  We put forward a method that allows the experimental determination of the
entire spatial mode spectrum of any arbitrary monochromatic wave field in a
plane normal to its propagation direction. For coherent optical fields, our
spatial spectrum analyzer can be implemented with a small number of benchmark
refractive elements embedded in a single Mach-Zehnder interferometer. We detail
an efficient setup for measuring in the Hermite-Gaussian mode basis. Our scheme
should also be feasible in the context of atom optics for analyzing the spatial
profiles of macroscopic matter waves.
