The law can be expressed in terms of attenuation coefficient, but in this case is better called the Bouguer-Lambert's law. The (Napierian) attenuation coefficient μ μ and the decadic attenuation coefficient μ μ 10 = μ μ ln ⁡ ⁡ 10 of a material sample are related to its number densities and amount concentrations as μ μ ( z ) = ∑ ∑ i = 1 N μ μ i ( z ) = ∑ ∑ i = 1 N σ σ i n i ( z ) , μ μ 10 ( z ) = ∑ ∑ i = 1 N μ μ 10 , i ( z ) = ∑ ∑ i = 1 N ε ε i c i ( z ) respectively, by definition of attenuation cross section and molar attenuation coefficient. Then the law becomes T = exp ⁡ ⁡ ( − − ∫ ∫ 0 ℓ ℓ μ μ ( z ) d z ) = 10 ∧ ∧ ( − − ∫ ∫ 0 ℓ ℓ μ μ 10 ( z ) d z ) , and τ τ = ∫ ∫ 0 ℓ ℓ μ μ ( z ) d z , A = ∫ ∫ 0 ℓ ℓ μ μ 10 ( z ) d z . In case of uniform attenuation, these relations become T = e − − μ μ ℓ ℓ = 10 − − μ μ 10 ℓ ℓ , or equivalently τ τ = μ μ ℓ ℓ , A = μ μ 10 ℓ ℓ .