Generalized approach for single-optical-access interferometric particle imaging

Interferometric particle imaging (IPI) enables the sizing of small transparent particles, from a few microns to several millimeters, across a large field of view, which is not feasible with typical imaging-based methods. However, current IPI setups typically require two separate optical accesses—one for coupling the illuminating laser beam into the measurement volume, and another to allow the scattered light to exit toward the camera—thereby limiting their applicability in constrained environments. This work presents a generalized IPI approach suitable across all scattering regimes—including backscatter for single-optical-access configurations—and is applicable to both droplets and bubbles. This study demonstrates how the scattering angle influences signal-to-noise ratio (SNR), measurable size range, and uncertainty. Polarization significantly affects signal quality, particularly in backscatter. Variations in light intensity with angular variations can introduce position-dependent uncertainties, and the sensitivity of the size measurement (transfer function) varies with scattering angle. Experimental validation with bubbles confirms the feasibility of backscatter IPI despite lower SNR. For droplets, the reduction in SNR is less pronounced; however, the transfer function behaves differently compared to bubbles, making both evaluation and uncertainty estimation more complex. Additional effects like glare-point splitting can add redundancy but increase complexity. Overall, this study demonstrates that IPI in backscatter is not only feasible but also a key step toward single-sided optical access, expanding the method’s practical applicability of the IPI method.