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Empirical relationships between extinction coefficient and visibility in fog
1Dipartimento di Elettronica e Informazione, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy. nebuloni@elet.polimi.it
Applied Optics
|July 2, 2005
Summary
Far-infrared radiation offers better visibility through fog than visible or near-infrared wavelengths, especially in low-visibility conditions. This advantage increases as fog density decreases, impacting how particle size affects extinction.
Area of Science:
- Atmospheric Optics
- Electromagnetic Propagation
Background:
- Fog significantly impacts visibility by scattering and absorbing electromagnetic radiation.
- Understanding extinction coefficients across different wavelengths is crucial for atmospheric sensing and communication.
Purpose of the Study:
- To derive relationships between visibility and fog-induced extinction coefficients in clear optical windows.
- To compare the performance of different infrared (IR) wavelengths for visibility through fog.
Main Methods:
- Analysis of extinction coefficients at visible (0.55 µm), near-IR (1.2 µm), mid-IR (3.7 µm), and far-IR (10.6 µm) wavelengths.
- Examination of fog's impact on optical windows free from molecular absorption.
Main Results:
- At visibilities below a few hundred meters, extinction coefficients at visible and near/mid-IR wavelengths are comparable to or double that of far-IR.
- The advantage of far-IR radiation over shorter wavelengths increases significantly as visibility exceeds 500 meters.
- The sensitivity of the extinction coefficient-visibility relationship to fog particle-size distribution increases with wavelength.
Conclusions:
- Far-infrared wavelengths provide superior visibility through fog compared to shorter wavelengths, particularly in conditions with reduced fog density.
- The particle-size distribution of fog plays a more critical role in determining extinction at longer IR wavelengths as visibility improves.