Darkfield microscopy uses a stop to block the center of the light beam, creating a bright sample against a dark background. It provides high contrast images but requires high-intensity light and may obscure some details. It can be a low-cost alternative and can be achieved by converting conventional microscopes or affixing a round disc.
A darkfield microscope is a microscope in which the sample appears bright against a dark background. Darkfield microscopy can be used to examine a wide range of samples, from fresh blood cells to live insects. Some microscopes are equipped for darkfield microscopy, while others can easily be equipped with the necessary supplies.
When someone uses a darkfield microscope, a stop is placed between the light source and the condensing lens under the microscope stage, where the sample is. The stop blocks the center of the light beam, allowing light to enter only along the sides of the stage. This light strikes the sample and scatters, passing through the optical lens and meeting the eye of the person using the microscope. Another stop between the stage and the optical lens prevents direct light from reaching the eye, making the background appear dark because the user can only see the scattered light from the specimen.
One major disadvantage of using a darkfield microscope is that it requires high-intensity light, because light levels tend to be low. Extremely bright light can damage some samples, which is a problem in cases where a sample cannot be easily replaced. Some aspects of the specimen may also be obscured in this type of microscope, such as naturally dark features such as veins in an insect, and this means the user may miss some important or interesting details.
The main advantage of this technique is that it provides high contrast images. For people who have limited access to expensive microscopy equipment, using a darkfield microscope can be a low-cost alternative that provides useful data for a fraction of the cost. In high school science labs, for example, students may work with darkfield microscopes because the school cannot afford expensive high-contrast microscopes. Sometimes it is also possible to see very small particles that are not seen in other types of microscopes.
Science stores sometimes sell darkfield microscopy kits, which allow people to convert conventional microscopes to darkfield microscopes. These kits are designed for use with microscopes that have slots for filters and accessories. For microscopes that do not have these features, a darkfield effect can still be achieved by affixing a round disc such as a coin or cardboard cutout to the bottom of the condensing lens.
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