How to make the best of tungsten-filament microscope bulbs

Tungsten-filament bulbs are becoming increasingly scarce, as manufacturers take them off the production line and microscope manufacturers switch to instruments with LED illumination systems. What bulbs are still available usually come from limited new-old-stock supplies still kept by some distributors and sellers like ourselves.

As a consequence of their increasing rarity, these bulbs are also becomimg expensive. In our experience, the simple rules outlined below may be useful in order to prolong their lifespan as much as possible, thereby reducing the risk of running out of working bulbs prematurely (and also saving a bit of money!)

1. Before switching the microscope lamp power supply on, DO check to see that the lamp voltage control is turned all the way down

2. After switching the power supply on, DO turn the lamp voltage control up slowly until the brightness is sufficient for routine observations

3. DO try to work routinely with the lamp voltage set below the bulb's nominal voltage. For example, when using a 6 V bulb try working with the voltage set at around 3.5 - 4 V 

4. When a bulb is powered below its nominal voltage, the colour temperature of the light it produces will decrease proportionately to the decrease in voltage. This phenomenon can be corrected by using appropriate coloured filters (for example, light blue or daylight) along the microscope's optical path (usually just below the condenser)

5. When maximum light intensity becomes necessary (as in photomicrography), DO turn the voltage up slowly to the lamp's nominal voltage and - only if absolutely necessary, providing the lamp power supply allows -  go slightly above that value (for example, 7 or 7.5 V for a 6 V bulb) but ONLY as briefly as possible (a few seconds max.) while setting up and taking the photo shot. In any case, always try to avoid keeping the voltage set at the bulb's nominal voltage for extended periods of time (see point 3. above)

6. Before switching off the lamp power supply, DO turn the lamp voltage control slowly all the way down

7. Switch off the lamp power supply and then the mains socket at the wall