Page 75 - Fundamentals of Light Microscopy and Electronic Imaging

58 LENSES AND GEOMETRICAL OPTICS

THE CARE AND CLEANING OF OPTICS

Maintenance and care are required to protect an expensive optical instrument and to
guarantee that optimal high-contrast images will be obtained from it. Neglect, such as
not removing immersion oil, forgetting to cover open ports and apertures, or acciden-
tally twisting or dropping an objective lens can ruin optical performance. Even if the
microscope is left unused but unprotected on the lab bench, image quality can deterio-
rate rapidly due to the accumulation of dust from the air. James (1976) and Inoué and
Spring (1997) provide detailed descriptions on the effect of dirt on the microscope
image and the cleaning of optical surfaces. Following are a few tips for maintaining the
performance and image quality of your microscope.

Dust

Keep the microscope protected with a plastic or cloth cover. Wipe dust off the micro-
scope body and stage with a damp cloth. Keep the objective lens turret spotless and free
of dust, immersion oil, spilled culture medium, and salt solutions. Hardened thread
grease, or additionally on inverted microscopes, dried immersion oil, buffers, and media
can weld objectives onto the rotating objective turret, making them difficult to remove
or exchange. If an objective is frozen fast, place a drop of water (if salts) or oil-
penetrating agent (if oil) at the lens-turret interface to loosen the material before trying
to remove the objective. Keep all openings covered with caps so that dust cannot enter
the microscope and soil inaccessible lenses, mirrors,and prisms in the microscope body.
Make use of the plastic caps designed to cover all objective ports, eyepiece sleeves, and
camera ports that are unoccupied and empty. The internal optical pathway should
always be completely protected from airborne dust.

Immersion Oil

When finished with an observing session, gently wipe off and clean away excess oil with a
high-quality lens tissue and then clean the lens surface with an agent designed for cleaning
microscope optics. Immersion oil is a slow-acting solvent that can weaken the cementing
compounds that act as a seal between the front lens element and the metal lens cap of the
objective. Residual oil should be removed with a lens tissue wetted with a mild lens cleaner
such as the solution sold by Edmund Scientific Company (Barrington, New Jersey). Com-
mercial glass cleaners such as Sparkle and Windex are also effective in removing immer-
sion oil, but these generally contain an acid or base that has the potential to erode the
antireflection coating on the front lens element and should be used only if a neutral cleaner
is not available. For more tenacious deposits, try, in order, ethanol or ethyl ether. However,
do not use toluene or benzene, as these solvents used over time will eventually dissolve the
front lens sealing compounds. Generally, it is advisable to remove immersion oil and con-
taminating liquids with the objective lens still mounted on the microscope, as this will avoid
mishandling or dropping, the worst accidents that can befall a valuable lens.

Scratches and Abrasions

Never wipe the surfaces of objectives with papers or cloths that are not certified to be free
of microscopic abrasives. All objectives contain an exposed optical surface that must be