Aerosol modulation transfer function model for passive long-range imaging over a nonuniform atmospheric path

Abstract

An aerosol modulation transfer function (MTF) model is developed to assess the impact of aerosol scattering on passive long-range imaging sensors. The methodology extends from previous work to explicitly address imaging scenarios with a nonuniform distribution of scattering characteristics over the propagation path and incorporates the moderate resolution transfer code database of aerosol cross-section and phase function characteristics in order to provide an empirical foundation for realistic quantitative MTF assessments. The resulting model is compared with both predictions from a Monte-Carlo scattering simulation and a scenederived MTF estimate from an empirical image, with reasonable agreement in both cases. Application to long-range imaging situations at both visible and infrared wavelengths indicates that the magnitude and functional form of the aerosol MTF differ significantly from other contributors to the composite system MTF. Furthermore, the image-quality impact is largely radiometric in the sense that the contrast reduction is approximately independent of spatial frequency, and image blur is practically negligible.

Aerosol MTF revisited

“…Recently, different views of the significance of aerosol MTF have been reported. In ref [6], on the basis of aerosol MTF modeling, it was concluded that "the image impact is almost completely radiometric: the image contrast is reduced uniformly at all spatial frequencies, and there is negligible image blurring". On the other hand, another recent paper [7] claims that aerosols "can have a non-negligible effect on the atmospheric point spread function".…”

Wave structure function and long-exposure MTF for laser beam propagation through non-Kolmogorov turbulence

“…We used the derived analytic expressions Eqs. (13) and (19) ), there is more degradation in the lower frequencies and less in the high frequencies compared to the Kolmogorov power-law exponent. Whereas 4 α → there is little degradation in the first few frequencies and more in the high frequencies compared to 3 α → .…”

“…From the results we observe that low turbulence brings less degradation on the imaging performance compared with high turbulence strength and this can be deduced directly from Eq. (13). This phenomenon can also be explained as in low turbulence the number of turbulence cells that bring the variation of optical wave phase decreases.…”

“…(13). For analysis purpose we normalized the spatial frequency by the aperture diameter, D in the form of…”

“…A laser beam propagating through unstable random atmosphere experiences phase and amplitude variations due to turbulence disturbances leading to such effects as beam wander, beam spreading, and intensity fluctuations better known as scintillations [6,8,10]. This turbulence is caused by refractive index inhomogeneities along the propagation path and it has significant degrading impact on the performance of free-space optical (FSO) communication systems as well as on the quality of imaging systems described by the modulation transfer function (MTF) [11][12][13]. In addition factors like aerosol scattering, molecular absorption caused by snow, fog, and rain can also severely degrade the quality of imaging systems [9].…”