Dark field microscopy
From Wikipedia, the free encyclopedia
Dark field microscopy is an optical microscopy illumination technique used to enhance the contrast in unstained samples. It works on the principle of illuminating the sample with light that will not be collected by the objective lens, so not form part of the image. This produces the classic appearance of a dark, almost black, background with bright objects on it.
[edit] The light's path
- Light enters the microscope for illumination of the sample.
- The condenser lens focuses the light towards the sample.
- A specially sized disc, the "patch stop" blocks some light from the sample, leaving an outer ring of illumination.
- The light enters the sample. Most is directly transmitted, while some is scattered from the sample.
- The scattered light enters the objective lens, while the directly transmitted light simply misses the lens and is not collected.
- Only the scattered light goes on to produce the image, while the directly transmitted light is omitted.
[edit] Advantages and disadvantages
Dark field microscopy is a very simple yet effective technique and well suited for uses involving live and unstained biological samples, such as a smear from a tissue culture or individual water borne single-celled organisms. Considering the simplicity of the setup the quality of images obtained from this technique is impressive.
The main limitation of dark field microscopy is the low light levels seen in the final image. This means the sample must be very strongly illuminated, and can cause damage to the sample.
Dark field microscopy techniques, due to the simplicity of the setup, are almost entirely free of artefacts. However the interpretation of dark field images must be done with care as common dark features of bright field microscopy images may be invisible, and vice versa.
[edit] External links and references
http://www.microscopyu.com/articles/stereomicroscopy/stereodarkfield.html
