Surface roughness measurement using OptiTopo technique

Purpose

Very detailed height variations are calculated on a 32x32 mm area of the paper surface using a patented shape from shading technique. The high-resolution height maps (4 million height values) gives a great advantage to traditional air-leak instruments like PPS and Bendtsen that has traditionally been used for print prediction on paper and paperboard. The possibility to filter the height map makes it possible to filter out non-interesting large-scale variations and only count the “dangerous” craters.

The OptiTopo measurements can be used to answer:

• How do the surface defects look like?
• Why did we get a customer complaint from this sample?
• In which way does my new product differ from my old?
• Detailed fingerprint features of the surface roughness can be obtained.
• How should the surface of my product look like to ensure the desired print quality level?
• Can I fine-tune the filtering settings to find craters or hills that are potentially dangerous to reach my desired print quality level?

Method

The shape from shading OptiTopo technique calculate a very detailed height map. The x/y resolution is 15.6x15.6 µm and the z-resolution < 0.1 µm. The height map is filtered using the image analysis technique Fast Fourier transform (FFT) to divide the height variation in spatial size classes and find craters and surface variation of the type we know correlate to the print results.

Deliveries

An Excel report consisting of:

• Surface variation in fine, medium and large scale.
• The amount of crater area below -1.5, -3 and 5 µm.
• Detailed surface variation spectra to reveal periodic variations like wire marks.
   

“A picture is worth a thousand words”.
It is difficult to describe everything with numbers but combined with images you get a better understanding of the relationship between the paper/board surface and the print quality.

Word report consisting of:

• Gradient maps that shows how the surface variation look like.
• Height maps (color coded)
• Crater maps where the “dangerous” craters are outlined