law of Bouguer-Lambert and Beer (A = lg(1/τ) = εbc) #01848OIML 1. Conditions for validity: A beam of parallel monochromatic radiation traverses, at normal incidence, an absorbing medium with planeparallel surfaces and which is homogeneous, isotropic, non-luminescent and non-scattering. 2. Adapted from ISO 6286, clause 3.3.