Seeing the invisible with SWIR Less scattering - better visibility

Light scattering effects can be noticed every day when we look at the sky or the atmosphere. The main reason why the sky is blue on a sunny day is Rayleigh scattering (after Lord Rayleigh) of sunlight, i.e., the scattering of electromagnetic radiation by particles much smaller than the wavelength. However, for cloud and haze scattering, the Mie theory (after Gustav Mie) is very important where diameter-to-wavelength ratios of the order of unity or larger are characteristic. According to Rayleigh, for particles small compared to the wavelength, the scattering is inversely proportional to the fourth power of the wavelength. Therefore, SWIR wavelengths will scatter much less than visible light, and therefore, SWIR imagers, like the Xenics Xeva camera, will be able to see through the particles (haze, smoke, dust, ...). However, when particle sizes grow (Mie theory), scattering is not strongly wavelength dependent, and eventually, the size of particles can become large enough to also obscure SWIR images.