Most of the commonly used optical non-invasive techniques for shape measurements are derived from industrial metrology, although in the Art Diagnostics field, the peculiarity of each artwork does not allow to straightforwardly apply many of them.
For conservation purposes, among the most interesting methods, there is the high resolution scan by means of optical techniques, like microprofilometry, which is becoming an useful tool to asses the object surface conditions.
Roughness measurement deriving from a shape survey is a new application in the Cultural Heritage field, where up till now scarce attention was paid to this kind of diagnostics.
Roughness knowledge is important to document the surface condition, to assess either changes due to restoration intervention or surface decay due to wearing agents, and to monitor the evolution in time in terms of shape variations.
To assess the roughness, a very accurate measurement of the surface is needed, with a high resolution, in the microscopic range.
This type of measure is often very difficult to perform, specially on large objects, when it must be executed in situ.
Generally speaking, the surface of an object can be described by three parameters, according to the spatial frequencies considered:
shape (low frequencies);
waviness (mid frequencies);
roughness (high frequencies).
The three frequency ranges depend on the object dimensions: for instance, for an optical component, shape is related to the designed form, waviness is related to the deviation between projected and manufactured form, and roughness is the surface irregularity that causes light scattering.