Tuesday, March 13, 2007

Differential interference contrast (DIC) microscopy is a beam-shearing interference technique in which the reference beam is sheared by a minuscule amount. This system produces a monochromatic image that effectively displays the gradient of optical paths for high and low spatial frequencies present in the specimen. Those regions of the specimen where the optical paths increase along a reference direction appear brighter (or sometimes darker), while regions where the path differences decrease appear in reverse contrast. As the gradient of optical path difference grows steeper, image contrast is usually dramatically increased. As with Phase Contrast Microscopes, DIC transforms the phase shift of light, induced by the specimen refractive index, into detectable amplitude differences. An advantage of interference-derived contrast is that an object will appear bright against a dark background but without the diffraction halo associated with phase contrast. Differential interference contrast microscopes are actually microscope interferometers in that they generate contrast within the specimen by exploiting phase differences between a specimen light ray and a reference ray.