We show that the focal modulation microscopy (FMM), which combines a spatial phase modulator with con-focal 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 16.4percent. When applied to saturable fluorescence, the half-width at half-maximum is improved by 33.6percent, 50.0percent, and 62.9percent, at demodulation frequencies 2omega, 4omega, and 8omega, respectively.
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