Height vs Intensity???

[chaseracing]

I am curious.....How different will the light intensity be for the different height in which you can hang your lights?

Is there a huge difference between 12" vs 8" off of the water?

Has anyone gone higher and lower and noticed a significant difference?

Thanks!

-=E=-

If it matters, I am thinking about the difference in a halide.

[GSMguy]

THERE WOULD BE A NOTICABL DIFFERENCE WITh a 4 inch drop i had my halides at 14 inches for the first couple weeks then lowered them to about 12 and now im going to 8 to stay

[CyclistMT]

Yes there can be a huge difference in the intensity of the light especially from a MH light source. How much difference is open to interpretation and if you don't use some sort of light meter you can only speak in generalizations not specifics. This is because water conditions will cause differences from tank to tank (yellowing agents, turbidity, use of ozone, etc.).

That being said, I have read that intensity can change anywhere from 25% - 50% by doubling the distance to the light source. I'm sure the range is due to different tank conditions as physics dictates a specific amount. As a point of reference, here is a link to an article on the subect and an excerp that states the law governing this:

The Properties of Light (http://reefkeeping.com/issues/2002-03/atj/index.php)

"The inverse square rule states that the light intensity will be in inverse proportion to the square of the distance from the light source. That is, if you double the distance from the light source, the intensity will be reduced to 25%. Figure 1 shows the inverse square rule in practice."

I hope that helps!

[chaseracing]

That is great information thanks. This is what I expected becuase I just installed my lighting setup on my 150, and I test drove the system on a 55 gal. When I put the lights on the 55, they were about 4" above the water and the lighting was so intense that the corals actually closed up. The lights are 12" above water on the 150.

Now I have the lights installed it is bright, but not what I would call "intense". I actually have some concerns on whether the lights would be bright enough for SPS.

I am going to lower them and see what happens.

Thanks!

-=E=-

[GSMguy]

what lights are you using what reflectors all that
??????

[chaseracing]

250w. Coralife with standard? reflectors. 2"x36"
Using ARO ballasts.

-=E=-

[chaseracing]

Quick Update:

I moved the lights down 4" and it made quite a significant difference. It now looks like I always pictured it to look.

Thanks for the info!

-=E=-

[GSMguy]

glad to hear it i do know what you mean about the light enterig the tank in that one perfect way

[pjf]

Here’s a good rule of thumb. Measure your tank from front to back. Divide that distance by 4. Mount your MH bulb at least that distance above the water. For example, if your tank is 24” from front to back, your MH bulb should be 6” or more above the water’s surface.

Below that height, some of your light will strike that water at greater than 60 degrees from vertical and pass through the upper portion of your tank. To insure total internal reflection (TIR), your bulb needs to be above that height.

You can easily tell if the MH bulb is too low. You will see “glitter lines” on the floor if the MH is the only light in the room. Commonly, you will see glitter lines on the floor next to the side of your tank. If you also see them on the floor in front of your tank, raise your lights.

You should mount your bulb higher until it begins to shoot over the tank rim and light up your room. Recessing a bulb inside a big reflector (Lumenarcs) helps to direct light into your tank even when the fixture is raised higher.

As for the inverse square law (1/r^2), it does apply to a MH bulb but not to its reflected light. Good parabolic reflectors reflect light rays that are almost parallel to each other. For tube lighting, the inverse law (1/r) is more appropriate. Individual tube reflectors will direct much of the reflected light downwards.

Dispersion is less when light enters the water. When the rays strike the water, they are bent closer to vertical so most of the light is refracted straight down.