Another Small Tank Lighting Issue

[Tong]

Another potential issue in the breakdown of the WPG rule for small tanks is the fact that smaller bulbs produce less light for a given number of watts.

Take for example a 48" 32 watt T8 bulb. A good one should produce 2800 lumens or 87.5 lumens/watt. Now, the same tube in 18" 15 watt T8 form should produce 1312.5 lumens. However, I have never run into an 18" bulb that produces this much light (If you do let me know!). The most I have ever seen is between 700 - 800 lumens (Hagens LifeGlo bulb might be higher as that is a pretty bright bulb!). This means that 18" 15 watt T8 actually produce around 47 - 53 lumens/watt. This is even lower than a 48" 40 watt T12, which is around 59 lumens /watt.

This may explain why some of the best small tanks has an insane amount of lighting...even if you take into account surface area. Using this kind of calculation (and surface area) a 10 gallon tank with 4 18" 15 watt T8 bulbs will still effectively only be ~2.5 - ~2.8 WPG.

Am I totally off base here?

 

[Zezmo]

This begins to touch on the idea of Lumens per Gallon as a more accurate estimator of light. The WPG rules assume a flat Lumens per Gallon ratio regardless of bulb size or type. Which of course is not the case.

 

[czcz]

Very interesting. We know lumens is not the perfect measure of usable light for our plants, but it being better than watts definitely puts you on the right track imo.

From a quick google search, even 17w T8 only has 70-78 lumens/watt, as opposed to the ~90lumens/watt of 4 32w T8 samples from Ivo Bucko's article and Phillips. It appears we do not see the typical lumen/watt ratings until reaching moderately high levels.

Quick google search also shows less lumens/watt for 13w CF as opposed to the ~80lumens/watt of samples between 55w-96w CF from Busko.

I was not aware of this at all, and as members get time we should collect samples of low wattage bulbs (typical for, say, 10gal tanks and lower) and see what happens after messing with the numbers. T12, T8, and regular (not screw-in/spiral) CF is probably as good a place to start as any.

This may explain why some of the best small tanks has an insane amount of lighting...even if you take into account surface area. Using this kind of calculation (and surface area) a 10 gallon tank with 4 18" 15 watt T8 bulbs will still effectively only be ~2.5 - ~2.8 WPG.

If we use the following assumptions:
15w T8 = 750lumens (just taking the simple mean from your search), so 4 15w T8 = 3000lumens
T12 = ~59lumens/watt (using simple deviation of the 17 samples between 36 and 115w from Busko's article)
then
4 15w T8 bulbs = ~50.8w T12 = ~5.1 equiv T12 wpg over a 10gal aquarium

If we use 3000lumens over a standard 10gal, the resulting 15 lumens per sq in is very close to the "High light" threshold of Ozz's article.

Indeed very surprising that 60w T8 over 10gal is so low using these standards.

Right now this points to 36w and 40w CF being even more desireable over 10gals, simply because they are in the range of more efficient lumen/watt output.