Dark- Field Microscopy

b. Dark- Field Microscopy

 Dark- Field Microscopy is a technique in which objects are brilliantly illuminated with hollow cone of light. This can be achieved by equipping the light microscope by special kind of condenser that transmits hollow cone of light from the source of light (illumination). The microscopic field appears dark in absence of specimen when specimen(sample) place on stage , it appears bright against dark background.
Light should pass through specimen to visualize under Bright Filed Microscope and passed light through specimen must change to interfere each others contrast and from a image. When refractive index of specimen and medium between objective lens and stage, image won't be seen. For such cases staining is necessary. For some instance, staining might kill specimen. In such case Dark-Field Microscopy is used.

Advantages

• Allow living microbes to viewed e.g. spirochetes.
• examination of unstained microorganisms suspended in fluid- wet mount and hanging-drop preparations.
• examination of specimen which are transparent and absorb little or no light.
• some specimens often have similar refractive index as their surrounding which make them hard to distinguish. Dark Field Microscope is useful in such cases.

Disadvantages

• Can't be used for stained specimens.
• images obtained from dark field are prone to inaccuracies, degradation and distortion.
•  High standard precaution should to be taken like, during preparation of slide, light source and condenser management.
• Not reliable for accurate management of specimens.
• Avoid air bubbles during the use on condenser or slide. 

1.  Fig: Schematic presentation of Darkfield an Brighfield Microscopy