Darkfield Microscopy

There are many types of microscopy and each has particular benefits and uses. Dark field microscopy is a kind of microscopy that can be used in light or electron microscopes and which provides you with a better microscope illumination for seeing your samples in greater focus. Essentially this works by excluding the unscattered beam from the image so that the area around your specimen is not lit up. This then in turn means that the lit up specimen against a black back drop stands out much more clearly. This form of microscope illumination then helps to increase the illumination of the specimen and to increase contrast in samples that have not been stained.

This works as so:

• The light enters through the microscope to illuminate the sample.
• A disk called a patch stop blocks some of this light from the light source so that there is an outer ring of microscope illumination.
• A darkfield condensor then focuses this light onto the sample. A dark field condensor works by adding a dark field stop inside the lens while concentrating the light onto the specimen.
• The light enters the sample and is transmitted while other light is scattered.
• The scattered light enters the objective lens. The rest of the light misses the lens and is not collected as a result of a direct illumination block.
• Only the scattered light shows the image, while the light that has come direct is omitted from the picture.

There are strengths and weaknesses of darkfield microscopy. On the one hand it means you will be getting a very impressive image with a simple set up that allows you to more easily focus on unstained items. However at the same time it will require large amounts of light in order to illuminate the specimen to a satisfying degree when compared to a brightfield microscope which lights up the whole microscope slide. This then means that a very powerful light must be shone through the darkfield condensor and that can in theory damage the specimen. When analyzing the results of darkfield microscopy it is important to do so carefully as some of the darkened areas that would normally show up on a brightfield microscope may be invisible – and the reverse is also true. It may be useful to have facilities to use a regular condensor or darkfield condensor in order to examine the specimen under the different kinds of illumination.

More forms of microscope illumination also exist. For instance a phase contrast microscope works by using small phase shifts in the light as it passes through a transparent specimen in order to change the contrast or amplitude in the light. This then creates a high contrast image that does not require staining in order to view the slide. This is a form of digital microscope which picks up phase changes that the human retina cannot quite observe, but which carries a lot of information. Phase contrast microscopy is what made it possible to study cell cycle.

There are lots of different types of microscope and things like a fluorescence microscope alter the way you perceive the specimen under inspection. Follow the links for more information.