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Using a Bright Field Light Microscope

Author(s): David R. Caprette, PhD

The Condenser

To obtain a sufficiently bright image at high magnification, we need a very intense light beam. The condenser lens gathers light from a wide cross-sectional area and concentrates it in the area of the specimen, giving us a far more intense light beam than we could obtain directly from the source. With a good quality illumination system, one usually has to reduce the intensity of the light source to view a specimen at low magnification.

As important as the condenser is for intensifying the light beam, its primary function is to condition the beam before it enters the specimen and objective lens. To obtain full resolution from a given objective, the condenser should be adjusted so that the back lens of the objective lens is filled with light. Such adjustment is accomplished using an aperture diaphragm control that is built into the condenser. The aperture diaphragm is similar to the aperture of a camera lens. The size of the opening is adjustable so that the diameter of the light beam as it passes through a particular plane can be varied.
For any given objective lens, there is an ideal position for the aperture diaphragm. "Stopping down" the aperture (making it smaller in diameter) beyond the ideal diameter dims and distorts the image. "Opening up" the aperture increases glare and washes out the image. Usually, to reduce glare, we sacrifice some resolution and stop down the aperture so that about 75% of the back lens of the objective is illuminated.

Inexperienced users may employ the aperture diaphragm control in the condenser to regulate the amount of light reaching the eye. However, misusing a condenser that way will sacrifice the resolving power of the microscope.