Surface metrology

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Surface metrology is the science of measuring small-scale features on surfaces, and is related to Metrology. Surface primary form, surface waviness and surface roughness are the parameters most commonly associated with the field. Surface metrology is important to many disciplines including:

It is particularly important in the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.

An instrument known as a profilometer often is used to measure a small-scale profile of the surface. These traditionally used a stylus and worked much like a phonograph. Newer versions often employ optical interferometry and digital holography. Depending upon the intended eventual use of a surface, metrology repeatability and accuracy may be desired to the level of microns or even nanometers.

Parameters used to describe surfaces include:

  • Ra, Roughness Average (Absolute value of the surface height averaged over the surface)
  • Rq, Root Mean Square (RMS) Roughness
  • Rv, Maximum Profile Valley Depth
  • Rp, Maximum Profile Peak Height
  • Rt, Maximum Height of the Profile (Rv+Rp)
  • Sm, Mean Peak Spacing
  • la, Average Wavelength
  • lq, RMS Average Wavelength
  • Da, Average Slope
  • Dq, RMS Average Slope
  • lr, Profile Length Ratio
  • Rsk, Skewness
  • Rku, Kurtosis

Most of these parameters originated from the capabilities of profilometers and other mechanical probe systems. In addition, new measures of surface dimensions have been developed which are more directly related to the measurements made possible by high-definition optical gauging technologies.

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