Re: How much light is reflected by a glass cover?

[Wright Huntley <jwwiii at pacbell_net>]

About 8% at normal incidence and even more at angles off of 90 degrees. 
Each surface reflects about 4%, and there are two. I'd guess average 
reflected light to be at least 15%-20% if the source is close to the 
glass. Some, but usually not much, is rerefected into the tank.

In the important red end of the spectrum, a 1/4" cover glass can also 
absorb another 15% of the useful light, again at normal incidence, and 
more at other angles. [If the glass looks green at the edges, it absorbs 
red light.] Make that another 20-30% loss of useful light. Since it is 
only in the red end, it might be 10% of the useful photosynthetic energy.

All told, I expect a clean cover glass to waste maybe 25% of the useful 
energy, so it is pretty significant. [Unfortunately, my fish are all 
killies, and air just doesn't keep them in the tanks. :-)]

Wright

> Date: Mon, 24 Feb 2003 20:59:39 -0500
> From: "Pete" <peteal at sympatico_ca>
> Subject: How much light is reflected by a glass cover?
> 
> Hello, I had a glass cover for my 90gal tank for evaporation and jumping
> fish protection for a long time.  Recently I'v tried it without one as I no
> longer have fish that are a high risk of bailing from the tank.
> 
> I now have noticed that my Rotala rotundifolia is now getting red at first
> 1/3 of the tank bottom instead of at the top 1/4 (where I'd have to soon
> prune it anyways) and other plants are bushier.  I have two 175W MH lamps
> pretty close to the surface so I thought with 350W of MH I had plenty of
> power to get light past the glass cover but since I see an improvement
> without the glass I guess not?!
> 
> So does anyone know about how much light I was losing with this cover
> reflecting light from the tank?
> 
> The MH lamps have a tempered glass shield also... the cover was more for
> fish and tank protection.
> 
> Thanks
> Pete.


-- 
Wright Huntley -- 209 521-0557 -- 731 Loletta Ave, Modesto CA 95351

    Q.  How many Frenchmen does it take to defend Paris?

    A.  Nobody knows - it's never been tried.

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