# Lighting efficiency: How much light actually makes it into the tank?



## Gomer (Feb 2, 2004)

This question is spurred on by a light project of mine.

I have a Tek light fixture and typically run 2x39T5HO GE Starcoats. These bulbs (ea)put out about ~3300 lumens/ ~92 lumens per watt. However, I HIGHLY doubt anything near 6600 lumens makes it into my tank. Infact, there is probably a ton of spill light and restrike loss.

Does anyone have a feel for what % of the light we actually get TO our plants? Has anyone tried mapping out the Lux (lumens per watt) for their tank to get this info?


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## Zapins (Jul 28, 2004)

http://www.thekrib.com/Lights/lights-wozniak.html

Pretty much everything here.


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## Brilliant (Jun 25, 2006)

I am interested in this too. I have all types of lighting I am just waiting for a certain person to come over with lux meter...ahem!...Jim if you see this cmon over bring that meter and pickup some of those A. bitaeniata. 

Ive been waiting patiently to brag about...opps share my results.


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## ruki (Jul 4, 2006)

I would like to see this too. I've tried more than a few fluorescent fixtures, and my 48 inch TekLight is the best so far, but I'd like to see exactly how much better it is.

The link to the Krib article, is useful, but is 10 years old and doesn't use modern fixtures or water in the tank.

It would be nice to do something similar to this, in the a water filled 75 gallon plant tank, switching between a number of commercially available fixtures. Also, one needs to measure the total power consumed by the fixture (with an actual meter) so one can compare them against each other.


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## Brilliant (Jun 25, 2006)

Yup take your pick I got NO, PC, HQI, T5, T5 HO...and a deep 24" tank...
I can hardly wait I might just buy one of these meters as a gift to myself.


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## hoppycalif (Apr 7, 2005)

It isn't the type of bulb that is important, but the reflector that is used. With a good enough reflector nearly all of the light makes it to the bottom of the tank. And, with no reflector much less than half does. One way to judge it is to look at the tank in a darkened room. That way you can see how much light is spilled out, through the glass, out the ends, etc. It aint science, but it is a good estimate. You can do quasi-science by then measuring the footprint of the light going to the floor compared to the tank cross section, which is a crude estimate of the percent of light going to the plants.


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## Dany (Oct 28, 2006)

This was done by a member of aquariumpros. Here's the link 
http://www.aquariumpros.ca/forums/showthread.php?t=23934&highlight=t5ho+lux



> I had a little time on my hands today, so I pulled out the light meter just to see how much punch these T5HO's really have over Power Compacts.
> 
> Here are the results:
> 
> ...


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## Zapins (Jul 28, 2004)

Interesting experiment, but the brands of the bulbs definitely matters. I don't know of many knockoff T5 bulbs (yet) but I know of dozens of crappy PC bulb companies like jebo bulbs vs coralife bulbs. So I think it really makes a heck of a difference when comparing the two kinds of lighting. 

Furthermore, T5 and PC are the same kind of lighting, except PC are bent, and T5 are straight, so such a huge difference seems odd to me.

You can calculate how efficient a reflector's shape is by taking into account the fact that an incoming ray of light bounces off a mirror at the exact same angle as it hits it. so if a light ray hits a mirror from a 30º angle it will leave the point of impact on the mirror at the same angle. If you draw a diagram of the reflector and start drawing straight lines and use a protractor for measuring correct angles you can get a pretty good idea of how much light re-strike there is from the light hitting the bulb surface again after bouncing off the reflector. This might take if drawn by hand, and most people probably wouldn't be interested in drawing it out anyway. Just a little FYI on how reflector efficiency is calculated.


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## hoppycalif (Apr 7, 2005)

Ray tracing, as Zapins suggested, is a lot of fun and very educational. Just remember that fluorescent type bulbs radiate from the inside surface of the glass envelope, which is coated with phosphors. That means every spot on the surface of the bulb radiates light over very nearly a half sphere. In other words you can't substitute a line for the bulb and trace rays from that line. It gets complicated, but that doesn't reduce the fun.


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## ruki (Jul 4, 2006)

hoppycalif said:


> Ray tracing, as Zapins suggested, is a lot of fun and very educational. Just remember that fluorescent type bulbs radiate from the inside surface of the glass envelope, which is coated with phosphors. That means every spot on the surface of the bulb radiates light over very nearly a half sphere. In other words you can't substitute a line for the bulb and trace rays from that line. It gets complicated, but that doesn't reduce the fun.


That's a very good point. For tubes, this means that there is enough "slop" that even if it looks like all the light will be sent straight down, some won't. That's OK if it's small, since this will act as fill.



danny said:


> ... interesting data ...


Please update this to include a description of the reflector. For example, I'd expect a TekLight to totally kick but over an Odyssea T5HO; partly due to using a more appropriate ballast but mostly due to the better reflector.

I'd also expect to see a small 10-to-20 percent difference between T5 HO and PC when both use similar poor shiny box reflectors, since PC blocks more light, and wastes some light by illumining itself. But predict that small difference would get much larger with a T5 optimal reflector.

Those are only expectations/predictions which can be proven/dissproven by observation.

The data I've read indicates that longer 4-foot tubes are a bit more efficient than shorter 2-foot tubes, so I'd just about bet money that your approximately 24 inch long T5 HO light has a much better reflector than for the 39 watt light.


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