OSD (OptiTopo Surface Deviation) correlates well with traditional air-leakage methods. However with the OptiTopo method small-, mid- and large-scale surface variations can be measured – which cannot be done with air-leakage methods. This difference is of great importance for acquiring a value that is relevant for printing, especially on surfaces with low roughness.
Crater value is a measurement that can help predict print quality and reduce the missing dots occurrences in flexo- and gravure. Deep craters in the paper’s surface will remain unprinted in the printing process, creating missing dots. L&W OptiTopo reports one crater value for three different crater depths, those are pre-set on delivery covering the majority of printing grades in the market, but can of course be configured later by the customer.
Topography measurements can either be done by optical methods using laser or mechanically with a stylus that touches the paper surface. The traditional optical, laser based, method is accurate but slow and requires expensive equipment. Traditional mechanical methods are often destructive and measures only the area being touched by the stylus, representing a fairly small area of the total surface.
Air leakage is a simple and inexpensive method that provides a single value in relation to the average value of the surface roughness of the area being measured, i.e. about 5-50 mm2, which only constitute a small area of the total printed surface. Air leakage according to various standards such as Bendtsen, Bekk, Sheffield or PPS is currently the most common methods, in the paper industry, for measuring surface roughness.