Brightfield, darkfield and phase contrast are three contrast-enhancement techniques in light microscopy.
- Bright field microscopy is the conventional technique. It is suitable for observing the natural colors of a specimen or the observation of stained samples. The specimen appears darker on a bright background.
- Darkfield microscopy shows the specimens bright on a dark background. Brightfield microscopes that have a condenser with a filter holder can be easily converted to darkfield by placing a patch stop filter into the filter holder. The specimens appear bright, because they reflect the light from the microscope into the objective. One disadvantage of darkfield is that it is very sensitive to dust. A small amount of dust will already light up on the dark background.
- Phase contrast microscopy requires special phase contrast objectives and a special phase contrast condenser. This technique is useful for observing unstained specimens that lack a color (eg. bacteria). The optics will convert the differences in refractive index of the specimen into brightness differences. This will cause transparent object to appear brighter or darker than their background. Phase contrast is expensive but sometimes necessary to see certain structures in living and unstained cells.
- Polarization microscopy also produces a bright specimen on a dark background. It can also be easily improvised with a brightfield microscope. A polarizing filter is placed above the lamp and another one is placed between the specimen slide and the objective. Parts of the specimen will then light up. Optically active crystals on the slide will produce nice colors.