts 601 and 701 describe the contact profilometer, using a diamond point to measure the surface with the assistance of a lateral scanning device.

Chromatic confocal gauge[edit]

Part 602 describes this type of non-contact profilometer, incorporating a single point white light chromatic confocal sensor. The operating principle is based upon the chromatic dispersion of the white light source along the optical axis, via a confocal device, and the detection of the wavelength that is focused on the surface by a "spectrometer.

Coherence scanning interferometry[edit]

Part 604 describes a class of optical surface measurement methods wherein the localization of interference fringes during a scan of optical path length provides a means to determine surface characteristics such as topography, transparent film structure, and optical properties. The technique encompasses instruments that use spectrally broadband, visible sources (white light) to achieve interference fringe localization). CSI uses either fringe localization alone or in combination with interference fringe phase.

Focus variation[edit]

Part 606 describes this type of non-contact areal based method. The operating principle is based on a microscope optics with limited depth of field and a CCD camera. By scanning in vertical direction several images with different focus are gathered. This data is then used to calculate a surface data set for roughness measurement.

See also[edit]

References[edit]

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