Physics of light and the reef tank

[PMc]

Hello,

I have posted lighting questions before and it may be somewhat presumptuous of me to post on the advanced topics boards but these questions fall outside of the normal "what light?" type. I am planning a new tank and considering LED lighting. Solid information is hard to come by. Extrapolation may be the only tool short of experimentation.

As I recall, radiation falls off by the cube of the increase in distance. Is this true for aquaria? Can I look at PAR readings at 12 or 24 inches and extrapolate to 36 inches?

Is there any source, that someone can point me to, that approximate light requirements for various species of soft corals and mushrooms?

People on the boards report that LED light appears dimmer to them than T5 or MH, yet they readily fry coral. Is this a particular property of the wavelengths produced? Or perhaps the lenses used to reduce scatter? (Is it scatter and reflection that our eyes are perceiving?). Would the creatures involved react differently to a given output of light at 24" vs double the output at 36".

Finally, I read alot about people preferring light at higher Kelvin numbers - 16K vs 10K. Is there anyway to correlate kelvin number to water depth in a "typical" reef environment? I would assume that shallower water would relate to a lower kelvin number but I may not understand the relative penetration of wavelength through water as it relates to K numbers?

I realize this is a little out of the way but available information is thin and my understanding a little sketchy. When trying to recreate an particular environment, it seems like these would be key questions. Instead, it appears we let the manufacturers do the research for us and then we swallow what they tell us, spin and all. I am trying to avoid the "pay and pray" technique. Thanks for your time.

Mc

 

[GroktheCube]

Generally, blue light penetrates water much better than red light, which is part of why people favor higher color temperatures. They are closer to what light would naturally be a dozen meters or so below the water's surface.

PAR doesn't quite work out like that, because you have factors like turbidity, refraction, and reflection from the glass and substrate that come into play. The angle of the reflectors also come into play. The same amount of light focused in a 30* beam vs a 70* beam makes a big difference in terms of PAR at a given depth for a given intensity. My understanding is that it's rather difficult to estimate a PAR measurement lower in the tank based on one higher in the tank, because there are too many variables. Beam angle, reflection, color of light, and turbidity among other factors come into play.

Truly recreating the lighting conditions over a tropical reef is a borderline impossible task in a home aquarium. Greenhouses get much closer than any kind of artificial lighting. That said, quality artificial lighting exceeds the bare minimum by a good margin.

 

[Ron Reefman]

You asked: As I recall, radiation falls off by the cube of the increase in distance. Is this true for aquaria? Can I look at PAR readings at 12 or 24 inches and extrapolate to 36 inches?
It's the inverse square law:
The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only one-quarter the energy (in the same time period).
More generally, the irradiance, i.e., the intensity (or power per unit area in the direction of propagation), of a spherical wavefront varies inversely with the square of the distance from the source (assuming there are no losses caused by absorption or scattering).

But many led fixtures use lenses, as you noted, that changes things:
Consider a point light source emitting light uniformly in all directions. The light energy through the sphere of radius r is therefore uniform, and the portion of energy through a circular area of radius a is then . For a lens of radius a at a distance d from a light source the energy flowing through it is. The brightness of the image depends on the size of the lens and the diameter of the aperture.

You asked: Can I look at PAR readings at 12 or 24 inches and extrapolate to 36 inches?
As grok stated above, you can't take PAR values at 12" and 24" and extrapolate it out to 36" or 48". There are too many variables. The inverse square law is in a perfect world (if you consider a near vacuum a perfect world). With 6-12" of air and then whatever depth of water you use, the calculations become nearly impossible without a lot of data on the water conditions. In general, PAR goes down dramatically as water depth increases. The easy solution is to get a PAR meter (about $300 new).

You asked: Is there any source, that someone can point me to, that approximate light requirements for various species of soft corals and mushrooms?
I don't think you are going to find much about light requirements of various corals other than super general terms. Corals all have varying tolerances and every species is different and what terms do you want the light requirement in (PAR values?). In general, zoas and palys need less, softies a bit more, lps more yet and sps the most. There is overlap and there are certainly exceptions, and we are only talking about corals that need light. If I'm not mistaken, about 20% of the world's corals don't use light to grow symbiotic algae.

You asked: People on the boards report that LED light appears dimmer to them than T5 or MH, yet they readily fry coral. Is this a particular property of the wavelengths produced? Or perhaps the lenses used to reduce scatter?
I don't know anybody who would say leds are 'dimmer' than t5. And your eye is a terrible 'meter' to gauge light intensity. Get a PAR meter and you will be much better served. I can look directly at t5 bulbs without issue. Both MH and leds are extremely difficult to look at directly. Led fixtures (especially newer ones) can bleach corals. They are intense lights and they are focused.
The wavelength of any particular color is the same, i.e. 440nm blue is the same whether it is produced by t5, MH, leds or the sun. Leds with lenses do focus the light down and that makes them more intense and reduces light radiated out to the side. MH and t5 very often use reflectors to collect and 'focus' light better as well.

You asked: Is there anyway to correlate kelvin number to water depth in a "typical" reef environment?
Color temperature is different than color or PAR. I won't try and explain it here, because my understanding is limited. But certain colors do penetrate water better than others and blues are the best at it. There is a good chart in the article linked below. In terms of color temperatures, 20K light tends to look more blue than 10K light. But Kelvin isn't a measurement of color, wavelength in nano-meters is.

Finally, consider reading the article at this site for more detail and understanding:
http://www.advancedaquarist.com/2012/10/aafeature

 

[PMc]

Thanks to both of you for the replies, and the link. Nothing like a little project to show you how little you really know. Physics was a long time ago.

Mc

 

[PMc]

I just finished the advanced aquarist article. Thanks for the heads up. It answers a lot of questions (and brings up lots of others). I never would have found it on my own.

Mc

 

[Ron Reefman]

Mc,

I was in the same boat as you about 5-6 months ago. I was solidly a MH user. I felt leds had just about hit the right level of development, but they were so hugely overpriced. I got a used Eco Tech Radion and used it over my frag tank and it worked great, but I'd need 4 or 5 for my 2 DT's and $3000 to $3750 is WAY too much. So I started reading and studying led light and coral needs. I spent hours talking to reps and others at MACNA in Dallas. Then I heard about EverGrow and their IT series (the Chinese led thread in the Equipment Forum here on RC).

I now have 2 IT2080's over my 180g and 1 IT2040 over my 70g anemone tank. All 3 together cost less than $1000 and they have a built in controller/timer rather than having to hook up a laptop to the Eco Tech Radion (what a PITA). I now have EverGrow D120 basic led fixtures over my 55g QT and 25g frag tank and I sold the Radion for enough to pay for all 3 D120's. My new led lights use 40% as much power as my old MH & t5 fixtures and my 2 chillers hardly run at all (in SW Florida). Oh, and not only do the tanks all look great, but the corals are growing like weeds. But in all honesty, I think setting up dosing pumps for my calcium and soda ash have made my chemistry a LOT more stable than when I tried to keep up with it manually. That's probably 50% or more of the improvement. Good luck.

 

[BeanAnimal]

As I recall, radiation falls off by the cube of the increase in distance. Is this true for aquaria? Can I look at PAR readings at 12 or 24 inches and extrapolate to 36 inches?

The inverse square rule (law) is ONLY for point sources (bare bulbs, leds, the sun, whatever). Add optics, a lense or collimaters and things change.

People on the boards report that LED light appears dimmer to them than T5 or MH, yet they readily fry coral. Is this a particular property of the wavelengths produced?

Yes.. .our eyes are more sensative to different wavelengths than the corals. What may be "dim" to us may be high energy to a coral.

Finally, I read alot about people preferring light at higher Kelvin numbers - 16K vs 10K. Is there anyway to correlate kelvin number to water depth in a "typical" reef environment? I would assume that shallower water would relate to a lower kelvin number but I may not understand the relative penetration of wavelength through water as it relates to K numbers?

A Kelvin number is a way to describe the color of a "light source" compared to the ideal black body radiation source of that same temperature. The Kelvin mumber really means nothing without also considering the CRI (color rendering index) and the CCT (color corrected temperature) of the source. In other words the Kelvin ratng of a light source really says nothing about the wavelengths that make it up and the CRI and CCT only point to how closely it resemebles the ideal black body temperatuer for a given set of predefined samples of color it illuminates... In other words, there are an infinite combination of wavelengths that can produce the same "K" light "source". The CRI and CCT indicate how closely a given combination compares to the ideal source when illuminating different surfaces. Buy 20 different brands of "10K" lamps and they will all look differently on one surface, but MAYBE look similar on another. Confusing? Sure, but the bottom line is that you can not convert the Kelvin rating to anything useable for our purposes.

 

[PMc]

I plowed through the discussion on K rating, CRI, and CCT in the advanced aquarist article. Even though it may not have much meaning for our purpose, consumers and manufacturers seem to use these K rating as some sort of touchstone. It is rare to find anyone who says they prefer the 10K rendition to the 16 K rendition in fixtures from the same manufacturer. Is it just a matter of aesthetics? Or is this the only reference we have so that is what we use?

Thanks, Bean, for the comments. I have pored over your thread here and some of the stuff on your website. And thanks to Ron as well. If you lurk long enough you begin to correlate certain names with well reasoned advice. It helps when you get into an area where you have no idea even what to ask.

Mc

 

[ReeferBatman]

I always learn something from these threads!

Anyway, to add to the topic...

Here are some graphs/charts that might help...

Par over reef at surface

And how different wavelengths are 'filtered' out @ depths [graphed lines represent certain depths]

And another chart showing that same information differently, with the red line (~100 ft down) being the rough boundry between PHOTOSYNTHETIC and NON-PHOTOSYNTHETIC corals below. We find that generally all photosynthetic corals are typically exposed from wavelengths from 300 nm - just over 570ish... and some corals [higher] on the reef will actually be exposed to light up to and over 600 nm, and will use chloryphyll [600-670 nm].

Some things to include... Chloryphyll is the best 'energy producer' for the coral, so if they can physically use it [exposed to enough light], they will prefer it (why most every terrestrial plant uses chlorophyll). I have a beautiful prism favia that supposedly utilizes chlorophyll in addition to other pigments.

However, that Red light fades fast, and so most corals come to rely upon other photosynthetic needs (think fluorescence) with pigments that rely upon lower nm spectrum of light.


What we then get, is a splattering of different photosynthetic pigments, each with their own particular lighting desires (the same 'strain' even having slightly different activation peaks in different corals/anemones!).

For example: P-486 in different corals...

Personally, I just try to cover as much of the spectrum as possible [with a declining emphasis on Blue-red light to imitate how light is filtered @ depths] in hopes that all the different photosynthetic pigments get what they need.


Some things to consider...

The corals 'eat' the light. However, too much light is bad. The extra light they don't need is de/reflected back. This can have the [human] desired effect of "extra glowiness". But we CAN give too much light, overwhelming the photosynthetic safeguards (de/reflecting), resulting in biological damage to organism [bleaching, death, etc.].

As in, the coral might be getting enough light already, but we add more to make it look 'extra pretty', which the coral can tolerate... but there is a point where the 'prettyness' peaks, and coral decline follows.


In General - As some said softies and zoas need less light, LPS more and SPS most.

I plowed through the discussion on K rating, CRI, and CCT in the advanced aquarist article. Even though it may not have much meaning for our purpose, consumers and manufacturers seem to use these K rating as some sort of touchstone. It is rare to find anyone who says they prefer the 10K rendition to the 16 K rendition in fixtures from the same manufacturer. Is it just a matter of aesthetics? Or is this the only reference we have so that is what we use?

As others mentioned, a true technical Kelvin rating only indicates the aesthetics of light, not the actual quality or nm spectrum within.

And in general, people prefer 'bluer' tanks [higher Kelvin].

And yes, each '10k' light from each brand will vary in actual spectrum within... there is no 'standard' because this Kelvin term is unrelated to other measurements of the quality of light.

But companies use Kelvin as a 'guideline' which which to tell the consumer the basic output color of the lamp.

Obviously, for aquatic purposes, we can imagine that chart going bluer and darker the higher up we get (with common bulbs around 14k and 20k for marine aquarists)

Don't forget quality of light [nm spectrum offered] doesn't correlate with Kelvin readings.

Let me explain it this way...

A tank with
1 blue T5

might look the same color 'Kelvin' [although less intense] as a tank with

4 blue T5
1 white T5

But despite a similar Kelvin appearance to us ["Both tanks appear the same color!" ] , the 2nd tank will actually have more varied [nm] 'light' for the corals to 'eat'...

People on the boards report that LED light appears dimmer to them than T5 or MH, yet they readily fry coral. Is this a particular property of the wavelengths produced? Or perhaps the lenses used to reduce scatter?

It is hard to speak in generalities because one LED is worlds different than the next... but IMO, LED bleaching is because LED's offer a very INTENSE amount of light, but not diverse enough in scope [breadth of nm offered]... either overwhelming the coral in intensity, or starving the coral of varied nm light (both can result in bleaching/coral decline).


Because of the possible dangers, dimable (sp?) LED's is the only way to go IMO...

And make sure you do your research to offer varied enough NM in the spectrum (LEDs often have many 'peaks and valleys'... make sure you overlap enough that there are no deep 'troughs' in your lighting system.


When in doubt, you can 'overlay' your light Par graphs (of lights) to give you an idea of your total light output using free internet software from a google search...

Here's what my personal lighting system offers (from my T5/LED setup including 'Blue Plus, 10k, Actinic, Coral Plus' T5 bulbs and 450nm Blue LED strips)

As you can see, i tried to offer a large variety of NM, with more intensity offered in the lower nm Bluer parts of the spectrum.



PS.
I keep saying intensity like that because is a QUANTITATIVE measure of light separate from Kelvin or Nm

 

[ReeferBatman]

double post

 

[PMc]

Thanks Batman. Lots to cogitate on.

Mc