Lighting specification for corals

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Hello, I`m new to this forum and have only been keeping a reef tank for 6 months. Can anyone help me understand the spectrum displayed by light manufacturers? I may have missed something, but I can`t find out whether the spectrum graphs shown by various light maufacturers are for air or water. I understand that corals need more of the blue end of the spectrum, and my lights peak at 460nm. But....if this is the peak wavelength in air, then when the light hits the water, it will be entering a medium with a different refractive index to air, and the wavelength will shorten. So should light designers be making lights with a slightly longer peak wavelength to compensate for this? Or...do these spectrum graphs already display the wavelengths as measured in water?

ajm83

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Reply #1

Hmm. That's a good question,  gut feel is that they are quoting the output of the emitters in air as I reckon they're just quoting based off the specs of the LEDs (which will have been measured in air).

I don't know for sure though.  Might be best to email the manufacturers directly? Put the reply up if you do! 👍


spiderreef

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Reply #2

Hi, thanks for the reply. I know it's not of vital importance because we have a vast array of lighting to choose from in our hobby, and we can get perfectly good results with them.

My thinking is that chlorophyll a has a peak response at something like 430nm, but we are providing our tanks with light spectrum peaking in the blue portion of visible light - by the time it hits the corals in the water, some of this spectrum will have shifted into the ultra violet portion of light that is not strongly absorbed by water. There has been a lot of research into the affects of ultra violet on xoozanthellae, and it may be beneficial at low intensity.....but I was considering a trial at home making my own led set up which shifts the peak output to somewhere around 580-600nm. Bearing in mind the refractive index of our tank water could be roughly taken to be 1.33, our corals would actually be receiving a spectrum of light peaking somewhere around 450nm.

The reason I asked the question is that I have probably missed something really obvious about why our lights are designed as they are, but it seems like they are optimised for plants grown out of water. Is it just that Cree leds with a certain output are commonly used, and this is what's available to manufacturers, and this is good enough?

If I ever get round to contacting any light manufacturers I will post any responses if anyone is interested.

ajm83

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Reply #3

Thinking more about this,  although the wavelength will change, because the speed of light in the medium is slower, the frequency (therefore colour) will still be the same.

spiderreef

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Reply #4

Ah....you might have just answered the question then! The reason I was focused on wavelength was because this is what is mentioned in all reef forums, research papers and lighting spec.

I've never seen anyone say "my tank lights are 400Thz"

But I get the feeling you might be right because I can't think of anything else!

spiderreef

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Reply #5

This now leads me on to thinking why aren't the light spectrum graphs for lights plotted against frequency? 

My original question would also have been  valid for people with glass or perspex lids affecting the wavelength of light....but I think you have answered it so thank you.

ajm83

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Reply #6

This now leads me on to thinking why aren't the light spectrum graphs for lights plotted against frequency? 

My original question would also have been  valid for people with glass or perspex lids affecting the wavelength of light....but I think you have answered it so thank you.

I'm not sure about that; coloured LEDs are usually labelled in terms of the wavelength.  I guess because 395nm is a bit easier to remember than 758968248hz.  lol Frequency would make more sense though, you're right. You could call it 759Ghz (is that right?)

edit: no it's not right, i've done the conversion wrong somewhere, hope you see what I mean anyway !
Last Edit: Mar 8, 2019 3:26:40 pm by ajm83

spiderreef

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Reply #7

Ok, so I'm alright with physics but my biology is really weak. I've been doing some reading and this topic of lighting has led me to read about photosystem 2, which is the starting point for photosynthesis. It requires 1.8 electron volts of energy from light to oxidize water and start the process. This means it is not the wavelength specifically that counts in any medium, it is the frequency of light (therefore the photon energy).  1.8 ev is 680nm, from the equation e = hf.  This is red light in the visible spectrum. All the blue light that we pump into our tanks is excess energy. Please see      Light Reactions or many other sites on photosynthesis/photosystem 1 and 2/biology. I know blue light makes our corals look nice, but energy in wavelengths shorter 680nm is lost as heat as part of the energy transfer process to activate chlorophyll a. In summary, it feels to me like we should be aiming to provide a slightly fuller white spectrum to our tanks, rather than just aiming for the bluest of blue all the time. Red light is absorbed by water in the first few metres, but I doubt many people have tanks metres deep. So in our homes and in very shallow corals, light from the red end of the spectrum will play a part. There is a Japanese company that is producing true full spectrum lights that cover the whole visible spectrum, and I would be interested to hear if anyone knows much about the quality and results from using them. The company is kyocera. I'm sorry for the ramble, but if there are any biologists here who could point me towards some further reading on how photosynthesis works for a non biologist, that would be great.


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