>  What is area roughness?  >  Evaluation Processing and Filtering

# Evaluation Processing and Filtering

## Process of Evaluation

The parameters used in ISO 25178 Surface Texture are calculated based on the following:

1. Obtain the primary surface by surface filtering (using the S-filter) the real surface.
2. Based on the evaluation results, perform further surface filtering (using the F-operator and the L-filter) to obtain the scale-limited surface.
3. Specify the evaluation area from the scale-limited surface.
4. Obtain the reference surface for the scale-limited surface and calculate the parameters.

## Filtering

The standards for filtering are still under review by the International Organization for Standardization (ISO).

This section introduces the filtering methods being evaluated by ISO for the S-filter and the L-filter.

##### Gaussian filter

The Gaussian filter (named for Carl Gauss) is one kind of smoothing filter that suppresses noise using the Gaussian function. Gaussian filters that are specified in JIS B 0632:2001 (ISO 11562:1996) and ISO 16610-21:2011 are applied to areal surface roughness measurements.

##### Spline filter

The spline filter is one kind of filter used to obtain a smooth profile by interpolating the sections between effective adjacent points using the spline function. Spline filters that are specified in ISO/TS 16610-22:2006 are applied to areal surface roughness measurements.

## S-filter and L-filter settings

##### Cutoff wavelength for S-filter

A value equal to or more than 3 times the measurement resolution for the XY plane (horizontal plane) is used. If the set value is not sufficiently effective, increase the set value until the scale-limited surface noise is removed. If a different kind of filtering was applied beforehand, this may not be used.

##### Cutoff wavelength for L-filter

The cutoff wavelength for the L-filter is difficult to uniformly specify based on lens magnification or stylus tip diameter; therefore, it must be adjusted with reference to the real surface. Set a value 5 times the XY-directional length of the profile that you want to remove as waviness.

Example:

Profile 1 Horizontal distance
Section 1 14.678 μm 0.58 Mil

In the example above, the cutoff value is 0.1 mm 0.004".

14.678 x 5 = 73.39 ≈ 0.1 mm 0.004"