Far- and near-field approximation for diffraction radiation

A new beam diagnostic technique based on the Optical Diffraction Radiation Interference (ODRI) has been recently proposed and experimentally investigated. The technique is based on the use of a two slits system. The far field approximation has shown rather accurate correspondence with experimental data when slits are centered with respect to the beam propagation line. Using formulas involving near field we point out in this work that a general treatment is needed with offset larger than 50 μm