The interference is essentially based on the same principle as that of phase contrast microscope.
In both the microscopes the principle of increased or decreased amplitude due to the interference between out of phase light waves and in phase light waves is used to produce contrast in the image.
While a single beam of light passing through the object is made use of in phase contrast microscope, there are two beams of light that combine after passing through the specimen. This produces an interference pattern.
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Interference microscopes are more versatile in comparison with phase contrast microscopes as they have objectives of higher NA and also plaque polarized light use color to increase the contrast.
One of the most frequently used interference microscope is a Nomarski Differential Interference Contrast (NDIC) Microscope which can produce a high contrast, almost three dimensional images of transparent objects.
The special features of a NDIC microscope are polarizing filler and interference contrast condenser and a prism analyzer plate. In the NDIC microscope, a beam of polarized light is split into two beams at right angles to each other; are made to travel through the specimen and are combined and forced through an analyzer.
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The resultant interference pattern will produce a pseudo three dimensional image. This is due to the fact that the two beams of light passing through the specimen produce a stereoscopic effect.
The degree or level of three dimensional configurations that is visualized is a function of the variations of refractive indices at the surface of the specimen.
In other words, higher the difference of refractive index at the surface, more will be the contrast in the image of the specimen.
Edges of specimen such as cell walls with a high refractive index- variation are very well defined under a NDIC microscope.
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Images seen through the NDIC microscope are normally brilliantly colored (non – stained) because of the phase changes of light waves that pass through the various components of an object such as a microbial cell.
Qualitative observations of specimens are best made through an interference microscope, because the images have a high contrast and high topographic relief due to the three dimensional effect.