  Scattering structure factors provide essential insight into material
properties and are routinely obtained in experiments, computer simulations, and
theoretical analyses. Different approaches favor different geometries of the
material. In case of lipid bilayers, scattering experiments can be performed on
spherical vesicles, while simulations and theory often consider planar membrane
patches. We derive an approximate relationship between the structure functions
of such a material in planar and spherical geometries. We illustrate the
usefulness of this relationship in a case study of a Gaussian material that
supports both homogeneous and microemulsion phases. Within its range of
applicability, this relationship enables a model-free comparison of structure
factors of the same material in different geometries.
