WPG, Lumens, LUX and LSI

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finz

AC Members
Apr 28, 2008
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bill roberts
First off, seems that per watt, a Compact Flouresent lamp produces 50 lumens.

So @ 65 watts, a lamp produces about 3250 lumens...

I suppose one could measure the total area of the lamp and say that x proportion of the lamp produces Y proportion of lumens.

One might suggest that, as a lamp is fixed above a tank, the point at which the radius of the lamp and the surface of the water are parrallel, one would suggest this condition to be the strongest projector of lighting into the tank and as the bulb curves away from the water surface, said intensity is dimished to the point where, when the lamp surface is orthoginal to the water surface, we're just about @ a Zero in terms of the light that is pushed into the tank. And again as both sides of the light curve upward, that light is essentially wasted and NOT benefitting the tank.

Fortunately, our lamps have above them a surface which acts to reflect this light back into the tank... Of course some of it goes back into the lamp per the angle of reflection and some is reflected into the lighting hardware.. etc... Point being it's difficult to get all produced light into the tank.

Anyway, a lot to consider whe determining LUX and I imagine I'm just scratching the surface...
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Lets assume you could focus all 3250 lumens into a square meter, LUX would be 3250!

Lets also assume now that you have not one but 4 lamps @ 65 watts producing 3250 with total combined output of 13000 lumens.

Lets however instead assume rather that 1 sq meter, the area is more like ohh, how about, 1.776710156 ? (lol)
That might give you a LUX of say 7316.894066.

If you instead wanted to see that in oh, i dunno, square inches (1 meter = 39.3700787 inches), you'd end up with 185.8491095 Lumens per Square Inches (LSI).

That number does seem high and then again, squeezing exactly 13000 lumens from 4 bulbs @ 65 watts (and therefore a base 13000 LUX) is mostly likely out of reach.

So lets assume you get half that amount, 6500 LUX... giving you 3658.447033 LUX when spread over 1.776710156 meters.
Factored to inches, you get an LSI of 92.92455474... still high...

Input?
 
Nice analysis, but for it to have any real meaning we would really need to know how efficiently the reflector in question reflects light back into the tank. Possible, with a relatively expensive light meter, but not necessarily practical (or necessary). You should also take PAR and color temperature into account - over say a 10gal tank, 15w of 6500K light will grow a lot more plants than say 50w of 3000K light.
 
finz said:
Lets assume you could focus all 3250 lumens into a square meter, LUX would be 3250!

Lets also assume now that you have not one but 4 lamps @ 65 watts producing 3250 with total combined output of 13000 lumens.

Lets however instead assume rather that 1 sq meter, the area is more like ohh, how about, 1.776710156 ? (lol)
That might give you a LUX of say 7316.894066.?
Click to expand...
Where did you get 1.777 m^2? I'm guessing you are using this number because you think it's the surface area of your tank. This would imply your tank is 1.333 meters long AND 1.333meters wide(that's 52"x52"((that's with a lot of rounding errors))). That could also be 72"x37". That's a big tank.

Let's assume for a minute that you meant your tank is 1.77 meters long, so that's, let's call it a 72" tank. Standard 72" tanks aren't 37" wide, at most 24". So, let's call that .6 m. Now, your 1.77 m x .6 m tank has a surface area of 1.062 m^2, not 1.77m^2.

finz said:
If you instead wanted to see that in oh, i dunno, square inches (1 meter = 39.3700787 inches), you'd end up with 185.8491095 Lumens per Square Inches (LSI).?
Click to expand...
1 meter = 39.37 inches, yes, but 1 square meter = 1550 square inches




Now, going with your original tank size, with surface area 1.77 square meters, and correcting your converstion error, it now works out to have 4.739 lumens per square inch, which is a low number...very low...and that's because you factored a freakin' huge tank. Not to mention the fact that, as we all know, lsi and wpg rules don't apply to tanks that big.



Additionally, why were you thinking in SI units in the beginning anyway? Based on your end calculation it appears you wanted to talk about LSI rather than LUX anyway. Point is, cut out the middle man. Needless converstions add confusion and rounding errors. If you want to think in LSI, just look at your total number of lumens and divide by your surface area(in inches).

Two reasons to do these converstions are either A)your bulb gave you lumen measurements in LUX rather than lumens or B)you were trying to convert for us if you naturally think in SI units and figured your audience here would be mostly English Stanard users.
 
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