A note about surface defects
Every object in the world has small “defects” on its surface, if by “defect” we mean a slight deviation from what we would expect to see on the surface. The deviations might be outwards, or inwards, or just a slight colour change, or sometimes a strange visual pattern that can be seen from a distance, but disappears once you get close up or look under magnification. If you find an object which looks perfect, and without any defects visible to your naked eye, then just look under a magnifying glass or a microscope and you will see hundreds of defects. There is no such thing as a defect-free surface, it just depends on how closely you look!
With our roughness comparison specimens, defects are hard to see because of the irregular nature of the roughness, and even if visible they rarely cause any problems in use unless they are extremely large and obtrusive. With precision reference specimens however, the situation is different. People expect to see a perfect surface and become worried if they can see small stains, scratches, or other marks. But because of the highly regular character of the surface profile or structure on all reference specimens, such defects stand out and are very easy to see even by naked eye. ISO 25178-73 Terms and Definitions for Surface Defects on Material Measures defines some terms for describing and for responding to, such defects. In what follows, these terms are in bold text.
In all cases it is important to consider the effect of the defect on any measurement which you want to make on your surface. There are three types of defect that can occur:
1. scratches that look like a straight, thin, shiny line;
2. stains such as brown patches or grey cloudiness;
3. strange visual effects, sometimes covering a large area, that can be seen from one angle but disappear when you look from another angle. In ISO 25178-73 these are called Gestalt defects.
Each of these 3 types of defect which are visible by naked eye turn out to be entirely cosmetic and completely ineffective, ie. they have no influence on measurements made over them by conventional roughness measuring instruments.
In the case of special applications, for example with surface measuring instruments that utilise optical or electrical properties of the surface in order to measure topography, some of these otherwise purely cosmetic defects may turn out to be effective. In such cases a customer must specify in advance, before buying a specimen, which types and size of surface defect would make the specimen unacceptable for their application.