Differential interference is a technique that utilizes the phenomenon of light interference for imaging, and its optical principle is based on the interference effect. When two beams of light through the optical system will interfere with each other, if the phase is the same, the result of interference is to enhance the brightness; on the contrary, they will cancel each other darker.

In differential interference, light from a light source is decomposed into two coherent beams of the same frequency after passing through a specific optical element, such as a Wollaston prism or DIC prism. These two beams of light are simultaneously focused and projected on the surface of the object in close proximity to each other, forming two spots. The optical range difference between the two coherent light beams depends on the relative heights of the two beams. Changes in the height of the surface of the object result in a change in the optical range difference, and by detecting this change in the optical range difference, the topography of the surface of the object can be measured.

In differential interference microscopy, the light source is usually plane-polarized light. The light is refracted by a prism and split into two beams, which will pass through adjacent parts of the sample at different times. When these two beams of light enter the prism again after being reflected from the sample surface, they will have different reflections and phase changes due to the unevenness of the sample surface, cracks, microholes, depressions, grain boundaries, and other features, which will result in the formation of new optical range differences. These light-range difference changes will eventually be converted into interference signals, and through signal processing and mathematical algorithms, the reconstruction and measurement of the surface topography of the object can be realized.

The principle of differential interference imaging has a wide range of applications in the fields of surface topography, optical microscopy, and nondestructive testing.