- optical path difference (OPD)
The relative path difference (or phase shift) traveled between two rays that pass through different mediums from the same object point. For a perfect optical system, the optical path, or distance, from an object point to a corresponding image point will be equal for all rays. In near-perfect systems, slight differences will exist between rays resulting in an optical path difference, usually expressed in fractions of the wavelength being analyzed. When rays pass through an optical system, the optical path difference between the wavefronts of ‘real’ and paraxial rays attribute to optical aberrations.