Differential interference contrast microscopy
Differential interference contrast (DIC) microscopy, also called Nomarski interference contrast (NIC) or Nomarski microscopy, is an optical microscopy technique used to enhance the contrast in unstained, transparent samples. DIC works on the principle of interferometry to gain information about the optical path length of the sample, to see otherwise invisible features.
Source: Wikipedia — Differential interference contrast microscopy (CC BY-SA 4.0)