In practice, the model is commonly modified to account for multiple absorption mechanisms present in a medium. The modified version is given by ε ε ^ ^ ( ω ω ) = ε ε ∞ ∞ + ∑ ∑ j χ χ j L ( ω ω ; ω ω 0 , j ) where χ χ j L ( ω ω ; ω ω 0 , j ) = s j ω ω 0 , j 2 − − ω ω 2 − − i Γ Γ j ω ω and