Real-time spatial frequency modulation imaging

Real-time spatial frequency modulation imaging (SPIFI) is demonstrated for various contrast mechanisms and applications. In linear SPIFI, it is used to monitor laser powder bed fusion, matching standard microscopy resolution while enabling real-time imaging with low numerical aperture optics in scattering environments. The SPIFI signal effectively tracks melt pool conditions, optimizing defect detection compared to camera feeds. Pushing the detector into saturation enhances signal-to-noise and resolution. Additionally, photon-counting, nonlinear SPIFI extends resolution limits beyond the classical cut-off by up to six times. A novel contrast-reversal regime is identified, and results for nonlinear SPIFI are presented.