ISO 25178


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

Structure of the standard

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

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, but can also be optical, such as chromatic confocal gauges and interferometric microscopes.

New definitions

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
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

Generalities

3D areal surface texture parameters are written with the capital letter S 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

These parameters involve only the statistical distribution of height values along the z axis.
ParameterDescription
SqRoot mean square height of the surface
SskSkewness of height distribution
SkuKurtosis of height distribution
SpMaximum height of peaks
SvMaximum height of valleys
SzMaximum height of the surface
SaArithmetical mean height of the surface

Spatial parameters

These parameters involve the spatial periodicity of the data, specifically its direction.
ParameterDescription
SalFastest decay auto-correlation rate
StrTexture aspect ratio of the surface
StdTexture direction of the surface

Hybrid parameters

These parameters relate to the spatial shape of the data.
ParameterDescription
SdqRoot mean square gradient of the surface
SdrDeveloped area ratio

Functions and related parameters

These parameters are calculated from the material ratio curve.
ParameterDescription
SmrSurface bearing area ratio
SdcHeight of surface bearing area ratio
SxpPeak extreme height
VmMaterial volume at a given height
VvVoid volume at a given height
VmpMaterial volume of peaks
VmcMaterial volume of the core
VvcVoid volume of the core
VvvVoid volume of the valleys

Parameters related to segmentation

These feature parameters are derived from a segmentation of the surface into motifs. Segmentation is carried out using a watershed method.
ParameterDescription
SpdDensity of peaks
SpcArithmetic mean peak curvature
S10z10 point height
S5p5 point peak height
S5v5 point valley height
SdaClosed dales area
ShaClosed hills area
SdvClosed dales volume
ShvClosed hills volume

Software

A consortium of several companies started to work in 2008 on a free implementation of 3D surface texture parameters. The consortium, called OpenGPS
later focused its efforts on an XML file format that was published under the ISO standard ISO 25178-72.
Several commercial packages provide part or all of the parameters defined in ISO 25178, such as MountainsMap from Digital Surf, SPIP from Image Metrology as well as the open source Gwyddion.

Instruments

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 : 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:
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

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

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. CSI uses either fringe localization alone or in combination with interference fringe phase.

Focus variation

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.