Optical reflectivity of an interface with random refractive-index-contrast patterns.

We develop simple models for the optical reflectivity of an interface in optical contact with random media consisting of discrete volumes of arbitrary form and different refractive indices. Examples of interest are surfaces sprinkled with microdroplets or an interface with biological cells adhered to it at random locations. We focus our attention to the case of internal reflectivity, in which the incidence medium has a larger refractive index than the refractive indices at the other side of the interface. Assuming an incident plane wave, we provide simple approximate expressions for the surface's coherent reflectance and for the surface's total reflectance. We compare predictions of the surface coherent-reflectance model with numerical simulations. Then we use the surface's reflectance models to interpret experimental measurements obtained with an optical prism and a thin vegetable tissue adhered to its base. In general, the surface reflectivity can be used to determine fractional contact area between the interface and microdroplets or biological cells and infer their refractive indices with an accuracy of about 0.5%.