In industrial processing it pays to know how much of what is in the mix, which is why on-line sampling of both the amount and type of chemical constituents is critical. Only in this way can an operator know whether the mix is correct, or intervention is required. With optical spectroscopy, manufacturers have the potential to perform such analyses with minimal interference to the process flow. For large volume applications, where lost time means lost revenue, this can amount to a huge advantage. Unlike laboratory applications such as forensic analysis or characterization of unknown samples, process monitoring relies on a priori knowledge of a sample's chemical makeup. Spectroscopic instruments differ in the physical processes they record, the spectral regions they cover and how fast they produce results. In some applications, instrument calibration may be needed to link spectral distributions to fundamental physical quantities.