Experimental comparison of turbulence modulation transfer function and aerosol modulation transfer function through the open atmosphere

Abstract

Although turbulence is usually considered to be the primary cause of image blur, simultaneous and independent measurements of overall atmospheric modulation transfer function (MTF) and turbulence MTF over fairly long horizontal paths at 15-m average elevation indicate that, even at midday, aerosol MTF deriving from forward scatter is usually more dominant than turbulence MTF. Three different experimental techniques are used, two passive and one active. Aerosol MTF measurements are accompanied by actual meteorological and coarse aerosol size distributions and scattering parameters at the times of MTF measurements. The wavelength dependence of aerosol and therefore of overall atmospheric MTF can be significant. This wavelength dependence and a usually well-defined knee can in no way be due to turbulence but can be explained by a significant aerosol MTF deriving from typical aerosol size distributions representative of other climates as well. Measurements confirm that the narrower the open-atmosphere aerosol scattering patterns and the greater the scattering densities, the greater the degradations of aerosol MTF and, consequently, of overall atmospheric MTF. Results imply that system design and image-restoration algorithms based on atmospheric turbulence only may often lead to image quality that is much poorer than if aerosol MTF is considered, too, with its proper proportional effect on imaging through the atmosphere. Whereas adaptive optics cannot correct for aerosol-derived blur, digital image restoration can.