# Cutoff สำหรับการวัดผิว

Definitions
In any discussion of this type, we need to start with a few definitions. The important ones here are: Roughness A quantitative measure of the process marks produced by the creation of the surface and other factors such as the structure of the material. Not to be confused with Waviness or Form, which have longer wavelengths.

Filter – A process to exclude wavelengths above or below a particular frequency. The measurement system is a mechanical filter. Software can perform mathematical filtering.
Cut-off – The wavelength at which a filter becomes effective. For surface parameters we normally analyse wavelengths between an upper and lower cut-off: these are referred to as Ls or λs (shortest) and Lc or λc (longest). “Cut off” is also used synonymously with Sample Length as the sample length is always set to Lc.
Bandwidth is the ratio of Ls to Lc.