  We present an apparatus that converts every pulse of a pulsed light source to
a pulse train in which the intensities of the different pulses are samples of
the spatial or temporal frequency spectrum of the original pulse. In this way,
the spectrum of the incident light can be measured by following the temporal
response of a single detector. The apparatus is based on multiple round-trips
inside a 2f- cavity-like mirror arrangement in which the spectrum is spread on
the back focal plane, where after each round-trip a small section of the
spectrum is allowed to escape. The apparatus is fibre-free, offers easy
wavelength range tunability, and a prototype built achieves over 10% average
efficiency in the near infra red. We demonstrate the application of the
prototype for the efficient measurement of the joint spectrum of a
non-degenerate bi-photon source in which one of the photons is in the near
infra red.
