Appendix: Comments on use of mercury arc lamps for fluorescence microscopy in a domestic environment.

Cost - each mercury arc bulb typically costs at least £90 in the UK and the 100W versions are typically rated for 300 hours. The 50W bulbs for the smaller lamphouses are often more expensive with shorter lives, ca. 200 hours.

Committed to long switch on times - the lives of these bulbs are reported to be significantly reduced if only used for short switch on periods; in the hobbyist setting this can be inconvenient as need to leave the bulb on for a safe minimum time which may be well beyond an intended working session.

Power supplies - the special expensive power supplies for the bulbs can be bulky and awkward to place in a space limited environment.

Safety - the mercury bulbs have potential safety issues:

At working temp. they are under high pressure*; although a small risk if they are used correctly, it is not unknown for the bulbs to explode. The lamphouse is designed to contain an explosion but can do a lot of damage to the lamp optics, as the photograph in Rost's book strikingly shows (2), the mirror and lamp optics were destroyed. (*Xenon lamps are always at high pressure, hot or cold.)

Although the bulb by inspection contains less mercury than a typical domestic thermometer, if the bulb explodes this mercury is in vapour form and potentially much more dangerous than the low vapour pressure of the cold liquid. A bulb exploding in a domestic environment can pose a dangerous safety hazard. Laboratories using mercury invariably have rigorous clean up and monitoring procedures after mercury splls, (especially for the old high vacuum diffusion pumps that used liters of boiling mercury!). The high air throughput of a lab was also a safety feature, unlike the stale air of a typical domestic environment.

Intensity and UV: The lamps emit intense light in both the visible and near UV and need great care in use; a correctly setup mercury lamphouse and dedicated epi-fluorescence head with the correctly matched dichromatic mirror and excitation / barrier filters need to be used. Unlike LEDS, the intensity cannot be reduced electrically.

1) 'Artifical lighting and the blue light hazard' by Dan Roberts.
2) F. W. D. Rost, 'Fluorescence microscopy', vol. I, CUP, 1992.
Ref. added July 22nd 2011. 3) Ely Silk, 'LED fluorescence microscopy in theory and practice', The Citizen Scientist*, July 19th, 2002.

*Magazine of the Society of Amateur Scientists, website (at the time of writing, the site's magazine archive doesn't seem to be working for 2002).


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