Dark field microscopy

Dark Field Microscopy

Dark field microscopy

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Description:
Dark field microscopy (dark ground microscopy) describes microscopy methods, in both light and electron microscopy, which exclude the unscattered beam from the image. As a result, the field around the specimen (i.e. where there is no specimen to scatter the beam) is generally dark.

Light Microscopy Applications

In optical microscopy, darkfield describes an illumination technique used to enhance the contrast in unstained samples. It 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 classic appearance of a dark, almost black, background with bright objects on it.

The light's path

The steps are illustrated in the figure where an upright microscope is used.
  1. Light enters the microscope for illumination of the sample.
  2. A specially sized disc, the patch stop (see figure) blocks some light from the light source, leaving an outer ring of illumination. A wide phase annulus can also be reasonably substituted at low magnification.
  3. The condenser lens focuses the light towards the sample.
  4. The light enters the sample. Most is directly transmitted, while some is scattered from the sample.
  5. The scattered light enters the objective lens, while the directly transmitted light simply misses the lens and is not collected due to a direct illumination block (see figure).
  6. Only the scattered light goes on to produce the image, while the directly transmitted light is......
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