Phase contrast

Phase contrast in images arises from differences in phase between scattered and unscattered rays in different parts of the image and interference between these rays. In a fully transparent (i.e. no variation in refractive index) object, there are no phase differences and, hence no phase contrast in the image

[img[1][EmbeddedImages]]

Illustration of electron wave passing through under phase object approximation. The phase changes (contrast) is imaged.

Defocusing, in which path lengths for scattered rays are changed more than for the unscattered rays, can be used to enhance phase contrast. Contrast due to phase differences is more important for thin objects and when working near the resolution limit than contrast due to amplitude differences.