(a) The structured illumination pattern is the convolution of the excitation beam intensity profile and a periodical modulation function, M(x), in real space. (b) The Fourier transform of the illumination pattern is the product of the Fourier transform of the excitation beam intensity profile and the Fourier transform of the modulation function, , in frequency space. The number of harmonics contained in the illumination pattern is determined by the excitation beam NAOD and the modulation period T. For the sheet-scan mode, T needs to be less than λ/(2NAOD). (c) The normalized intensity profile of each modulation harmonic with excitation wavelength 488 nm, NAOD = 0.5 and T = 1.8 μm. The DC harmonic contains highest axial spatial frequency information, but provides the worst optical sectioning capability. All AC harmonics provide better optical sectioning capability but lower axial frequency limits compared with the DC harmonic. (d) The corresponding OTF of each modulation harmonic, which is the convolution of the wide-field detection OTF with each modulation harmonic in frequency space. A gamma value of 0.5 is applied to the DC harmonic OTF to make the high-frequency contents visible. The Bessel SR-SIM OTF is the sum of all OTFs at the corresponding position of each OTF in frequency space. The fundamental harmonic OTF gives the best optical sectioning, and the highest frequency harmonic gives the maximal resolution extension in the x direction.