Optical properties of semi-transparent sensor covers determined from their spectral intensity distribution function

The optical properties of semi-transparent components used as multifunctional coatings or in various non-imaging devices are not always completely known a priori. The quality of plastic covers used in night sky brightness monitors, such as Sky Quality Meter (SQM), can deteriorate with age, causing initially transparent windows to become semi-transparent media that absorb and scatter light. We demonstrate here that the asymmetry parameter, single-scattering albedo, and volume extinction coefficient of such a diffusing optical element can be determined by measuring the light escaping from its outer interface.

Here we develop a simple model that allows for retrieval of the above parameters and can be applied to characterize various semi-transparent materials. The method is demonstrated for a particle-doped polycarbonate and Poly(methyl methacrylate) flat-plates of different thicknesses illuminated by a white light source. The spectral intensity of transmitted radiation is analyzed at discrete scattering angles for two samples of nearly the same optical properties except for the scattering asymmetry parameter. The samples sharing a number of similarities provide an ideal tool to test the method, because the angular structure of the scattered light can be interpreted in terms of a single optical parameter (while holding all other factors equal or nearly equal).

The method is useful for the optical diagnosis of covers on non-imaging devices as they age (e.g., SQMs). The optical properties of a plastic cover obtained from its measured spectral intensity distribution function are needed to interpret and correct the data gathered by still non-retired night sky scanners. Additionally, the method can assist in selecting the optimal optical covers for solar and other applications.