2-4 watts/gal of what kind of light? - New to Planted Aquariums - Aquatic Plant Central

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Old 10-08-2007, 06:28 PM   #1 (permalink)
 
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Default 2-4 watts/gal of what kind of light?

Recommendations for lighting (based on what particular plants require in the aquarium) are generally given in "watts per gallon" Well, I did some research on how much light is produced per watt by different types of lighting systems as follows:

Tungsten Incandescent: 5-17.5 Lumens/Watt
Quartz Halogen: 24
T12 Fluorescent: 50
Compact Fluorescent: 45-60
White LED: 26/70 (some prototypes are running up to 150)
Metal Halide: 85-95
T5 Fluorescent: 104

So the question when trying to apply such recommendations is "watts of what kind of light?"

Example from my aquarium: Ludwigia Repens calls for 2-4 watts per gallon, per variou sites I've seen. 58-116 watts in a 29 gallon tank (my tank size). The problem 116 watts of T5 fluorescents is about six times as much light as 116 watts of Tungsten incandescents. If the recommendation referred to incandescents, then 20 watts of T5 fluorescent would serve. (Modulo variations on spectra, of course.)

One thing I note, the metal halide lamps, they are less efficient than the T5's, but they do pack a lot more watts into a smaller package. Which just goes to show that "efficiency" is not the be-all-end-all of the matter.
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Old 10-08-2007, 06:50 PM   #2 (permalink)
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Default Re: 2-4 watts/gal of what kind of light?

Generally people use florescent or halides, don't think plants use the incandescent or led.
florescent all use the same wpg rules, not sure about the halides and halogen.
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Old 10-08-2007, 07:39 PM   #3 (permalink)
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Default Re: 2-4 watts/gal of what kind of light?

Generally the watts per gallon people talk about refer to bulbs comparable to AH Supply bulbs, and with reflectors as good as those used by AH Supply. T5 bulbs provide more light per watt. T12 bulbs provide less light per watt. And lights that don't use good reflectors provide still less light. This is a moving target though. As time rolls on people tend to shift to the latest and greatest as the standard, but generally stay with about 2 watts per gallon as what is "needed".

Even if we all use the same "standard" light configuration we still don't all agree on what constitutes high light. I consider anything around 2 watts per gallon to be high light. Others consider that to be moderate light.
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Old 10-08-2007, 08:22 PM   #4 (permalink)
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Default Re: 2-4 watts/gal of what kind of light?

There was a quite long thread on this not all that long ago.

Can't remember if it was on this forum or another. I'll have to look for it, but the title "watts per gallon rule revisited" sounds familiar.
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Old 10-08-2007, 08:50 PM   #5 (permalink)
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Default Re: 2-4 watts/gal of what kind of light?

This might shed some "light" on the subject....
http://woo.gotdns.com/Aquarium/Lighting.htm
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Old 10-09-2007, 07:39 AM   #6 (permalink)
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Default Re: 2-4 watts/gal of what kind of light?

Watts per gallon has never been more than a very rough estimate of how much lighting to get for a given tank size. And, that is all it will ever be. It is very easy to debunk it as a "rule". Take a 15 inch deep tank, add 2 watts per gallon. Now make the tank a third deeper. Light is just barely attenuated in 5 inches of water, but the watts per gallon rule would say you need a third more light for the same intensity. We could shift to a PAR based rule, but how would a light manufacturer be able to assign a PAR value to their lights? PAR would depend on too many variables. Then, we would still use watts per gallon for our first crude guess at how much light we need.
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Old 10-11-2007, 08:53 PM   #7 (permalink)
 
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Default Re: 2-4 watts/gal of what kind of light?

There are people that are trying to figure out many of the variables. Check out this site http://www.reeflightinginfo.arvixe.com Right now its only halides. I have seen presentations from him and seen the comparison of spread using t5, led, and halides.

All lights have their advantages and equal disadvantages
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