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Using a Bright Field Light Microscope

Author(s): David R. Caprette, PhD

Dirty Lenses?

Unless you are working in an ultra-clean laboratory, none of your optical surfaces will be spotless. A small amount of dust shouldn't interfere with viewing at optimum resolution and contrast in bright field. However, when you have persistent shadows in the field of view, it probably is time to clean one or more of your lenses. Rather than spend the time to clean every optical surface on the microscope, you might try to determine which lens is the culprit.

Move the slide. Obviously, anything moving with the slide itself is on it and not on one of your lenses. If the slide appears to be clean, rotate an ocular in the eyepiece tube. Do the shadows move? If so, your ocular should be cleaned. If not, try the other one. Are there shadows that don't move? Is there a persistent shadow in the same place with all objectives? Chances are, your condenser lens is dirty. If you can adjust the position of your condenser, move it down. If the shadow changes, the condenser exit lens is the culprit. Otherwise, try cleaning the condenser and see if the marks go away.

If the contamination cannot be identified with condenser, slide, or oculars, then the answer by default is to clean your objective. Always clean gently, and dab rather than rub the surface. Use a 100% cotton applicator or good quality lens tissue with distilled water or very dilute acetic acid if necessary. Sometimes, a user will mistakenly leave oil on a lens surface, possibly even the high dry lens. Dried oil should be loosened by removing the objective and placing it on a surface with lens up.  Place a drop of distilled water or dilute acetic acid on top and dab off after a minute or two.

If no surface can be identified as the source of the contamination, there may be internal contamination. In this case, it is time to take the instrument to the repair shop. Taking apart a lens is not recommended. A user should clean only the outside surface of any lens, including objectives, condenser, and oculars. If there is contamination inside a lens, it should be taken for professional cleaning.