## Download a PDF version of the Surface Texture Parameters Glossary

# 3D Legacy Parameters (WYKO 3D Parameters)

**Rpm, Rvm, Rz****(X,Y) Slope Rq****NormVolume****SAI**

## Rpm, Rvm and Rz (Average Max Peak Height, Valley Depth and Height of Surface)

The **Rpm**, **Rvm**, and **Rz** parameters
are evaluated from an average of the heights and depths of a number
of extreme peaks and valleys.** Rpm **the** **Average
Maximum Peak Height, ** **is found by averaging the
heights of the ten (10) highest peaks found over the complete 3D image. **Rvm, **the** **Average
Maximum Valley Depth, is found by averaging the depths of the ten (10)
lowest valleys found over the complete 3D image. **Rz, **the Average
Maximum Height of the Surface, is found from **Rpm**-**Rvm**.
Note that in determining the peaks and valleys, the analysis software eliminates
a grid of 11 x 11 pixels around a given peak/valley before searching for
the next peak/valley, thus assuring that significantly separated peaks/valleys
are found.

Application

**Rpm** may be useful in establishing the height
of the dominant peaked structures which may easily be plastically deformed
under contact or penetrate a coating. **Rvm** provides an estimate of the average
valley depths and may be useful in understanding fluid flow through an
interface and space for debris entrapment.
**Rz** provides an estimate of the overall peak to valley magnitude of a surface
and may serve to predict the thickness of coating needed to completely cover
and level a surface.

## X Slope Rq and Y Slope Rq (X and Y Root Mean Squared Slope)

**X Slope Rq** and **Y Slope Rq**,
the Root Mean Squared X Slope and Root Mean Squared Y Slope are
found by calculating the standard deviation (i.e. rms) of the slopes of
the surface along the X and Y directions respectively. The slope is found
by taking the derivative of the surface profiles along each direction,
using the lateral resolution of the measurement area as the point spacing. Analytically, **X
Slope Rq** and **Y Slope Rq** are given by:

Where the brackets, < >, represent the average value of all slopes in the relevant direction.

Application

Slopes may be used to investigate the behavior
of various surface texture forming techniques in that material and process
conditions may change the slopes considerably, whereas the height parameters
such as **Ra** are relatively unaffected. The surface slope
may also be useful in relating the nature of the surface when used in coating
and visual appearance applications.

## NormVolume (Normalized Surface Volume)

**NormVolume**, the Normalized
Surface Volume, is found by calculating the volume contained by
the surface and a “plane” that is placed near the top of
the surface. The placement of the reference plane is done on a statistical
basis to ensure that the very high peak locations are not used as the
reference point for the plane. Once the volume is calculated (e.g.
in units of cm3), the volume result is “normalized” to the
cross sectional area of the plane (e.g. units of m^{2}). Other units
of **NormVolume** are BCM which is an acronym for “Billions
of Cubic Microns per Inch Squared”. The BCM units are typically
used in the printing industry.

Application

**NormVolume** finds application in the printing industry,
by providing a measure of the fluid holding properties of printing device
surfaces (e.g. anilox rolls, Lithoplates, etc. ) and the printed media
(e.g. paper). Other applications, such as in the tribology field use the
NormVolume to establish the volume of space a surface provides for lubricant
containment. The measurement of various wear patters may result in a structure
in which NormVolume may quantify the amount of material removed from a
surface or displaced along a surface.

## SAI (Surface Area Index)

**SAI**, the Surface Area
Index, is the surface area at the lateral resolution of the measured/filtered
surface as compared to that of a perfectly flat/smooth surface. The calculation
involves fitting triangular patches between the measured points and adding
up the total area of all patches. A ratio is then formed of the total
surface area measured and the nominal flat area of measurement. This
analysis is a precursor to a complete fractal analysis of the surface.^{i}
Since **SAI** is a ratio, it is a unitless quantity.

Application

The combination of different surface texture amplitudes and spacings are further manifested in the overall surface provided by the texture. The SAI parameter finds application for issues relating to surface wetting, fluid flow, coating adhesion, printing etc.

^{i} C.A. Brown, P.D. Charles, W.A. Johnsen, S. Chester,
fractal analysis of topographic data by the patchwork method, Wear 161
(1993) 61-67.