>I have heard and read warnings that Quartz-Halogen lamps which are
>unprotected (through plain glass) are a source of damaging ultra
>violet light. Damaging in the sense that it might cause skin cancer.
There are two sorts of lamps that you might be referring to.
One sort of "halogen" bulb uses a tungsten filament (just like a normal
incandescent light) with a halogen added inside the envelope... they are
"socket compatible" with ordinary incandescent light bulbs. The halogen
helps keep the tungsten filament from evaporating (actually, it
sequesters the evaporated tungsten and redeposits it on the filament).
This permits the filament be run at a higher-than-usual temperature,
giving a "bluer" spectrum and more "useful" light per watt. As far as I
know, though, these bulbs do _not_ run hot enough to radiate
significantly more ultraviolet than an ordinary incandescent bulb. Like
normal incandescents, they radiate a large percentage of their energy in
the infra-red, and aren't all that efficient compared to fluorescent
lights. Probably not a good choice for plant lights.
Then, there's the "metal halide" or "high intensity discharge" halogen
lamp. These use a quartz envelope containing a halide vapor; they do
not have a filament. They require a transformer/ballast, and cannot
simply be plugged into a 120-volt socket.
HID lamps have an outer glass bulb, which protects the inner quartz
envelope from damage AND protects you from the large amounts of UV that
the arc/discharge produces. With the outer bulb intact, I believe that
their UV content is very roughly equivalent to that of sunlight (perhaps
With the outer bulb broken or removed, HID lamps are VERY DANGEROUS
INDEED. They emit enough shortwave UV to cause serious skin burns or
eye damage within a VERY short period of time. Never, never operate a
HID bulb which has taken damage to the outer protective glass!
>This brings up the possibility that they might be a good source of
>light for indoor plants: nursery, hotbeds and just plain replacement
>of the traditional flourescent light sources commonly used for plant
My recollection is that the efficiency of a HID bulb in good working
condition is roughly equivalent to that of a new fluorescent tube. It's
not significantly better, and the HID bulb/ballast combination costs a
whole lot more than a fluorescent "shop fixture".
HID lamps do have the advantage that they can deliver a lot of light in
a very small space. They can provide daylight-intensity lighting
indoors, while it's difficult to get that sort of light intensity with
fluorescent fixtures. If you're growing a plant which really craves
high light intensities (some orchids, I guess, or cactus) then a
HID fixture might be a better solution than a bank of fluorescents.
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