Re: Light intensity and water depth

>From: George Booth <booth at hpmtlgb1_lvld.hp.com>
>Date: Fri, 17 Nov 1995 08:11:36 -0700
>Subject: Re: Light intensity and water depth
>From: Jayme Donnelly <Jayme_Donnelly%BOND__NOTES at notes_worldcom.com>
>> One other question.  If depth is not important, then why are salt
>> water/reef aquariasts concerned with depth and what type of lighting
>> they use to "insure sufficient lighting reaches the bottom"??
>I, for one, have not yet been convinced that depth is not important.
>Perhaps Wright could reenter the discussion, but my impression was
>that the "AquaMyth #1" was only debunked in the case of a plain tank
>where light was free to reflect off the sides. 
>A heavily planted tank usually has plants covering 3 of the 4 sides
>and the light does indeed drop off dramatically from water surface to
>substrate.  Many of the shorter plants are shaded by the taller
>plants.  Stem plants especially tend to grow in an inverted pyramid
>shape and cast a lot of shade as they get taller.  
>A taller tank DOES need proportionally higher light at the surface if
>low foreground plants are too receive enough light.  IMHO, the
>AquaMyth is NOT a myth, at least for a typical planted aquarium. 

OK George, consider me re-entered.

My original post was to get people to pay attention to what *is* 
important, and quit assuming there was a) significant water absorption 
and b) a spreading loss like in air (1/R^2 loss). I agree completely 
with the view you express here, but defy you to express it in any 
useful "Watts/unit depth" rule of thumb. It's highly dependent on the 
individual tank, and 95% of those tanks out there are *not* heavily 
planted. Hell, most of them don't suffer the thick cover of duckweed, 
salvinia, and water sprite that makes my tanks look downright gloomy, 
half the time. :^) As you say, yourself, it's not the depth but the 
planting density that absorbs the light. My biggest losses are often 
from just above the surface to just below the duckweed -- not really a 
tank-depth-dependent phenomenon at all.

By the way, Charley, I believe the salt water absorption is also mostly 
insignificant unless the reef uses energy out at 360nm or less. (I know 
from nothing about reefs. I quit killing them in 1957). The density, et 
al, arguments certainly are significant at the ocean depths, but not in 
our shallow home aquaria. Pale reefs bounce light around inside the 
tank a lot (a small plus) and reflect a lot of light out of the tank (a 
big minus), so the amount of light should be determined by how much 
wattage at what wavelength is important to the reef surface. Most of 
the losses getting it there occur out in the air (spreading loss), at 
the reflector, at the water surface (reflection, particularly at high 
incidence angles) and in the wall scum. Water depth is way less 
important than those until it gets so cloudy or tinted you have trouble 
seeing the tank's contents clearly. Otherwise, you only lose about half 
the visible light for every 20' or so of depth from normal salt-water 
absorption. Thank goodness, tho, for we would otherwise miss all those 
wonderful Natl. Geographic/Nature/Nova reef flicks! Shots of 40' whales 
would be quite impossible.

Wright Huntley (408) 248-5905 Santa Clara, CA USA huntley at ix_netcom.com