6.1 Absorption Parameters
The absorption coefficient α applies to irradiance (power per area), while the extinction coefficient κ applies to electric field (force per charge). The transmittance T applies to the irradiance:
T = IT/I0 = exp(−αz)
where I0 is the incident irradiance and IT is the transmitted irradiance.
The absorption coefficient α is derived from the extinction coefficient κ as
α = 4πκ/λ.
The absorption coefficient α is derived from the molar extinction coefficient αρ as
α = ραρ,
where ρ is the molar concentration. The units of molar concentration are moles per liter (mol/L). The units of molar extinction coefficient are concentration per distance [moles per liter per centimeter (mol·L-1cm-1)]. It is important to correctly define these similar metrics: absorption coefficient α, extinction coefficient κ, and molar extinction coefficient αρ.
The absorbance for a small α is defined as
A = 1−T = αz.