What is Dark Microscope?
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dark field microscopy of sugar crystals-Dark Field illumination is a technique used to observe unstained samples causing them to appear brightly lit against a dark, almost purely black, background.Pictured right: Highly magnified image of sugar crystals using darkfield microscopy technique,When light hits an object, rays are scattered in all azimuths or directions. The design of the dark field microscope is such that it removes the dispersed light, or zeroth order, so that only the scattered beams hit the sample.The introduction of a condenser and/or stop below the stage ensures that these light rays will hit the specimen at different angles, rather than as a direct light source above/below the object.The result is a "cone of light" where rays are diffracted, reflected and/or refracted off the object, ultimately, allowing you to view a specimen in dark field.
Dark Microscope TECHNOLOGY:
Darkfield microscopy creates contrast in transparent unstained specimens such as living cells. It depends on controlling specimen illumination so that central light which normally passes through and around the specimen is blocked. Rather than light illuminating the sample with a full cone of light (as in brightfield microscopy) the condenser forms a hollow cone with light travelling around the cone rather than through it.
This form of illumination allows only oblique rays of light to strike the specimen on the microscope stage and the image is formed by rays of light scattered by the sample and captured in the objective lens. When there is no sample on the microscope stage the view is completely dark.
Care should be taken in preparing specimens as features above and below the plane of focus can also scatter light and compromise image quality (for example, dust, fingerprints). In general, thin specimens are better because the possibility of diffraction artifacts is reduced.
What is Advantages of Dark Microscope?
A dark field microscope is ideal for viewing objects that are unstained, transparent and absorb little or no light.
These specimens often have similar refractive indices as their surroundings, making them hard to distinguish with other illumination techniques.
You can use dark field to study marine organisms such as algae and plankton, diatoms, insects, fibers, hairs, yeast and protozoa as well as some minerals and crystals, thin polymers and some ceramics.
You can also use dark field in the research of live bacterium, as well as mounted cells and tissues.
It is more useful in examining external details, such as outlines, edges, grain boundaries and surface defects than internal structure.
Dark field microscopy is often dismissed for more modern observation techniques such as phase contrast and DIC, which provide more accurate, higher contrasted images and can be used to observe a greater number of specimens.
Recently, dark field has regained some of its popularity when combined with other illumination techniques, such as fluorescence, which widens its possible employment in certain fields.
What is Disadvantages of Dark Microscope?
A dark field microscope can result in beautiful and amazing images; this technique also comes with a number of disadvantages.
First, dark field images are prone to degradation, distortion and inaccuracies.
A specimen that is not thin enough or its density differs across the slide, may appear to have artifacts throughout the image.
The preparation and quality of the slides can grossly affect the contrast and accuracy of a dark field image.
You need to take special care that the slide, stage, nose and light source are free from small particles such as dust, as these will appear as part of the image.
Similarly, if you need to use oil or water on the condenser and/or slide, it is almost impossible to avoid all air bubbles.
These liquid bubbles will cause images degradation, flare and distortion and even decrease the contrast and details of the specimen.
Dark field needs an intense amount of light to work. This, coupled with the fact that it relies exclusively on scattered light rays, can cause glare and distortion.
It is not a reliable tool to obtain accurate measurements of specimens.
Finally, numerous problems can arise when adapting and using a dark field microscope. The amount and intensity of light, the position, size and placement of the condenser and stop need to be correct to avoid any aberrations.
Dark field has many applications and is a wonderful observation tool, especially when used in conjunction with other techniques.
However, when employing this technique as part of a research study, you need to take into consideration the limitations and knowledge of possible unwanted artifacts.
Dark Microscope APPLICATIONS
• Viewing blood cells (biological dark field microscope, combined with phase contrast)
• Viewing bacteria (biological dark field microscope, often combined with phase contrast)
• Viewing different types of algae (biological dark field microscope)
• Viewing hairline metal fractures (metallurgical dark field microscope)
• Viewing diamonds and other precious stones (gemological microscope or stereo dark field microscope)
• Viewing shrimp or other invertebrates (stereo dark field microscope)
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