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A brief discussion of units might be helpful. Back in the 1960s, when scatterometry started as a useful way to monitor surface roughness, it was common to express the rms roughness of mirrors in angstroms, spatial wavelengths in microns, and spatial frequency in inverse microns. Angstroms have given way to nanometers (ten times larger), and microns are now more properly called micrometers. Machine shop surface roughness in the United States has been expressed in microinches using a coarse geometric scale of 2, 4, 8, 16, 32, ..., but as pointed out in Chapter 2, machine slope roughness is usually expressed as an arithmetic average (a.a.) rather than rms (see Table 2.2).

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