Dry powders reflectance model based on enhanced backscattering: case of hematiteα-Fe2O3.

By performing bidirectionnal reflectance distribution function (BRDF) measurements, we have identified backscattering as the main phenomenon involved in the appearance of dry nanocrystallized powders. We introduce an analytical and physically based BRDF model that relies on the enhanced backscattering theory to accurately reproduce BRDF measurements. These experimental data were performed on optically thick layers of dry powders with various grains' morphologies. Our results are significantly better than those obtained with previous models. Our model has been validated against the BRDF measurements of multiple synthesized nanocrystallized and monodisperse α-F e 2 O 3 hematite powders. Finally, we discuss the ability of our model to be extended to other materials or more complex powder morphologies.

Evaluating edge loss in the reflectance measurement of translucent materials.

In many commercial instruments for measuring reflectance, the area illuminated on the measured object is identical to the area from which light is collected. This configuration is suitable for strongly scattering materials such as paper, but issues arise with translucent materials, because a portion of the incident light spreads around the illuminated area by subsurface transport and escapes the detection system. This phenomenon, referred to as edge loss, yields erroneous, underestimated reflectance measurements. In the case of colored and opalescent materials, the impact of edge loss on the measured reflectance varies with the wavelength, which is a significant issue for spectrophotometer and colorimeter users. In the present study, we investigate the edge-loss phenomenon with an emphasis on human skin measurement. In particular, we use a mathematical model to estimate the PSF of translucent materials, relying on the diffusion approximation of the radiative transfer theory, to predict edge-loss measurement error. We use this model to discuss the suitability of several commercial spectrophotometers to accurately measure the translucent materials of various optical properties and show that not all devices can adapt to all translucent materials.

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

Strongly scattering supports coated with thick transparent medium display a bright halo with a characteristic ring shape when illuminated in one point by a thin pencil of light. The halo, whose size is related to the coating thickness, is due to the Fresnel internal reflections of the light scattered by the diffusing support at the coating-air interface. The angular distribution of the reflected light strongly varies over the halo according to the distance from the point initially illuminated, a fact that cannot be observed when a large area of the surface is illuminated as in usual reflectance and bidirectional reflectance distribution function measurements. By considering a Lambertian background and a transparent layer on top of it, both of them being possibly absorbing, we develop a bidirectional subsurface scattering reflectance distribution function model, based on analytical equations and matrix numerical computation, which enables a detailed description of the spatial and angular distribution of the scattered light including the multiple reflections between the background and the coating-air interface. Some applications in which this subsurface scattering phenomenon can be an issue are addressed, such as the reflectance measurement, which can be undervalued when the geometry is not adapted to the coating thickness, or the impact of the phenomenon on heterogeneously colored surfaces such as coated or laminated halftone prints.