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Measurement Parameters:
Ra = Roughness Average. The arithmetic average height of roughness irregularities
measured from a mean line within the evaluation length (L)
Rz = Mean Roughness Depth. The mean of 5 maximum peak to valley roughness depths in 5
successive sampling lengths
Rq = Maximum Roughness Depth. The largest peak to valley roughness depth in the evaluation
length
Rq - Root-Mean-Square Roughness
The root-mean-square (rms) average roughness of a surface is calculated from
another integral of the roughness profile:
For a pure sine wave of any wavelength and amplitude Rq is proportional to Ra; it's about 1.11 times larger. Older instruments made
use of this approximation by calculating Rq with analog electronics (which is easier than calculating with analog
electronics) and then multiplying by 1.11 to report Rq. However, real profiles are not simple sine waves, and the approximation often
fails miserably. Modern instruments either digitize the profile or do not report Rq. There is never any
reason to make the approximation that is proportional to Ra. Rq has now been almost completely superseded by Ra In metal
machining specifications. Rq still has value in optical applications where it is more directly related to the optical quality of a surface.
Rt, is the vertical distance from the deepest valley to the
highest peak. (sum of maximum height of the profile peak and max. depth of the profile valley for
the evaluation length)
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