Differential interference contrast microscopy

The differential interference contrast (also: differential interference contrast, differential interference contrast or Nomarski contrast, abbreviated DIK or DIC from English- differential interference contrast) is a method of imaging light microscopy, converts the differences in the optical path length in the object being viewed in differences in brightness of the image. Characterized transparent phase objects can be made visible. In reflected light, the image contrast the changes in the surface morphology are again. In transmitted light caused vivid pictures of the object. Here, the image contrast is due to local variations in optical path length of light in the sample. The image so (hence the term " differential " image contrast ) corresponds to the local change (gradient ) of the refractive index of the sample.

History

DIK was developed in the 1950s by Georges Nomarski in Paris; the CNRS held the license for this procedure. The first standard application built the company Carl Zeiss in Oberkochen. In the following period only the large microscope manufacturers have adopted the sophisticated process in their program.

Principle

The basic configuration of a microscope with Köhler'scher lighting is needed. In addition, depending Nomarski setup a Nomarski prism and one polarization filter in front of the condenser and behind the lens to be installed. The condenser prism provides for splitting the illumination beam into two parallel, orthogonally polarized beam paths, having a displacement below the resolution limit of the microscope objective. Both beams are recombined after passing through the specimen and objective lens located behind the prism. The polarization directions are then combined by the Analyzer and can interfere. The image contrast is produced by the interference of the two partial beams that have passed through different optical path lengths. Such path length differences can be caused by a varying thickness of the object, or by variations in the refractive index. Since the partial beams are polarized perpendicular to each other, different representations of the object are possible with polarizing samples by rotation of the microscope stage. By incorporating a 4 plate a color contrast can also be generated.

In construction by de Senarmont a compensator comes from a 4 plate and the analyzer used.

For adjustment of the microscope ( Nomarski ) first the polarizers and prisms are removed from the beam path and set the Koehler illumination. After the polarizer and analyzer are used again (crossed). The position of the polarizer and analyzer is optimized for maximum darkness (dark cross when using an auxiliary microscope or a Bertrand lens). Then both prisms are used again and moved if necessary to optimize the image contrast. The setting of aperture and field diaphragm be in accordance with Kohler illumination.