  We calculate the optical diffraction radiation generated by a bunch of high
energy particles as they pass through a round hole within an annular metallic
ring. We derive expressions for the differential angular spectrum in the
far-field and the intensities of the horizontal and vertical polarizations. The
sensitivity of the spectrum to changes in beam size and position is shown. The
total photon yield from the bunch is calculated and used to set limits on the
detectable wavelengths.
