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Transmitted-light imaging with an optical microscope in which the directly transmitted illumination contributes the bright background. An absorbing specimen can reduce that background locally. The ideal lossless phase object calculation in coherent zero-mode model of phase contrast shows why ordinary brightness detection need not reveal every optical variation.

A phase object for which the relevant relative phase variations are much smaller than one radian. The Taylor expansion $e^{i\varphi }=1+i\varphi +O({\varphi }^2)$ separates a direct reference term from a small specimen term. A constant overall phase can be removed before making this approximation: phase-contrast microscopy measures spatial variations, not an absolute common phase.