Overthinking coral location???

truth954

New member
At the FMAS meeting last night which was nice presentation (thank you Tony Vargas). The question was brought up about the depth of the water for some of the photographs. Tony mentioned the depth which i forgot.
Soooo... I was thinking about the depth of corals in the aquariums and why is it important that the coral be either at the top or middle or bottom. When in the ocean there is low tide and the sun is more intense and the other way around high tide is opposite. So why would it make a diff. in the location of some corals when we cannot adjust the intensity of light in the aquarium other than left to right and the timers being utilized. On the other hand the moon has a lunar cycle with a full moon and new moon and the some lighting companys have the led lights programmable to be on the same cycle. The moon lights have less affect other than to help with some spawning. So why not try to get a lighting system that will have the same affect as the intensity of the sun....
Or maybe me overthinking?

whew i almost lost myself with that question.

thanks
Gerald:confused:
 

tperk9784

New member
I think the reason that people talk so much about coral placement is that even the best aquarium lighting cannot even come close to natural sunlight.

thats why people have given the corals the low light, medium light, and high light labels. This is more of a reference to the aquarium not naturally occuring light.

I once read an article on a scientific study of sunlight on natural reefs and the author compared sunlight in something like 20 feet of water with being 1 inch from a 400W MH bulb. No way we can compete with that.

Tim
 

jeffbrig

Premium Member
I have played around with a lux meter and my lighting, and although lux isn't a terribly useful measure for artificial light sources (PAR or PPFD is much better), it gives us an idea of relative light intensities.

My MHs at the time were 250w Ushio 10k in PFO pendants. To see how they compared to the sun, I first measured the outdoor sun. Back in January, this yielded ~110 near midday. Then, in my canopy, I measured the light output from the pendant. At the absolute hottest point under the center of the pendant, the intensity peaked at 170. Yes, actually higher than what I measured outdoors. Can I trust that the meter is comparing apples to apples? No, since I don't know how closely my lights match the spectral output of the sun. But, for purposes of measuring lux, I'm reasonably confident that the MHs intensity rivals the sun right up against the glass.

Does this mean the tank is as bright as a real reef. No, not even close. The problem is that light from the sun comes from millions of miles away. Even though the sun is a point source of light (from our perspective), after traveling that distance, light rays are essentially parallel. Light from my MHs are not. Sure, it's a point source, but the rays are diverging. Move a few inches away from the pendant, and the intensity drops significantly. Move a few inches further from the sun, and the intensity is unchanged.

So, how much does it drop? With my lights just 6" or so above the water, the lux meter drops to 50 at the center of the pendant, right at the water level. So, that's less than half the intensity of the sun already, plus we still don't know if my lighting spectrum is as efficient as the sun (hint: it probably isn't).

The next question is how much light penetrates the water, vs. how much is reflected away. From a physics perspective, this depends on angle of incidence of the light rays. Light rays perpendicular to the water's surface will penetrate the best. The sun has a distinct advantage because when it's directly overhead, every ray is straight down and achieves maximum penetration through the water. With my MHs directly overhead, I still have a lot of light spread and scatter.

Even after the light enters the water, my MHs continue to lose ground on the sun. Since the sun's rays are parallel, they don't spread any further, they just go straight. Sure, there's some attentuation from the water itself, which affects both. But the light from the MHs are still diverging, and losing intensity in the process.

Another concern is water clarity. Obviously, clearer water transmits light better. For most tanks, organics and particulates in the water column further attenuate light when compared to a natural reef.

Put all of these factors together, and it's easy to see why the sun can sustain corals at depths far beyond what we can do in an aquarium. Because artificial light sources fall off so much faster, we're forced to think about how shallow we have to put something to provide it with the amount of light it requires. But, in our tanks, placing something high in the tank might approximate 20' of water on the reef, while the middle is closer to 40-50', and the bottom could be even deeper. The intensity drops off much faster than it would with natural sunlight.

That's my 2¢ on the subject,

Jeff
 

gasman059

The OG mad scientist
<a href=showthread.php?s=&postid=9810628#post9810628 target=_blank>Originally posted</a> by jeffbrig
I have played around with a lux meter and my lighting, and although lux isn't a terribly useful measure for artificial light sources (PAR or PPFD is much better), it gives us an idea of relative light intensities.

My MHs at the time were 250w Ushio 10k in PFO pendants. To see how they compared to the sun, I first measured the outdoor sun. Back in January, this yielded ~110 near midday. Then, in my canopy, I measured the light output from the pendant. At the absolute hottest point under the center of the pendant, the intensity peaked at 170. Yes, actually higher than what I measured outdoors. Can I trust that the meter is comparing apples to apples? No, since I don't know how closely my lights match the spectral output of the sun. But, for purposes of measuring lux, I'm reasonably confident that the MHs intensity rivals the sun right up against the glass.

Does this mean the tank is as bright as a real reef. No, not even close. The problem is that light from the sun comes from millions of miles away. Even though the sun is a point source of light (from our perspective), after traveling that distance, light rays are essentially parallel. Light from my MHs are not. Sure, it's a point source, but the rays are diverging. Move a few inches away from the pendant, and the intensity drops significantly. Move a few inches further from the sun, and the intensity is unchanged.

So, how much does it drop? With my lights just 6" or so above the water, the lux meter drops to 50 at the center of the pendant, right at the water level. So, that's less than half the intensity of the sun already, plus we still don't know if my lighting spectrum is as efficient as the sun (hint: it probably isn't).

The next question is how much light penetrates the water, vs. how much is reflected away. From a physics perspective, this depends on angle of incidence of the light rays. Light rays perpendicular to the water's surface will penetrate the best. The sun has a distinct advantage because when it's directly overhead, every ray is straight down and achieves maximum penetration through the water. With my MHs directly overhead, I still have a lot of light spread and scatter.

Even after the light enters the water, my MHs continue to lose ground on the sun. Since the sun's rays are parallel, they don't spread any further, they just go straight. Sure, there's some attentuation from the water itself, which affects both. But the light from the MHs are still diverging, and losing intensity in the process.

Another concern is water clarity. Obviously, clearer water transmits light better. For most tanks, organics and particulates in the water column further attenuate light when compared to a natural reef.

Put all of these factors together, and it's easy to see why the sun can sustain corals at depths far beyond what we can do in an aquarium. Because artificial light sources fall off so much faster, we're forced to think about how shallow we have to put something to provide it with the amount of light it requires. But, in our tanks, placing something high in the tank might approximate 20' of water on the reef, while the middle is closer to 40-50', and the bottom could be even deeper. The intensity drops off much faster than it would with natural sunlight.

That's my 2¢ on the subject,

Jeff
WOW that's a statement and a half. In my cawse i just drop them sukers in and let them figure it ou LOL
that's why i have 3x400 b/c I'm not as smart as Jeff LOL
 

jeffbrig

Premium Member
<a href=showthread.php?s=&postid=9810917#post9810917 target=_blank>Originally posted</a> by gasman059
b/c I'm not as smart as Jeff LOL

Oh, I'm really not that smart. . .

but I did stay at a Holiday Inn Express last night. :D
 
Last edited:

tperk9784

New member
<a href=showthread.php?s=&postid=9810962#post9810962 target=_blank>Originally posted</a> by jeffbrig
Oh, I'm not really that smart, but I did stay at a Holiday Inn Express last night. :D

LMAO
 

truth954

New member
<a href=showthread.php?s=&postid=9810628#post9810628 target=_blank>Originally posted</a> by jeffbrig
\ Move a few inches away from the pendant, and the intensity drops significantly. Move a few inches further from the sun, and the intensity is unchanged.

Jeff thanks for the explanation, the above quote is interesting

Thanks
Gerald
 
Top