We investigate the near field distribution of light diffracted from deep grooves of sub-wavelength lateral dimensions, carved on a flat metallic surface. It is shown that, in spite of multiple interaction, the reflected field presents, forspolarization, peaks very localized on the groove positions, and whose amplitude increases with the groove depth. Forppolarization, however, the near field does not follow the surface profile. This permits us to establish forspolarization a threshold in order to distinguish signals produced on reflection from grooves having different profiles. This is of guidance in diffractive modeling of readout systems of high density optical disks. ©1997 American Institute of Physics.