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| Advanced Optics for Differential Interference Contrast (DIC) |
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As far as the components used are concerned, this very powerful contrasting technique is based on polarization contrast. Its function is related to DIC in incident light, but it is slightly more complicated: first, two birefringent prisms are used, and second, the path difference in the objective lens is different.
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| Ray path for DIC in transmitted light. |
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Fig. 1 shows the beam path. Initially, it corresponds to the beam path of polarized transmitted light. The two birefringent prisms (2) are inserted into the condenser and near the objective pupil (6). The condenser prism (2) performs a vectorial dispersion of the previously linearly polarized light into two vibration directions perpendicular to each other and laterally shifts these partial beams in such a way that a lateral displacement of Δx = k · λ. λ is the wavelength of the light used and k is a number which is normally smaller than 1.
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| Interference |
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If the two partial beams now pass through exactly the same structures, no further path difference will occur in the specimen (cases A and C in Fig. 2). However, if the two partial beams experience slightly different conditions, each one will have a different path difference which accompanies it on the rest of the way to the intermediate image (case B in Fig. 2). The second prism (6) cancels the splitting process again behind the objective.
Analyzer (7) selects those components from the now phase-shifted wave trains which lie in its direction of vibration. Only then, with their common vibration plane, can the two partial rays interfere with each other. Therefore, path differences are converted to intensity differences which can be seen by the eye.
A λ-plate (7a,λ) permits color contrast to be additionally produced. The resulting images appear to display relief because this method exhibits only ”lateral” changes. DIC is therefore also ideal for the optical sectioning of unstained, thick objects.
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| Structure in transmitted light | DIC in transmitted light |
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