Reducing light by raising height?

[FacePlanted]

When talking about reducing one's lighting or WPG for a tank that is overlit, will raising the height that the fixture hangs above the tank accomplish this?

I ask this because I acquired an outer orbit fixture that has CF (2 x 65w) and
MH (1 x 150w) and LED lights and it is about 4 times too much light for the 29 gal tank that I have. I need to use this fixture because it is the only one I have and the overspill of the light into the room is not a concern because the room is dark and needs to be lit. I like the shimmering and intensity from the MH, but I need the coverage of the CF lamps.

My thinking was that I could just hang the fixture WAY above the tank and therefore the light hitting aquarium would be would be about right.

Is there some way to know how much light the tank is receiving based on the total wattage, tank dimensions, and the height above the substrate or water level?

My fixture is currently about 25 inches above the surface of the tank and 38-40 inches above the substrate.

Will this work to reduce the amount of light in the tank to reasonable levels?
I would rather keep raising the fixture than replace it or use less bulbs (though I could unhook one of the CF bulbs).

Should I raise the fixture even higher?

I am 4 days into a redone 29 gal. tank with aquasoil, this light fixture at the mentioned height, pressurized co2 with a green drop checker and just starting the EI dosing, plus water changes every couple of days......yet I am already starting to get algae threads all over. Its gotta be the light at this point, right?
I'm trying to crank the co2 since there are no fish/livestock in the tank at the moment because of ammonia & nitrite. The tank is still cycling right now, but is proceeding quickly because I used mulm in the substrate, and a small established internal filter to supplement the lifeguard mechanical/chemical combo filter filled with activated charcoal until that gets going also.
----anyways, I'm getting off on a tangent-------

sorry for such a long post......this is just a question that has bugged me for a long time and it seems that I cannot find a direct answer to the hanging light question.

thanks a bunch,
-mike b-

 

[VaughnH]

I can't answer the question, but here is how I would find an answer it I had that problem. Lower the fixture to about 6" above the water. Turn on the lights and in a darkened room measure the approximate length and width of the light pattern at the substrate level - hold a card at that level parallel to the substrate and see where the approximate edge of the light is. Do the same on all 4 sides, then calculate the area of the lighted rectangle. Raise the lights to where you want them and repeat. The light intensity in watts per gallon will be the area with the lights low divided by the area with the lights high times the actual wattage of the bulbs divided by the tank volume. If you do this please let us know your results.

 

[FacePlanted]

That is very interesting and could be the exact answer I need to confirm the height I need for this fixture.
I will try to do this, but the area of light cast by the fixture is fairly large and am not exactly sure I could distinguish the edges of the light cast by the lamp.

I posted this thread because a while back I found some random article about lighting and intensity and the amount needed for a planted tank.
It gave a formula to use to find the total watts needed for a particular tank size, at a certain height, based on a low-med-high light tank/plants.
This is recalled from what I wrote down in my notebook and are not exact quotes, but it said:

- Multiply the surface area(SA) of the aquarium (all in inches) by the
distance from the light source to the top of the gravel (H).

- Then multiply this by one of these constants (P):
-low light plants: .08
-low-med light plants: .12
-med-bright light plants: .18
-bright light plants: .27

- This total would equal the ideal watt-hours of flourescent lighting needed.
And dividing this number by 11 gives the approx. total wattage of lights
needed for the particular surface area, height, and type of plants

- you would then try to match this total wattage with a number of fl. tubes
(ex. formula gives 105 total watts - use 5 20watt tubes for an Actual Total
Wattage of 100 watts)
Dividing the watt-hours by the actual total wattage would give the
lighting duration at this level.

this is the equation as I read it:
SA * H * P
------------- = Total Wattage Needed
11

SA * H * P = ideal watthours needed

I thought that the normal WPG rule was for lights pretty much sitting on top of a tank. So H in that equation would JUST BE THE HEIGHT OF THE TANK AND NO MORE.
My tank dimensions are 30"wide X 12" deep X 17" high
I tried to check the ability to calculate WPG using this equation and the constants for P by using my 29 gal. tank as an example (30"*12")for SA and H being directly on top of the tank and no more (17")
I then plugged in each one of the constants (P) for each type of plants and found that when i divided the total wattage given by the equation by the 29 gallons of my tank (thus the watts-per-gallon) that the particular constants for P gave:
low light plants to be 1.48 WPG
low-med: 2.25 wpg
med- bright: 3.33 wpg
bright: 5.17 wpg
this seems to be about right, but a little high for each category. However, the constants for P could easily be reduced and checked again to get more like 3 wpg for bright, 2 for med, etc.
So it seemed that the equation worked and would help determine the height I needed if I plugged in the 280 total watts that I have and the other variables, and solved for H.

If i wanted 2.25 wpg in this tank (which many here say is plenty to grow any kind of plant, even high light ones) I would use P=.12 And solving for H using the total of my 280 watt fixture would equal like 71.29.
So it would seem like I need to hang the 280w fixture 71.29 inches above the substrate to equal 2.25 WPG in my tank.

Even if this is not exactly correct, a general approximation would be sufficient for me, and when I raise the fixture to these heights, the brightness and intensity in the tank sure seem to be about right.

If I can try your method correctly, Vaughn, I could compare and see if they give similar heights/results.

I just dunno if an equation that simple could yield accurate results (assuming an appropriate value for P is used).
maybe others could plug in their variables and known wattages/wpg, etc. and see if the variables/constants match up.
Any thoughts on this would be eagerly welcomed.
If I happen to come across the article that I found all this in, I will post the link. But it was months ago, and very obscure - so I doubt it.

thanks,
-mike b-

 

[FacePlanted]

OMG.....Duhh.....I found the article where I got this equation from.....
Its at the Krib, under plant lighting FAQ

here is the link to the page:
FAQ: Plant Lighting

check it out........
can I use this to determine how high to hang my lights, regardless of the total watts of the fixture?

on a side note: is it possible to use 36w CF tubes in the 65watt square pin sockets? I know that it is ok to put the 55w bulbs in them, but is 36w too low?

switching to 2 36 watt tubes would allow me to hang my fixture MUCH lower.

 

[VaughnH]

All ballasts are not equal, but I use 36 watt bulbs in my AH Supply fixture which is made for 55 watt bulbs. It works fine. As far as the lighting goes, it isn't any where near an exact science. Whatever method you use will just give you a good starting point for what you need. You can also assume the light intensity falls off with the square of the distance, and adjust according to that. It will be fairly accurate if your reflectors aren't very good. If you had perfect reflectors you would get very little drop off in intensity with distance.

 

[Tom Barr]

Remember the inverse square law for light.
The light decreases the square of the distance.

So it's a bit like a log scale curve for decreasing intensity.

That does not include glass lid, water and other things, just plain old air.
Regards,
Tom Barr

 

[VaughnH]

Remember the inverse square law for light.
The light decreases the square of the distance.

So it's a bit like a log scale curve for decreasing intensity.

That does not include glass lid, water and other things, just plain old air.
Regards,
Tom Barr

Light only decreases with the square of the distance from it if the light is emitted equally in all directions, such as from a point source. If you have a perfect parabolic reflector, an impossibility, the light wouldn't fall off at all with distance other than by absorption and diffusion by the air/water. So, a light with a terrible reflector will come close to dropping off as the square of distance directly under the light.

 

[aquabillpers]

Note that the term "fluorescent" can refer to at least 4 different kinds of bulbs.

Here, I am told, the term usually refers to CF lighting. The cited Krib article refers to T12's, I believe.

Bill

 

[VaughnH]

Just a little logic shows you that the closer you are to a light source, compared to the size of the light source, the greater the deviation from the inverse square relationship of intensity to distance. If you have a 4 foot by 2 foot source of light over a 4 foot by 2 foot top surface area tank, and it is only a few inches above the top, the drop off in intensity will be far less than an inverse square relationship. It could easily be a few percent per inch of distance. That is because as you get further away you get light from portions of the source that are further to the side, which offset the spread of light from that directly above. But, if you are far away (like 100 times the diameter of the light source) from any light the drop off is proportional to the square of the distance. I still like the idea of approximating the area of the cone of light at the two distances, then using the ratio of those areas to approximate the drop off.

 

[FacePlanted]

Bill, your right about the T-12s....and I'm lumping together CFs and a MH.....so I'm already "bending" the rules there....

All I really do need is an approximation or a starting place just so I know I'm not WAY off on what the light in my tank is at, and so I can see that I am not getting too much. I just wanted to be on the right track.

The intensity dropping off in proportion to the square of the distance does make sense and I believe the reflectors in my fixture are very basic so as not to skew this. However, I cant think right now of how to apply this with actual numbers and units to my lights and tank.

That is ok, because Vaughn I like your method above because I can actually see how this can find the different WPGs at relative heights. It makes sense to me, and I can picture in my head how it works. And I can put some numbers to it and compare how things measure out. The only downside for me is that there really isnt any overhang of the reflectors past/below the bulbs and the light just flies out and spills all over the room. The MH directs a beam of light, but the CFs throw more of an ambient, all over kinda light. I think it would be a careful task finding the edge/cross section of their "beam"...at any height.
Maybe I could put it REALLY low and see if I can find the edges that way, and then slowly raise it up and keep my eye on where the edge is at.
With the equation from the Krib, I dont really see how/why it works or where it came from, thus I cannot guess at its accuracy. But it is easy to use and I only need to plug in numbers of measurements I already know or that are easy to find.

I guess I cannot really compare the two methods' results until I try yours, Vaughn. When I do, or if I can, I will post back and say whether they came up with similar heights to hang the fixture.

Thanks for everyone's help and responses, I really appreciate it. Its nice to finally know what to look at when trying to figure this out, and I have enjoyed the conversation.

To all a good night,
-mike b-