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Optical surface roughness measurement

 

While the widespread stylus based roughness measurement procedure provides results according to the standards, the measurement is slow and not very flexible. Fast 100% in-line measurements can therefore not be made using these instruments.

This requirement can be met by optical methods, but there are some problems to be aware of:

• high sensitivity for dirty workpiece surfaces
• more or less bad correlation to standardised tactile measurements
• often limited to constant machining methods

 

3 measurement methods having a relevant practical meaning for in-line measurements are known:

 

Scattering method

There have been continuous efforts since 1980 to correlate the scattered light from a surface with the roughness of the surface. With some success this method can be used for a constant machining process and within defined (small) roughness values. The parameters obtained by this optical method can be compared with standardised parameters only within limits and only for statistical parameters like Ra. More relevant parameters like Rz cannot be measured. The size of the light spot on the surface doesn’t correspond to the evaluation length of a stylus instrument and workpiece surface slopes have a negative influence.



Speckle contrast

This method is scientifically more meaningful than the scattering procedure but also more demanding with respect to the design of the instruments. This method provides the best correlation possible with regard to statistical parameters.



Stripe projection

Within the physical limits which are given for all optical surface profilometers this method provides results which are closest to those of tactile pick ups. Light stripes are projected onto the workpiece surface and these are imaged onto a camera. From the bending of the stripes the surface contour can be deduced.
The measurement field size can be adjusted to the object and the measurement conditions with a tactile pick up and there is in principle no limitation with respect to certain workpiece surfaces. (Too) steep profile flanks cannot be measured with good fidelity.

Due to the reasons given above we sell a measurement instrument using the method of speckle contrast and also an instrument using the stripe projection technique. In the end the measurement task determines which one is preferred.