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Differential interference contrast microscopy converts small sideways differences of optical path length into visible intensity differences. A polarizer and birefringent prisms produce slightly displaced beams with perpendicular polarizations; after they pass through neighboring specimen positions, the beams are recombined and analyzed so that their relative wave phase controls brightness.^[ref]

For beam displacement $\mathbf{s}$ small compared with the scale of specimen variation, the phase difference is approximately $\mathbf{s}\cdot \nabla \varphi$. Thus a transparent object can appear shaded along the shear direction. Rotating the specimen changes that shading: the relief appearance does not by itself measure three-dimensional shape.