The following discussion is presented in terms of solutal concentration. It is equally applicable to imaging thermal convection.
Interferometry
For a light source of wavelength  the change in concentration required per fringe shift in the infinite fringe setting is given by the equation
The fringe positions are to be determined from interferogram analysis. In the wedge fringe setting, it can be shown that the fringe displacement from the initial position is proportional to the change in concentration with respect to the portion of the solution where the fringes are undisturbed. These results hold under the approximation that refraction effects are small.
Schlieren
Image formation in a schlieren system is due to the deflection of light beam in a variable refractive index field towards regions that have a higher refractive index. In order to recover quantitative information from a schlieren image, one has to determine the cumulative angle of refraction of the light beam emerging from the growth chamber as a function of position in the  plane. This plane is defined to be normal to the light beam, whose direction of propagation is along the  coordinate. Using principles of ray-optics, the total angular deflection  can be expressed as
where  is the refractive index at any point in the solution. The change in the intensity field  relative to the background  can now be related to the refractive index field directly as
|
