Enhancing Contrast

This article briefly outlines some contrast enhancing techniques that are used in microscopy.

Many microscopic specimens are low in contrast. Many naturally pigmented specimens are very thin and therefore too transparent for easy observation. Other specimens are simply not pigmented enough. It is necessary to enhance the contrast of these specimens. A range of techniques can be applied:

  • Optical techniques: The use of phase contrast is a very popular technique to increase contrast in research labs, but it is probably too expensive to be used in schools. Phase contrast optics transform transparent objects into a black-white image, depending on their refractive index.
  • Staining techniques: Transparent specimens, such as bacteria, can be heat-mounted on the slide and then stained with specific chemicals.
  • Use of filters: Colored filters can be used to enhance the contrast of certain objects. If the object already possesses a certain color, then a filter with a complimentary color will result in the specimen to appear darker.
  • Use of dark-field illumination: A dark-field ring can be placed into the filter holder of the condenser. Specimens will then appear bright on dark background. This system does not simply invert the colors, but makes specimens with a refractive index different from the medium visible.