A trace that looks jagged and rough to the eye is really a distorted view of a nearly flat surface with moderate ripples across it. There is no difference between the 40X vertical magnification and a straight line with tile resolution of a laser printer. Always keep in mind the extreme vertical magnification when looking at surface profiles produced by a practical instrument. In a profile measurement the evaluation length, the length over which data may be collected, is shorter than the physical traverse length because of end effects in the motor control and settling times for optional electronic filters. An evaluation length consists of one or more sample lengths.
Many roughness parameters are statistical averages of values for the individual sample lengths.
A standard roughness evaluation length comprises five sample lengths. The sample length is always equal to the filter cutoff length. A single sample length is denoted l. There is often not a clear distinction made between the sample length and the evaluation length, even within a particular instrument manufacturer. Be very careful to decipher what is meant when any of these three terms is used.