Light vs. Depth

To: Aquatic-Plants at actwin_com
Subject: Light vs. Depth
From: Erik Olson <eriko at wrq_com>
Date: Sun, 22 Oct 1995 11:39:55 -0700 (PDT)
In-Reply-To: <199510220739.DAA06049 at looney_actwin.com>
> From: huntley at ix_netcom.com (Wright Huntley )
> Date: Sat, 21 Oct 1995 14:10:59 -0700
> Subject: AQUAMYTH #1 More Light for Deep Tanks -- NOT!

> This is one of a (proposed) series of posts aimed at exploding (or at 
> least questioning) some of the mythology that is frequently repeated 
> here.
...
> We aren't talking the Monterey aquarium here, but a tank 1m deep and a 
> tank 0.5m deep will have virtually identical illumination at the 
> substrate.  Check it out with any light meter, keeping the 
> lamp-to-water distance constant, changing only depth.
> 
> A distributed source, like fluorescents, couples light into the water. 
> with inside-the-tank propagation within what is called the critical 
> angle. Light hitting the sides of the tank is totally reflected, for it 
> is beyond the critical angle. It is absorbed or reflected by objects in 
> the tank, but keeps going until it finds such objects, like a huge 
> light pipe.

I checked this myself empirically.  The rest of you can do this at home
too. Put your eye close to the front glass of your tank and look up at the
water surface.  Move your head down until you can see the light from
above.  That's where you hit the critical angle, and at depths below this
the light will start to escape through the tank front and back. For my
tanks, this happens at about 18" below the surface.  So while I agree with
Wright's point for tanks of 18" or less depth, I would tend to disagree for
tanks of 20" or more (which is what the old timers are usually cautioning
folks about). 

Another point worth noting is that we don't have perfectly-reflecting 
tank walls here!  There's algae, crud, possible flat black paint, 
etc, which are all helping to ABSORB much of that light instead of 
reflecting it back into the tank.  The deeper the tank, the taller the 
walls, the more surface area potentially covered in crud to eat the light.

(This is certainly a good argument for keeping your tank walls shiny clean 
and algae-free!)

A simpler way of looking at it is that you're trying to distribute all 
the light (wattage) to all 5 walls of the tank (bottom + 4 sides).  As 
the tank gets taller, the total surface area increases and the same light 
must be diluted over this increased area.

Unfortunately, I don't have a light meter, but I think George Booth does. 
In fact, I recall a posting remotely (though not directly) concerned with
this effect... Perhaps he can be coerced into a simple little depth 
experiment? :)

...
> planted tank may use some more Watts (NOT lumens, by the way),

(yeah, but Watts doesn't refer to light output on the bulbs; it refers to 
the consumption, right?)

     - Erik
---
Erik D. Olson					         amazingly, at home
eriko at wrq_com