Technical Analysis of Surface Profile Measurement in Smoothness Testing

Expanding on the airflow-based measurement of smoothness, advanced Paper Smoothness Testers and related surface metrology instruments can employ direct surface profile measurement techniques to provide an even more detailed technical analysis of paper surface texture.
 

These instruments utilize non-contact optical profilometers or contact stylus profilometers to directly measure the microscopic variations in the height of the paper surface.
 

Optical profilometers typically use interferometry or confocal microscopy to create a three-dimensional map of the paper surface topography without physically touching it. These techniques offer high resolution and can capture very fine surface details. The instrument directs a light beam onto the paper surface, and the reflected or scattered light is analyzed to determine the height of different points on the surface.
 

Contact stylus profilometers involve dragging a very fine diamond stylus across the paper surface under a controlled force. A sensor measures the vertical movement of the stylus as it follows the contours of the surface, generating a two-dimensional profile of the surface roughness along the trace. By scanning the stylus across the surface, a three-dimensional map can also be created.
 

The data acquired by these surface profiling techniques goes beyond a single smoothness value. It allows for the calculation of various surface roughness parameters as defined in standards like ISO 25178. These parameters include:

Ra (Arithmetic mean roughness): The average of the absolute values of the height deviations from the mean line over the evaluation length.

Rq (Root mean square roughness): The root mean square average of the height deviations from the mean line over the evaluation length.

Rz (Maximum height of the profile): The vertical distance between the highest peak and the lowest valley within the evaluation length.

Rp (Maximum peak height): The height of the highest peak above the mean line.

Rv (Maximum valley depth): The depth of the lowest valley below the mean line.

These parameters provide a much more comprehensive characterization of the paper surface texture than a single airflow-based smoothness value. They can reveal information about the height and spacing of surface features, which can be critical for predicting print quality issues like mottling or missing dots, as well as the tactile feel of the paper.
 

The resolution of the profilometer is a key technical specification, determining the smallest surface features that can be accurately measured. Optical profilometers typically offer nanometer-level resolution.

The sampling length and evaluation length are also important parameters that define the area of the paper surface being analyzed and the length over which the roughness parameters are calculated. These parameters are often specified in testing standards or chosen based on the application.

In conclusion, advanced Paper Smoothness Testers employing surface profiling techniques provide a highly detailed technical analysis of paper surface texture by directly measuring the microscopic height variations. The resulting surface roughness parameters offer a more comprehensive understanding of the paper's surface characteristics than airflow-based methods, enabling a more accurate prediction of its performance in various applications.