However, the diffraction pattern, from which the parameter R_q can be extracted, contains additional information. The surface can be considered to be a fingerprint of the machining process which produced this surface. This information is reflected in the optical diffraction pattern. Simple calculations show what information can be extracted from the diffraction pattern. It is assumed that the surface profile consists of a periodic, a random and a tool scar component. The periodic component is a function of the tool shape and feed rate. The random component is caused by machine vibrations. The calculations lead to the following conclusions.

1. (i) The light intensity in the centre of the diffraction pattern (zeroth order) is affected by the surface roughness parameter R_q.

2. (ii) The light intensity at the extremes of the diffraction pattern (high orders) contains information about the tool scars.

3. (iii) The peaks in the low order diffraction contain tool shape information.

4. (iv) The continuous diffraction pattern around the zeroth order contains information about the random component of the surface, which can be caused by faulty spindle bearings.

5. (v) Scattering and fragmenting of the diffraction pattern leading to loss of even symmetry of the pattern is caused by waviness of the surface. This waviness can be caused by slideway errors in the machine tool.

This leads to the conclusion that optical diffraction techniques are not solely useful for the determination of the surface roughness but are also a powerful tool for monitoring the machining process in general.