  Dark matter consisting of very light and very weakly interacting particles
such as axions, axion-like particles and hidden photons could be detected using
reflective surfaces. On such reflectors some of the dark matter particles are
converted into photons and, given a suitable geometry, concentrated on the
detector. This technique offers sensitivity to the direction of the velocity of
the dark matter particles. In this note we investigate how far spherical
mirrors can concentrate the generated photons and what this implies for the
resolution in directional detection as well as the sensitivity of discovery
experiments not aiming for directional resolution. Finally we discuss an
improved setup using a combination of a reflecting plane with focussing optics.
