 Phase-contrast microscope

A phase-contrast microscope makes small specimen-induced wave phase changes visible through wave interference. An illumination annulus is imaged onto a phase ring in the microscope objective's rear focal plane. The ring changes the relative phase and amplitude of largely undeviated illumination compared with specimen-diffracted light, producing intensity differences in the image.^[ref]

Correctly matching and centering the annulus and ring is an apparatus requirement. A transparent unstained specimen can then acquire strong image contrast, but halos and image contrast reversals can occur. Comparing ordinary illumination with the aligned phase system demonstrates that image brightness is not simply a map of optical absorption.