Towards a mobile quantitative phase imaging microscope with smartphone phase-detection sensors

Quantitative phase imaging (QPI) enables the visualization and quantitative extraction of optical phase information from transparent samples. However, conventional QPI techniques typically rely on multi-frame acquisition or complex interferometric optical setups. In this work, we propose Quad-Pixel Phase Gradient Imaging (QP²GI), a single-shot quantitative phase imaging method based on commercial quad-pixel phase detection autofocus (PDAF) sensors that are now commonly utilized in modern smartphones. PDAF sensors include an array of small microlenses, wherein each microlens covers a 2 × 2 pixel group on the sensor. The sample's phase gradients induce focal spot displacements beneath each microlens, which in turn result in intensity imbalances across the four constituent pixels. By deriving the phase gradients of the sample from these imbalances, QP²GI reconstructs quantitative phase maps from a single exposure. We establish a light-propagation model to describe this process and evaluate its performance in a customized microscopic system. Experiments demonstrate that quantitative phase maps of microbeads and biological specimens can be reconstructed from a single acquisition. Furthermore, low-coherence illumination improves robustness by suppressing coherence-related noise. These results reveal the potential of quad-pixel PDAF sensors as cost-effective platforms for single-frame QPI.