Darkfield Microscopes

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What is a dark field microscope?

Dark field microscopy describes the microscopy methods used in light and electron microscopy, excluding the unscattered beam produced by the image, resulting in a dark field around the specimen. When it comes to optical microscopy, the term dark field is used to describe an illumination procedure that is used to improve the contrast in samples that have not been stained. It does this by illuminating the sample using light that cannot be collected by the lower lens, hence part of the image will not be formed. This way, a background that has a classic dark appearance and bright objects is formed.

The first step that takes place is the entrance of light into the dark field microscope to allow for the illumination of the sample. A specially sized disk then blocks some of the light from the source forming an outer illumination ring. After this, the condenser lens makes the light focus towards the sample. Once it enters the sample, a large amount is transmitted directly as some is spread from the sample.

The scattered light gets into the objective lens as the light that is directly transmitted misses the lens and is therefore not collected because of the direct illumination block. The only light that is used to produce the image is the one that gets scattered. The directly transmitted one is left out. The image produced normally has a dark background. This simple technique used in dark microscopy is very effective when used to examine live or unstained biological samples like smears from tissue culture or water borne single celled organisms. The image produced using this method is of high quality.
 
Problems with dark field microscopes

One major disadvantage of the dark field microscopy is that the final image has low levels of light. Therefore, the sample has to be illuminated strongly, which can lead to the destruction of the sample. The interpretation of the images has to be done very carefully because most of the features of the images produced using the dark light microscopy may not be visible.

In brief, the usual dark field imaging involves slanting the incident illumination until a diffracted instead of the incident beam gets through the small objective aperture situated in the objective lens. The images permit you to map the diffracted concentration produced by a collection of diffracting planes.