Microscope Cabinet
for the Prevention of Fungus Growth

by Frank Weithner

Fungus growth
In hot and humid climates laboratory microscopes often show an unclear, dull or milky image. If this happens, even when the eyepieces and objectives are regularly cleaned, it is very likely that the microscope is affected by fungus.
Fungus grows on all glass surfaces in warm climates where the humidity is permanently above 60%. This applies to all laboratories in tropical areas without air-conditioners.
In the case of the microscope, all eye-piece lenses, objectives and the prism are affected. Once the fungus has grown, removing it is very difficult. In the early stage it could be removed easily, however, fungus is hardly noticeable. Later, when the image becomes dull, it becomes very difficult or even impossible to remove the fungus. Mechanical polishing, which requires special skills, is necessary but this destroys the lens coating. In the final stage of fungus growth, the lenses surfaces are etched and irreversibly destroyed.
The fungus growth process takes time, months or years, but precautions should be made as early as possible.

The factors which support fungus growth are:

  warm and humid climates (> 60% relative humidity)
  unventilated storage
  storage in dark rooms

Particularly bad are closed (and dark) microscope storage boxes or plastic dust covers where no air flow takes place.
An air-conditioned laboratory would solve this problem, because an air-conditioner not only cools the room down but also reduces the humidity. It is, by the way, enough that the A/C runs during working hours. A 24-hour air-conditioning is not necessary.
When an A/C is not present, and thus the humidity is high during working hours, then at least the storage over night should be 'air-conditioned' i.e. the microscope is put in a special cabinet with a heater and ventilation holes. The heat creates a humidity drop inside the box and a continues airflow. Under these conditions fungus can not build up any longer.

How it works
In order to achieve a considerable drop in humidity, the temperature inside the cabinet has to be increased by a minimum of 5C. This can be done easily by mounting a standard light bulb in the top of the cabinet. Additional ventilation holes in the top and in the bottom create an air flow inside the cabinet.
A light bulb (incandescent light bulb with a wire filament) should be avoided for lighting purposes nowadays because the luminous efficiency is relatively small compare to the power consumption. The major part of the electrical power is not converted into light but into heat. Energy saving bulbs are much more efficient and they should be taken only. But in our case the old fashion light bulb is an advantage because we use the light bulb as a heating element which is extremely cheap and (still) available everywhere.
Depending on the size of the container which we want to use as a microscope cabinet, a 40W or a 60W type can be used. It has to be taken into account that the temperature in the interior of the cabinet should not exceed 50C.
I bought a big plastic bucket at the local market and did some tests where I measured the humidity and temperature inside and outside the cabinet.

Light bulb Temperature rise Humidity drop
40 W 6C 20%
40 W with reflector 7C 25%
60 W 8C 25%

In the end I decided to use a 40W bulb with a metal reflector. This reflector is nothing else than a metal plate, also found at the market.

The construction
I drilled holes in the bottom of the bucket (the top of the cabinet) and around the rim (the bottom). The sizes and the number of holes are not critical. But do not make them too small. Too big is no problem because the ventilation depends more on the power of the lamp than on the size of the holes. The system controls itself. An additional fan is not needed.

The usage is easy. Instead of covering the microscope with the plastic cover at the end of a working day, the microscopist now puts the bucket over the the microscope, switches on the light and keeps it switched on over night. The temperature in the cabinet rises, the humidity drops, an air flow comes into being and fungus can not grow.

OK, the bucket does not look elegant, but the solution is simple, cheap and easily done. A custom-made plywood cover hood, perhaps with a front door would be, of course, much nicer and smaller.