The utility of a schlieren image lies in the fact that the refractive index is dependent on the density of the medium it passes through. Knowledge of this relationship permits one to obtain quantitative **data** from such images. To make this lucid, let ρ (x, y) be the spatially varying density of a fluid surrounding an object of interest; it is expressed as a scalar-field with a horizontal coordinate x and a vertical coordinate y. It has a linear relationship with the refractive index n (x, y),