EXPERIMENTAL COMPARISON OF TURBULENCE MODULATION TRANSFER-FUNCTION AND AEROSOL MODULATION TRANSFER-FUNCTION THROUGH THE OPEN ATMOSPHERE

Although turbulence is usually considered to be the primary cause of i mage blur, simultaneous and independent measurements of overall atmosp heric modulation transfer function (MTF) and turbulence MTF over fairl y long horizontal paths at 15-m average elevation indicate that, even at midday, aerosol MTF deriving from forward scatter is usually more d ominant than turbulence MTF. Three different experimental techniques a re used, two passive and one active. Aerosol MTF measurements are acco mpanied by actual meteorological and coarse aerosol size distributions and scattering parameters at the times of MTF measurements. The wavel ength dependence of aerosol and therefore of overall atmospheric MTF c an be significant. This wavelength dependence and a usually well-defin ed knee can in no way be due to turbulence but can be explained by a s ignificant aerosol MTF deriving from typical aerosol size distribution s representative of other climates as well. Measurements confirm that the narrower the open-atmosphere aerosol scattering patterns and the g reater the scattering densities, the greater the degradations of aeros ol MTF and, consequently, of overall atmospheric MTF. Results imply th at system design and image-restoration algorithms based on atmospheric turbulence only may often lead to image quality that is much poorer t han if aerosol MTF is considered, too, with its proper proportional ef fect on imaging through the atmosphere. Whereas adaptive optics cannot correct for aerosol-derived blur, digital image restoration can.