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The ultimate in sophistication:
Differential Interference Contrast (DIC) in transmitted light |
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This highly efficient contrasting technique is based on the Pol contrast technique as far as the components used are concerned. In its function, it is indeed related to DIC in reflected light. However, DIC in transmitted light is slightly more complicated than in reflected light because, firstly, two birefringent prisms are used, and, secondly, the path difference in the object is created in a different way.
The accompanying illustration shows the beam path which, initially, is identical to that of polarized transmitted light. Additionally, the two birefringent prisms (2) are inserted in the condenser and near the objective pupil (6). The condenser prism (2) performs a vectorial decomposition of the previously linearly polarized light into two vibration directions which are perpendicular to each other, and laterally shifts these partial beams in such a way that a lateral displacement of Δx = k · λ occurs in the specimen. λ is the wavelength of the light used and k is a number which normally is smaller than 1.
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. below). However, if the two partial beams “see” slightly different conditions, each of them will “experience” its own path difference which accompanies it on the remaining path to the intermediate image (case B in Fig. below). The second prism (6) cancels the splitting process again behind the objective, and analyzer (7) selects those components from the now phase-shifted wave trains which lie in its vibration direction.
It is only now – with a common vibration plane – that the two partial beams can interfere with each other and therefore convert path differences to intensity differences which can be seen by the eye. A λ-plate (7a,λ) permits additional color contrast to be produced. The resulting images look like reliefs because this method displays only “lateral” changes. DIC is therefore also ideal for the optical sectioning of unstained, thick objects.
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