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DIC

Nomarski Differential Interference Contrast (DIC) microscopy is a technique invented by Georges Nomarski in the mid 50s. This method is used for subject, living or colored, which contain little or no optical contrast when viewed in bright field.
The proceedings of this theoretical method is quite complicated, the light from the light source is passed through a polarizer located under the condenser, similar to polarized light microscopy. The polarized light then passes through a modified Wollaston prism (below the condenser) which divides the beam into two beams traveling in slightly different directions, but perpendicular to each other, and therefore not able to recombine to cause interference. The distance between the two beams is called away "shear" and is always less than the objective resolution and capacity, to avoid the appearance of double images. The beams of light, divided, pass through the sample where their paths are altered due to the different thickness of the preparation and its refractive index. When parallel rays entering on the objective, are concentrated above the focal plane, where they enter into a second modified Wollaston prism, which recombines the two beams in a defined distance outside the prism. This eliminates the "shear" and the path difference between the original pair of rays. However, the parallel beams no longer have the same length because of the path changes caused by the sample. In order to make the parallel beams interfere with each other, the vibrations of the rays of different lengths have to be taken on the same plane and axis. This is achieved by placing a second polarizer (analyzer) above the upper Wollaston prism. DIC microscopy makes it appear brighter (or colored) one side of the object, while the other appears darker (or a different color). This shadow effect gives it a pseudo three-dimensional model, but it is not an accurate representation of the geometry of the sample, because it relies on an optical depth. The pseudo three-dimensional aspect of the sample can also be profoundly influenced by its position, for example the rotation of the sample of 180 degrees can change a hill in a valley or vice versa. Therefore, the DIC microscopy is not suitable for accurate measurement of actual height and depth of the subject.
Using this technique brings many advantages over other techniques and, in particular, the phase-contrast microscopy.
DIC microscopy allows greater use of the numerical aperture of the system because, unlike the phase-contrast microscopy, there is not the phase ring to restrict the numerical aperture of the objective, in addition, the Köhler illumination can be used properly.

Images can also be viewed in color (lambda compensating) with a 3 dimensions and with excellent resolution. There are not halos, such as may be encountered in the phase images. DIC is also superb for the study of living cells, because it is not invasive and offers the possibility, in real time, to easily follow the movement of small organelles within cells.
However, there are several disadvantages in DIC microscopy, the necessary equipment for this technique are very expensive because of the many prisms that are necessary, birefringent specimens, such as those relating to many types of crystals, may not be suitable because of their effect on light polarized. Similarly, slides in plastic culture vessels, petri dishes, etc., may not be fit for purpose.
They lend themselves well to be seen with this technique, cells, protists, diatoms, bacteria, microrganisms, etc..