Differential interference contrast
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 , which converts differences in the optical path length in the viewed object into differences in brightness of the image. This allows transparent phase objects to be made visible. In reflected light , the image contrast reflects the changes in the surface morphology. Vivid images of the object are created in transmitted light. The image contrast is based on local variations in the optical path length of the light in the sample. The image corresponds to the local change (gradient) in the refractive index of the sample (hence the term "differential" image contrast).
history
DIK was developed by Georges Nomarski in Paris in the 1950s and patented by the CNRS . The first series application was built by the Carl Zeiss company in Oberkochen. In the period that followed, only the major microscope manufacturers adopted this demanding process in their program.
principle
A microscope with Koehler illumination is required as the basic configuration . In the Nomarski design, a Nomarski prism and a polarization filter are also installed in front of the condenser and behind the lens. The condenser prism splits the illuminating beam into two parallel, perpendicularly polarized beam paths which are offset below the resolution limit of the microscope objective. After passing through the specimen and objective, both beams are brought together again in the objective prism behind. The polarization directions are then combined again by the analyzer and can interfere. The image contrast is created by the interference of the two partial beams that have passed through different optical path lengths. Such differences in path length 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 in polarizing samples by rotating the microscope stage. By installing a λ / 4 plate , a color contrast can also be created.
In the de Senarmont design , a compensator consisting of a λ / 4 plate and an analyzer are used.
To adjust the microscope (according to Nomarski), the polarizers and prisms are first removed from the beam path and the Koehler illumination is set. Then the polarizer and analyzer are reinserted (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 reinserted and, if necessary, shifted to optimize the image contrast. The aperture diaphragm and field diaphragm are set according to Köhler's illumination.
See also
Web links
- Polarization and interference contrast microscopy , website at the University of Hamburg
- Differential Interference Contrast , detailed website of Olympus with interactive applets (in English)
- Köhler's lighting , website of the Microbiological Association Munich e. V.
Individual evidence
- ↑ Patent US2924142 : Interferential polarizing device for study of phase objects. Applied on May 11, 1953 , published February 9, 1960 , applicant: CNRS , inventor: Georges Nomarski ( also on Google Patents ).