  Optical transition radiation interferometry (OTRI) has been shown to be a
very useful technique to measure the divergence of electron beams with energies
in the range of 15-100 MeV. However, application of this method to low energy
or very high quality beams is limited by scattering in the front foil of the
interferometer. To overcome this limitation we propose to use a perforated
front foil. For the beam energy and hole sizes we are considering, the
unscattered beam electrons passing through the holes will produce diffraction
radiation (ODR). The total radiation produced from the first and second foils
then will be a spatially coherent sum of ODR and OTR from unscattered and
scattered electrons. By controlling the number and size of the perforations,
the inter-foil spacing, the thickness of the first foil and the wavelength and
band pass of the observed radiation, the coherent interferences due to the
unscattered portion of the beam can be isolated and observed. The visibility of
these interferences can then be used to determine the rms beam divergence. We
have developed a general computer code which can be used to calculate
diffraction radiation from any type of perforation and another code to compute
the ODR-OTR interference pattern for a given set of beam and optical
parameters. These codes are employed in the design of an interferometer to
measure the divergence of the ATF accelerator operating at 30 MeV. This beam
will be used in an initial proof of principle experiment for the ODR-OTR
interferometer. We present the results of our code calculations which show that
the expected divergence, 200 micro radians, can be easily measured.
