We propose the implementation of a ballistic auto-correlation interferometer (BAI) for the assessment of biological tissues. We develop the theory of the interferometric technique, and demonstrate that a method to isolate ballistic from forward-scattered contributions is of central importance. We, therefore, propose theoretically and experimentally a non-linear grating-based angular filter. A monochromatic source incident on a ruled grating, positioned at grazing diffraction, is followed by a narrow slit to conform the proposed system.