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New way of measuring light intensity?


Cyber-goby

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What do you think of this?

 

I've been trying to think up a new way of quantifying light intensity for aquariums. The watts per gallon definition obviously doesn't work, so I came up with this: the watt per inch; simply divide the total system watts by the length of the setup. This takes into account the fact that certain light and light setups concentrate more wattage into an area than other setups. For instance, my 75 gallon would have an W/in. rating of about 9 W/in., whereas my 30 gallon would have a rating of about 2.5, and my 10 would have a rating of 4.8 (the 30 and the ten both have 96 watt fixtures). Metal halides would obviously have very high watts/inch ratings, so the "formula" observations seem to hold true for reality. In addition, one can take the percent attenuation of light intensity (these are observed experimental values) and find the penetration of the different lights by multiplyng that percent by the watt/in. These results would also hold true with observed reality. The formulas are still rough, but what do you think? Am I on to something (I'm considering doing a watt/sq. inch)? The best method will always be lux meter use, but do you think this might be a good quick way of estimating?

[Constructive] criticism is appreciated, because I think having a working quick (non-lux-oriented) definition of high intensity, low intensity, etc. would save a lot of headache as people figure out what is best for their system.

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though your idea is an interesting spin on an old idea, i'm not sure that really helps any at all. watt/sqaure inch is pretty much the same as watts per gallon, i.e. power per unit volume. that being said, we know that as the depth of the water increases the less light makes it to the bottom. i think that is the key to determining the kind of lighting requirements you have, as well as the types of coral you want to keep. this is, of course, why intensity is just as important as power. just my 2 cents.

 

gr

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Actually, watt/sq inch inch is a power per unit area, which is actually the scientific definition of intensity (I wonder why that didn't occur to me before). What my idea takes into account is the fact that, for instance, an x-watt light that is y-inches long produces more intensity for a coral than a light that is also x-watts, but is 2y-inches long. The idea is still very rough, but it could be hammered into working.

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It's an interesting notion, but I don't think it would be a useful tool. There are too many variables with each individual tank to try and use a 'rule of thumb' to guestimate one of the most important aspects of reefkeeping. When setting up a tank, you should know what you will be keeping from fish to corals and even cleanup crew and what all the care requirements are. Lighting can be tricky, but it's also integral to your success as well as being downright expensive. I know of MANY people who wasted money on lighting that just wasn't adequate for their needs.

 

But hey, if you think you are on to something, then go with it - just make sure to verify and document any experimentation you do so it will be credible.

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If you think about it, most of the variables (depth of tank, "concentration" of light, etc.; everything except clarity of water-still working on that) are taken into account. This would even allow you to consider where in a given tank a certain coral can go, if anywhere. The only thing we lack is a definition (in W/sq. inches) of what is high light intensity, what is low light intensity, etc. I am coming up with a plan to verify my idea. Again, though, all thoughts are appreciated.

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yeah, sorry about that, i was thinking you said watts/inch cubed. anyway, intensity is the real issue, maybe you might stumble on to something.

 

gr

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If we can just work on a simple but accurate-enough definition, I think that would really help people. It might not be doable in one's head, but the math is still pretty straightforward. And anything is better than wpg. I will keep thinking about it.

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If you think about it, most of the variables (depth of tank, "concentration" of light, etc.; everything except clarity of water-still working on that) are taken into account.
I don't get it. How is depth of tank taken into account when comparing electricity consumption to surface area?

 

And how is "concentration of light" taken into account? Watts is a measurement of electricity consumed, not light produced. How much of that is converted into light is going to differ from light to light. 70w of PC does not put out the same amount of light as 70w of metal halide.

 

What about reflector quality? Restrike? Distince between lamp and water? Will these all be taken into account?

 

If there is a method of determining wattage required and it is even remotely simple, I'm sure someone like Sanjay Joshi would have come up with it a long time ago.

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Depth of the tank itself is not taken into account in the W/sq inch. It is taken into account later when I said you could multiply the intensity by the percent loss with depth.

 

"Concentration" is just a term I used because I didn't have a better term. What I mean is that it takes into account the fact that certain systems concentrate more light into a given area. This would of course predict that metal halides give you more concentration than the same wattage of anything else, because the anything else's wattage would be spread over a greater area.

 

The formula assumes that the reflector is of good quality. Distance between the lamp and the water is taken into account, and here's why: if you take x-lamp, and have it y-inches away from the water, then your intensity will be z. However, if you move it out to 2y inches, the new intensity will be .25z. The new intensity is the old intensity divided by the radius squared. Elementary geometry.

 

One thing that needs to be changed is that lights really need to be in candelas, not watts. However, for the purpose of A QUICK ESTIMATE, watts actually seem to do okay. Ok, though, is not good enough.

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There are far too many variables for this to actually be practical. You would need to consider the surface area, the depth of the tank, the specs of the specific bulb (if you've seen Dr. Joshi's work, you'll know that there is a VAST difference between the various halide bulbs and ballasts and that's not even considering other types of lighting), the reflectivity of the reflector, the design properties of the reflector, restrike (based on the design of the reflector), bulb distance from the surface, salinity (to figure out the level of refraction), suspended particulate in the water and focus (ie, how much of the light hits the tank and how much spills out). And I'm sure I'm missing at least a few variables.

 

Watts/gallon is used mainly by/for noobs who don't have a solid understanding of aquarium lighting. It is a simple (although not always too useful) guideline for lighting an aquarium. It's used because it's simple. And figuring out all those variables in each and every tank is far from simple... And if/when you do come to a solution, how will you know how much light (in whatever unit you come up with) each species of coral needs?

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Like I've said multiple times, I'm not trying to come up with a totally accurate model. I'm trying to come up with a model that will quickly estimate the intensity of light at a given point. The W/sq. inch (or candela/sq inch), though far from perfect, is nonetheless somewhat accurate, especially if you are a newbie trying to plan a tank. It's certainly far, far better than watts per gallon, and it allows you to predict, to some degree, where, if anywhere, a particular coral can go, once someone (possibly myself, if I get the grant that I'm seeking) finds with experimentation what defines low light, high light, etc.

If you want to get exact, you need a lux meter, bottom line. The only problem with that, as I see it, is that you can only test a certain system once it's set up, which means there's not much you can do to change it (of course, you can still use a lux meter to determine what areas in a tank are appropriate for what corals). With all the headache involved in finding an appropriate light source, and generalization, however imperfect, that still models reality somewhat, will be of benefit.

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I've considered a similar idea in the past, but it was found not to be too practical.

 

WPG fails horribly when applied to smaller tanks. (I have a tank with 18.66 "wpg"). A better "standard" would be great, but I haven't seen one yet. How does your idea work for huge tanks? Medium tanks? Small tanks? Is it consistent enough for all? (I'm too lazy to work out your math ;))

 

Still probably wouldn't work out decently for the huge differences between NO fluoros, T5, PC, and MH.

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It seems to work out in such a way that it models reality. That is, it predicts that metal halides haved the highest penetration (and intensity), followed by, I believe T5 and PC (lots of little tubes in a small area), then VHOs (lots of wattage, but it's fairly spread out - not as good penetration), and finally NOs.

The beautiful thing is that it doesn't matter what size tank it is. To illustrate this, consider the following problem:

You have a 900000000 gallon tank (absurdly huge). Over this tank is a single 150 W metal halide (the corals in the rest of the tank are probably glad this is only a thought problem). Though the tank is certainly underlit using a wpg, in the column below that metal halide, we can predict that there is enough light for corals, and we can predict where in the water column the light dies off to an unnacceptable level.

Now let's say you have a much, much smaller tank (a 30 cube, for instance) with the same single metal halide. We predict(using the formula that this water column (assuming the lamps were at the same height) will have the same exact light intensity as the one in the huge tank. That is a more accurate representation of the way light works. Light doesn't care how big the tank is; it is concerned only with the size of the area it is illuminating.

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followed by, I believe T5 and PC (lots of little tubes in a small area), then VHOs (lots of wattage, but it's fairly spread out - not as good penetration)
Watt for watt, my VHOs blow my PCs away. From my understanding, VHOs have a higher output than T5 as well. The advantage with T5 is that they are small enough that you can jam many in a small space and because of the thin tube, with a good reflector you can direct a lot of the light around the tube, reducing restrike. And how is it more spread out? In many cases, it is more focused because some bulbs have an internal 180 degree reflector.

 

is, it predicts that metal halides haved the highest penetration (and intensity)
Yes they do, and the amount of PAR produced by a halide is extremely varied, depending on the bulb and ballast combination. A 6500k Iwasaki 250w SE running off a PFO HQI ballast puts out 950par, while a Sun 20k on the same ballast puts out less than 250 PAR. An XM 10k on an Icecap ballast puts out 530 PAR, while a Blueline 10k puts out 263. How are you going to take this into account? I don't see how you can say a tank that X" x Y" x Z" needs however many watts, when one bulb/ballast combination may put out 4x the useful light as another combination. You are trying to simplify something that cannot be simplified.
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Whoops, did the math wrong; VHOs do produce a higher output. I'll have to look at how I made that mistake. Probably punching too many buttons too quickly. That puts them ahead of most PCs, but I can't give a single value for T5s because, like you said, you can just cram so many of them into a given space.

This is an interesting article I just found. I can no longer claim my idea as original, but I now have backing (cut and paste; my links never work right):

 

http://www.c-sea.org/members/brad/articles...light/#lumanpar

 

So, instead of watts/sq. inch, you just have PAR (or lumens)per square inch. Fairly simple; you just have to find the PAR rating for your light/ballast combination. That shouldn't take too long given the wealth of information online.

Note, however, that I am NOT trying to come up with a way to say a tank of dimensions xyz will need either a or b light (that would be called the watt-per-gallon "rule"). What I'm giving people is a way to say, "ok, this light will give me x intensity at y depth, allowing me to keep z coral in this part of the tank." without a lux meter

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the wattage rule is completely useless. think of it this way. if you have a total of 180 watts of pc light compared to 150watts of mh, which is better? But wait, the pc have more watts... The best way to measure light is by par. Power compact ligting is good for shallow tanks, or begginers. VHO uses very high wattage to put out par, more for shallow to medium depth tanks, and they let you have more choices of what you want. T5 ho use less wattage but due to excelent indevidual reflectors they put off a tremendace amout of par. good for medium to deep tanks, depending on what you want to keep, they also are very enegy saving, and very cool running. MH is for and tank depth, it allows you to have any number things.

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neanderthalman

The reflector shouldn't influence how much of the light produced is PAR. PAR is determined directly the spectral output of the bulb. From what I've read, bulbs with a higher color temperature have a lower PAR, so a 20,000k MH isn't as good as a 10,000K MH, it just looks better. It also depends on what type of bulb it is, as different bulbs, at the same color temperature, will have slightly different spectra.

 

I think that the solution might very well be in a neat new idea I've had called watt's per gallon. Now, the thing is, it isn't a number, like before, where 5wpg was good for softies, and 10wpg was good for..... It's completely different.

 

Lets ignore for a moment the differences in quality of light (PAR) from different types of sources, and focus purely on the wattage of the lights. The amount of light that you need over a tank does depend on the size of the tank. Nobody here can argue that 200g aquarium needs less light than a 2g pico.

 

The difference with this, is that it is not a linear relationship between the watts you need and the size of your aquarium. You'd likely find that there's more of a difference in need between a 1g and a 10g, than there is a difference in need between a 100g and a 110g. No, they're not nanos, but that's not the point.

 

What you would need, as a "rule of thumb" is a fairly simple equation that starts out increasing quickly for small tanks, but levels out for larger tanks. Logarithmic functions are like that, as are equations of the form f(x) = (1-exp^x)

 

If I had some data as to what is considered enough light over a tank of different sizes, then an equation can be found that matches the data, hopefully fairly closely. If a few ppl could post their opinion on what is sufficient lighting over the following tank sizes, I'll see what equation fits best.

 

1g

2.5g

5g

10g

20g

30g

40g

50g

 

Basically, what it would do is take everybody's opinion, and turn it into an equation that could be used to calculate the light requirements of any particular size of tank. It isn't based on any physical property, only your experiences. No, it wouldn't be perfect, but it would be more accurate than the old wpg rule.

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Bulb brands and color temps make such a difference that you cannot overlook that and go upon wattage alone. Also what is being kept in the tank will really determine what lighting intensity will be needed. There really can be no easy "rule" that will apply here.

 

Again I direct you to Sanjay's site.

http://www.reeflightinginfo.arvixe.com/

He and a few others have done the research to prove what different bulbs, temps, ballasts, and yes reflectors can produce in terms of light intensity.

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neanderthalman

I checked out Sanjay's site, and while he's done some impressive work with MH, I was not able to find, at least in a few minutes of poking around, any information relating to light sources other than MH.

 

That aside, it is true that different bulb/ballast/reflector combinations will result in different light intensities. My argument, however, is that for newbies, that minor distinction isn't as important as knowing a ballpark figure to shoot for.

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