Quantitative phase imaging (QPI) takes another step forward with this interesting study on the influence of annular illumination. Understood in its broadest sense, QPI has made important contributions to optics over many decades, for a variety of radiation and matter-wave fields. This family includes visible light, electrons, x-rays and neutrons. The current paper, by Kulkarni and coauthors, studies annular illumination in two-dimensional QPI for visible-light microscopy. Particular attention is paid to the comparison between annular illumination and disk illumination. Defocus is the method of choice for this study. In particular, defocus generates the phase contrast that is subsequently decoded to achieve QPI. As is the case with Gabor's immensely fruitful concept of inline holography, the QPI imaging protocol is a two-step process. First, multiple defocused images are measured, that contain coded phase information. Second, the measured focal series of images is decoded, to provide quantitative phase information. Stated differently, this is phase-retrieval computational imaging. Optical software constitutes an intrinsic part of the QPI imaging system, whose optical components are therefore both actual and virtual. The work is important and interesting. I warmly recommend it to your attention.
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