Salmon, E. D. and Tran, P.
High-resolution video-enhanced differential interference contrast (VE-DIC) light microscopy.  Methods in Cell Biology 81: 335-364 (2007).

Differential interference contrast (DIC) light microscopy was an immediate success after its introduction in the 1960s (Allen et al., 1969) because it could produce high-contrast optical images of the edges of objects and fine structural detail within transparent specimens without the confusion induced by the interference fringes typical of phase-contrast microscopy. DIC contrast depends on gradients in optical path (OP = nd; refractive index, n, × thickness, d) and not absolute values of OP. As a result, DIC methods can produce clear optical sections of relatively thick transparent specimens. Resolution in DIC microscopy is also superior to phase contrast. In transmitted light microscopy, lateral resolution, r, depends on the wavelength of light, λ, and the numerical aperture, NA, or both the objective and condenser:

r = λ/(NAobj + NAcond).