The degradation in performance of imaging or beam projection systems, due to turbulence in the atmosphere, can be evaluated or predicted in terms of a path-position weighted value of Cn2, the optical turbulence structure constant. The degradation also depends on the wavelength and range. At visible wavelewth, for paths of a few km, serious degradation occurs for Cn2 of the order of 10-14 m-2/3. Such turbulence is not uncommon for paths over the ocean and occurs frequently over land. The appropriately weighted value of Cn2 can be measured experimentally for a given path, with a slit scanning telescope, imaging a point source at the far end of the range. A portable system will be described that is capable of these measurements. This is coupled directly to an on-line data processing minicomputer to predict the performance of a given system, using Fourier and Abel transform techniques applied to models by Lutomirski and Fried. The results can be presented in a variety of forms, including immediate hard copy plots of the MTF of the atmoshpere, of the overall system being tested, or plots of predicted radial distribution of intensity on target. For non-uniform turbulence, the proper weighting of Cn2 as a function of path-position is crucial. For example, Cn2, obtained from scintillation, weights path-position differently and yields results of marginal value to determination of image resolution or spot size.