The roughness of surface can contribute to the extent of the diffused light, and rougher surface leads to increased light scattering or decreased transparency. Maximum scattering occurs when the roughness is comparable with the wavelength of the light. Light is scattered into different angles at each rough interface. This results in a prolonged average optical path-length and hence an enhanced light absorption in the layers. Scattered light beams fall on the interfaces non-perpendicularly which results in higher angular-dependent reflectances at the interfaces than in case of a normal incidence. Enhance reflections at internal interfaces lead to a more efficient light trapping which additionally increases absorption of light.

You can find more information in: Thin Solid Films 426 (2003) 296–304

If we consider the intensity of the incident beam to be "I" and that for the reflected beam to be "R", the transmitted beam to be "T", and the absorption portion to be "A", then transmitted beam intensity is T = I - (R + A). Obviously, R and A are impacted by the angle of incident, changes in the index of refraction at the interface, diffused reflectance due to surface roughness, and diffraction due to porosity and sample roughness as well as changes in the absorption coefficient of the coating used. Moreover, thickness of the film and how it is correlated with the wavelength (quarter- or half wavelength) also impact the transmission much like that done in anti-reflection coating.