Differential schemes for particle detection
Using the coherence of some light sources, one can use clever illumination and detection schemes to achieve high sensitivity to particle size/refractive index, using just the scattering signal. However, if the background varies over time, its fluctuations can overcome the signal from the detected objects. In some cases, background can be made constant by improving the hardware (i.e. making it more stable over time), but it will never be able to go beyond shot-noise.
References: [@Faez2015Fast, Label-Free Tracking of Single Viruses and Weakly Scattering Nanoparticles in a Nanofluidic Optical Fiber]
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