Darkfield Microscope: Definition, Principle, Application.

Written by SouravBio · 3 minutes read >

Darkfield Microscope Definition

  • Darkfield microscope allows a viewer to observe the specimen bright whereas the surrounding appears as dark.
  • Using a dark field microscope we can see the living and unstained cells.
  •  The darkfield microscope can reveal considerable internal structure in microorganism.
  • For a dark field microscope, we can use three types of condenser such as abbe condenser, cardioids condense, paraboloids condenser.
  • The condenser lens of a dark field microscope creates a hollow cone of light which is opposite of a bright field microscope.
  • An opaque disc is located underneath the condenser lens, which only allows the scattered lights from the object, and prevents the straight lights which are transmitted through the specimen. So that we can see a dark background and the bright object.
  • Most of the compound and stereo microscope can be converted into dark field microscope.

Principle of Darkfield Microscope

A hollow cone of light is focused on the specimen in such a way that unreflected and unrefracted rays do not enter the objective. The only light that has been reflected or reflected by the specimen forms an image.  The field surrounding a specimen appears black or dark while the object itself is brightly illuminated.

Path of light in Darkfield Microscope

Image: Light Path of Darkfield Microscope | Image Source: en.wikipedia.org
  1. Light enters the microscope to illuminate the specimen.
  2. A special disc, called patch stop blocks some light from the light source, leaving an outer ring of illumination.
  3. A wide phase annulus can also be reasonably substituted at low magnification.
  4. Now the condenser lens focuses the light towards the specimen.
  5. The light enters the specimen.
  6. Most of the light rays are directly transmitted, while some of them are scattered from the sample.
  7. Only Scattered light started to enter the objective lens and creates an image of the specimen.
  8. While the directly transmitted light simply misses the lens and is not collected due to a direct-illumination block and they omitted.
Darkfield Microscope
Image: Darkfield Microscope | Image Source: www.microscopeworld.com

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Application of Darkfield Microscope

  1. The darkfield microscope is used to identify bacteria like thin and distinctively shaped.
  2. We Can observe the living and unstained cells by using a dark field microscope.
  3. Considerable internal structure in microorganisms can be revealed by the dark field microscope.
  4. A biological darkfield microscope used to observe the blood cells.
  5. Used to identify algae.
  6. A metallurgical darkfield microscope is used to observe hairline metal fractures.
  7. Stereo darkfield microscope or gemological microscope used to study the diamonds and other precious stones.
  8. A stereo dark-field microscope used to observe the shrimp or other invertebrates.
  9. In the study of crystals and imaging of individual atoms, Dark-field studies in transmission electron microscopy play a powerful role.
  10. It helps to characterize the embedded nanomaterials in cells by combined with hyperspectral imaging.

Advantages of Darkfield Microscope

  1. No need to stain the specimen.
  2. It is useful for those specimens that are transparent and absorb little or no light.
  3. Marine organisms such as algae, plankton, diatoms, insects, fibers, hairs, yeast, and protozoa can observe under a dark-field microscope.
  4. Dark-field microscope can use to study for minerals and crystals, thin polymers, and some ceramics.
  5. It is useful to study the external structure of the specimen in great detail.
  6. It can be used for the research of live bacteria and mounted cells and tissues.

Disadvantages of Dark-field Microscope

  1. Dark-field microscope creates prone to degradation, distortion, and inaccuracies images.
  2. If the specimen’s density differs across the slide or is not thin enough, it can create artifacts throughout the image.
  3. The slides of bad quality can grossly affect the contrast and accuracy of a dark field image.
  4. Before use make sure slide, stage, nose, and light source are free from dust.
  5. An intense amount of light is required for a dark field microscope to work.
  6. It cannot measure the specimen accurately.
  7. Liquid bubbles can be formed during uses of oil or water on the condenser and/or slide, which is almost impossible to avoid.

Reference

  • https://www.microscopemaster.com/dark-field-microscope.html
  • https://en.wikipedia.org/wiki/Dark-field_microscopy
  • https://www.microscopyu.com/techniques/stereomicroscopy/darkfield-illumination
  • https://www.slideshare.net/abhishekindurkar/dark-field-microscopy-81857005
  • https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/3%3A_Microscopy
  • https://www.ruf.rice.edu/~bioslabs/methods/microscopy/dfield.html

Writer and Founder of Microbiologynote.com. I am from India and my main purpose is to provide you a strong understanding of Microbiology. Microbiologynote.com shares notes related to different branches of microbiology.

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