Introduction
This article aims at highlighting microscopic surface conditions treatment into Ocean™. Microscopic surface conditions can be created at the surface of a material with various technics, mainly depending on the considered materials as well as the expected roughness scale : acid etching, laser structuring or roll printing.
We will not enter into the detail of the various existing techniques. What interests us here is the result of these different techniques, namely the microscopic surface conditions created at the surface of the material, and how to simulate it with Ocean™.
In this article, we will focus on the microscopic surface state of glass, which is of interest in a variety of fields: automotive, architecture/building, product design, displays…In the following, we describe how to simulate a microscopic surface state with Ocean™, and illustrate this with two case studies: one in the automotive segment and one in the building segment.
Simulations within Ocean™
Ocean™ is a spectral ray tracer that provides high accuracy illumination calculations. Thanks to its specific spectral features, it allows a highly physically accurate description of materials and provides physically true predictive images. Relying on a valid and strong physical approach, it guarantees a perfect match between virtual simulations and the real world.
As with most rendering software, microscopic surface states can be modeled and simulated with a BRDF (Bidirectional Reflectance Distribution Function) using a distribution of microfacets. There is a multitude of theoretical models to be used according to the type of state to be simulated: Beckmann, Phong, Ward…
These models use different parameterization to describe the distribution of facet orientations. Figure 1 presents a cross sections view of a high and low roughness surface, linked to the variation of the facet orientations. We see that the higher the angle of the facet, the rougher the surface.
Figure 1 - Example of two distribution of facet orientations. (a): High variation of facet orientations leads to rougher surface. (b): Small variation of facet orientations leads to smoother surface.
The global surface is thus represented by a distribution of facets (statistical representation of the orientation of the facets) and a BRDF describing how the light is scattered by the individual microfacets.
Example of Beckmann parameterization
where α=√2 σ with σ the root mean square (RMS) of the slope of the microfacets, and θh being the angle between the facet normal and the surface normal. Figure 2 shows the isotropic Beckmann distribution as a function of θh, for α=0.5.
In Ocean™, the roughness parameter corresponds to σ, the RMS of the slope.
Figure 2 – Isotropic Beckmann distribution as a function of θ_h, for α=0.5
We have shown an example of a theoretical microfacet model. But how to simulate a measured microscopic state of surface in Ocean™? A dedicated model exists, the Isotropic Table.
Measured microscopic state of surface
Figure 3 – Left: 3D view of the microscopic state of surface of a etched glass. Right: corresponding 2D view.
As we see, this microscopic state of surface is isotropic, and thus we can exploit the Isotropic Table model in Ocean™.
Figure 4 – Distribution of the slope angle as a function of the slope angle.
Case Studies
Automotive application
Over the years, significant efforts have been deployed to increase the comfort but also the safety of drivers at the steering wheel. This includes for example the improvement of Advanced driver-assistance systems (ADAS), but also the use of specific materials for the dashboard and its components, such as the displays.
Nowadays, several technologies exist to reduce the reflection of light from the dashboard displays so as not to dazzle the driver while driving. In this case study, the idea was to study and evaluate the efficiency of two of them: a Physical Vapor Deposition (PVD) anti-reflective (AR) coating and an ion-implanted anti-reflective coating. Both were coupled with a microscopic state of surface that was treated as described in the previous section. The optical behavior and efficiency of the two coatings are simulated with Ocean™. Figure 5 shows aesthetic simulations of a flat/etched cover glass of a display. The display is split in two areas, the left one with a flat surface and the right one with an etched surface.
Figure 5 – Illustration of flat and etched glass. A display is split in two areas, the left one with a flat surface and the right one with an etched surface. Left: display off. Right: display on.
We see that the use of an etched cover glass on a display allows to reduce the specular reflection (intense, sharp reflections visible on the left part of the display), which ease the readability of the display screen because of the reduction of parasitic specular reflections.
Thanks to the high capabilities of Ocean™, we are also able to extract quantitative information. We have used Ocean™ to virtually reproduce an optical measurement device, allowing to extract reflection spectrum of the different configurations and have shown an agreement in 0.1% between Ocean™ measurements and lab measurements.
Note: we also emphasize that thanks to the high modularity of Ocean™, its measurements were available six months before any lab measurements due to the intrinsic difficulties of manufacturing the real samples.
During this project, we have also conducted in-situ studies. We have thus simulated both cases (PVD/ion-implanted) when used for HUD application: the transparent element on the dashboard which let pass the light from the HUD system to the windshield is composed of glass. The reflection of the sun on this piece of glass could dazzle the driver while driving. This is why we conducted simulations where we applied the two previously mentioned coatings on the dashboard piece of glass to evaluate the best configuration in terms of glaring and color stability. Figure 6 presents the setup used during the studies.
Figure 6 – Setup used during the studies.
As previously mentioned, thanks to the high modularity of Ocean™, we can easily assemble different optical properties together, and namely combined different type of products for example check their complementarity. This was done to evaluate the performance of the ion-implanted coating applied on the dashboard assembled with a polarized HUD system (polarized emitter and dedicated coated windshield).
Building application
As previously mentioned in the blog post Light optimization study of a Greenhouse, one of the current topics of study in the world of agronomy is the study of agricultural greenhouses with the aim of optimizing the amount of light entering the greenhouse to maximise the agricultural yield. As discussed in the article, several compositions of glass were studied, namely from a microscopic state of surface point of view. We have simulated two different microscopic states of surface on glass, see Figure 7. Left image represents a low strength of surface state. Right image represents high strength of state of surface. Three targets are placed behind the glass to assess the impact of etching on the transmission of light through the glass. We clearly see the impact: the higher the strength of the state of surface (i.e altitude and frequency of the structure at the surface level), the hazier the transmission.
Figure 7 – Simulations of different microscopic state of surface on glass. Left: low strength of state of surface. Right: high strength of state of surface. Three targets are placed behind the glass to assess the impact of etching on the transmission of light through the glass.
Irradiance simulation
The effect of surface conditions can also be observed by performing an irradiance analysis. Here the irradiance simulation is run on a single hour for a single day to show the impact of double AR or etching.
Aesthetic renders
In conjunction with the irradiance analysis, we also simulate the aesthetics of the greenhouse in its environment to visualize microscopic surface conditions..
Read more details about this study in our use case: Light optimization study of a Greenhouse.
Conclusion
As a conclusion, we have shown that if we can obtain a characterization of the state of surface of a transparent material (such as glass), we can propose a faithful representation of it which allows appearance and quantitative studies.
These studies can be carried out in situ, on the final application and no longer on lab samples.
To go further, we have shown through the different case studies that the main idea is to scatter the light. The scattering of the light here is thus due to a surface property, which differ from the scattering of the light due to volume diffusion
Figure 9 - Aesthetics simulation. Left column : Clear Glass, Right column : High etching Glass
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