Performance of the backscattering laser dark-field microscope. (a) Scattering cross section of gold particles from experimental data (circles) and Mie scattering theory (line). Each data point shows mean and standard deviation from 32 independent nanoparticles. The experimental cross sections were calculated as the total laser power in the image of a single particle at the CMOS camera divided by the illumination intensity at the particle. The theoretical line was calculated assuming that particles were illuminated by parallel light along the objective’s optical axis, and that all scattering angles with 0.25<N.A.<1.3 were collected (this criterion is for light to be collected by the objective and also pass the rod mirror). (b) The power spectral density (PSD) of noise in the calculated *x* position (black, left axis) and *y* position (gray, right axis) of a 100 nm gold particle stuck on the coverslip, illuminated by 3 μW∕μm^{2} light intensity, at a sampling rate of 110 kHz, corresponding to a Nyquist frequency of 55 kHz. Power spectra of nine sequential traces, each 0.1 s long, were averaged. (c) The light intensity dependence of the total positional noise for a 100 nm gold particle, over the 1.1–55 kHz bandwidth.

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