Halo and subsurface scattering in the transparent coating on top of a diffusing material

Area of Science:

• Optics
• Materials Science
• Surface Science

Background:

• Thick transparent coatings on scattering surfaces exhibit a distinct halo effect under point illumination.
• This halo is caused by Fresnel internal reflections at the coating-air interface.
• The halo's size correlates with the coating thickness.

Purpose of the Study:

• To develop a model describing the spatial and angular distribution of light scattered from coated surfaces.
• To analyze the impact of subsurface scattering on reflectance measurements and colored surfaces.
• To account for absorption within the background and transparent layer.

Main Methods:

• Development of a bidirectional subsurface scattering reflectance distribution function (BSSRDF) model.
• Utilizing analytical equations and matrix numerical computation.
• Considering a Lambertian background and a transparent top layer, potentially absorbing.

Main Results:

• The model accurately describes the spatial and angular distribution of scattered light, including multiple reflections.
• The halo's angular distribution varies with distance from the illumination point.
• Subsurface scattering can lead to undervalued reflectance measurements and affect the appearance of halftone prints.

Conclusions:

• The developed BSSRDF model provides a comprehensive understanding of light interaction with coated scattering surfaces.
• Subsurface scattering is a critical factor in accurate reflectance measurements and visual appearance assessment of coated materials.
• This phenomenon has practical implications for industries dealing with coated and laminated printed surfaces.