Characterizing the bulk angular distribution of metal flakes in pigmented coating systems leveraging RTI

A key factor in the appearance of metal flake-pigmented coatings (MF-PCs) is the orientation of the metal flakes, which affects the coating's luster (at large distances) and visual texture (up close). This study introduces a dome-based reflectance transformation imaging (RTI) system to estimate metal flakes' orientation using intensity multi-light image collections (MLICs) and an algebraic reflectance map inversion algorithm. The flake's angles and 2D/3D probability density functions (PDFs) were derived from this, and a new orientation index (OI) for MF-PCs. The impact of metal flakes' orientation on appearance was assessed using the lightness flop index (LFI), sparkle (SG), and graininess grades (GG). Results show that the LFI and GG increase as the OI decreases, while SG at 15° rises with higher orientation but decreases at 45° and 75°. Additionally, a non-linear negative exponential relationship between the OI and LFI was observed, highlighting orientation as a significant dimension in predicting MF-PC's appearance.