  It is an intriguing possibility that the cold dark matter of the Universe may
consist of very light and very weakly interacting particles such as axion(-like
particles) and hidden photons. This opens up (but also requires) new techniques
for direct detection. One possibility is to use reflecting surfaces to
facilitate the conversion of dark matter into photons, which can be
concentrated in a detector with a suitable geometry. In this note we show that
this technique also allows for directional detection and inference of the full
vectorial velocity spectrum of the dark matter particles. We also note that the
non-vanishing velocity of dark matter particles is relevant for the conception
of (non-directional) discovery experiments and outline relevant features.
