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Light Microscopy: Comparison of Optics

Author(s): David R. Caprette, PhD

Bright Field Microscopy

Any light microscope requires a fairly intense light source, preferably built into the microscope and preferably with an intensity control. A typical light source is a tungsten lamp, which produces a yellowish light compared to daylight. On a good quality microscope, light passes through a blue or polarized filter to remove some of the light of yellow wavelength selectively, so that the colors we see are true colors.
Light from the source is collected by a component called a condenser, which serves to focus the light toward the specimen. By concentrating the light, the condenser increases the intensity of illumination. Lenses and/or devices in a condenser permit the use of specialized optics if they are available. On a bright field microscope, a device in the condenser, called an aperture diaphragm, allows a user to optimize contrast and resolution. With some microscopes, the position of the condenser is adjustable so that it can be centered in the light path or moved up and down. 
The light path is through the condenser lens, directly through the specimen, and into an objective lens. The user can select from several objective lenses, adjust the condenser, and vary the intensity of illumination at the source.
The expression "bright field" refers to the fact that absorbance or scattering of light causes the object to show up against a bright background, called the field of view, or simply the field. The specimen absorbs light due to natural pigmentation, because we deliberately stain it, and/or because it is dense enough to scatter a significant amount of light. Many specimens, especially live specimens, are invisible or nearly so in a bright field microscope. Viewing them requires staining or the use of specialized optics. The first order of business, then, is to consider the amount of contrast you can expect from a given specimen, including whether or not it is stained or naturally pigmented.