EFFECTS OF PRACTICAL AEROSOL FORWARD SCATTER OF INFRARED AND VISIBLE-LIGHT ON ATMOSPHERIC COHERENCE DIAMETER

A correction to the definition of the atmospheric coherence diameter i s suggested here, based on the existence of a practical instrumentatio n-based aerosol modulation transfer function (MTF), which is often the dominant ingredient of the atmospheric MTF. As defined classically by Fried about 25 yr ago, atmospheric MTF and coherence diameter were re lated to turbulence MTF only. Lutomirski considered diffractive aeroso ls, too, but did not consider effects of instrumentation on scattering angles actually recorded in the image. These are limited in the real world by instrumentation to milliradians, rather than by the broad ang ular spread of diffraction to radians. In the case of a Gaussian appro ximation of the practical aerosol MTF, an analytical expression is der ived for the practical aerosol-derived coherence diameter. This parame ter is related to the practical aerosol MTF's cutoff frequency, and to its asymptotic value at high spatial frequencies. Thus, a more genera l concept of atmospheric coherence diameter is proposed here, which is relevant to actual real-world imaging systems, whether they are passi ve or active. Quantitative validation of the theory is presented, base d on both simulations and actually measured atmospheric MTFs in both t he visible and thermal infrared spectral ranges. Overall atmospheric c oherence diameter is determined generally by the smaller of the turbul ence and practical aerosol coherence diameters, depending on optical d epth. The results here appear applicable particularly to cost-effectiv e thermal imaging system design, although applications are considered, too, for the visible and near infrared. For example, blur deriving fr om aerosol scatter should have much less effect in coherent detection laser radar (LIDAR) than in direct detection imaging.