Illustration of the line depth test. Top: Number of lines included in each mask as a function of their intrinsic depth. Bottom left: LSD I profiles associated with each mask. Bottom right: LSD V profiles (magnified by a factor 5 for clarity) associated with each mask. The black curve stands for the raw synthetic spectrum. The green curve corresponds to the mask used in this study. The blue curve corresponds to a depth limit of 0.8. The red curve corresponds to a depth limit of 0.4.