We report a simple, rapid, and quantitative wide-field technique to measure the optical extinction $sigma_{rm ext}$ and scattering $sigma_{rm sca}$ cross-section of single nanoparticles using wide-field microscopy enabling simultaneous acquisition of hundreds of nanoparticles for statistical analysis. As a proof of principle, we measured nominally spherical gold nanoparticles of 40,nm and 100,nm diameter and found mean values and standard deviations of $sigma_{rm ext}$ and $sigma_{rm sca}$ consistent with previous literature. Switching from unpolarized to linearly polarized excitation, we measured $sigma_{rm ext}$ as a function of the polarization direction, and used it to characterize the asphericity of the nanoparticles. The method can be implemented cost-effectively on any conventional wide-field microscope and is applicable to any nanoparticles.
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