The absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium is proportional to the absorption path length, l, and to the concentration, c, or — in the gas phase — to the pressure of the absorbing species. The law can be expressed as: \[A = \text{log}_{10}\left ( \frac{P_{\lambda }^{0}}{P_{\lambda }} \right ) = \varepsilon \:c\:l\] or \[P_{\lambda }=P_{\lambda }^{0}10^{-\varepsilon \: c\: l}\] where the proportionality constant, ɛ, is called the molar (decadic) absorption coefficient. For l in cm and c in mol dm-3 or M, ɛ will result in dm^3 mol^-1 cm^-1 or M^-1 cm^-1, which is a commonly used unit. The SI unit of ɛ is m2 mol-1. Note that spectral radiant power must be used because the Beer–Lambert law holds only if the spectral bandwidth of the light is narrow compared to spectral linewidths in the spectrum.