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 = I_T/I₀ = exp(−αz)

where I₀ is the incident irradiance and I_T 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 α_ρ as

α = ρα_ρ,

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 α_ρ.

The absorbance for a small α is defined as

A = 1−T = αz.