lighting theory - very long post!

[cellodaisy]

This whole "watts per gallon" thing wasn't making sense to me. I mean, we all know that fluorescent bulbs are more efficient than regular incandescent bulbs, so how could there be a blanket "watts per gallon" rule when different kinds of bulbs would put out different amounts of light for the same wattage? So I went researching. (Big thanks to Nolapete and TwoTankAmin for pointing me in the right direction.)

DISCLAIMER: I'm a total newbie to all of this. I think I get the theory, but I have no experience to back it up. I am NOT trying to tell ANYONE what to do, I'm just hoping to get feedback on this so I'll know if I'm on the right track or if I've got it totally wrong. Now back to our regularly scheduled programming. Here goes nuthin'.

First some definitions as they pertain to aquarium lighting:

watts - the rate at which a bulb consumes energy
lumens - a measurement of the amount of light output
lux - a measurement of light over a given area (1 lux = 1 lumen per square meter); 1 lumen over 1 square meter is a lot different from 1 lumen over 100 square meters---it matters how "thin" the light is spread
lumens/watt - the amount of light that a bulb puts out per watt (the bulb's efficiency)

Different bulbs have different lumens/watt. That's why you see things like "60 watt equivalent" on a compact fluorescent bulb, even though it consumes far less than 60 watts. Because it is more efficient (has a higher lumens/watt output), you can get more light for the same wattage OR the same light for less wattage.

For example, let's say we have a typical incandescent (regular) light bulb that gets 10 lumens/watt and a much more efficient compact fluorescent that gets 40 lumens/watt. Let's say they're both 100 watt bulbs.

incandescent: 100 watts x 10 lumens/watt = 1000 lumens
fluorescent: 100 watts x 40 lumens/watt = 4000 lumens

But what if you only wanted 1000 lumens, but you wanted to switch to a fluorescent to save electricity?

(1000 lumens)/(40 lumens/watt) = 25 watts

That's why, in our scenario, a 25 watt fluorescent would give us the same light as a 100 watt incandescent. (Note: Lumens/watt depends on a lot of factors and each bulb is different. I just chose plausible numbers that were easy to work with.)

You can see that with the myriad different kinds of bulbs available now with widely varying efficiencies, a "watts per gallon" rule doesn't work. Your tank doesn't care how much electricity you use, it cares how much light it gets.

So if we assume that the old rule works for mid-sized tanks (and I think that's not a bad assumption since so many people still use it and their tanks do quite well), let's use a 55g as an example. By the old rule, "low light" is 2WPG, so a 55g would require 110 watts. Most people use fluorescents, so we're assuming this rule means 110 watts of fluorescent light, and we'll just go with an average efficiency for fluorescent tubes (different tubes will have different efficiencies).

Rex over at rexgrigg.com discusses this. He dismisses tank depth as irrelevant and I'm just going along with that. You should really go visit because he has some great things to say on other topics as well. (I just have to note, because I'm anal about things like this, that Rex uses the terms watts, lumens, and lux very loosely, so you won't find them used the same way in my write up. Who am I to question more experienced fishkeepers? I dunno. Nobody, I guess. Please feel free to ignore everything in this post! )

On to our 55g/110w example:

55g surface area = 48in x 13in, or about 0.4 sq.meters

fluorescent lumens/watt = 50 (conservative estimate)
light output of 110 watt (fluorescent) = 5500 lumens
5500 lumens/0.4 sq.meters = 13750 lux

So if we use the old 2WPG rule and 55g/110w as a starting point, we get a new low-light rule of 13750 lux. (We did a lot of estimating to get this number, so it's a rough guide only.)

To calculate the your low-light wattage needs by this new rule:

1) Find the surface area of your tank and convert it to square meters
2) Multiply your surface area by 13750 to get the number of lumens you need.
3) Find the lumens/watt rating of the lighting you want to use. (Keep in mind that the housing/reflector will impact the ultimate lumens/watt efficiency.)
4) Divide your required lumens by the lumens/watt rating of your lighting type. Voila! You have your wattage requirement according to the new rule.


Example: I have a 10g that I want to set up for low light.

1) 10g surface area = 20in x 10in
20in x 10in = 200sq.in
200sq.in = 0.129sq.meters
2) (0.129sq.meters)x(13750 lumens/sq.meter) = 1774 lumens
3) We'll use our 50 lumens/watt estimate for now.
4) (1774 lumens)/(50 lumens/watt) = 35 watts

So you see we end up with 35 watts, where the old rule would say 20 watts.

So my question to all of YOU more experienced fishkeepers is this: does it work? I can't find any problems with the theory, but I have no experience to back it up.

Comments, criticisms, etc. all welcome. Thanks for reading!

 

[jpappy789]

One thing that bothers me is that lux is a measurement based on the human eye, therefore lumens are as well...can someone explain how that corresponds to the amount of lighting needed for plants? It seems like you could easily just insert foot candles in instead, although it would be obvious that any measurement in foot candles would be utterly useless...

 

[cellodaisy]

One thing that bothers me is that lux is a measurement based on the human eye, therefore lumens are as well...can someone explain how that corresponds to the amount of lighting needed for plants? It seems like you could easily just insert foot candles in instead, although it would be obvious that any measurement in foot candles would be utterly useless...

Yes, I thought about that, especially since human eyes are more sensitive to green, which is almost useless to plants. I'm not quite sure how to do it, but I know I need to work color temperature (degrees Kelvin) into my calculations somehow. I'd love your suggestions for doing it!

 

[jpappy789]

Haha I wish I could help but unfortunately you'd need to turn to the heavy duty plant experts (Rex Grigg, Tom Barr etc.) for that answer...unless another AC member has a suggestion?

 

[jmhart]

So,

All of your concerns about the watts per gallon "rule" are accurate. There are many flaws with calling it a "rule", that's why I like to refer to it as a guideline. There are many exceptions to the rule, for example:

1)"Rule" was developed for NO T12 bulbs, so all other bulbs fall outside the rule. That's where this comes in handy:http://www.plantedtank.net/articles/Light-Bulb-Comparison/29/

2)"Rule" doesn't apply to tanks smaller than 20g or larger than about 80g. Read this: http://www.rexgrigg.com/mlt.html

3)Lumens is indeed a slightly more appropriate view of light, but PAR is an even more appropriate measurement. Unfortunately, most manufacturers do not yet provide PAR values for bulbs, so you'll have to rely on fellow hobbyist to supply this information.


So, to wrap up, your thinking is correct. You put a lot of thought into, perhaps more than required, but you arrived at the same conclusion that many people before you have arrived at.


One more thing to add: This website broke down light over dozens of Amano tanks, and performed a mathematical analysis, resulting in a nice formula that would indicate, perhaps, what Amano would, or has, used over tanks of particular size: http://www.fitchfamily.com/lighting.html

 

[jpappy789]

I feel dumb...what does PAR stand for again?

 

[jmhart]

PAR=Photosynthetically Active Radiation

 

[jpappy789]

Thanks! :thumbsup:

 

[cellodaisy]

Maybe if I figure out more specifically how lumens are measured, there could be some kind of conversion from lumens to total energy output?

 

[jpappy789]

But I still think there is a gray area that encompasses what our lights give out and what our plants actually need to take in...