# intensity loss after raising fixture?



## davemonkey (Mar 29, 2008)

I have a 2x65 Watt Coralife PC sitting over my 50 gal (48x12x18 )...about 2.6 WPG.

I also had 2x15 watt normal flourescent tubes that came with the tank (no idea what kind of light, it just said "Aquarium Bulb, 15 Watt" )...don't know how much light intensity the accounted for, maybe another 0.3 - 0.6 WPG.

I had a glass cover over the tank and the PC fixture sat on 1/4" rubber feet where the actual bulb was about 2" from the surface of the water (and was filtered through the glass). The other lights were probably the same distance from water surface.

I just put 2" tall legs on the PC fixture and removed the glass top, and did away with the 2x15's. So, now the bulbs are about 4" from the water surface and light only has to pass through the acrylic fixture cover.

1. Should I still assume I'm getting 2.6 WPG? 
2. Any idea how much my intensity is reduced by raising the fixture?
3. If it were you, would you bother trying to adjust ferts for this change?

-Dave


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## Bunbuku (Feb 10, 2008)

davemonkey said:


> 2. Any idea how much my intensity is reduced by raising the fixture?
> 
> -Dave


Assuming a point source, the intensity of light decreases by the square of the distance (1/r^2).


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## davemonkey (Mar 29, 2008)

Bunbuku said:


> Assuming a point source, the intensity of light decreases by the square of the distance (1/r^2).


Forgive my lack of understanding, but does that mean the distance from bottom of tank to light, or is this the distance by which I moved the lights?

I'm guessing that to get a true answer I would have needed a light measurement to start with? Hoppy, ya wanna take a trip to Abilene and show me how that light meter works?


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## bartoli (May 8, 2006)

davemonkey said:


> I'm guessing that to get a true answer I would have needed a light measurement to start with?


You may want to try HydroFarm's Light Intensity Meter:

http://www.amazon.com/Hydrofarm-Light-Meter/dp/B000BWCIZW


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## Bunbuku (Feb 10, 2008)

davemonkey said:


> Forgive my lack of understanding, but does that mean the distance from bottom of tank to light, or is this the distance by which I moved the lights?
> 
> I'm guessing that to get a true answer I would have needed a light measurement to start with? Hoppy, ya wanna take a trip to Abilene and show me how that light meter works?


It's the distance to the "detector". For example, if you moved your detector 1 unit of distance to 2 units from the light source the intensity would decrease by 1/4th.


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## hoppycalif (Apr 7, 2005)

If you are only interested in the intensity at the substrate, where a carpet plant would grow, raising the fixture 2 inches should affect the intensity as follows:
Assume you have 4 inches of substrate. The original distance from the substrate to the light bulbs was 18" + 2" - 4" = 16". You raised it two inches, so it is now 18" above the substrate. The intensity at the substrate will be between [16/18] squared, or 80% and, 16/18 or 90% of what it was before. You had 160 watts and 50 gallon, for 3.2 watts per gallon. Now you have the equivalent of 130 times .85 [from raising the lights] over 50 gallons, or 2.2 watts per gallon. This is only at the substrate. The reduction in intensity at the top of the tank is a lot more, but our tanks always have far too much light there anyway.


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## davemonkey (Mar 29, 2008)

hoppycalif said:


> If you are only interested in the intensity at the substrate, where a carpet plant would grow, raising the fixture 2 inches should affect the intensity as follows:
> Assume you have 4 inches of substrate. The original distance from the substrate to the light bulbs was 18" + 2" - 4" = 16". You raised it two inches, so it is now 18" above the substrate. The intensity at the substrate will be between [16/18] squared, or 80% and, 16/18 or 90% of what it was before. You had 160 watts and 50 gallon, for 3.2 watts per gallon. Now you have the equivalent of 130 times .85 [from raising the lights] over 50 gallons, or 2.2 watts per gallon. This is only at the substrate. The reduction in intensity at the top of the tank is a lot more, but our tanks always have far too much light there anyway.


YES!! Thank you hoppy! \\/ That's exactly what I needed to know. So, I still have plenty of light for the plants I'm growing (they are all medium light tolerant or lower). Thanks bunbuku and bartoli for your info as well.

Just as a side note, I have always had "too warm" water (80 degrees F, without a heater). With the glass cover removed and the light raised, the temp has dropped to 77, just about where I want it. Now all I have to do is keep my 3 yr old daughter from throwing pizza into the aquarium now that I've removed the glass cover.

-Dave


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