Modeling and analysis of the spectral transmittance of droplet-adhered windows via morphology reconstruction

Condensation and droplet adhesion on optical windows severely degrade the spectral transmittance and, thus, impair the performance of sensors, infrared detectors, and solar photovoltaic systems. Related optical researches mostly idealize the droplets as spherical caps, neglecting the surface tension and gravitational deformation. A morphology-resolved framework is developed, coupling surface evolver (SE)-based droplet reconstruction with Monte Carlo ray tracing, to predict the spectral transmittance of droplet-covered windows over 0.4–6.0 μm. The differences in geometrical and optical properties are firstly discussed between the spherical-cap model and the SE-reconstructed model. The deviations and droplet diameters are both exponentially increasing. The height deviation reaches 1.0 mm (100%) at diameter of 4.0 mm; meanwhile, the transmittance overestimations reach up to 44% at wavelength of 2.5 μm. The effects of droplet size, incidence angle, and surface coverage are then examined. Results show that surface coverage and incidence angle dominate the spectral transmittance, especially in the strong water-absorption waveband (2.0–3.0 μm). Additionally, the droplet diameter has a weak influence on the transmittance property in the visible and far-infrared wavebands.