Fourier ptychographic microscopy aided with transport of intensity equation for robust full phase spectrum reconstruction

Fourier ptychographic microscopy (FPM) is a pivotal computational imaging technique that achieves phase and amplitude reconstruction with high resolution and a wide field of view, using low numerical aperture objectives and LED array illumination. Despite its unique strengths, FPM remains fundamentally limited in retrieving low spatial frequency phase information due to the absence of phase encoding in all on-axis and slightly off-axis (bright-field) illumination angles. To overcome this, we present a hybrid approach that combines FPM with the transport of intensity equation (TIE), enabling robust phase retrieval across a wide spatial frequency range without compromising system simplicity. Our method extends standard FPM acquisitions with a single additional on-axis defocused image, from which low-frequency phase components are reconstructed via the TIE method, employing large defocus distance to suppress low-frequency artifacts and enhance robustness to intensity noise. High-frequency phase details are recovered through FPM processing. To additionally compensate for defocus-induced magnification variations caused by spherical wavefront illumination, we employ an affine transform-based correction scheme upon image registration. Notably, by restoring the missing low-frequency content, our hybrid method allows for more reliable quantitative phase recovery than standard FPM. We validated our method using a quantitative phase test target for benchmarking accuracy and biological cheek cells, mouse neurons, and mouse brain tissue slice samples to demonstrate applicability for in vitro bioimaging. Experimental results confirm substantial improvements in phase reconstruction fidelity across spatial frequencies, establishing this hybrid FPM + TIE framework as a practical and high-performance solution for quantitative phase imaging in biomedical and optical metrology applications.