Surface metrology is the measurement and characterization of surface topography, and is a branch of metrology. Surface primary form, surface fractality, and surface finish (including surface roughness) are the parameters most commonly associated with the field. Surface metrology is a fundamental measurement science critical across diverse manufacturing and engineering disciplines. While historically associated with precision machining and mechanical assemblies, it now plays essential roles in industries ranging from medical devices and electronics to aerospace and energy systems. Applications include ensuring biocompatibility of implants, optimizing semiconductor wafer quality, controlling paint adhesion in automotive manufacturing, enhancing solar panel efficiency, and managing thermal performance in electronic components. The field encompasses measurements from nanometer-scale surface features to large industrial components, making it indispensable for quality control, performance optimization, and failure prevention across modern manufacturing. Surface finish may be measured in two ways: contact and non-contact methods. Contact methods involve dragging a measurement stylus across the surface; these instruments are called profilometers. Non-contact methods include: interferometry, digital holography, confocal microscopy, focus variation, structured light, electrical capacitance, electron microscopy, photogrammetry and non-contact profilometers.
Overview The earliest ways of surface measurement relied on subjective tactile and visual inspection via fingernail and eye respectively. The quantitative way of measuring surfaces were based on this rough concept. This created two main branches of instrumentation that follow the contact and optical form coming from these rudimentary techniques. Contact techniques use physical probes to map the surface topography and optical methods employ light based systems to characterize surface features and geometry. The most common method is to use a diamond stylus profilometer. The stylus is run perpendicular to the lay of the surface. The probe usually traces along a straight line on a flat surface or in a circular arc around a cylindrical surface. The length of the path that it traces is called the measurement length. The wavelength of the lowest frequency filter that will be used to analyze the data is usually defined as the sampling length. Most standards recommend that the measurement length should be at least seven times longer than the sampling length, and according to the Nyquist–Shannon sampling theorem it should be at least two times longer than the wavelength of interesting features. The assessment length or evaluation length is the length of data that will be used for analysis. Commonly one sampling length is discarded from each end of the measurement length. 3D measurements can be made with a profilometer by scanning over a 2D area on the surface. The disadvantage of a profilometer is that it is not accurate when the size of the features of the surface are close to the same size as the stylus. Another disadvantage is that profilometers have difficulty detecting flaws of the same general size as the roughness of the surface. There are also limitations for non-contact instruments. For example, instruments that rely on optical interference cannot resolve features that are less than some fraction of the operating wavelength. Another limitation is that it cannot take reliable measurements of edge cross-sections. This limitation can make it difficult to accurately measure roughness even on common objects, since the interesting features may be well below the wavelength of light. The wavelength of red light is about 650 nm, while the average roughness, (Ra) of a ground shaft might be 200 nm. The first step of analysis is to filter the raw data to remove very high frequency data (called "micro-roughness") since it can often be attributed to vibrations or debris on the surface. Filtering out the micro-roughness at a given cut-off threshold also allows to bring closer the roughness assessment made using profilometers having different stylus ball radius e.g. 2 μm and 5 μm radii. Next, the data is separated into roughness, waviness and form. This can be accomplished using reference lines, envelope methods, digital filters, fractals or other techniques. Finally, the data is summarized using one or more roughness parameters, or a graph. In the past, surface finish was usually analyzed by hand. The roughness trace would be plotted on graph paper, and an experienced machinist decided what data to ignore and where to place the mean line. Today, the measured data is stored on a computer, and analyzed using methods from signal analysis and statistics.
Equipment
Contact (tactile measurement)
Stylus-based contact instruments have the following advantages:
The system is very simple and sufficient for basic roughness, waviness or form measurement requiring only 2D profiles (e.g. calculation of the Ra value). The system is never lured by the optical properties of a sample (e.g. highly reflective, transparent, micro-structured). The stylus ignores the oil film covering many metal components during their industrial process. Technologies:
Contact Profilometers – traditionally use a diamond stylus and work like a phonograph. Atomic force microscope are sometimes also considered contact profilers operating at atomic scale.
Non-contact (optical microscopes) Optical measurement instruments have some advantages over the tactile ones as follows:
no touching of the surface (the sample can not be damaged) the measurement speed is usually much higher (up to a million 3D points can be measured in a second) some of them are genuinely built for 3D surface topography rather than single traces of data they can measure surfaces through transparent medium such as glass or plastic film non-contact measurement may sometimes be the only solution when the component to measure is very soft (e.g. pollution deposit) or very hard (e.g. abrasive paper). Vertical scanning:
Coherence scanning interferometry Confocal microscopy Focus variation Confocal chromatic aberration Horizontal scanning:
Scanning laser microscope (SLM) Structured-light scanning Non-scanning
Digital holographic microscopy
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