Grating-free autofocus for single-pixel microscopic imaging

As a computational technology, single-pixel microscopic imaging(SPMI) transfers the target's spatial information into a temporal dimension. The traditional focusing method of imaging before evaluation is not applicable to the SPMI system. We propose a grating-free autofocus strategy derived from the physical mechanism of optical defocus. Maximizing the amplitude information of just one high-frequency point in the spectrum is all that is needed to achieve fast autofocus with the SPMI system. Accordingly, only four patterns need to be cyclically projected, enabling efficient localization of the focal plane based on the measurement data. We demonstrate SPMI autofocus experiments at micrometer and even nanometer depths of field. The proposed method can be extended to achieve SMPI autofocus with invisible optical pattern illumination.