Do you want the best imaging experience each time you use a microscope? Well, this is a rhetorical question, as we all desire that these delicate optical instruments are clean, free from immersion oil and correctly aligned.

From the routine checking of slides and, more importantly, for capturing images for presentations and publications, microscopes need to be cared for by all users. This is especially important in a shared microscope facility. Although there may be a dedicated microscope technician present, such facilities are typically very busy and you should leave the microscope as you expect to find it.

I’ve got it covered

Image clarity’s main enemy is dust. Laboratories are reasonably clean environments, but dust can be carried in from outside and will ultimately find its way to the optical components of a microscope. The easiest way to prevent microscopes from becoming dust collectors is to use the dust cover which came with the microscope. These often-neglected plastic covers can sometimes be found stuffed under the bench or tucked away in a forgotten corner of the lab, where they are keeping a section of the bench nicely dust- and dirt-free. If the microscope doesn’t have a dust cover, replacement ones can be purchased from microscopy suppliers, or the simplest (and cheapest) option is to use a thick bin liner. Get into the habit of replacing the dust cover after each imaging session and persuade other users to do the same.

Know what you are doing

If you sit down and find yourself faced with a dirty microscope, or an incorrectly aligned light source, routine cleaning and microscope maintenance can be carried out even by a relatively inexperienced beginner. However, be warned! Don’t be tempted to strip down and clean optical components unless you know how to do this correctly. Improper cleaning of lenses and components can do more harm than good. Some of the corrected and immersion objectives can cost as much as a complete research-grade microscope, so always follow the manufacturer recommendations (or those of the microscope technician/lab manager). If in doubt—ask! For a more in-depth look at cleaning optical components, follow this link.

One routine maintenance task that can be carried out by the user is the replacement and alignment of the microscope bulb. Before we cover this task, let’s look at common light sources in optical microscopy.

Common microscope light sources

Incandescent bulbs (those which produce light via heating a wire filament) have been used for many years as reliable sources in light microscopy. Earlier bulbs employed tungsten or carbon filaments, but these are being replaced with tungsten-halogen bulbs. Carbon filament bulbs need to be used at low voltages, as the filament degrades at high temperatures and as a consequence, light output has a relatively low colour temperature. Although tungsten bulbs can withstand a higher operating temperature, the filament continually produces vaporised tungsten which can produce a sooty deposit on the inside of the glass.

Tungsten-halogen bulbs

The first patent for tungsten-halogen lamps was issued in 1959. Their production started in the early 1960’s and development continues to this day.

In a tungsten-halogen bulb, vaporised tungsten reacts with halogen gas, which instead of depositing on the glass, is recycled to the filament of the lamp through a process known as the ‘halogen regenerative cycle’.

Tungsten-halogen light sources have many advantages over other bulbs; they produce a continuous distribution of light across a wide range of the visible spectrum, are easy to replace and align and, importantly, they are relatively inexpensive.

The average life-span of a tungsten-halogen bulb is dependent on a number of characteristics which include the filament size as well as the rated voltage and wattage. For example, a 10 watt bulb with a filament size of 1.5 x 0.7 mm can last up to 300 hours compared to a 50 watt bulb with a filament size of 3 x 3 mm which can last for 1,100 hours.

Tungsten-halogen bulbs are amongst the most common light sources in research-grade and standard microscopes, and the variety and specification of bulbs available covers most applications. They are well-suited for brightfield microscopy and for capturing digital images of both living and fixed specimens. These bulbs are also suitable for stereomicroscopy, from the basic to research-grade instruments. In contrast microscopy (including differential interference contrast and phase contrast), one of the most common light sources is the 12 volt, 100 watt tungsten-halogen lamp.

For further information on tungsten-halogen bulbs and their characteristics, see this in-depth article.

How to replace the halogen bulb

Below is an excellent video tutorial on replacing a tungsten-halogen bulb in a microscope.

Some microscopes (or separate lamp power units) have timers that keep track of the hours since the bulb was replaced. If there is no timer, you can always keep a log book next to the microscope to keep track of the hours used. But, remember, the bulb lifespans quoted by manufacturers are averages and certain scenarios, such as repeated switching off and on of the bulb, will reduce this time.

If you don’t know the age of the bulb, then signs that it needs replacing include: when it doesn’t produce light, only produces very red light, or the bulb takes longer to either increase or decrease in light intensity.

The tools you will need for bulb replacement are:

  • A new halogen bulb which matches the voltage and wattage of the halogen unit
  • A bulb replacement tool
  • Safety goggles
  • Nitrile or latex gloves

Replace the bulb by following these seven steps:

  1. The lamp housing should be cool before handling, so if a bulb blows during an imaging session, allow at least 15 minutes for the lamp and housing to cool down.
  2. Turn off the microscope and the power supply (there may be a separate power supply for the bulb).
  3. Unplug the housing from the microscope body, making note of the correct port (you may not need to unplug the housing to fully access the bulb). Put on your safety goggles just in case the bulb is cracked or damaged.
  4. There should be an unlock button on the housing which will open the spring-loaded tray. Gently pull the tray out and place on a flat surface. Press down on the spring-loaded clips which hold the bulb in place and use the bulb replacement tool to grasp the old bulb and pull up and out of the housing.
  5. Pick up the new bulb, but only with gloves on! Any oil from your hands and fingers can reduce the life-span of the new bulb. Hold the bulb in the replacement tool, depress the spring-loaded clips and insert the new bulb into the socket on the housing. Release the spring-clips and the replacement tool.
  6. Slide the lamp tray back into the housing until you hear it click into place. If you unplugged the housing, replace the lead back into the correct port.
  7. Switch on the microscope (and power supply for the bulb if separate). Turn up the bulb intensity.

The process of bulb replacement doesn’t end there. You will now need to correctly align the new bulb.

How to align the halogen bulb

Below is a follow-up tutorial on correct alignment of a new bulb.

Even illumination and contrast are crucial to achieve optimal imaging results, especially for Köhler illumination (which will be covered in Part 2 of these articles).

The following will be needed to correctly align the bulb:

  • 3 mm hex or Allen tool
  • Blank wall approximately three metres away from the microscope

Align the bulb by following these steps:

  1. As above, make sure the lamp housing is cool before handling.
  2. Grasp the housing in your left hand (if you are right handed) and loosen the 3 mm hex screw which holds the housing to the body of the microscope. In this alignment, you will need power to the bulb, so don’t unplug the lamp housing from the microscope.
  3. Turn on the microscope (or power unit for the lamp if it is separate). Point the lamp housing at the blank wall and turn up the lamp power slightly.
  4. You should see two images of the bulb filament projected onto the wall. If the lamp housing gets too hot to hold, place the lamp on the bench and turn down the intensity slightly.
  5. Insert the hex key into the screw marked with a double pointed horizontal arrow on the side of the housing. Turn this screw until both images of the filament appear sharp.
  6. There are two remaining adjustment screws which will move the filament image- one marked with a 45-degree double-pointed arrow and the other with a vertical double-pointed arrow. Adjust these in turn so that both filament images are the same size and that one filament image precisely fills the gaps in the other.
  7. Once the filament images are aligned, turn the lamp intensity down and re-insert the lamp housing into the back of the microscope. Make sure the surfaces are flush and tighten the 3 mm screw to securely hold the lamp housing in place.

Those were some basic tips in caring for and replacing light microscope bulbs. In part two of these articles, we will learn how to correctly align the microscope for Köhler illumination.