Classification

Optical techniques can be classified as linear or nonlinear, depending on the relationship between an attribute of the optical signal generated and the flow property to be measured. For example, LDV is a linear device since the Doppler shift in frequency scales linearly with the fluid velocity. Interferometry is a linear instrument since the fringe shift scales with temperature difference. In the presence of refraction errors, linearity is lost since nonlinear effects related to light refraction should be accounted for. As shown in module 4, these nonlinearities, to a first approximation, are quadratic with respect to temperature. Similarly, schlieren and shadowgraph are linear devices since, under some approximations, the change in intensity scales with temperature or its derivatives (modules 4 and 5). This reasoning holds even when temperature derivatives or its Laplacian is involved. Spatial derivatives can be resolved by integration with respect to the spatial coordinate and the linearity in the relationship between temperature and change in intensity is recovered. PIV can be considered a linear measurement technique for fluid velocity if it viewed as tool for measurement of the particle displacement. Methods such as tomography for three dimensional reconstruction of the temperature field can also be viewed as linear devices since the extraction of data is in space and mathematical operations are temperature independent.

Nonlinear measurements were encountered in liquid crystal thermography where the temperature-color (equivalently, temperature-hue) information had to be generated from a calibration experiment. As discussed in the following sections, scattering techniques are invariably nonlinear and the relationship can be established, generally, from first principles.

Scattering techniques are classified as elastic or inelastic depending on the changes taking place in the wavelength of the photon leaving the particle with respect to that incident. Mie and Rayleigh are examples of elastic scattering. Invariably, inelastic scattering is the norm and the majority of techniques such as Raman, Bragg, Compton, and Brillouin are inelastic. Fluorescence is inelastic since the emitted wavelength is material-specific. Methods that rely on absorption in a material medium can be seen as special cases of scattering techniques. Since the absorption coefficient is wavelength-dependent, the outgoing radiation is spectrally distorted with respect to the incoming and the method classifies as inelastic.