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Surface finish

Surface finish is a measurement in a special category of surface roughness, which includes a determination of the qualities of a surface in three dimensions, to ascertain that it falls within acceptable parameters to ensure a finished surface is suitable to an intended purpose, which may be esthetic, optical, chemical, wear-related or mechanical.

Processing methods that are common to metal parts include casting or rolling (to create a basic form) cutting or sawing (to fit a general size), machining or turning (to reshape or create structure), grinding and polishing (which are often used to refine the surface finish).

Why is surface finish measured?

In some cases, surface finish is primarily a matter of appearance, and it is measured for purposes of ensuring consistency between finished parts. Indeed, surface finish comparators have been available for many years that are strictly visual and tactile in nature, and are designed to provide a rough estimate of the degree of roughness present on a worked surface.

However, surface finish is also a concern for other reasons, such as optical qualities. For example, a ‘brush finished’ metal surface might be used as the reflector in a light fixture. But, depending on the microstructure of the finish, the surface may reflect light well, might direct the light, or may absorb or scatter the light and make the fixture darker. Which would be more appealing to consumers?

Surface finish: the measurement

Like surface roughness, surface finish measurement is concerned with the highest-frequency features of the surface. In order to measure surface finish, underlying structure must be removed from measurement results.

Surface finish concerns

Surface shape is the overall geometry of the area of interest. “Area of interest” varies according to the application. For example, if you were measuring the topography of a lens, “shape” might be described as a measurement of the lens’s curvature, as compared to a manufacturing specification. Ultimately, this is a measure of the lens’s ability to focus correctly. Other shapes of interest in materials sciences and engineering are planes, spheres, toroids, cylinders, parabolas and aspherical and free-form curves.

Measuring surface shape requires a field of view large enough to include shape geometries.

Surface finish typically refers to a level of polishing or texture intended for, or resulting on, a surface.

On a lens, it is usually desirable to have as little roughness on a lens surface as possible, if light is being guided, so that light is scattered as little as possible — but this roughness (or smoothness, ideally) doesn’t include the general curvature of the surface shape.

In other applications, roughness may need to be optimized: an adhesive may need a certain amount of roughness to permit air pockets for hardening, while presenting enough surface area to bond.

Optical profilers and stylus profilers are suitable for measuring most surface roughness applications.

Determining surface finish with optical profilers

  • Optical profilometers measure area as well as height
  • Measurement accuracy that reaches picometer repeatability (0.1 Angstrom)
  • No diamond-tipped stylus to damage or alter fragile or soft surfaces
  • 3-D measurements can calculate volumes of bumps, mesas or voids
  • Is better able to measure co-planarity of discontiguous areas in the field of view
  • Large field of view of Filmetrics profilometers produces results of multiple long scans