Surface water movement and lighting intensisty

[Pete_the_Puma]

Hello,

I originally posted this in the SPS forums but I guess it would be more appropriate here.

I was thinking about this a lot and I think there is another parameter that is NEVER discussed when people talk about lighting their tank and that is how choppy the water surface is. Unfortunately that is harder to measure than "how many inches off the surface" or "what percentage do you run the LEDs at".

Here is the original post:

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Hello,

I have this theoretical question/idea and was wondering if anyone had any thoughts about this.

Tanks with a strong water movement on the surface have much more light dispersion versus tanks with a relatively smooth Air/water interface.

So the tanks with wavelets/waves/choppiness on the surface have more light dispersion, which I would guess is a good thing in a way as it softens the light penetration causing more dispersion. I guess that could explain why for example some people I talk to say they run their LEDs at 100% with no problems and great color/growth whereas other people say you would be crazy to run them at 100% because they "burned" their corals at 40%...

I was wondering is this was something anyone else frequently thought about, for what it's worth I have 2 Gyres XF150's at opposite ends of the tank on pulse mode with different frequency wavelengths and the surface of the water is almost never flat, I am ramping up my LEDS slowly to 100%.


Just trying to start a discussion about this as it seems to not be a factor anyone discusses when talking about lighting for their SPS tanks.


Pete


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And after some research my own first reply

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Well i found this good article about this:

"Underwater Lighting conditions"
by Andrew Trevor-Jones


but unfortunately this is all he says about this:

"Effects of the Air-Water Interface

When light hits the water surface, some light penetrates, but some light is reflected. The amount of light that is reflected is greater when the angle between the light rays and the water surface is small. When light is coming from a point source, such as the Sun, the percentage of reflectance can be determined using Fresnel's law (Weinberg, 1976). Figure 10 shows the theoretical reflection of sunlight based on the elevation of the Sun. However, Fresnel's law assumes an optically flat water surface which would be rare for the ocean surface, and so actual reflectance may be greater. Figure 10 also includes measured values of reflectance for both smooth and rough water surfaces."


Anyone have any thoughts about this? Is this something other people think about when discussing/setting up lights?



Pete

 

[Member No. 1]

I would guess that over our tanks it's negligible.
But if you wanted to be sure, take some par readings at different water depths throughout the tank first will all pumps off, then under normal pump operation.

 

[Ron Reefman]

I have a PAR meter and a 180g sps/lps reef. I took PAR readings with everything on, return pump is a DC12000 running at 3000+ gph, a CL with a 2nd DC12000 at 3000+ gph and thru an Ocean Motion with 3 out of 4 lines open, and finally, 2 Jebao WP40 wave makers set to make waves at full power. And the return from the sump has 2 lines with anti-siphon. The holes for the anti siphon is just below the waterline and they have John Guest elbows threaded at one end and screwed into the 3/4" return line. They can be adjusted up or down to get the siphon break ASAP or after the water level goes down about 2". I have them turned just above horizontal so I get a fairly quick siphon break and I get a lot of fine water movement on the surface. That mixes with the waves created by the Jebao WP40 to make a fairly turbulent surface to the water.

OK, all that explaining to say that With everything running I got PAR readings of 400-410 at 3" deep and 180 to 200 at 20" deep. I turned off the CL which had very little effect on the surface agitation. Next I turned off the WP40's and let the waves settle out. The anti-siphon pipes were still creating a nice amount of small water movement on the surface, so I turned them straight down. In a couple of minutes there was a very smooth water surface. PAR reading didn't change at all. 400 to 410 at 3" and 180 to 200 at 20".

Short answer, the wave action in a 180g tank had virtually no affect on the PAR readings.

 

[jdamon]

It may not change the PAR, but it could change the incidence angle, so maybe it's not blasting the coral from the same direction at all times?

 

[Pete_the_Puma]

Wow Thanks for running the test Ron,


I have to say I am incredibly surprised by those results. I think the laws of optics would dictate some level of difference, I would have guesstimated at 20-30% difference in light intensity.

I do not know enough about PAR vs whole light spectrum and guess I will need to educate myself. I'm also going to try to get my hands on a PAR meter to replicate this.


Pete

 

[Ron Reefman]

The surface agitation does cause light to shift and bend, and that's what makes for 'shimmer' on the surfaces of sand, rock and corals. But the light is still going down to the bottom one way or another. It's not being deflected at such an angle that it doesn't still get in the water. Maybe big waves in the ocean would have more affect on the light, but in our aquariums, it's just not enough angular change in the light to have a big impact... IMHO.