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Re: Lighting and Lux
- To: Aquatic Plants Digest <Aquatic-Plants at actwin_com>
- Subject: Re: Lighting and Lux
- From: Wright Huntley <jwwiii at pacbell_net>
- Date: Fri, 14 Jun 2002 07:57:07 -0700
- User-agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:0.9.4.1) Gecko/20020314Netscape6/6.2.2
> Date: Thu, 13 Jun 2002 18:21:48 EDT
> From: Billinet at aol_com
> Subject: Re: Lighting and Lux
> Hi, Wright,
> OK, I misapplied the inverse square "law", and there are other things in an
> aquarium that interfere with the transmission of light energy, and there are
> other variables, but, darn it, it is a fact that deep tanks are noticeably
> darker at the bottom given the same watts per gallon as shallow tanks, and
> some bottom dwelling plants that do fine in shallower tanks just don't make
> it in deeper tanks. I think we've all witnessed that.
Of course, Bill. Bigger plants provide a *lot* more shade, for example.
The loss of blue light with depth is negligible for the first few feet,
and the loss of reds is not nearly as significant as reflector
differences, which can easily be 40-50% or even more. The tank is a true
"light pipe" and the light will propagate forever until absorbed or
scattered out through the glass by hitting something in the tank.
In a 9'-deep swimming pool, the loss of reds is enough to make the white
plaster bottom look a pastel blue as the 18' round trip absorbs some of
the red light. It is still *really* bright at the bottom of the pool,
though. [Gee. Wonder if that is enough effect to make water look blue in
a white bucket? <VBG>]
> What I would like to see would be a table that would quantify this
> difference. It wouldn't contain "thousands" of entries. Maybe just about
> 100 cells. In spreadsheet terms, column A would contain the distance in
> inches from the light source, perhaps at four inch increments. Across the
> top would be the description of the light source, such as "Brand A, 20 watt,
> one tube," "Brand A, 20 watt, 2 tubes," "Brand B . . .. ," etc. For each of
> the three or four depths the lux would be shown.
I repeat, lux is not an important factor in most plant growth. It weighs
the green as 10X more valuable than red or blue (the photosynthesis A
and B peaks), yet the plants usually are reflecting away much of that
green light (i.e., most don't use much of it). There are *no* simple
conversions from psychophysical units (lux, Lamberts, lumens) to
energetic physical radiation units (Watts, Einsteins, and even PAR). You
need detailed spectral information to make those conversions, and even
then there often is a lot of guesswork involved.
> Perhaps other light source variables might be included, such as the
> efficiency of the reflector ("high" or "regular"). Not all kinds of lights
> would be need to be included. I'd guess that no more than 100 spreadsheet
> cells would be required.
> So, Wright, I'll design the spreadsheet form and e-mail it to you, and you
> can coordinate the gathering of the data. Then as a group here we can
> estimate the lux requirements of various plants and revolutionize the hobby!
> How about that?
Not a chance, Bill. IMHO, the results would not only *not* be worth the
effort, they would be just as misinterpreted as our present W/G rules of
thumb. :-) [My revolutionary days are over (if you can ignore my blatant
Here's a better way to think of it.
A normal 2' deep tank will have, down at the substrate, about 70% or
more of whatever red light was available entering the surface and 90%+
of the blue light. This is deeper than I like to work on. "Black Water"
and other absorbers may reduce those numbers (particularly blue) by a
few percent, too. Intermediate colors will have intermediate values.
Dirty glass may even cause some scatter and slightly reduce the amount, too.
Shading and layout will have a far greater effect than optical losses in
most tanks. Those just don't fit any simple spreadsheet.
Concentrate your efforts on getting a pleasing mix of viewing light and
photosynthetically useful light (the right phosphors), and then on
getting as much of that light entering the water as you can (the right
size and shape tube(s) in the best-designed reflector). Then do your
layout so high-light plants are not in the shade. Effort placed there
will have many, many times as much effectiveness as your proposed
spreadsheet numbers, IMO.
Wright Huntley -- 290 521-0557 -- 731 Loletta Ave, Modesto CA 95351
"If you aid and abet the enemy, whether you're a citizen or not,
you're not entitled to the right of due process,"
-- Sen. Charles E. Schumer,
New York Democrat, member of the Senate Judiciary Committee.
So much for the (late) constitution he swore to uphold!