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Old 28-09-2007, 01:57 PM posted to rec.gardens.orchids
jadel jadel is offline
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First recorded activity by GardenBanter: Jan 2007
Posts: 57
Default Reality Check on an Orchid Light Solution?....

On Sep 27, 11:38 pm, Steve wrote:
Al wrote:
Yes, you did. And I agree. Although I think I understand more clearly
now and can rephrase the idea we both agree with so it says this:


given two bulbs that produce the same desired lumen output at one foot
from the bulb, the first bulb being a fist-sized incandescent and the
second being a 4 foot long tube, the difference in the size of the lumen
footprint between the two bulbs is what makes one more desirable than
the other. The first bulb produces a 1 foot by 1 foot space of desired
lumens, the second produces a 1 by 4 foot space of desired lumens.


However, what really through me for a few minutes in what I read he


HOWEVER, from a large or diffuse source such as a fluorescent tube,
the falloff is less drastic because the tube is, in effect, a large
number of overlapping point sources. The handy rule for such a source
is that the intensity falls off as the inverse of the distance, i.e.
twice as far ,1/2 as intense, three times as far, 1/3, etc.


was the difference between "inverse" and "inverse square". One makes
the statement false; although not so far wrong in the short distances we
are talking about under a bulb, that it was worth jumping to this lovely
thread. The other thing that threw me was the need to state how distance
and intensity are related in the equation. "Twice" and "Three times"
mislead me, (again the distances we are talking about are small, so as a
handy rule over distances of a foot or two, it works well enough).
Anyway, I managed to puzzle out the paragraph and make sense of it to my
own satisfaction. Sorry to do it in front of everybody, I should have
just roamed around the greenhouse talking to myself until I figured it
out. That would have been the more peaceable choice.


This is an interesting thread. Don't stop on my account.


OK, I won't.
Think about this. A fluorescent tube is pretty much an infinite
collection of points. Place a plant one foot under the center, then move
it to two feet. For the one point directly over head, the plant will be
twice as far away and the inverse square rule works. For a point way
over at the end of the tube, the plant has barely moved a little farther
away from that point. Sure, the point directly over head provides more
light than the others but the others all contribute too.
So, the points that contribute the most light fade the most as the plant
moves farther away. The points that contribute the least light nearly
stay the same as the plant is moved farther away.
My conclusion? I don't know, now I'm lost....


There's no reason to be lost. The rule used by lighting engineers
is that for large or diffuse sources the intensity of light falls off
inversely to the distance.

If they followed the classical physics inverse square rule, they'd
need a hell of a lot more lights to brighten up the Walmart.

The real trick is to change the bulbs every few monthsbecause their
output decreases over time.

J. Del Col