Dark-field describes an illumination technique used to enhance the contrast in unstained organisms or any samples. Dark Field Microscopy works by illuminating the sample with light that will not be collected by the objective lens and thus will not form part of the image. This produces the appearance of a dark, almost black, background with bright objects on it.
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It creates a contrast between the object and the surrounding field so that the background is dark and the object is bright. The objective and the ocular lenses are used in this microscope are the same as ordinary microscopes but a special condenser is used that prevents the transmitted light from directly illuminating the specimen. Only oblique scattered light reaches the specimen and passes on to the lens and causing the bright objects against a dark background (Fig).
It has a dark ground condenser that focuses only the oblique rays of light onto the specimens, a high-intensity light lamp, and a funnel stop that reduces the apparatus of the objective to less than 1.
- It is used for the diagnosis of Syphilis.
- Viewing bacteria, algae, and blood cells.
- Viewing hairline metal fracture.
- Dark Field Microscopy is ideal for viewing unstained, transparent, and little absorbed objects.
- It is ideal to study marine organisms such as diatoms, algae, plankton, etc.
- It is used for research on the live bacterium, mounted cells, and tissues.
- It is used to examine external details like outlines, edges, grain boundaries, etc.
- The images are prone to degradation and distortion.
- It needs an intense amount of light to work.
- It is not a reliable tool to obtain an accurate measurement of specimens.
- If oil or water is used on the condenser then it is impossible to avoid air bubbles on slides.
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