Re: UGF and reflecting tubes

To: Aquatic-Plants at actwin_com
Subject: Re: UGF and reflecting tubes
From: rjw at aluxs_att.com (Ronald Wozniak)
Date: Tue, 26 Dec 95 12:45:16 EST
>From: gomberg at wcf_com (Dave Gomberg)
>Date: Fri, 22 Dec 95 16:03:14 PST
>Subject: Re: UGF and reflecting tubes
>
>Re: Reflecting tubes
>
>My cut on this is to put the time and money into your light fixture. 
>That way you reap the benefit not only for the next tube, but all
>subsequent ones.  Get some aluminized mylar and line your reflector
>with it.  Even better, try to make a reflector like this (excuse
>terrible ascii art):
>
>
>
>
>   __     ___
>  /  \   /   \
> /    \ /     \
>/      |        \
>|      O         |
>|                 |
>
>The O is the bulb, and you have a McDonald's effect reflector, double
>arch.
>I have though a lot about the right shape and I think this is it. 
>Dave
>
>Dave Gomberg, Experimenta      San Francisco CA USA   gomberg at wcf_com

Dave, I think your right on the shape of reflector being a "McDonald's
effect" shape. I came across a good book in regard to light reflector design:

_Optical Design of Reflectors_ by William B. Elmer, Wiley 1980 
ISBN 0-471-05310-4

In Chapter 10, the book talks about the case when you have a large lamp in
a small reflector. (A situation typically found in most multi-tube fluorescent
aquarium hoods.)  Elmer defines a large lamp in a small reflector situation
when the diameter of the lamp is more the 15 to 25% of the diameter of the the
reflector.  The book calls these "McDonald's effect" type of reflectors: 
"divided convergent reflectors."  Basically, what one is trying to do is to
try and reflect the light that would be behind the lamp back around and down
to the aquarium.

It appears that this design may improve the amount of light making down towards
the aquarium from the 60-70% range to around 85-90% range. To build the
"perfect" divided convergent reflector may be impractical, but by using these
ideas it seems that one can design a cheap home-brew reflector and get fairly
close to the optimum.  Also, the reflector can be designed so that most of the
light from the fluorescent lamp is directed down (perpendicular) to the surface
of the aquarium water. (Light directed perpendicular to the surface of the water
has the least amount of it reflected back.)

The book has a very good table (pg. 272) on the reflection characteristics 
of various types of aluminum, glass and baked white enamel surfaces.  
Also a very good discussion on the reflector characteristics - specularity
(mirror like) and diffusion of different materials such as silver mirrors,
polished steel, and white porcelain enamel.  It seems that it is better to
use white enamel than polished aluminum for metal halide bulbs in order to
help spread out the sharp peak in light intensity arising from the relatively
small filament.

I don't think one has that problem is using fluorescent tubes since the tube
is much longer (spread out and more uniform) than a metal halide bulb.  

I've been debating on which surface is better for fluorescent tube reflectors 
used in fresh water aquariums polished aluminum or white enamel?  

Polished aluminum seems to have better than 90% reflection characteristics over
the visible light range and is mirror like (as opposed to being diffusive - like 
white enamel) so that one can use it to reflect most of the light back down into
the aquarium.  Saltwater folks (Hamilton Technology Corporation) say that the
aluminum reflector gets dull from the salt water and in the long run white enamel
doesn't degrade its reflection characteristics as much.  White enamel has around
80-88% reflection characteristics over the visible light range and most of the
light gets diffused (spread out as opposed to reflected.)

Any one have any recommendations?

Ron Wozniak  Allentown PA, USA
rjw at aluxpo_att.com