Methodology for BRDF Measurements in Outdoor Conditions with Solar Illumination

Understanding and modeling the reflectance properties of materials is essential in various fields, for example during the development and evaluation of camouflage materials. An important concept for this purpose is the Bidirectional Reflectance Distribution Function, BRDF, which describes how light is reflectedfrom a surface as a function of both the incident and viewing angles. While BRDF is typically measured in indoor laboratories, there is significant value indeveloping methods for outdoor measurements under natural illumination, particularly for studying large or context specific samples. However, this introduces challenges in managing environmental variables. The aim of this project was to design a first useful methodology for measuring BRDF using the sun as the light source. A theoretical model was proposed to isolate the direct sunlight component by comparing radiometric measurements of both reference and sample materials under sunlit and shaded conditions. Measurements were performed with a high dynamic range camera sensitive to the visible spectrum, mounted onto an adjustable boom to alter the zenith angle. Images of the sample materials and the reference material were captured and the data was then used to calculate the BRDF of the different sample materials. Ingoing and outgoing angles were determined using a solar position algorithm program together with a mechanical angle measurement device on the boom. The results showed strong agreement between outdoor BRDF measurements and indoor lab-oratory measurements, performed with a 633 nm laser scatterometer, with only minor deviations. A limitation of the project was the boom that only allowed measurements within a fixed plane, which did not align with the incident plane of the sunlight. As a result, the measurement predominantly captured diffuse reflection, limiting characterization of specular materials. For future work, a drone based system flying in a hemispherical pattern could enable a broader angular sampling, allowing for detailed BRDF mapping of more specular or inhomogeneous materials in three dimensions.