  Smith-Purcell radiation and Transition Radiation are two radiative phenomenon
that occur in charged particles accelerators. For both the emission can be
significantly enhanced with sufficiently short pulses and both can be used to
measure the form factor of the pulse. We compare the yield of these phenomenon
in different configurations and look at their application as bunch length
monitors, including background filtering and rejection. We apply these
calculations to the specific case of the CLIO Free Electron laser.
