Fig. 1. Schematic diagram of structured illumination microscopy. (a) In sinusoidal illumination microscopy, interference between multiple beams (usually generated by a diffraction grating or spatial light modulator) creates a 2D or 3D striped pattern with spatial frequency illuminating on the sample. This pattern shifts the sample's spatial frequency spectrum to and , translating high-frequency SR information into the diffraction-limited detection passband with the spatial cutoff frequency . After computational processing, the sample's highest detectable frequency can be extended to . (b) Spot-scanning illumination microscopy where fluorescence is collected by an array detector, and pixels offset by a distance from the excitation spot detect a shifted but higher-resolution, low-signal confocal image. The reconstruction algorithm corrects the shift and restores the signal by reassigning the detected fluorescence toward the illumination axis, with the final resolution determined by the product of the excitation PSF ( ) and the emission PSF ( ). After deconvolution, this process improves resolution similar to that obtained with sinusoidal illumination microscopy

Fig. 2. The schematic diagram of TIRF-SIM (a) and instant SIM (b). Adapted from Kner et al.^[35] and York et al.^[40]

Fig. 3. Schematic diagram of early implemented 2P SIM (a), 2P-ISIM (b), and 2P SIM with the resonant scanner (c). Adapted from Ingaramo et al.^[42], Peter et al.^[43] and Gregor et al.^[44]

Fig. 4. (a) Schematic diagram of 3D STED-SIM. (b) The cross-section comparison of lateral PSF (top, left), axial PSF (bottom, left), lateral OTF (top, right), and axial OTF (bottom, right) of the widefield microscopy (red) and 3D STED-SIM (blue). Adapted from Xue et al.^[49]