For the case of absorption (Beer), this later quantity is called the absorptivity [ ϵ ϵ ], which is defined as "the property of a body that determines the fraction of incident radiation absorbed by the body". The Beer–Lambert law [ log 10 ⁡ ⁡ ( I 0 / I ) = A = ϵ ϵ ℓ ℓ c ] uses concentration and length in order to determine the number of particles the beam encounters. For a collimated beam (directed radiation) of cross-sectional area S , the number of particles encountered over a distance ℓ ℓ is N A c σ σ ℓ ℓ S , where N A is the Avogadro constant, c the molar concentration (in mol/m), and σ σ the particle cross section.