Abstract: An optical configuration and related analysis techniques are presented that provide a practical method for determining the Rytov parameter based on the difference of variances for the differential image motion (average wavefront gradient or wavefront tilt) between two receiving apertures. The Rytov parameter is the log-amplitude variance predicted by an approximate solution to Maxwell's equations for propagation through media with random index of refraction (Rytov theory). It is a useful metric of the optical effects for extended turbulence propagation and is a leading indicator of the performance limitations of adaptive optical compensation devices not related to the transverse coherence diameter. A time-duplex alternative apparatus and a single source alternative apparatus are also disclosed.