We show that the focal modulation microscopy (FMM), which combines a spatial phase modulator with confocal microscopy, results in an improvement in spatial resolution. This technique was introduced to increase imaging depth into tissue and rejection of background from a thick scattering object. A theory for image formation in FMM is presented, and the effects of detecting the in-phase modulated fluorescence signal are discussed. Compared with conventional confocal microscopy, the width of the point-spread function for the in-phase fluorescence signal is improved by 16.4%. When applied to saturable fluorescence, the half-width at half-maximum is improved by 33.6%, 50.0%, and 62.9%, at demodulation frequencies , , and , respectively.
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