FIGURE 5.

Irradiance of light and the physical cross section and scattering cross section of a particle. (A) Irradiance E is the power of light $P_L$ per unit area A in the focus (blue oval) of the laser beam. (B) Physical cross section ${\sigma }_{\mathbf{p}}$ (dotted oval) of a particle (circle) oriented perpendicular to the incoming laser beam. For clarity, ${\sigma }_{\mathbf{p}}$ and A (panel A) have similar dimensions, but in reality ${\sigma }_{\mathbf{p}}$ for extracellular vesicles is much smaller than$A$. (C) The scattering cross section ${\sigma }_s$ (red oval) is a hypothetical area defining which fraction of the incoming light is scattered by a particle. (D) The effective scattering cross section ${\sigma }_{\mathrm{\Omega }}$ (red oval) is a hypothetical area defining which fraction of the power of the incoming light is scattered by a particle towards a lens with solid collection angle $\mathrm{\Omega }$.