I wish some one would come up with a better formula than the watt per gallon

Great info on this thread! I would agree PAR is probably our best bet at the moment. And if you stick with fairly broad spectrum light, it is fairly good measurement. Sometimes tricked out or narrow spectrums can make things tougher to gauge. Most people don't have access to PAR meters, but you can still guesstimate what PAR reading will be at the surface using information from sources such as Sanjay's data:
Manhattan Reefs - Sanjay's Lighting Guide

Also, if you do research you can find people who have tested PAR in their tanks and overlay the readings on an image of their tank. As you look at these, you can build intuition regarding what the PAR will be from a given bulb/reflector/ballast combo at a given depth.

We don't have the specific spectrum, but we know the spectrum of the bulb and know that most of the red is filtered out in the first meter or so, and the shorter wavelengths such as blue will penetrate deeper.

While it takes some time to build intuition about lighting, most experienced reefers have done so already, so, you can always ask and a lot of 3reefers will be able to give you a very good answer.


Also, just to add one comment on leds. A huge benefit to leds and one reason they are seen as more efficient is that they are able to focus light better than MHs or t5s. So, they don't waste as much light out of the tank as they can be fit to the shape of the tank better. A MH reflector for example is best suited for a square or circular tank, but most tanks are rectangles, so they tend to waste a lot of light or need to be put low and use more of them, but then more of them means more energy and lower means more heat. The downfall to leds is they tend to produce somewhat different and often narrower, or more peaked spectral outputs and therefore it may be tougher to gauge how much lighting is needed. Also, there is a lot of confusion about optics and spread and less data available on individual configurations.

Also, khowst, an led should last around 100000 hours, which at 10 hours a day, would be about 27 years. However, that depends on a number of factors such as electronics, how hard they are being driven and cooling. So, 5-10 years probably is a safer guesstimate.

 

Be careful the lower watt leds as I understand may be ok for a shallow tank, but they may not have the penetrating power for a deeper tank, even if using more of them.

 

Thats not how LED's work. more LED's are a lower watt isnt going to equal out, especially when you start talking a depth.

 

So it wouldn't penetrate to the full 18 inches? That given that the height of the sandbed at the lowest point.

 

You can look at it like this: Different wavelengths of light have different energies. The energy is absorbed by the water molecules as the photons pass through and beside it. Now the wavelengths like red, orange, yellow do not travel very far at all through the water molecules because their energy is sucked up real fast. Blue light however has the least amount of resistance as it passes through the water molecules and that is why it penetrates further than any other color.

The reason that MH is better than T5 is because the photons are emitted in a more linear pattern and there have more potential to carry energy farther in the water (penetration). LEDs are even more linear yet so they have the potential for father penetration. Now the downside of LEDs is that the best spectrums cannot be met perfectly. A part of this is due to the lack of UV light and and increase in red/orange wavelength.

I actually was discussing this the other day with Sanjay over lunch and he has been telling me the same thing as always, that he doesnt forsee LEDs being where halides are for a few years yet. He did however mention a new led fixture that will be coming out within the next year that has some potential to meet most of the spectral needs we desire.
I would think seriously before going with LEDs if you are concerned about growth... Especially with sps.

 

It is a function of intensity.
Par is measured by the amount of photons passing a specific point per second. With higher wattage more photons are released so more photons pass the point in a given second.

 

So no then?

 

more watts means more light is put out. more light put out is more intensity.
the more linear the photons are then the farther they travel in water... so if you have high intensity (from more watts) then you will have a higher number of photons at farther depths.

thus you can have 250 watts of t5 and 250 watts of MH. The MH will be putting out a more realistic wavelength pattern. The for arguments sake we will say that the intensity will be very close between the two types of bulbs but the halide photons will travel farther and be more efficient than the t5 photons

 

Yes I get that part. But what I was asking is 30 one watt leds will be fine for the job but an equal wattage amount of smaller wattage leds will not make it to the bottom? I'm not trying to grow sps. The light only has to penetrate 18 inches of water max.

 

No, don't use anything less than 1W. 1W is really the minimum for a shallow tank. They are pretty weak though, your really better off with 3W CREE leds. These are the most widely used and their properties are the most widely understood of any of the leds on the market. This is a very important point when trying to figure out how many and so forth.

Output per watt is not consistent even among leds, so, you need to be able to reliably determine how many to use. Also, the spacing is important, while you cna buy more small leds, you cant neccessarily fit enough of them in a given area to provide the correct coverage. To answer your question though, I don't think anything less than 1W would be capable of penetrating 18". 3W will with not problem. For a shallow tank, you can use less by using wider optics. You could probably get away with 80deg, but I would probably use 60deg for an 18" tank.

If you know how to solder, they are very easy to build:
Results for DIY Retrofit Kits