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from what i could find, most of what's being said about fluorescent lighting refers to W/G as a measure of how much light is being used.
this is an awkward measure, because it reflects only how much electricity being used without taking into consideration how much light is produced. a quite look at Osram's tables shows that the relations between Watts and Lumens is not linear at all.
so how many lumnes should be used?

furthermore, light intensity changes with distance from the light source. i assume that cylinder of light produces light in a close to cylindric shape, and so the "isoluminent" surfaces are also cylindric in shape. the surface is 2*pi*d*h, where d-distance from source, and h=length of lamp. this means that plants planted 30cm deep will have twice as much light as 60cm deep plants. it also says that a 20" long tank with, say, 5000lumen lighting will enjoy twice as much light as a 40" long tank with the same 5000lumen. both parameters are taken cared of by the lumen/G, but only approximately.

my question is how many lumens/vertical-surface (length*depth) are optimal?
what's yours?

i can see people reading this, but say nothing.
do you think i'm wrong?
can you compute that measurement?

lumens/height*length-of-lamp

lumens arwe written on the lamps themselves.

what kind of light do you use? is it good for your plants?

The WPG “rule” like the Inch per gallon “rule” is not really a rule at all , just a general guideline. You are correct in that intensity or brightness is the most important aspect and that lumens is a better measurement. It’s simply that watts is a more familiar measurement to most people then lumens so therefore it’s just simpler to explain lighting requirements in watts. Especially so as most flourescent bulbs come in standard wattage at standard lenghts. There is somewhat of a relationship between watts and lumens is that a bulb using 20 watts of electricity is going to produce more lumens then one using 10 watts of electricity. Yes, a high grade tri-phosphor 20 watt bulb is going to produce considerably more lumens than a standard 20 watt bulb, and a 10 watt bulb on a 5 gal is the 2 wpg but is not as good as it sounds as a 10 watt bulb is not going to be that light intensive anyway but at the end of the day it’s just more convenient to talk about watts per gallon.

As to how many lumens should be used, I have seen guides but I can’t remember. If you ever did find out you could do an exercise. An average 20watt tube produces about 1,000 lumens. If you did get an answer on how many lumens were needed you would have to work out how many bulbs you need for your tank, based on it’s size. I bet the answer would be the same number of bulbs you would have calculated using the WPG rule ! Lets face it we are limited by the standard length and wattage of bulbs available.

As regard to the depth of the tank, yes this is an important consideration. Light is deflected and breaks up as it travels down thru the water , so while it may start as 2 wpg at the top it ends up as wpg at the bottom ! So in deeper tanks more light is needed to make sure enough gets to the bottom. This can be done in a number of ways, by increasing intensity, the use of reflectors to ensure all the available light is pointed downwards into the water rather then bouncing all over the place or using a different color temp/spectrum like 10,000k bulbs as blue light has a much easier job of getting thru the water. In my own case I have a “show” tank which is tall and wide rather then long, so I use reflectors and 10,000k bulbs. Plants do fine, but still lose leaves at the bottom of the plant because of lack of light. This is due more to the fact that the tank is so heavily planted that they shade one another. I get around this by layering the plants or putting low light plants, rocks or wood in front of the stem plants so the bare stems are not visible.

In my own set up I use 144 watts of PC lighting with 10,000k bulbs, no idea of lumens (wasn’t stated on bulb packing) but guessing about 10,000. This over about 65gals of water. Plants do great.


[span class="edited"][Edited by bensaf 2004-09-05 22:13][/span]

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