Firstly, I apologise for the
photos here...they're less than great. I underestimated how
tricky getting the lighting right to take photos of something this long
and transparent was going to be (and don't even get me started on the
fingerprints!).
This
lamp isn't actually intended as a light source (though as you can see
from the photos below, it does produce light), it's instead intended as
a source of shortwave ultraviolet radiation (UV-C).
Just in
case you missed the huge great yellow warning at the top of the
page...do not, whatever you do, run one of these things in open air -
they are designed to be dangerous to living cells - and as well as
killing bacteria, will quite happily do really nasty things to the
cells in your skin and your eyes. This basically manifests
itself as a severe sunburn, but can lead to other far nastier
conditions such as skin cancer, and degradation of the cornea in your eyes. One thing which makes this
particularly hazardous is that symptoms of UV exposure can be delayed,
often not reaching a peak until as much as half an hour after the
event. Okay, so the cautionary bit's over...you want to know
what's so special about this thing...and probably why it looks like a
half finished fluorescent tube...
Well...the
main reason it looks like a half finished fluorescent tube, is that
both physically and electrically, it is identical to a fluorescent tube
- minus the phosphor layer, and with a quartz envelope instead of a
conventional glass one. The gas filling is exactly the same,
the electrode construction is exactly the same, the end caps are
exactly the same, and they even work in conventional fluorescent
fittings.
The low
pressure mercury vapour discharge in a fluorescent tube is actually
extremely inefficient when it comes to producing visible
light. What it does however, is produce a whole heap of
shortwave ultraviolet radiation (in addition to a small amount of pale
bluish green visible light). While no use for lighting a
room, this ultraviolet radiation is extremely useful for exciting a
phosphor, which then fluoresces - producing copious amounts of visible
light very efficiently. The shortwave UV that is not absorbed
by the phosphor layer is normally stopped by the glass wall of the
tube. Shortwave UV has a relatively low ability to make its
way through materials, hence normal soft glass and most plastics being
able to stop it.
While in
a normal fluorescent lamp, this shortwave UV is an integral part, but
nothing of any real use to the outside world, it does have its
uses. UV-C (UV radiation shorter than around 300nM in
wavelength) is extremely effective at destroying bacteria, hence
bombarding water with it is an effective way of removing bacteria which
may be living in there (you will find these lamps in a number of
commercial water coolers and "purifiers" by the way). It also
serves as the means by which EPROM chips are erased (as the shortwave
radiation bombarding the surface of the silicon increases its
conductivity). Hence being able to produce these wavelengths
without the need for messy things like open air carbon arcs is actually
desirable, hence the development of these lamps. They are
developed specifically to let all of the UV-C out of the lamp, with no
phosphor layer, and a quartz tube.
These
lamps are available in a number of standard fluorescent tube sizes
(both linear and compact), easily identifiable by the "F" prefix in the
model number being replaced with a "G" to indicate a germicidal lamp,
and of course a lack of lamp colour indicator - as there's no phosphor
to give details on.
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