  Dark matter may consist of light, very weakly interacting bosons, produced
non-thermally in the early Universe. Prominent examples of such very weakly
interacting slim particles (WISPs) are axions and hidden photons. Direct
detection experiments for such particles are based on the conversion of these
particles into photons. This can be done in resonant cavities, featuring a
resonant enhancement, or by using suitably shaped reflecting surfaces that
allow for broadband searches. In this note we want to elucidate the relation
between the two setups and study the transition from resonant to broadband
searches. This then allows to determine the sensitivity of off-resonance cavity
searches for cavities much larger than the wavelength of the generated photons.
