Roughness is the condition of an object's surface. Let's call it the "unevenness". Now, most surfaces of components are not flat, but are primarily determined by their functional form. The first step is to remove this shape so that the texture of the surface remains. After all, the shape itself may have heights and depths, but these are geometrically intended to be so. No matter how small they may be, they have nothing to do with the roughness. Therefore, the shape is subtracted for the time being. But that is not all, because the surface is characterized by high and low frequency waves. High-frequency waves are the roughness you want to measure. That means you still have to get rid of the low-frequency ones, the so-called waviness. And for that, in turn, we need a filter.

This is a question you are now legitimately asking yourself when you want to perform a surface roughness measurement. Or not, because you already work with Bruker Alicona measurement technology and therefore know that this is done with a few mouse clicks. Examples of geometric shapes are: 

• Cone
• Level
• Cylinder
• Ball
• Polynomial (2nd - 8th degree)

After form removal, a flat surface remains for evaluation.

In order to be able to deduct the waviness after the shape, there are two different methods in roughness measurement, depending on the preceding level of knowledge. Both procedures have one thing in common: they are based on Lc (LambdaC). Lc is the cutoff wavelength, the length that separates waviness from roughness. Or to put it more simply: higher frequencies remain in the surface, lower frequencies are filtered out. But how do you get the right Lc value? There are two possibilities:

1. Lc can be seen in the drawing.

2. The drawing does not give any information about Lc. In this case, assume your nominal roughness value and the Setting Class and look for your corresponding Lc in the table. See figure below.

Now this all sounds insanely complicated. However, the good news is that Bruker Alicona's software (MetMaX) already gives instructions on how to perform a roughness measurement in compliance with the standard. The table for determining Lc according to ISO 21920 is also implemented.

Actually, it should be called profile roughness measurement and areal roughness measurement, because it is about the way of measuring. They are two different measurement methods for determining roughness, although different results are obtained. The name already reveals what is involved: profile roughness measurement takes its values from a line, while surface roughness measurement evaluates an entire surface.

Profile roughness measurement is widespread and thus better established than areal surface determination. This is because roughness on the profile has been measured for decades. The result values may be Ra (mean roughness), Rq (root mean square) and Rz (roughness depth) to name just a few. The standards behind these are extensive, but we will go deeper to the standards in a different article. To get meaningful results, the metrologist must already have some knowledge of the surface and have an idea of what they are doing.

Surface measurement is newer. In the past, roughness was measured with tactile systems. Tactile means that a probe moves over the component and thus determines where there are highs and lows. With such a roughness measuring device, it is not possible, or possible only with extremely high effort, to perform a surface measurement. Only non-contact methods make areal surface measurement possible. The results are more complex and comprehensive. In addition to Sa (mean arithmetic height), Sq (mean square height) and Sz (maximum height), you receive many other parameters, such as the volume parameters, which also allow better conclusions to be drawn about the function of the surface. With these parameters, you can make statements about haptics, friction or sliding features. The outcome is also more reliable and repeatable. Furthermore the surface measurement is safer against user influences. The figure below shows you, why in most cases the areal roughness measurement is way more meaningful.

So that you also know which values are behind which terms, we provide you with a short glossary. You have already heard some of them: