The optical properties of an antiferroelectric liquid crystal (AFLC) cell slightly deviating from the ideal orthoconic condition, i.e., 45° tilt angle and fully horizontal surface stabilization (the molecular tilt plane parallel to the glass plates), have been investigated using the fully leaky guided mode (FLGM) technique. In contrast to standard optical investigative methods, such as polarization microscopy, the FLGM data give information about the orientation of the index ellipsoid or the molecular tilt plane, not only in the plane of the cell but in three dimensions. The sensitivity of the FLGM technique thus allows a detailed optical characterization on a mesoscopic scale and, in particular, has allowed us to discriminate, in the investigated cell, a tilt out of the substrate plane in the anticlinic state, thus a deviation from the horizontal surface stabilization condition. Orthoconic surface-stabilized AFLCs are, in the ideal case, uniaxial negative with the optic axis perpendicular to the glass plates and therefore provide a perfect dark state between crossed polarizers even if the direction of the smectic layer normal is not homogeneous in the plane of the cell. Small deviations from the ideal case lead to the appearance of a very small birefringence in the cell plane which, however, should not compromise the attractive optical properties of this class of AFLCs. Using laser diffraction we have verified that, even with the deviation from the horizontal surface stabilization detected by the FLGM study, a horizontal chevron geometry, clearly visible in the synclinic state, is not optically detectable in the anticlinic state of orthoconic AFLCs. We conclude that the attractive optical properties are preserved in a reasonably large interval around the ideal orthoconic conditions allowing for the practical exploitation of these materials as key components in liquid crystal displays and other electro-optic devices. © 2002 American Institute of Physics.