Incident plane-parallel light I_c (dotted lines) is passing through two boxes, that can be filled with (identical) absorbing material (trianges, squares) and is detected by a spectrometer. The observed spectrum is plotted as solid line at the bottom of each panel. Left panel: the total line profile is the sum of the two individual profiles caused by only material 1 (I₁/I_c, dashed line) and 2 (I₂/I_c, dot-dashed line), respectively. Right panel: if box 1 contains both absorbing materials, the residual depth is calculated using Eqs. (10), (11), and cannot exceed a value 0.5, since half of I_c always passes through box 2. Saturation effects are visible in the center of the line.