Nomarski's DIC system for transmitted light consists of two birefringent prisms, one of which is located behind the microscope objective, while the other is placed below the condenser. The latter is used as a compensator; it is similar to the former and oriented in such a manner as to cancel out the optical path differences between lightwave components slightly sheared and brought into mutual interference. Due to the sub- condenser birefringent prism, this system suffers from some limitations when anisotropic objects are examined. These limitations can be removed if a subtractive combination of two birefringent prisms is placed behind the microscope objective and the sub-condenser prism is replaced by a slit diaphragm. If the required wavefront shear is equal to or less than the resolving power of the objective (this is the case of submicron objects), then the slit of the sub-condenser diaphragm is very large or even equal to the diameter of the condenser aperture. Moreover, a variable wavefront can be achieved easily by a suitable combination of two birefringent prisms installed behind the microscope objective.
|