Particle extinction coefficient
= Particle extinction coefficient
{title2=$\beta_{\rm ext}=\int Q_{\rm ext}(r)\pi r^2n(r)\,dr$}
The total extinction cross-section per unit volume of particles is a length-inverse coefficient. Its path integral is <optical depth>. It combines scattering and absorption and differs from the dimensionless imaginary part of a <complex refractive index>. For liquid droplets with size-independent $Q_{\rm ext}$, $\beta_{\rm ext}=3Q_{\rm ext}w/(4\rho_wr_e)$.