In this paper, a non-destructive method is proposed for measuring the density of a verythin lubricant layer (weight and spatial) on an industrial surface. We consideredspectroscopic ellipsometry measurements on rough tinplated steel substrates protected by alubrication layer. The thickness of the coating was less than the roughness parametercharacterizing the metallic surface. As the optical properties of the substrates could notbe modelled in a conventional way due to the roughness and the complex structure of themetal, the variations of one of the ellipsometric angle (Δ) were evaluated as a functionof the lubricant film surface density. After identification of the potential outliersusing a multivariate analysis technique based on the Mahalanobis distance, we interpretedthe data using the Drude’s approximation for thin dielectric films. The values of Δlinearly decrease with the lubricant surface density, allowing us to evaluate locally thelubricant surface density and its point-to-point variations.