**2.2.1 Time domain characterization**

Most surface quality analysis including three-dimensional analysis has been traditionally based upon the surface tracing or surface profile. Most analysis of the surface profile generated by the stylus system has been evaluated using time domain parameters such as height deviations and asperity spacing or wavelength. Whitehouse (1982) gives a brief history of the development of surface quality evaluation techniques and the confusion that has developed due to new developments in measurement technology, lack of coordinated efforts between countries, changes in manufacturing processes resulting in different surface textures for a given part, and economic considerations affecting instrumentation development. King and Spedding (1983) discussed three categories of approaches that have been used to characterize a surface:


The figure below (figure 3) shows a common example of time domain measurements. The measurements include a measure of the average roughness, Rq (second moment, root mean square), a measure of "extremes" Rtm, a measure of whether the surface defects are above or below the average surface, a measure of skewness, Rsk (third moment), and a measure of the shape of the surface defects, Rku kurtosis (4th moment).
