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Re: Light Levels and Glass Lids

>Date: Tue, 2 Nov 1999 09:23:44 -0500
>From: Peter_Bradley at hc-sc_gc.ca
>Subject: Light Levels and Glass Lids
>Hello all,
>>I have recently started a plant tank and was wondering if anybody out there has
>any comments regarding what the effect of a glass top on my 55 gallon (LxWxH:
>36x18x20) has on light penetration into the water column. I currently have 4x30
>W FL tubes for a total of 120 W of light ( ~2.2 W/gal). I have heard both
>extremes: it has a significant effect and it has very little effect. Has anybody
>actually measured the PAR with and without a glass top? Thanks in advance.
>Ottawa, Canada

Check out :


The article basically says that you loose about 10-15% of the light when
using a glass top.  A more critical aspect is how good is the bulbs
reflector in efficiently transmitting the light into the aquarium water.

Most glass used for aquarium glass tops (green - boron) attenuate most
of the light in the non-visible UV range. Most FL tubes don't transmit much
in this range anyways. Other than the 10-15% attenuation of the FL tubes light
output, I would not expect that a glass top would significantly shift the 
FL tubes PAR measurement. Maybe someone who exactly did some measurements
could comment on this.

Ron Wozniak  Allentown PA, USA
rjwozniak at lucent_com
AGA member