**1.** In a medium of constant refractive index, *n* , the product of the geometric distance and the refractive index.

**2.** In a medium of varying refractive index, the integral of *n* δ*s* , where δ*s* is an element of length along the path, and *n* is the local refractive index.

*Note:* Optical path length is proportional to the phase shift that a lightwave undergoes along a path.

Source: from Federal Standard 1037C and from MIL-STD-188