Bright Field, Phase Contrast and Dark Field: Three Microscope Illumination Options
One of the key elements of any microscope system is illumination. No matter what sort of object or specimen is to be viewed, it must be illuminated in such a way that it can be clearly seen, and so that the details the viewer wishes to examine stand out well. Since some types of illumination are better than others for particular applications, a variety of different lighting methods and equipment types have been developed through the years.
Bright Field Illumination
The original means of microscope lighting – and still the most familiar and most common – is called bright field illumination.
In this method, the light source is positioned so that it passes through the object to be viewed and into the instrument’s objective lens (and then on to the eyepiece or camera). In the most common microscope design, this means that the light source is positioned below the microscope’s stage, shining up through the stage toward the objective lens. When viewed through the microscope’s eyepiece, this arrangement gives a brightly-lit background. The object or objects being viewed must be thin enough to allow light through. For example, bacteria or blood cells viewed this way show up as darker objects against a bright background, or field.
Many of the objects viewed in this way are transparent, or nearly so, so that they must be stained in order to be seen clearly and in detail.
Phase Contrast Illumination
The second of the most common types of microscope illumination is called phase contrast. This method takes advantage of the fact that different substances refract (bend) light in different ways. A phase-contrast microscope uses a precisely-arranged combination of optical elements which make visible the differences in light refracted by various parts of objects being viewed.
In this way, for example, different structures within cells – which are transparent and colorless, but refract light in different ways – are clearly visible, even though they could not be distinguished at all with direct (bright field) illumination. One viewing characteristic which phase contrast microscopes share with bright field instruments is a brightly-lit field of view.
Because of the precision and delicacy of their optics, high-quality phase contrast microscopes are considerably more expensive than bright field microscopes. For this reason, their use is most often limited to health care and high-level research applications.
Dark Field Illumination
The third common method of microscope lighting is called dark field illumination. Because some objects (or details of objects) are better viewed against a dark background, rather than a bright one, dark field illumination was developed. Instead of passing through the object, the area directly beneath or behind the object is unlit, and light is directed in such a way that it illuminates the object from all sides. The result is a brightly-lit image against a black background. Objects viewed in this way may seem to be glowing, or emitting a light of their own. External structures and detail show up particularly well with this method.
Because dark-field illumination systems are considerably less complex and delicate than phase contrast systems, but allow viewing of similar details of many objects, dark field microscopes can be an attractive and affordable alternative to more expensive phase contrast systems.
One increasingly common application of dark field illumination is live blood analysis. Live blood microscopy is employed by some health care practitioners to examine fresh, live blood, in order to assess the condition and relationships of the cells, fluid and other objects that make it up – and to detect and identify objects or conditions that may indicate or explain health problems.
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