Subwavelength structures of sufficient height-to-width aspect ratio are adequate for reduction of the Fresnel reflection of surfaces. In the case of low-pressure plasma-treated polymers, the self-organized features are stochastically distributed on the surface area with unequal shapes. Common descriptions use effective medium theory along with a continuously altering volume filling factor that together lead to a lateral homogeneous layer with a depth-dependent refractive index. We use a fixed gradient function for the refractive index for reverse engineering of in situ measured broadband optical data, and we compare the results with nonoptic methods.