Quantitative phase imaging: introduction.

Quantitative phase imaging (QPI), propelled by advancements in digital holography and computational imaging, has revolutionized the ability to retrieve phase delays with high precision. Over the past two decades, the field has seen tremendous growth, contributing to numerous applications in biomedicine and material metrology, including live cell monitoring, material structure profiling, and defect inspection, among others. This special issue commemorates Prof. Gabriel "Gabi" Popescu, a former faculty member at the University of Illinois at Urbana-Champaign and a pioneer in QPI and label-free biological imaging, who passed away on June 16, 2022, in his hometown of Prundu, Romania. The issue honors Gabi's legacy with a collection of articles exploring QPI methodologies and their diverse applications.