Photos Copyright©, Toomas Tamm
Used with permission.  Not to be reproduced.

In countries or regions with hot and humid climates, it is very common for fungus to grow on optical components of medical equipment such as on the lenses and mirrors of microscopes, slit lamps, fundus cameras, etc. The photographs above show fungus growth on the surface of a lens. In its first stages, fungus as pictured would not be perceivable by the clinician. With time the fungus covers the lens surface in a web like manner. Initially there will be a very slight loss of image brightness, followed by decreased contrast due to light reflecting off the fungus. In its final stages, the lens outer coatings are etched by the fungus and image sharpness deteriorates. You will probably be able to use the lens for many months; depending on how humid the environment is, before it reaches this stage.

Fungus Removal

Removing fungus from lenses is extremely difficult and rarely yields good results. You can use ultraviolet radiation (sunlight or an ultraviolet lamp) or paraldehyde to kill it. Once killed, the fungus may be easier to remove but the delicate outer coatings of the lens will most likely have irreversible damage.

Prevention

To prevent fungus growth on optical components the following should be observed:

• Keep the optics in a dry place with plenty of air circulation (air conditioners and dehumidifiers are very helpful)
• Do not cover optics with plastic drape covers (commonly supplied by medical equipment manufacturers) since these will retain humidity. If you need to drape the equipment to protect against dust, use a porous cloth cover.
• If you need to keep certain optics in sealed containers, place silica gel packs with it to absorb any humidity. It is important, though, to check the silica for discoloration or moisture to know when to replace it.

Tip

If it is difficult to keep the environment dry, then you may want to construct a drying box (see diagram below) where you can store the optical components of your equipment when not in use. This box consists of a heater or light bulb used to heat up and dry the air. Openings on the top and bottom permit air exchange in the box with air flowing from bottom to top. Mesh filters placed on the openings prevent any dust from entering the box.

Thanks to Lutz Heinrich of Zeiss for his help with this article.

Written by:  Ismael Cordero