How do you find out how much light you have per inch

[RedDragon]

I wanted to know the way you find out how much light per square inch, I have been :confused: and was wondering, I have 1 400w MH and 3 65w Pc's so how much light do I have in a 120g tank

[Jimnorris]

Jaime,
Do you mean watts per gallon? Your have 4.95 watts per gallon. Do you mean watts per square inch???? You have 55.85 watts per cubic inch?
Jim

[RedDragon]

thanks Jim, but now how did you find that out?

[z28pwr]

Jaime do the math

400 Watts
+ (65X3) 195
= 595 Watts Of Light / 120 (Tank Size) = 4.95 Watts/Gallon

[RedDragon]

oh did not know it was done so easy, I thought it was some long problem :hammer:

[newtosa]

Jaime,

Your intuition was right. What you really need to know is light intensity (not necessarily watts) per square inch of surface.

Watts per gal is an ok way to measure, but not great. Consider a 20L tank: (30"L x 12"W x 12"deep) vs. a 20H tank (24"L x 12"W x 16"deep).
If both have a single 175w MH, that's 8.75w/gal. Great, right?
The problem is that the 175w of light is getting progressively less intense as you go each inch lower into the water. So the w/gal metric does account for tank depth (which is good), but not very well for surface area (which is really the more important factor).

Say a 175w MH bulb puts out 10,000 lumens (this is the same as lux). That means the lumens per square inch for each tank are:
20L 10,000 / (30*12) = 27.78
20H 10,000 / (24*12) = 34.72

Real sunlight lumens per square foot is about 10,000. That's about 70 lumens per sq. in.

So the 20L ends up being only 40% of "real" sunlight values, while the 20H is 50% of "real" sunlight values.

That difference is small, but keep in mind that the surface area of the different tanks changes much more rapidly in reef-size tanks (65 vs 58, 70 vs. 75, 90 vs. 100, etc.). So this difference is much more pronounced as your tank configuration changes (even though the gallon capacities are similar).

One more piece of theory that (IMO) everyone should read once per year:
http://community-2.webtv.net/deflizard/doc/page8.html

Now, on Jaime's question:
120 tank probably 48x24x24 (?)
Surface area is 48*24 = 1152 sq. in.
Lumens depends on manufacturer; let's assume 22,000 lumens for the 400w MH, and also assume 3,000 lumens per 65w CF. Total = 31,000.
Light intensity = 31,000 lumens / 1152 sq. in. = 26.91 lux/sq.in.
26.91 / 70 = 38% of sunlight.

IMO, we should strive for 75% - 85%, so my opinion would be no, not enough light for SPS, clams, or Pacific host anemones. This would put your lumens target at around 60,000.

These are my opinions, and no one should think I'm saying this is the only way. Keep in mind that using a surface-area-based calculation, you still need to account for tank depth when you place the animals. Anyway....

Dean

[newtosa]

Baby's great; she's getting fat and cute, just like she's supposed to. It's so much fun.

All this w/gal and lux/sq.in. discussion has no bearing on a coral placed 5" directly under the same bulb. (What I mean is the metric you use does not matter; the coral is going to receive the same light in either situation.) The reason I like lumens/lux per sq. in. is that you at least know whether you're close to "sunlight" values, where the w/gal metric has no way of telling you that.

Dean

[matt]

Dean;

You posted some interesting data, but if I'm not mistaken, lumens are not a great measure of the type of light needed by corals for photosynthesis. For that, you need either PAR (photosynthetic available radiation, I think) or PPFD, (photosynthetic photon flux density, or something) These measure light in the specific wavelengths that supply corals with energy for photosynthesis. There's a good, brief discussion of this in the Borneman coral book.

Also, your comparison of the 20 long vs 20 high is really interesting, because although it appears as if the 20 high is getting a better percentage of light with regards to sunlights, this is on the surface of the tank. Since light penetration into sea water decreases geometrically, even a few more inches of tank depth can result in less intensity for the animals. So maybe the 20 long would be a better choice for animals that need alot of light, like clams. But you're right on by saying that watts/gallon is basically irrelevent for raising corals. A coral in an aquarium receives the same light from the same bulb in the same place no matter what size aquarium it's in.

I've read several times that light intensity at the surface on a reef is WAY beyond our ability to realistically produce in an aquarium, and so, the "more light, the better" is the lesson. But, most reef animals live at a depth which far exceeds our aquariums; what I'd like to see is some data of light intensity at typcial reef depths, like 3 to 10 meters.

Of course, there are lots of people who can get carried away by discussing light. I've done a bit of reading, but I'm really not that interested in the details. Basically, I would try to compare the PAR or PPFD performance of various bulbs, choose something that's adequate, and shoot for a color that you enjoy looking at. For example, Iwasakis outperform higher temp bulbs by ALOT in terms of PAR. But I use ushios because I like the way they look, even though I lose some intensity.

There's a guy, Sanjay Joshi, who's written many articles and done lots of tests on mh lighting. He's teaching a MACO course on lighting this summer. It sounds interesting.

Matt

[newtosa]

Matt,

I'd like to take a course from Sanjay. As you point out, he's where a lot of this knowledge surfaced originally.

Yes, lumens as a measure are skewed toward "brightness" perceptible to the human eye. And you're right; PAR is the right measure to use. As soon as I can get published PAR values across all the MH bulb options (we may already be there; it's been a couple years since I checked), I'll switch to that measure of evaluating lighting schemes.

The problem I still have in my mind is adjusting intensity at the surface over reefs (it doesn't matter whether you're measuring lumens or PAR) to intensity from a MH bulb envelope, and tracking the decrease in light as you move down through the water (something like an intensity reduction per foot in the ocean vs. per inch in our tanks). The formula is somewhere in one of these articles, but the main idea is that light intensity decreases with the square of the distance, according to the original intensity. You can see immediately that a couple extra inches' depth is really going to matter in our tanks, where a couple feet's depth doesn't make much difference in the ocean. Anyone thought about that enough to teach us all something on this subject?

Dean