# Does glass top cut par or spectrum compared to open top?



## The Big Buddha (Jul 30, 2012)

I was curious.

Thanks


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## [email protected] (Jan 8, 2010)

Yes


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## jimbo662 (Aug 4, 2013)

yep, that's why most reef tanks have screen tops.


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## The Big Buddha (Jul 30, 2012)

So is there a rough percentage of drop?...... Hoppy? Lol


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## Xirxes (Aug 18, 2008)

Just posted half a second ago elsewhere:

Any barrier or distance between light source and desired destination reduces quantity of visible radiation.

The amount of reduction is determined by optical clarity of the glass, cleanliness of glass, incident angle of light entering and exiting both sides of the glass in respect to where it is that you want the light to end up.

Sum it up: if you must use glass, thinner, clearer, ultra clean low lead glass is best, but how much PAR drop depends on how thin, clear, and clean the glass stays.


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## mattinmd (Aug 16, 2014)

Rough percentage drop for clean glass is 10-20%... or 90-80% transmissive.

This is consistent with my recent testing where my versatop took par from 43 to 36, a drop of 7 PAR, or 16%.

http://www.plantedtank.net/forums/showthread.php?t=842665&highlight=

Of course, this is all assuming *clean* glass, not crudded-up glass with algae and calcium deposits all over it....


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## Seattle_Aquarist (Jun 15, 2008)

Hi The Big Budda,

I responded to a similar question on another forum. GSAS has a PAR meter so I used it to do some quantitative measurements on your subject. The results were interesting. 

I used my standard 30 gallon (36" length) with an 2X36 watt (6700K) AH Supply kit over a Aqueon Versa-Top. With new bulbs in the fixture and no glass top the PAR = 110; with "dirty"* Versa-Top in place PAR = 96; with cleaned Versa-Top in place PAR = 101. All readings were taken at the substrate level about 13 inches below the fixture.

dirty* = lots of dried water spots on top, condensation; dust

So in answer to your question it does block light; a dirty* glass top reduces light by about 13%; a clean glass top reduces it by about 8%.....not a much as some might think.

30 gal w/Versatop and AH Supply DIY 2X36 watt


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## mattinmd (Aug 16, 2014)

Interesting, I didn't know it was possible to get beyond 90% light transmittance without resorting to anti-reflective coatings and/or low iron glass...

Looks like I need to re-run my par meter experiments (admittedly I'm using a Hoppy DIY meter, so that has its own accuracy limits).

edit:

Thinking about this, I realize fixture type is going to be a major factor in how much light you loose... A large portion of what isn't transmitted by glass is actually reflected back towards the fixture, and re-reflection back to the tank is possible.

If your fixture is one like a t5ho with nice big, polished reflectors, sitting down close to the glass, a lot of the light bouncing off the glass is going to get bounced right back at the glass, cutting your losses.

If your fixture is one like my planted+, which is a basic LED strip with white backer sitting up about 1" from the glass, a lot of what bounces off goes right past the sides of my fixture. The light that does strike it is hitting a flat white strip, which scatters it and reflects a small portion back into the tank...

Worse by a small amount would be something like the Sat+, which has LEDs on a black background.. pretty much everything reflected by the glass is going to either miss the fixture, or be absorbed by it.

I'll have to play with that idea some, as I do have a hydrofarm t5ho reflective fixture around..


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## The Big Buddha (Jul 30, 2012)

Thanks guys, I was curious because I just changed up my 40b with 3 ikea 10 inch foto pendants with 15w 6500k CFLS that are 14 inches from the substrate in the back and a current satelite + 36 inch over the front. Lights are on 8 hours a day. Dosing 45 mins before lights come on.
I am dosing PPS Pro daily with 3.5ml of metricide. I have an aqueon glass top, relatively clean, just a lot of condensation on back panel.
Should I increase the metricide? I do have some BBA, I pull out the leaves or spot treat with the excel doses. I seem to get small BBA patches on the glass an inch above the substrate. The substrate is MGOC capped with black diamond.

Thanks


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## jeffkrol (Jun 5, 2013)

mattinmd said:


> Interesting, I didn't know it was possible to get beyond 90% light transmittance without resorting to anti-reflective coatings and/or low iron glass...
> 
> Looks like I need to re-run my par meter experiments (admittedly I'm using a Hoppy DIY meter, so that has its own accuracy limits).












Achieving >90% in the vis spectrum isn't too difficult for glass..90% is a pretty good cutoff point though.

http://www.shimadzu.com/an/industry/ceramicsmetalsmining/chem0501005.htm

What I find a bit amusing is the fact that any Screening will also cut light..so it seems to be a wash. Screen will give you more gas exchange I suppose.


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## mattinmd (Aug 16, 2014)

Fair, although I wonder if "clear glass 2" is a low-iron type of glass.

It is quite spectrally flat in the visible range (which is really just a small part of the graph), which suggests a lack of "green tint" effect from iron.

Maybe it's not as low iron as we see in expensive low iron aquariums, but it certainly seems to be "less green" than most.

Regardless, it is a good lead-in for the second part of the asker's question... spectrum...

Glass of the normal sort does attenuate blues and reds slightly more than greens. That's why glass looks a bit green when you look at its edges.


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## jeffkrol (Jun 5, 2013)

mattinmd said:


> Fair, although I wonder if "clear glass 2" is a low-iron type of glass.
> 
> It is quite spectrally flat in the visible range (which is really just a small part of the graph), which suggests a lack of "green tint" effect from iron.
> 
> ...











There..

Pyrex









seems most glass is fairly "flat" in the vis. spectrum.

Quartz glass


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## mattinmd (Aug 16, 2014)

That soda lime is hardly flat... It is hump-shaped in the visible end, and well matches clear glass 1. 

As for pyrex, that is a very clear glass, not typical of aquarium lids (and nor is low iron)


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## jeffkrol (Jun 5, 2013)

mattinmd said:


> That soda lime is hardly flat... It is hump-shaped in the visible end, and well matches clear glass 1.
> 
> As for pyrex, that is a very clear glass, not typical of aquarium lids (and nor is low iron)


Hard to tell how humped since the scale is so skewed.. 

Better???


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## roadmaster (Nov 5, 2009)

The Big Buddha said:


> Thanks guys, I was curious because I just changed up my 40b with 3 ikea 10 inch foto pendants with 15w 6500k CFLS that are 14 inches from the substrate in the back and a current satelite + 36 inch over the front. Lights are on 8 hours a day. Dosing 45 mins before lights come on.
> I am dosing PPS Pro daily with 3.5ml of metricide. I have an aqueon glass top, relatively clean, just a lot of condensation on back panel.
> Should I increase the metricide? I do have some BBA, I pull out the leaves or spot treat with the excel doses. I seem to get small BBA patches on the glass an inch above the substrate. The substrate is MGOC capped with black diamond.
> 
> Thanks


 Lot's of theory's as to causes of BBA but no definitive cause(s) that I have ever heard of.
Would just keep cleaning the glass and removing manually what I could.
Would not be inclined to up the dose of metricide with fish or invert's present.
Keep filter's cleaned regularly,larger water changes for a while is what I do.
Have noticed with heavy plant mass ,mature tank,that many algae cannot thrive.


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