Re: Re[2]: Halide lamps (short)

Jon Singer (
Wed, 09 Feb 94 19:08:19 PST

Hi, Marc!

Remember, that's your ordinary cool white tube, measured when new.
There's a fairly straightforward way to measure fc (footcandles) with
the meter in your camera, but I don't quite recall it. It's around,
though. Check through the archives of the CP list and the ORCHIDS list
-- I'm pretty sure I got it from one or the other.

As an approximation, if you are spang in the middle of a regular
fluorescent fixture with a pair of 40-watt tubes in it, about 6" apart
from each other, and you are 6" away from the tubes, you are getting
roughly 500 fc. _IF_ the tubes are new. They get dimmer with age, as
do many lightsources. If you want a comparison, I have a 400-watt MH
lamp in the greenhouse, and the "hot spot" directly below it, at a
distance of maybe 20-24", seems to receive 2500 fc, which is 1/4 of
full desert sunlight! (I variously hear 9000 or 10,000 fc for that.)

Needless to say, I _like_ my MH lamp!


Remember, lumens relate only indirectly to fc, because there's a
distance-from-source issue, and with line sources or distributed
sources, it's not so simple as "inverse-square law". There's also
the fact that the ends of fluorescent lamps are quite dim compared
with the middles, and of course the dark space beyond the end doesn't
light up at all, thus compounding the problem. Even if you put a
reflector at the end, it's still pretty dim there.