The effects of turbulence on the performance of imagers or on beam forming optical systems are well expressed by the optical transfer function or its essential equivalents, the modulation transfer function or the mutual coherence function for the atmosphere. These quantities can be adequately expressed in terms of the Fried model by means of a single number, the turbulence structure constant for optical index, (C sub n) squared, provided that a properly path-weighted value is obtained. The behavior as a function of wavelength and range is also well understood. The quantity (C sub n) squared, which varies with the micrometeorology, can be determined in its properly path-weighted form with a slit-scanning telescope system developed at NPS. A portable system is available which includes an on-line data processing system that gives immediate numerical and graphical results for the properties of the atmosphere, together with performance prediction for a given optical system. The direct optical measurement of weighted (C sub n) squared for the path avoids the necessity of making a large number of measurements of (C sub t) squared at points along the path. Measurement of (C sub n) squared by the slit-scanner method also obtains a direct result independent of assumptions as to the statistical form of the turbulence.