A dark field compound microscope is a type of optical microscopy that allows sample specimens to be viewed through peripheral illumination from a low angle.  This makes the background appear dark against the object being observed, which appears bright.  The contrast allows the observer to view the specimen in greater contrast and to see particles that would otherwise remain invisible when used with standard brightfield microscopes.  Because of its unique function, the dark field compound microscope can be best used with certain observation techniques and requirements.

How dark field microscopy works
The use of light makes all the difference in a dark field compound microscope.  Unlike other conventional compound microscopes where light is directed in such a way that it passes through the specimen, the dark field compound microscope uses light applied from the sides of the sample specimen.  This is because the instrument is equipped with a dark field condenser which blocks the path of transmitted light, allowing only light from the sides to reach the specimen.  The light used in a dark field compound microscope is cone-shaped and intense and the instrument’s objective lens is found in the dark part of this cone.  The lens then picks up the scattered light emanating from the object.  Whatever clear portions there are in the specimen will therefore appear dark.

Best uses for a dark field compound microscope
The dark field compound microscope is best used with unstained specimens and for viewing objects that would otherwise be unseen when used with normal illumination.  This means that users don’t need to stain the specimen in order to view it.  This allows live organisms to be observed, most of which will appear invisible under a brightfield microscope.

Using dark field microscope with ‘invisible’ organisms
There are certain organisms that are best viewed with a dark field microscope.  Organisms such as algae, protists, metazoans, many types of bacteria and cell suspensions are difficult to view using standard brightfield microscopy.  Not only are the details lost, the specimen will also look dull.  Furthermore, many of them will be virtually invisible with other optical microscopy techniques.

With a dark field compound microscope, however, not only are these organisms readily observed, there is also a significant improvement in how they look.  Pond water samples, for example, will already be interesting in conventional compound microscopes but with a dark field microscope, the array of life visible in a single drop will often offer a stunning view.  Organism structure and movement will also be easy to observe.  This makes dark field compound microscopes best even for specimens that require low contrast.

Using dark field microscope with living organisms
Other types of optical microscopes rely on fixing agents or stains to allow the observer to view microorganisms.  Staining provides excellent details and contrast but it has its limitations.  Staining agents are often toxic to many organisms so they won’t last long in order for the observer to view them live.  Once they are killed, any changes in growth or movement will no longer be possible to view.

However, with a dark field compound microscope, organisms don’t need to be stained in order to be observed, which makes the instrument perfect for viewing live organisms such as algae and protozoa and those found in bacteria cultures.  Dark field microscopy can also help distinguish internal structures in many living specimens.

Using dark field microscope for live blood cell analysis
Even Antoni van Leeuwenhoek, one of the first to develop the modern microscope, could have used the dark field technique to view blood samples.  Although Leeuwenhoek did not reveal his observation methods, some of his letters suggest that he might have begun using dark field illumination in order to view specimens.  He once described blood corpuscles against a ‘black silk’ background.

Dark field microscopy, especially one that uses high power magnification, is one of the most common techniques used for live blood cell analysis.  This type of analysis cannot be performed with standard optical microscopy such as brightfield because they usually require staining or might not provide sufficient contrast.  Dark field microscopy does not make the blood cells appear translucent and so will not affect its clarity.  It also will not require staining so any biological processes in the blood remain intact, allowing observers to view the blood in its natural condition.

Using this technique, health professionals will be able to evaluate the status of individual cells, allowing them to determine if there are certain conditions that may pose a risk for a patient’s health as indicated in the blood cell analysis.  The appearance of red and white blood cells, plasma, fungi, crystals and microbial activity will become apparent.  This makes dark field compound microscopes an excellent tool that can be used for detecting any disorders in the blood even if they are still in the early stages.